Volume 58, Issue 5-6
May 2002, pages 713-1229
pp 713-714 May 2002
pp 715-723 May 2002 Strongly Correlated Electron Systems
The transport properties of the semimetallic quasi-one-dimensional S=1/2 antiferromagnet Yb4As3 have been studied by performing low-temperature (T≥0.02 K) and high magneticfield (B≤60 T) measurements of the electrical resistivity ρ(T, B). For T ≿ 2 K a ‘heavy-fermion’-like behavior Δρ(T)=AT2 with huge and nearly field-independent coefficient A ≈ 3 µΩ cm/K2 is observed, whereas at lower temperatures ρ(T) deviates from this behavior and slightly increases to the lowest T. In B>0 and T ≾ 6 K the resistivity shows an anomalous magnetic-history dependence together with an unusual relaxation behavior. In the isothermal resistivity Shubnikov-de Haas (SdH) oscillations, arising from a low-density system of mobile As-4p holes, with a frequency of 25 T have been recorded. From the T- and B-dependence of the SdH oscillations an effective carrier mass of (0.275±0.005)m0 and a charge-carrier mean-free path of 215 Å are determined. Furthermore, in B≥15 T, the system is near the quantum limit and spin-splitting effects are observed.
pp 725-730 May 2002 Strongly Correlated Electron Systems
The temperature dependence of the rare-earth Δ2-bulk band has been regarded as an exemplary case which realizes the simple Stoner behavior. We examined the evolution of GdΔ2 bulk bands with temperature in the range 0.5≤T/TC≤1 with spin-resolved, photoemission spectroscopy. The direct observation of the spin-dependent spectral line shapes reveals a complex temperature dependence and manifests a clear inadequacy of the Stoner model to the description of the magnetism in rare earths.
pp 731-736 May 2002 Strongly Correlated Electron Systems
YbCu5−xAlx provides the possibility to tune ground state properties by a change of the valence due to the Cu/Al substitution, by pressure as well as by the application of a magnetic field. Near to the critical concentration xcr≈1.5 non-Fermi-liquid properties (NFL) are obvious, obeying hyperscaling. If magnetic order sets in for x>1.5, the application of moderate magnetic fields quenches order and again NFL features become evident. Hyperscaling in this case indicates strongly interacting spin fluctuations.
pp 737-742 May 2002 Strongly Correlated Electron Systems
Electron-doped metallic states of Ca1−xYxVO3 and Ca1−xYxTiO3 change into non-metallic states around x∼0.4 and 0.6, respectively. The residual resistivity in the metallic states increases with increasing effective magnetic moment or coefficient of T2 term of resistivity. The effective moment reaches ∼ 0.5 µB/molecule in Ca1−xYxVO3 and also in Ca1−xYxTiO3 near the metal-nonmetal phase boundary. In these metallic states. ∼ 10% of 3d atoms seem to have large localized magnetic moments. In electron-doped metallic sample of Ca1−xYxVO3, the temperature dependence of resistance shows no resistance-minimum. However, weak negative magneto-resistance is observed for the sample with x=0.2 up to 50 Tesla at 4.2 K.
pp 743-754 May 2002 Strongly Correlated Electron Systems
Zero-temperature magnetic phase transitions exhibit an abundance of nearly critical magnetic fluctuations that allow to probe the traditional concepts of the metallic state. For the prototypical heavy-fermion compound, CeCu6−xAux, a breakdown of the Fermi-liquid properties may be tuned by Au concentration, hydrostatic pressure, or magnetic field. The d-electron weak itinerant ferromagnet ZrZn2, on the other hand, was recently found to display superconductivity in coexistence with ferromagnetism.
pp 755-760 May 2002 Strongly Correlated Electron Systems
NMR and NQR studies on two interesting systems (URu2Si2, CeTIn5) were performed under high pressure. (1) URu2Si2: In the pressure range 3.0 to 8.3 kbar, we have observed new 29Si NMR signals arising from the antiferromagnetic (AF) region besides the previously observed 29Si NMR signals which come from the paramagnetic (PM) region in the sample. This gives definite evidence for spatially-inhomogeneous development of AF ordering below T0 of 17.5 K. The volume fraction is enhanced by applied pressure, whereas the value of internal field (∼91 mT) remains constant up to 8.3 kbar. In the AF region, the ordered moment is about one order of magnitude larger than 0.03 µB. (2) CeTIn5: The pressure and temperature (T) dependences of nuclear spin-lattice relaxation rate 1/T1 of 115In in CeTIn5 have shown that the superconductivity (SC) occurs close to an AF instability. From the T dependences of 1/T1 and Knight shift below Tc. CeTIn5 has been found to exhibit non-s wave (probable d wave) SC with even parity and line nodes in the SC energy gap.
pp 761-767 May 2002 Strongly Correlated Electron Systems
The formation of induced 5d magnetic moment on Ir in Fe100−xIrx (x=3, 10 and 17) and Co100−xIrx (x=5, 17, 25 and 32) alloys has been investigated by X-ray magnetic circular dichroism (XMCD) at Ir L2,3 absorption edges. Sum rule analysis of the XMCD data show that the orbital moment of Ir is in the range of −0.071(2)µB to −0.030(1)µB in Fe-Ir alloys and −0.067(2)µB to 0.024(1)µB in Co-Ir alloys. We find that the total moment of Ir in Fe-Ir alloys is approximately 1/5 of the total 3d moment on Fe at all the three compositions. In contrast, the total moment on Ir in Co-Ir alloys varies between 1/6 to 1/16 of the 3d moment on cobalt. The observed trends of Ir moments and the role of interatomic exchange interactions in 5d moment formation are discussed.
pp 769-771 May 2002 Strongly Correlated Electron Systems
Effect of Al substitution on the magnetic properties of Ce(Ga1−xAlx)2 (x=0, 0.1 and 0.5) system has been studied. The magnetic state of CeGa2 is found to be FM with a TC of 8 K, whereas the compounds with x=0.1 and 0.5 are AFM and possess TN of about 9 K. These two compounds undergo metamagnetic transition and the critical fields are about 1.2 T and 0.5 T. respectively at 2 K. These variations are explained on the basis of helical spin structure in these compounds.
pp 773-776 May 2002 Strongly Correlated Electron Systems
The magnetic-field-induced valence transition in rare-earth systems has been investigated using the periodic Anderson model supplemented by the Falicov-Kimball term. This model has been solved by first decoupling the Falicov-Kimball term as proposed by Khomskii and Koharjan and then taking the limit of infinite intra-site Coulomb repulsion. The valence transition both in the absence and in the presence of magnetic field as a function of temperature is studied. It has been found that the system makes transition from non-magnetic to magnetic state when the magnetic field increases beyond a critical value Hc. The phase boundary defined in terms of reduced field Hc(T)/Hc(0) and reduced temperature T/Tv (Tv being the valence transition temperature in the absence of field) is almost independent of the position of the localized level. The results are in qualitative agreement with experimental observations in Yb- and Eu-compounds.
pp 777-782 May 2002 Strongly Correlated Electron Systems
Recently, the solid solution Ce2Au1−xCoxSi3 has been shown to exhibit many magnetic anomalies associated with the competition between magnetic ordering and the Kondo effect. Here we report high pressure electrical resistivity of Ce2AuSi3, ac susceptibility (X) and magnetoresistance of various alloys of this solid solution in order to gain better knowledge of the magnetism of these alloys. High pressure resistivity behavior is consistent with the proposal that Ce2AuSi3 lies at the left-hand side of the maximum in Doniach’s magnetic phase diagram. The ac X data reveal that there are in fact two magnetic transitions, one at 2 K and the other at 3 K for this compound, both of which are spin-glass-like. However, as the Co concentration is increased, antiferromagnetism is stabilized for intermediate compositions before attaining non-magnetism for the Co end member.
pp 783-786 May 2002 Strongly Correlated Electron Systems
Kβ-to-Kα X-ray intensity ratios of Fe and Ni in pure metals and in FexNi1−x alloys (x=0.20, 0.50, 0.58) exhibiting similar crystalline structure have been measured following excitation by 59.54 keV γ-rays from a 241Am point source, to understand as to why the properties of permalloy Fe0.2Ni0.8 is distinct from other alloy compositions. It is observed that the valence electronic structure of Fe0.2Ni0.8 alloy is totally different from other alloys which may be attributed to its special magnetic properties.
pp 787-789 May 2002 Strongly Correlated Electron Systems
ac Conductivity measurements are carried out across the metal to insulator transition in NiAl0.7Cr0.7Fe0.6O4. The low frequency data is analyzed using Summerfield scaling theory for hopping conductivity. The exponent of the scaling behavior has significantly different values in the conducting and insulating regimes. The hopping frequency and the zero frequency conductivity are found to increase with temperature, slowly in the metallic regime and rapidly in the insulating regime.
pp 791-797 May 2002 Superconductivity
The effect of substitutional disorder on the superconducting properties of YNi2B2C was studied by partially replacing yttrium and nickel by Lu and Pt, respectively. For the two series of (Y, Lu)Ni2B2C and Y(Ni, Pt)2B2C compounds, the upper critical field Hc2(T) and the specific heat cp(T, H) in the superconducting mixed state have been investigated. Disorder is found to reduce several relevant quantities such as Tc, the upper critical field Hc2(0) at T=0 and a characteristic positive curvature of Hc2(T) observed for these compounds near Tc. The Hc2(T) data point to the clean limit for (Y, Lu) substitutions and to a transition to the quasi-dirty limit for (Ni, Pt) substitutions. The electronic specific heat contribution γ(H) exhibits significant deviations from the usual linear γ(H) law. These deviations reduce with growing substitutional disorder but remain even in the quasidirty limit which is reached in the Y(Ni1−x, Ptx)2B2C samples for x=0.1.
pp 799-802 May 2002 Superconductivity
Microwave absorption studies have been carried out on MgB2 superconductor using a standard X-band EPR spectrometer. The modulated low-field microwave absorption signals recorded for polycrystalline (grain size ∼ 10 µm) samples suggested the absence of weak-link character. The field dependent direct microwave absorption has been found to obey a ✓H dependence with two different slopes, which indicated a transition from strongly pinned lattice to flux flow regime.
pp 803-808 May 2002 Superconductivity
A metal-insulator crossover appears in the experimental data for in-plane resistivity of underdoped cuprates and a range of superconducting cuprates in the presence of a strong magnetic field suppressing superconductivity. We propose an explanation for this phenomenon based on a gauge field theory approach to the t-J model. In this approach, based on a formal spin-charge separation, the low energy effective action describes gapful spinons (with a theoretically derived doping dependence of the gap ms2 ∼ δ¦ ln δ¦) and holons with finite Fermi surface (ɛF ∼ tδ) interacting via a gauge field whose basic effect on the spinons is to bind them into overdamped spin waves, shifting their gap by a damping term linear in T, which causes the metal-insulator crossover. The presence of a magnetic field perpendicular to the plane acts by increasing the damping, in turn producing a big positive transverse in-plane magnetoresistance at low T, as experimentally observed.
pp 809-815 May 2002 Superconductivity
Both RuSr2GdCu2O8-δ (Ru-1212) and RuSr2Gd1.5Ce0.5Cu2O10-δ (Ru-1222) exhibits magnetism and superconductivity, as seen by magnetization vs. temperature behavior measured in 5 Oe field. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization data show branching at around 140 K and 100 K with a cusp at 135 K and 80 K and a diamagnetic transition around 20 K and 30 K in the ZFC part, for Ru-1212 and Ru-1222, respectively. The isothermal magnetization possesses a non-linear contribution due to a ferromagnetic component at low temperatures below 50 K for both samples. The resistance vs. temperature behavior of the samples in applied fields of 0, 3 and 7 T confirmed superconductivity, with a different type of broadening of the superconductivity transition under magnetic fields for Ru-1212 from that known for conventional high-Tc superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature at 135 K. Below the Ru magnetic ordering temperature, MR displays a positive peak at low fields and becomes negative at higher fields for Ru-1212. For Ru-1222, MR remains negative both above and below the ordering temperature. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for the Ru-1212 sample shows two types of superstructure: one has a weak spot at the centre of the a–b rectangle, and the other only along the b direction. Interestingly, Ru-1222 shows only clean a–b and a–c planes, without any superstructures.
pp 817-825 May 2002 Superconductivity
We have measured the in-plane resistivity of Bi2Sr2CaCu2O8+δ and Tl2Ba2 CaCu2O8+δ single crystals in the temperature range 70–300 K. The thermodynamic fluctuations in the conductivity of both the samples start around ∼ 125 K. We find the Lawrence and Doniach  model to be inadequate to describe the fluctuation conductivity in these materials. The modification suggested by Ramallo et al  where by the conductivity is enhanced due to the presence of two superconducting layers in each unit cell is also not adequate. We suggest the fluctuation conductivity to be reduced due to the reduction in the density of states (DOS) of the quasiparticles which results due to the formation of Cooper pairs at the onset of the fluctuations. The data agrees with the theory proposed by Dorin et al  which takes into account this reduction in DOS.
pp 827-837 May 2002 Superconductivity
We review the observation of novel charge density wave (CDW) transitions in ternary R5Ir4Si10 compounds. A high quality single crystal of Lu5Ir4Si10 shows the formation of a commensurate CDW along c-axis below 80 K in the (h, 0, l) plane that coexists with BCS type superconductivity below 3.9 K. However, in a single crystal of Er5Ir4Si10, one observes the development of a 1D-incommensurate CDW at 155 K, which then locks into a purely commensurate state below 55 K. The well-localized Er3 moments are antiferromagnetically ordered below 2.8 K which results in the coexistence of strongly coupled CDW with local moment antiferromagnetism in Er5Ir4Si10. Unlike conventional CDW systems, extremely sharp transition (width ∼ 1.5 K) in all bulk properties along with huge heat capacity anomalies in these compounds makes this CDW transition an interesting one.
pp 839-841 May 2002 Superconductivity
The nominal (Hg1−xRex)Sr2CuO4+δ (x=0.10 and 0.20) samples were synthesized at ∼ 920°C in partial vacuum. The compound with x=0.10 exhibits superconductivity at ∼ 54 K while the composition x=0.20 is non-superconducting down to 5 K. On cooling below 10 K in an applied field of 4 kOe, the former causes a noticeable upturn in the field cooled (FC) magnetization signal. Such a change in magnetic response is also reflected in the magnetic hysteresis loop generated at 9 K. We attribute this effect to a paramagnetic contribution arising from Re in (Hg,Re)-1201 phase.
pp 843-848 May 2002 Superconductivity
High-Tc superconductors are EPR silent but on a little deoxygenation of the high-Tc materials and their constituents, they yield rich but complex spectra. Spectra of (1) CuO, (2) BaCuO2, (3) CaCuO2, (4) Y2Cu2O5, (5) La2CuO4, (6) La2−xMxCuO4 (M=Sr, Ba), (7) Y based-123, (8) Bi based-2201, 2212, 2223, (9) Tl based-2223 and (10) Hg based-1212, 1223 have been studied. One thing common to all these materials is the CuO2 plane which gets fragmented on deoxygenation and the inherent antiferromagnetic coupling is partially destroyed which results in the appearance of the spectra.
The spectra recorded have been identified to be due to (1) Cu-monomer, (2) Cu-dimer, (3) Cu-tetramer, (4) Cu-octamer and (5) one signal at very low field which could not be identified because there was no structure in it and may be due to fragments higher than octamers. Very big fragments do not give any spectra because the original AF order probably remains intact in them. It is expected that when the fragments become magnetically isolated from the bulk, they produce EPR spectra. Most of the spectra have been analyzed and their spin-Hamiltonian parameters determined. The spectra of these species vary a little in terms of g-value and fine-structure splitting constant from sample to sample or even in the same sample and this may be attributed to some extra oxygen attachments retained with these species. Most frequently occurring species is the Cu-tetramer, (CuO)4. As (CuO)4 represents the unit cell of the all important two-dimensional CuO2 plane of the high-Tc materials, its spectra have been argued to provide some clue to the mechanism of high-Tc superconductivity. The tetramer (CuO)4 is a four one-half spin system and is essentially 16-fold degenerate by Heisenberg isotropic exchange, it is split into 6 components: one pentet, three triplets and two singlets. In superconductors the pentet appears to be the ground state and in the non-superconducting constituents the singlets seem to form the ground state as revealed by the temperature variation studies. In the case of La1.854Sr0.146CuO4 we have found the signature of quantum stripe formation. The high-Tc superconductivity theories involving spin bag, antiferromagnetic spin fluctuations and magnons can be explained on the basis of Cu-tetramers.
pp 849-853 May 2002 Superconductivity
Our well-recognized pseudopotential is used to investigate the superconducting state parameters viz: electron-phonon coupling strength λ. Coulomb pseudopotential μ*, transition temperature Tc, isotope effective exponent α and interaction strength N0V for the In1−xZnx and In1−xSnx binary alloys. We have incorporated six different types of local field correction functions, proposed by Hartree, Taylor, Vashistha-Singwi, Ichimaru-Utsumi, Farid et al and Sarkar et al to show the effect of exchange and correlation on the aforesaid properties. Very strong influence of the various exchange and correlation functions is concluded from the present study. The comparison with other such theoretical values is encouraging, which confirms the applicability of our model potential in explaining the superconducting state parameters of binary mixture.
pp 855-859 May 2002 Superconductivity
Recently, angle-resolved photoemission spectroscopy measurements on Bi2Sr2CaCu2O8+δ, which possesses two CuO2 layers in the same unit cell, have yielded very interesting results. For the overdoped samples, these results show a splitting of electronic states near k=(π, 0) point of Brillioun zone. On the other hand, no splitting is observed in the underdoped samples. In view of this, the detailed studies including the doping and temperature dependence of the spectral properties become desirable. In this paper, we consider cuprates possessing two CuO2 layers per unit cell. Each layer in the system is described by the t-t1-J model and the two layers are coupled via an intrabilayer hopping term (t⊥) and an intrabilayer exchange coupling (J⊥). A self-consistent perturbation approach is used to calculate the electronic spectral function for different values of hole density, hole momentum and temperature. We find that the imaginary part of the self energy is strongly momentum dependent which contradicts the suggestion that the Fermi surface of cuprates may be described by marginal Fermi liquid theory. We have calculated the spectral function for various values of intrabilayer parameters t⊥ and J⊥. For larger values of intrabilayer interactions we observe the splitting in the quasi-particle peak at k=(π, 0) which is in agreement with the recent observations. The splitting is also found to be sensitive to the hole concentration as well as the temperature of the system. We have also discussed the reasons why the splitting is absent in underdoped bilayer cuprates at low temperature.
pp 861-866 May 2002 Superconductivity
In the present work, we report the interplay of single particle and Cooper pair tunnelings on the superconducting state of layered high-Tc cuprate superconductors. For this we have considered a model Hamiltonian incorporating the intra-planar interactions and the contributions arising due to the coupling between the planes. The interplanar interactions include the single particle tunneling as well as the Josephson tunneling of Cooper pairs between the two layers. The expression of the out-of-plane correlation parameter which describes the hopping of a particle from one layer to another layer in the superconducting state is obtained within a Bardeen-Cooper-Schriefer (BCS) formalism using the Green’s function technique. This correlation is found to be sensitive to the various parameter of the model Hamiltonian. We have calculated the out-of-plane contribution to the superconducting condensation energy. The calculated values of condensation energy are in agreement with those obtained from the specific heat and the c-axis penetration depth measurements on bilayer cuprates.
pp 867-870 May 2002 Superconductivity
The MgB2 superconductor, synthesized using solid-state and liquid-phase sintering methods, have been characterized for various properties. The upper critical field, irreversibility line and critical current density have been determined using magnetization data. The current-voltage characteristics recorded under an applied magnetic field revealed the existence of vortex glass transition. The surface analysis using X-ray photoelectron spectroscopy shows that MgB2 is sensitive to atmospheric degradation.
pp 871-875 May 2002 Superconductivity
We have investigated the superconducting properties of the Bi1.7 Pb0.3Sr2Ca2−xCexCu3O10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high Tc (2 : 2 : 2 : 3) phase decreases and low Tc phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca2+ → Ce4+ replacement changes the hole carrier concentration. Hole filling is the cause of lowering Tc of the system.
pp 877-880 May 2002 Superconductivity
A new series of mixed oxide superconductors with the stoichiometric composition La2−xDyxCayBa2Cu4+yOz (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La2−x(Y/Er)xCayBa2Cu4+yOz series, show a strong dependence of Tc on hole concentration (psh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum Tc is obtained for x=0.5, y=1.0 sample (Tc ∼ 75 K), for La1.5Dy0.5Ca1Ba2Cu5Oz (La-2125).
pp 881-884 May 2002 Superconductivity
Results of ab initio electronic structure calculations on the compound MgB2 using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated D (EF) and the experimental specific heat data. The Tc is found to be 24.7 K.
pp 885-888 May 2002 Superconductivity
Coexistence of superconductivity and magnetic order has been one of the exciting aspects of the quaternary borocarbide superconductors, So far, RNi2B2C (R=Tm, Er, Ho and Dy) are the only known magnetic superconductors in this family. Here, we present our resistivity, magnetization and heat capacity studies on NdPt2B2C (nominal composition, NdPt1.5Au0.6B2C and NdPt2.1B2.4C1.2). We find superconductivity in both samples with Tc,onset ∼ 3 K. Bulk magnetic order is found to occur below 1.7 K. We suggest that NdPt2B2C is a possible magnetic superconductor.
pp 889-891 May 2002 Superconductivity
We calculate the longitudinal ultrasonic attenuation rate (UAR) in clean d-wave superconductors in the Meissner and the mixed phases. In the Meissner phase we calculate the contribution of previously ignored processes involving the excitation of a pair of quasi-holes or quasi-particles. There is a contribution α T in the regime kBT《QvF《Δ0 and a contribution α 1/T in the regime QvF《kBT《Δ0. We find that these contributions to the UAR are large and cannot be ignored. In the mixed phase, using a semi-classical description, we calculate the electronic quasiparticle contribution to the UAR which at very low T, has a T independent term proportional to √H.
pp 893-898 May 2002 Physics Of Vortex State
Using a novel differential magneto-optical imaging technique we investigate the phenomenon of vortex lattice melting in crystals of Bi2Sr2CaCu2O8 (BSCCO). The images of melting reveal complex patterns in the formation and evolution of the vortex solid-liquid interface with varying field (H)/temperature (T). We believe that the complex melting patterns are due to a random distribution of material disorder/inhomogeneities across the sample, which create fluctuations in the local melting temperature or field value. To study the fluctuations in the local melting temperature/field, we have constructed maps of the melting landscape Tm(H, r), viz., the melting temperature (Tm) at a given location (r) in the sample at a given field (H). A study of these melting landscapes reveals an unexpected feature: the melting landscape is not fixed, but changes rather dramatically with varying field and temperature along the melting line. It is concluded that the changes in both the scale and shape of the landscape result from the competing contributions of different types of quenched disorder which have opposite effects on the local melting transition.
pp 899-901 May 2002 Physics Of Vortex State
We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, when the flux lines in low Tc type II superconductors are caused to move by a transport current. Using small-angle neutron scattering by the flux line lattice (FLL), we are able to demonstrate directly the alignment by motion of the nearest-neighbor FLL direction. This tends to be parallel to the direction of flux line motion, as had been suspected from two-dimensional simulations. We also see the destruction of the ordered FLL by plastic flow and the bending of flux lines. Another technique that our collaboration has employed is the direct measurement of flux line motion, using the ultra-high-resolution spectroscopy of the neutron spin-echo technique to observe the energy change of neutrons diffracted by moving flux lines. The muon spin rotation (μSR) technique gives the distribution of values of magnetic field within the FLL. We have recently succeeded in performing μSR measurements while the FLL is moving. Such measurements give complementary information about the local speed and orientation of the FLL motion. We conclude by discussing the possible application of this technique to thin film superconductors.
pp 903-905 May 2002 Physics Of Vortex State
We compare the results of small angle neutron scattering on the flux line lattice (FLL) obtained in the borocarbide superconductor LuNi2B2C with the applied field along the c- and a-axes. For H‖c the temperature dependence of the FLL structural phase transition from square to hexagonal symmetry was investigated. Above 10 K the transition onset field. H2(T), rises sharply, bending away from Hc2(T) in contradiction to theoretical predictions of the two merging. For H‖a a first order FLL reorientation transition is observed at Htr=3–3.5 kOe. Below Htr the FLL nearest neighbor direction is parallel to the b-axis, and above Htr to the c-axis. This transition cannot be explained using nonlocal corrections to the London model.
pp 907-911 May 2002 Physics Of Vortex State
We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi2B2C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, TF of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at TN, but no changes are seen at TF.
pp 913-917 May 2002 Physics Of Vortex State
We have performed extensive small-angle neutron scattering (SANS) diffraction studies of the vortex lattice in single crystal YNi2B2C for B‖c. High-resolution SANS, combined with a field-oscillation vortex lattice preparation technique, allows us to separate Bragg scattered intensities from two orthogonal domains and accurately determine the unit cell angle, β. The data suggest that upon increasing field there is a finite transition width where both low- and high-field distorted hexagonal vortex lattice phases, mutually rotated by 45°, coexist. The smooth variation of diffracted intensity from each phase through the transition corresponds to a redistribution of populations between the two types of domains.
pp 919-924 May 2002 Physics Of Vortex State
Commensurability between inter-vortex distance and crystal lattice constant is investigated by angular dependent magnetization in very pure twinned and twin-free NdBa2 Cu3 Oy single crystals. With increasing temperature the incommensurate states split up and become finally commensurate with half the vortex distance. These new commensurate states are related to a substructure of the intrinsic pinning potential within the unit cell and discussed with respect to temperature, field, anisotropy, and twin structure.
pp 925-935 May 2002 Physics Of Vortex State
The weakly pinned single crystals of the hexagonal 2H-NbSe2 compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe2, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.
pp 937-947 May 2002 Physics Of Vortex State
Results of an investigation of the path dependence of the critical current density Jc due to the plastic deformation of the flux line lattice in a weakly pinned YBa2Cu3O7-δ (YBCO) crystal for H‖c are reported. The procedure of minor hysteresis loops has been used to explore the path dependence of Jc and the metastability effects. Contrary to the behavior observed in low Tc systems. in YBCO it is found that at low temperatures, the multivaluedness in Jc(H) could persist beyond the notional peak field Hp, at which the anomalous variation in Jc(H) reaches its maximum value.
pp 949-954 May 2002 Physics Of Vortex State
The vortex dynamics at microwave frequencies in YBa2Cu3O7-δ (YBCO) films have been studied. We observe a peak in the microwave (4.88 and 9.55 GHz) surface resistance in some films in magnetic fields up to 0.8 T. This is associated with the ‘peak-effect’ phenomenon and reflects the order-disorder transformation of the flux line lattice near the transition temperature. Introduction of artificial pinning centers like columnar defects created as a result of irradiation with 200 MeV Ag ion (at a fluence of 4×1010 ions/cm2) leads to the suppression of the peak in films previously exhibiting ‘peak effect’.
pp 955-958 May 2002 Physics Of Vortex State
A pronounced peak in the microwave (at frequency 9.55 GHz) surface resistance, Rs vs. T plot (where T is the temperature) has been observed in epitaxial DyBa2Cu3O7−y superconducting thin films in magnetic fields (parallel to c-axis) in the range 2 to 8 kOe, and temperatures close to the superconducting transition temperature Tc(H). Our data suggest that the nature of peaks observed in the two films is different, thereby indicating different defect structures in the films.
pp 959-963 May 2002 Physics Of Vortex State
We report the observation of a propounced peak in surface resistance at microwave frequencies of 4.88 GHz and 9.55 GHz and its disappearance after irradiation with swift ions in laser ablated DyBa2Cu3O7-δ (DBCO) thin films. The measurements were carried out in zero field as well as in the presence of magnetic fields (up to 0.8 T). The films were irradiated using 90 MeV oxygen ions at Nuclear Science Centre, New Delhi at a fluence of 3×1013 ions/cm2. Introduction of point defects and extended defects after irradiation suppresses the peak at 9.55 GHz whereas no suppression is observed at 4.88 GHz. These results and the vortex dynamics in the films at microwave frequencies before and after irradiation are discussed.
pp 965-969 May 2002 Physics Of Vortex State
Understanding the dynamics of vortex matter subjected to random and correlated pinning disorders in layered superconductors remains a topic of considerable interest. The dynamical behavior of vortices in these systems shows a rich variety of effects due to many competing interactions. Here, we study the ac response of as-grown as well as heavy-ion-irradiated Tl2Ba2CaCu2O8 (Tl-2212) thin films by using a micro Hall-probe susceptometer. We find that the dynamics of vortices in the high-temperature, low-field regime of the H-T phase diagram investigated here depends on the nature of pinning defects. While the decay of screening currents J(t) indicates a glassy behavior in both types of samples, the nature of the glassy phase is different in the two cases. Samples with columnar defects show distinct signature of a Bose glass in the measurement of J(t) and the angular dependence of the irreversibility field (B irr).
pp 971-974 May 2002 Physics Of Vortex State
Epitaxial multilayers of YBa2Cu3O7-δ and P1Ba2Cu3O7-δ have been deposited on (100) cut SrTiO3 substrates using the technique of pulsed laser deposition. Standard ϑ-2ϑ X-ray diffraction measurements on the films showed excellent superlattice reflections. The mixed state of these superlattices has been probed through measurements of radio frequency penetration depth (λ) as a function of temperature (T), magnetic field (H) and it’s orientation (ϑ) with respect to the planes of the superlattices. These data reflect the two-dimensional nature of the mixed state in these systems.
pp 975-978 May 2002 Physics Of Vortex State
We have studied the peak effect (PE) phenomenon in single crystals of weakly pinned superconductors CeRu2 and 2H-NbS2. 2H-NbS2 is iso-structural and iso-electronic to 2H-NbSe2, whose similarity with CeRu2 as regards the PE representing the order-to-disorder transformation of the flux line lattice was claimed some time ago. We report on the step change in equilibrium magnetization across the peak effect in CeRu2. We also present the vortex phase diagram of 2H-NbS2 obtained from the magnetization data, and compare the PE phenomenon in 2H-NbS2 and 2H-NbSe2.
pp 979-983 May 2002 Physics Of Vortex State
A comparative study of the thermomagnetic memory effects of Jc in two weakly pinned low Tc superconductors, Ca3Rh4Sn13 (CaRhSn) and Yb3Rh4Sn13 (YbRhSn), is presented. In both the systems, the peak effect (PE) phenomenon appears as an order-disorder transformation through stepwise amorphization of the flux line lattice (FLL). However, in CaRhSn. we can witness another disorder-driven transition (Bragg glass (BG) to a vortex glass (VG) in a distinct manner as in a single crystal of high Tc YBa2Cu3O7-δ for H‖c.
pp 985-993 May 2002 Physics Of Vortex State
We present a novel ac susceptibility technique for the study of vortex creep in superconducting thin films. With this technique we study the dynamics of dilute vortices in c-axis oriented Y-123, Hg-1212, and Tl-1212 thin films, as well as a axis oriented Hg-1212 thin films. Results on the Hg-1212 and Tl-1212 thin films indicate that dislocation-mediated plastic flux creep of single vortices dominates at low temperatures and fields. As the temperature (or the field) is increased, the increasing vortex-vortex interactions promote a collective behavior, which can be characterized by elastic creep with a non-zero μ exponent. Also, in some of these samples effects of thermally assisted quantum creep are visible up to 45 K in some of these samples. In Y-123 thin films, creep is found to be collective down to the lowest temperatures and fields investigated, while the quantum creep persists only up to 10–11 K.
pp 995-1002 May 2002 Colossal Magnetoresistance And Other Materials
Ferrites, combining insulating and ferrimagnetic properties, have long been used in technology. The aim of this paper is to focus on new features in these materials. In the classical theory of ferrimagnets, Néel had predicted the unusual thermal variation of the spontaneous magnetization, such as, the disappearance of the magnetization at a temperature which was not the Curie temperature but at a point where there was compensation of the spontaneous magnetization of the two sublattices. We show experimentally that temperature (TK) in spinel oxide is different under the ZFC and FC magnetization method. To our knowledge, only limited attempt has been made to study TK as very few systems exhibit such behavior. In general, some of the ferrites have specific semiconducting properties, e.g., a very low carrier mobility. We discuss the anomalies of the magneto-resistance in ferrites that occur at order-disorder and order-order magnetic phase transition along with our ac and dc conductivity data near the spin compensation temperature. Another notable feature of the ferrites is that, upon irradiation of heavy ions, one can tune the magnetic ordering on bulk sample without destructive effects, i.e., irradiation-induced magnetization. It is interesting to note that spinel ferrite (nano) particle is an ideal small particle magnetic system as the crystal chemistry issue can be controlled, unlike pure metal particle systems where the crystal chemistry issues are basically fixed. In relevance to this, we will also discuss the future prospects, namely, the effect of irradiation on small particle magnetism, as, so far, only a limited attempt has been made in this field.
pp 1003-1008 May 2002 Colossal Magnetoresistance And Other Materials
In this paper we report synthesis of phase-pure highly resistive magnetoelectric BiFeO3 thin films on Pt/TiO2/SiO2/Si substrate by using pulsed laser deposition technique. For the first time saturated ferroelectric hysteresis loop has been observed. It has confirmed the presence of ferroelectricity in BiFeO3 compound. The films exhibit dielectric anomaly near Neel temperature. This anomaly is related to the influence of vanishing magnetic order on the electric order. In situ domain alignment occurs during observation of the films under transmission electron microscope.
pp 1009-1012 May 2002 Colossal Magnetoresistance And Other Materials
Electron-doped (Ba1−xLax)MnO3 compounds were prepared for x=0−0.5. Measurements of X-ray diffraction (XRD) at room temperature and temperature variation of dc electrical resistivity down to 20 K were carried out. Samples with x=0.2–0.5 exhibit metal-insulator (M-I) transition. The maximum M-I transition temperature (Tc) of 289 K was observed for 30% of La doping (x=0.3). XRD patterns of these samples (x=0.2−0.5) were analyzed using Rietveld refinement. These samples are found to be mostly in single-phase form with orthorhombic symmetry (space group Pbnm). We have found strong correlation between Mn-O-Mn bond angles and Tc of M-I transition. The resistivity data below Tc could be fitted to the expression ρ=ρ1+ρ2T2 and this shows that double exchange interaction plays a major role even in Mn4+-rich compound. Above Tc the resistivity data were fitted to variable range hopping and small polaron models.
pp 1013-1017 May 2002 Colossal Magnetoresistance And Other Materials
We report polarized Raman scattering in single crystals of Nd0.7Sr0.3MnO3. The temperature dependence of the MnO6 octahedral bending and stretching modes observed in the XX spectra points to the existence of local lattice distortions, possibly polarons. The XY spectra have been analyzed using a collision-dominated model, which allows the extraction of the carrier scattering rate.
pp 1019-1026 May 2002 Colossal Magnetoresistance And Other Materials
Mn site is substituted with closed shell ions (Al, Ga, Ti, Zr and a certain combination of Zr and Al) and also with Fe and Ru ions carrying the magnetic moment (S=5/2 and 2 respectively) at a fixed concentration of 5 at %. Substitution did not change either the crystal symmetry or the oxygen stoichiometry. All substituents were found to suppress both the metal-insulator and ferromagnetic transition temperatures (Tp(ρ) and TC, respectively) to varied extents. Two main contributions identified for the suppression are the lattice disorder arising due to difference in the ionic radii between the substituent (rM) and the Mn3+ ion (rMn3+) and in the case of the substituents carrying a magnetic moment, the type of magnetic coupling between the substituent and that of the neighboring Mn ion.
pp 1027-1030 May 2002 Colossal Magnetoresistance And Other Materials
In this paper, we report for the first time finite size effects on Néel temperature (TN) of magnetoelectric BiFeO3 system. Novel wet chemical route has been developed to produce fine particles of BiFeO3 with controlled size and size distribution. Unlike other oxide systems, lattice volume contraction has been observed with decrease in particle size. The decrease in TN is co-related to unit cell volume contraction occurring with reduction in particle size.
pp 1031-1034 May 2002 Colossal Magnetoresistance And Other Materials
In order to study the effect of substitution of Fe3+ by Al3+ and Cr3+ in Li0.5Fe2.5O4 on its structural and magnetic properties, the spinel system Li0.5AlxCrxFe2.5−2xO4 (x=0.0, 0.2, 0.4, 0.5, 0.6 and 0.8) has been characterized by X-ray diffraction, high field magnetization, low field ac susceptibility and 57Fe Mossbauer spectroscopy. Contrary to the earlier reports, about 50% of Al3+ is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on magnetic sextet in the Mössbauer spectra (x>0.5).
pp 1035-1039 May 2002 Colossal Magnetoresistance And Other Materials
The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO3 perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La1−2xPrxCax)MnO3 (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the Tc varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.
pp 1041-1044 May 2002 Colossal Magnetoresistance And Other Materials
Electrical resistance (R) measurements of a bulk La0.33Nd0.33Ca0.33MnO3 perovskite in magnetic fields up to 40 kOe have revealed anomalous temperature hysteretic effects both in 0 Oe and 20 kOe magnetic fields. The sharp peak observed in the R vs. T plot indicates the occurrence of metal-to-insulator (M-I) transition at a temperature of TMI=110 K and 140 K, for cooling and warming paths, respectively. An applied magnetic field of 20 kOe reduces the resistance and shifts TMI to 160 K and 185 K for cooling and warming, respectively. We have observed a much higher resistance in the cooling path than in the warming path leading to the hysteretic resistance ratio (Rcool/Rwarm) of 200 at 110 K and 1.8 at 160 K for 0 Oe and 20 kOe, respectively. Record values of colossal magnetoresistance (CMR) have been achieved. The CMR value reaches nearly 99% in the temperature ranges of 90 K to 140 K and 90 K to 170 K for 20 kOe and 40 kOe magnetic fields in the cooling mode, respectively. The observed unusual behavior is attributed to the co-existence of La-rich and Nd-rich domains assumed to be distributed randomly in the compound.
pp 1045-1049 May 2002 Colossal Magnetoresistance And Other Materials
The structural and electronic transport properties of La1−xCexMnO3 (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration up to 50% doping. The system La0.5Ce0.5MnO3 also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition.
pp 1051-1059 May 2002 Colossal Magnetoresistance And Other Materials
We present studies of novel nanocomposites of BiNi impregnated into the structure of opals as well as inverse opals. Atomic force microscopy and high resolution elemental analyses show a highly ordered structure and uniform distribution of the BiNi filler in the matrix. These BiNi-based nanocomposites are found to exhibit distinct ferromagnetic-like ordering with transition temperature of about 675 K. As far as we know there exists no report in literature on any BiNi compound which is magnetic.
pp 1061-1063 May 2002 Colossal Magnetoresistance And Other Materials
From the transport studies in the bilayer manganites LaSr2−xCaxMn2O7, we have found the variable-range hopping model proposed by Viret et al to be inadequate to describe the transport of charge in these materials. The polarons appear to hop to their nearest neighbors with an activation energy, which in part is dependent on the magnetic interactions in the lattice.
pp 1065-1067 May 2002 Colossal Magnetoresistance And Other Materials
Epitaxial La1−xPbxMnO3 (LPMO) thin films, grown on (100) SrTiO3 substrates by laser ablation technique at different temperatures between 600 and 850°C, have been characterized for electrical and magnetic properties. The temperature dependence of resistivity showed that the metal-insulator transition temperature (TMI) decreases with increasing substrate temperature, which has been attributed to decrease in Pb content in the filsm. The YBa2Cu3Ox/La1−xMnO3 heterostructures, exhibiting both superconductivity and ferromagnetism, have been fabricated.
pp 1069-1073 May 2002 Colossal Magnetoresistance And Other Materials
We have grown single crystals of the psuedo-one-dimensional compound Sr3CuIrO6, a K4CdCl6-derived monoclinic structure with Cu-Ir chains along the  direction. We present the ac and dc magnetization behavior of the single crystals in comparison with that of the polycrystalline form reported earlier. There is a distinct evidence for at least two magnetic transitions, at 5 K (T1) and 19 K (T2), with different relative magnitudes in the single and polycrystals. The low temperature magnetic relaxation behavior of both the forms is found to be widely different, exhibiting unexpected time dependence.
pp 1075-1078 May 2002 Colossal Magnetoresistance And Other Materials
We report measurements of non-linear charge transport in epitaxial (La1−xPrx)0.7Ca0.3MnO3 thin films fabricated on (100) oriented SrTiO3 single crystals by pulsed laser deposition. The end members of this series, namely Pr0.7Ca0.3MnO3 and La0.7Ca0.3MnO3 are canonical charge-ordered (CO) and ferromagnetic manganites, respectively. The onset of the CO state in Pr0.7Ca0.3MnO3 is manifested by a pronounced insulating behavior below ∼ 200 K. The CO state remains stable even when a large (∼ 2×105 V/cm) electric field is applied across the thin film samples. However, on substitution of Pr with La, a crossover from the highly resistive CO state to a state of metallic character is observed at relatively low electric fields. The current-voltage characteristics of the samples at low temperatures show hysteretic and history dependent effects. The electric field driven charge transport in the system is modelled on the basis of an inhomogeneous medium consisting of ferromagnetic metallic clusters dispersed in a CO background.
pp 1079-1083 May 2002 Colossal Magnetoresistance And Other Materials
We report here the low-temperature resistivity of the chemical solution deposited La1−xCaxMnO3 (x=0.2, 0.3 and 0.33) thin films on LaAlO3 substrates. The films were post-annealed in atmosphere at 850°C. The low temperature resistivity data has been studied in order to understand the nature of low-temperature conduction processes. The data showed T2 dependence from 60 K to 120 K consistent with the single magnon scattering process. The deviation from this quadratic temperature dependence at low temperatures is attributed to the collapse of the minority spin band. The two-magnon and electron-phonon processes contribute to scattering of carriers in the temperature range above 120 K.
pp 1085-1088 May 2002 Colossal Magnetoresistance And Other Materials
The layered manganite Nd2−2xSr1+2xMn2O7, with x varying between 0 and 0.5, has been synthesized using solid-state reaction method. We have found that Nd2−2xSr1+2xMn2O7 do not form the single-phase layered compound with A3B2O7 structure. Instead, mixtures of various phases, such as, orthorhombic perovskite, i.e., Nd1−zSrzMnO3, layered manganite and unreacted starting compounds, have been obtained. Except for x=0.4, which is found to be an antiferromagnetic insulator, all other x values yielded metal-insulator transition and ferromagnetic ordering.
pp 1089-1092 May 2002 Colossal Magnetoresistance And Other Materials
Epitaxial La0.2Nd0.4Ca0.4MnO3 thin films have been deposited at 800°C on LaAlO3 substrate using pulsed laser deposition technique. The structural and magnetotransport properties of the films have been studied. The sharp peak in the temperature dependence of the resistance corresponding to metal-to-insulator transition (Tp) has been observed at a temperature of Tp=82 K, 97 K and 110 K for 0 Oe, 20 kOe and 40 kOe magnetic fields, respectively. The film exhibits a large nearly temperature-independent magnetoresistance around 99% in the temperature regime below Tp. The zero field-cooled (ZFC) and field-cooled (FC) magnetization data at 50 Oe shows irreversibility between the ZFC and FC close to the ferromagnetic transition temperature Tc=250 K. The ZFC temperature data of the film displays ferromagnetic behavior for higher temperature regime Tc=250 K>T>Tp=82 K, and a decrease in magnetization with decreasing temperature up to 5 K below 82 K exhibiting a sort of antiferromagnetic behavior in the low temperature regime (T<82 K=Tp=TN).
pp 1093-1100 May 2002 Colossal Magnetoresistance And Other Materials
Heavy ion irradiation in the electronic stopping power region induces macroscopic dimensional change in metallic glasses and introduces magnetic anisotropy in some magnetic materials. The present work is on the irradiation study of ferromagnetic metallic glasses, where both dimensional change and modification of magnetic anisotropy are expected. Magnetic anisotropy was measured using Mössbauer spectroscopy of virgin and irradiated Fe40Ni40B20 and Fe40Ni38Mo4B18 metallic glass ribbons. 90 MeV 127I beam was used for the irradiations. Irradiation doses were 5×1013 and 7.5×1013 ions/cm2. The relative intensity ratios D23 of the second and third lines of the Mössbauer spectra were measured to determine the magnetic anisotropy. The virgin samples of both the materials display in-plane magnetic anisotropy, i.e., the spins are oriented parallel to the ribbon plane. Irradiation is found to cause reduction in magnetic anisotropy. Near-complete randomization of magnetic moments is observed at high irradiation doses. Correlation is found between the residual stresses introduced by ion irradiation and the change in magnetic anisotropy.
pp 1101-1104 May 2002 Colossal Magnetoresistance And Other Materials
New R2Ti3Ge4 (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm5Ge4-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.
pp 1105-1113 May 2002 Colossal Magnetoresistance And Other Materials
Rare earth element substituted bismuth ferrites (BiFeO3) are of enormous importance as magnetoelectric materials. The polycrystalline samples of BixLa1−xFeO3 (x=0, 0.2, 0.4, 0.6, 0.8) were prepared by solid-state reaction using standard ceramic method. The single-phase formation of these compounds was confirmed by X-ray diffraction (XRD) studies. The samples with x=0, 0.2, 0.4, 0.6 are found to be orthorhombic while the sample with x=0.8 is triclinic. The dielectric constant (ε′) and dissipation factor (tan δ) were measured in the frequency range 100 Hz to 1 MHz at room temperature and as a function of temperature at certain fixed frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz). All the samples showed dielectric dispersion. The dielectric constant with temperature shows a broad peak; the peak temperature shifts with frequency which reflects the relaxor-type behavior. The peak above 600 K in the measured temperature range corresponds to antiferromagnetic ordering temperature (Néel temperature). The broadness of the peak changes with composition. The ac conductivity as well as ε′ are found to be maximum for the sample x=0.2 at room temperature.
pp 1115-1124 May 2002 Colossal Magnetoresistance And Other Materials
CoFe2O4-BaTiO3 composites were prepared using conventional ceramic double sintering process with various compositions. Presence of two phases in the composites was confirmed using X-ray diffraction. The dc resistivity and thermoemf as a function of temperature in the temperature range 300 K to 600 K were measured. Variation of dielectric constant (ɛ′) with frequency in the range 100 Hz to 1 MHz and also with temperature at a fixed frequency of 1 kHz was studied. The ac conductivity was derived from dielectric constant (ɛ′) and loss tangent (tan δ). The nature of conduction is discussed on the basis of small polaron hopping model. The static value of magnetoelectric conversion factor has been studied as a function of magnetic field.
pp 1125-1129 May 2002 Thin Film And Devices
The response of giant magnetoresistance (GMR) devices depends critically on the film microstructure, with parameters such as layer thickness and interfacial abruptness being crucial. This paper presents results obtained using high resolution electron microscopy (HREM), chemical mapping and atom probe microanalysis. Local variations in the magnetic properties are induced by the microstructure and also when the films are patterned to form small elements. These lead to changes in the magnetization reversal mechanism. Some results of the studies of the magnetization reversal carried out using in situ in Lorentz transmission electron microscopy (LTEM) magnetizing experiments are also included.
pp 1131-1139 May 2002 Thin Film And Devices
Shape memory alloys are the generic class of alloys that show both thermal and mechanical memory. The basic physics involved in the shape memory effect is the reversible thermoelastic martensitic transformation. In general, there exists two phases in shape memory alloys, viz., a high-temperature phase or austenitic phase (A) and a low-temperature phase or martensitic phase (M). In addition, an intermediate R phase exists in some special cases. The M↔A transformation is associated with a recoverable strain of about 6.5–8% and the R↔A transformation is associated with a recoverable strain of about 1%. The former transformation has been widely used in the applications like antenna deployment of satellite, aerospace couplings, orthodontic arch wires, medical guide wires for diagnostic and therapeutic catheters and other industrial applications. Our group has been giving emphasis to the characterization techniques for R phase, using differential scanning calorimetry (DSC), electrical resistivity probe (ER) and thermomechanical analyzer (TMA). R phase is found to have attractive features like stability against thermal cycling, a small thermal hysteresis and a negligible strain recovery fatigue. DSC has been used successfully to characterize the recoverable strain parameters, apart from the determination of transformation temperatures. ER is used, for the systematic study of the dependence of various phases on heat-treatment temperatures. TMA has been effectively employed for the study of the mixed phases. A space-rotating platform is designed and fabricated, using an actuator of shape memory spring, for obtaining controlled rotations. The efficiency and the reliability of this actuator has been tested, over a million thermal cycles.
pp 1141-1145 May 2002 Thin Film And Devices
Aging, which manifests itself as an irreversible increase in electrical resistance in island metal films is of considerable interest from both academic as well as applications point of view. Aging is attributed to various causes, oxidation of islands and mobility of islands followed by coalescence (mobility coalescence) being the main contenders. The effect of parameters like substrate temperature, substrate cleaning, residual gases in the vacuum chamber, ultrasonic vibration of the substrate, suggest that the mobility coalescence is responsible for the aging in island metal films. Electron microscopy studies show evidence for mobility of islands at high substrate temperatures. The comparison of aging data of island silver films deposited on glass substrates in ultra high vacuum and high vacuum suggests that the oxidation of islands, as being responsible for aging in these films, can be ruled out. Further, under certain conditions of deposition, island silver films exhibit a dramatic and drastic fall in electrical resistance, marking the insulator-metal transition. This interesting transition observed in a conservative system — after the stoppage of deposition of the film — is a clear evidence for mobility coalescence of islands even at room temperature. The sudden fall in resistance is preceded by fluctuations in resistance with time and the fluctuations are attributed to the making and breaking of the percolation path in the film.
pp 1147-1151 May 2002 Thin Film And Devices
We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La0.7Ca0.3MnO3 (LCMO) and YBa2Cu3O7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO3 (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 µm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, Ic, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a Jc of ∼ 2 × 106 A/cm2, the LCMO side showed about half the value for the same thickness (1800 Å). The difference in Jc indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of Jc at two different thicknesses (1800 Å and 1500 Å) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 Å. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO3 deposited by PLD on YBCO did not show any evidence of self-injection.
pp 1153-1157 May 2002 Thin Film And Devices
The generation of the magnetic flux quanta inside the superconductors is studied as a new effect to destroy superconductivity using femtosecond (fs) laser. The vortices are successfully generated in the YBa2Cu3O7-δ thin film striplines by the fs laser. It is revealed that the vortex distribution in the strip reflects the fs laser beam profile.
pp 1159-1164 May 2002 Thin Film And Devices
A program has been developed and initiated at the Indira Gandhi Centre for Atomic Research (IGCAR) for the utilization of SQUID sensors in various application areas. DC SQUID sensors based on Nb-AlOx-Nb Josephson junctions have been designed and developed inhouse along with associated flux-locked loop (FLL) electronics. A compact low field SQUID magnetometer insertible in a liquid helium storage dewar has also been developed inhouse and is in use. Efforts to build a high field SQUID magnetometer, SQUID-DAC system, are in progress. A planar gradiometric DC SQUID sensor for non-destructive evaluation (NDE) application to be used in relatively unshielded environment has been designed and developed. An easily portable NDE cryostat with a small lift-off distance, to be used in external locations has been designed and tested. The magnetic field produced by a given two-dimensional current density distribution is inverted using the Fourier transform technique.
pp 1165-1170 May 2002 Thin Film And Devices
La0.67Ba0.33MnO3 (LBMO) thin film is deposited on a 36.7°C SrTiO3 bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.
pp 1171-1177 May 2002 Thin Film And Devices
Microwave surface resistance (Rs) of silver-doped YBa2Cu3O7-δ (YBCO) thin film, deposited by laser ablation technique on 10 mm × 10 mm LaAlO3 substrate, has been measured by resonant techniques in the frequency range from 5 GHz to 20 GHz. The geometrical factor of the sample and the resonator has been determined theoretically by the knowledge of the electromagnetic field distribution in the resonators. The microwave surface resistance of the superconducting sample is then extracted from the measured Q value as a function of temperature. The sensitivity of the Rs measurement, that is, the relative change in the Q value with the change in the Rs value is determined for each resonator.
pp 1179-1182 May 2002 Thin Film And Devices
Hole-doped rare-earth manganite La0.7Ca0.3MnO3 and the electron-doped manganite La0.7Ce0.3MnO3 both show a metal-insulator transition around 250 K associated with a ferromagnetic transition and colossal magnetoresistance. In an earlier publication we have reported the rectifying characteristic of La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3 tunnel junction at room temperature, showing that it is possible to fabricate a diode out of the polaronic insulator regime of doped manganites. Here we report the magneto-transport properties of such a tunnel junction above and below the metal-insulator transition. We show, from the large positive magnetoresistance of the tunnel junction at low temperature, that La0.7Ce0.3MnO3 could be a minority spin carrier ferromagnet. The implication of this observation is discussed.
pp 1183-1190 May 2002 Thin Film And Devices
Metal/superconductor/semiconductor (Ag/Hg-1212/CdSe) hetero-nanostructures have been fabricated using pulse-electrodeposition technique and are characterized by X-ray diffraction (XRD), full-width at half-maximum (FWHM) and scanning electron microscopy (SEM) studies. The junction capacitance of Ag/Hg-1212, Hg-1212/CdSe and Ag/Hg-1212/CdSe heterojunctions is measured in dark and under laser irradiation at room temperature. The nature of the junction formed and built-in-junction potentials were determined. The increase in carrier concentration across the junction due to photo-irradiation has been observed.
pp 1191-1198 May 2002 Thin Film And Devices
One of the innovative technological directions for the high-temperature superconductors has been persued by fabricating the heteroepitaxial multilayer structures such as superconductor-semiconductor heterostructures. In the present investigation, metal/superconductor/semiconductor (Ag/Tl-2223/CdSe) hetero-nanostructures have successfully been fabricated using dc electrodeposition technique and were characterized by X-ray diffraction (XRD), full-width at half-maximum (FWHM) and scanning electron microscopy (SEM) studies. The measurement of junction capacitance as a function of biasing voltage was used for the estimation of junction built-in-potential (VD) and to study the charge distribution in a heterojunction. The Mott-Schottky plots were measured for each junction in dark and under the photo-irradiation. The effect of laser irradiation on C-V characteristics of hetero-nanostructure has been studied.
pp 1199-1201 May 2002 Thin Film And Devices
We report here injection of spin-polarized carriers from a half-metallic La0.3Ca0.7MnO3 (LCMO) colossal magnetoresistive (CMR) thin film into a high-temperature superconducting YBa2Cu3O7-δ (YBCO) thin film studied using a micro-bridge. The LCMO and YBCO films were grown on 〈100〉 LaAlO3 (LAO) substrate sequentially using pulsed laser deposition (PLD). I-V measurements carried out at 77 K show that while normal critical current, Icn, of the micro-bridge is 80 mA, the critical current, Icp, through the micro-bridge when injected from the CMR layer is 38 mA. This clearly shows that spin-polarized quasiparticles injected from the the CMR layer into the YBCO layer suppress the critical current of the superconductor via the pair-breaking phenomena.
pp 1203-1206 May 2002 Thin Film And Devices
Analysis of a TE011 mode composite sapphire-rutile dielectric resonator has been carried out to study the temperature variation of resonance frequency, close to the Cs atomic clock hyperfine frequency of 9.192 GHz. The complementary behavior of dielectric permittivity with temperature of the composite has been exploited to obtain the desired turning point in the resonant frequency. The frequency of the composite structure is found to be independent of the shield diameter beyond four times the puck diameter.
pp 1207-1217 May 2002
pp 1218-1229 May 2002
Volume 93 | Issue 6
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