• R Vijayaraghavan

      Articles written in Pramana – Journal of Physics

    • Crystallographic and magnetic phase transition in TlMnCl3

      R Vijayaraghavan M D Karkhanavala S D Damle L C Gupta U R K Rao

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      The results of our NMR, EPR and magnetic susceptibility measurements in the paramagnetic state of TlMnCl3 are reported here. The NMR paramagnetic shift of thallium is found to be small but positive. Mn2+ EPR line is exchange narrowed. The susceptibility measurements indicate an antiferromagnetic transition. The heat of crystallographic phase transition ΔH, in TlMnCl3 has been measured using differential scanning calorimetry. The crystallographic phase transition appears to be first order and ΔH is unusually low viz. 10 cal mole−1. In the case of KMnF3 ΔH, which is reported here for the first time, is determined to be 2 cal mole−1.

    • Crystal field effects on the saturation magnetic moment of Sm3+ ion in ferronagnetic samarium compounds

      S K Malik R Vijayaraghavan

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      The effects of cubic crystal fields on the saturation magnetic moment of Sm3+ ion in ferromagnetic compounds have been investigated. In samarium compounds with magnetic elements, the exchange fieldHex acting on Sm3+ ion is taken to be proportional to the sublattice magnetization of the magnetic element, while in compounds with nonmagnetic elementsHex is taken to be proportional to the spin average of the Sm3+ ion and is determined self-consistently. In both types of compoundsHex is assumed to be along [001] direction. The saturation magnetic moment is calculated by taking into account the admixture of excited (J=7/2 andJ=9/2) levels into the ground (J=5/2) level of Sm3+ ion by crystal fields and exchange fields. It is shown that depending upon the strength, the crystal fields quench or enhance the magnetic moment from the free ion value, and in some cases force Sm3+ ion to behave effectively like an (L+S) ion rather than an (LS)ion. The crystal fields may have important bearing on the performance of samarium compounds as permanent magnet materials.

    • Nuclear quadrupole resonance studies of209Bi in Bi4(GeO4)3 and Bi4(SiO4)3

      K V Gopalakrishnan L C Gupta R Vijayaraghavan

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      Nuclear quadrupole resonance (NQR) of209Bi has been studied in Bi4 (GeO4)3 and Bi4 (SiO4)3 using a wide band coherence-controlled superregenerative oscillator-detector. All the four allowed (ΔMI=±1) transitions are observed. In both cases the electric field gradient (EFG) tensor is axially symmetric (η=0.0). The quadrupole coupling constante2qQ is measured to be 490.8±1 MHz and 470.4±1 MHz respectively. It is pointed out that the purely ionic model is inadequate to understand these results. With the available experimental accuracy and the strength of the applied electric field (∼ 6 KV/cm), no field-induced effects on the NQR spectrum could be observed in the case of Bi4 (SiO4)3.

    • Studies on the valence state of Ce and Eu in some new boron and silicon containing rare earth intermetallic compounds

      S K Malik S K Dhar R Vijayaraghavan

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      Compounds of the formula RPd3Bx (R=rare earth with 0⩽x⩽1) and RPd3Six (R=La, Ce, Eu with 0⩽x⩽0.3) can be prepared by alloying boron or silicon with parent RPd3 compounds. Addition of boron (silicon) does not change the structure but results in lattice expansion. The valence state of Ce in CePd3 and that of Eu in EuPd3 is strongly influenced by boron and silicon. Ce is known to be in a valence fluctuating state in CePd3 while Eu is trivalent (J=0) in EuPd3. The increase in the lattice parameter as a function of boron concentration is observed to be larger in CePd3Bx and EuPd3Bx compared to that in other RPd3Bx alloys giving the first indication of the change in the valence state of Ce and Eu. This is confirmed from susceptibility measurements. With the addition of boron, susceptibility increases and the effective paramagnetic moments approach the values corresponding to Ce3+ (J=5/2, μeff=2.54 μB) and Eu2+ (J=7/2, μeff=7.94 μB) in the two alloy systems CePd3Bx and EuPd3Bx respectively. In the case of europium alloys,151Eu Mössbauer studies point out the importance of near-neighbour environment effects. Further, in EuPd3B, where all the europium ions are crystallographically equivalent, a single Mössbauer line, with an isomer shift characteristic of europium ions in valence-fluctuating state, is observed at 300 K. However, at 88 K the Mössbauer absorption splits into two lines corresponding to europium ions in two valence states,e.g. divalent- and trivalent-like. Such a behaviour indicates thermally-induced charge ordering of europium ions. Addition of silicon to CePd3, like boron, results in unusual lattice expansion and changes the valency of cerium towards 3+. the valence change is further corroborated by susceptibility measurements. In EuPd3Six alloys, susceptibility and Mössbauer studies indicate that in the limiting single phase alloy EuPd3Si0.25 the europium ions are on the verge of valence instability. Susceptibility results on CeRh3Bx alloys are also presented.

    • Spin fluctuation effects in substituted CeRu2Si2 and YbPd2Si2 alloys

      A M Umarji C Godart L C Gupta R Vijayaraghavan

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      Effects of chemical substitution in CeRu2Si2, a well-studied heavy fermion system and YbPd2Si2 have been investigated through magnetic susceptibility and x-ray diffraction in the systems CeRuxSi2, CeRu2−xOsxSi2, CeRu2Si2−xGex and YbPd2Si2−xGex. Replacing silicon by germanium generates normal chemical pressure effect, namely, Ce and Yb atoms in CeRu2Si2 and YbPd2Si2 became more and less magnetic respectively. With increasing Ge concentration, CeRu2Si2−xGex exhibits larger susceptibility at low temperature, goes to an antiferromagnetic state and finally becomes ferromagnetic. In YbPd2Si2−xGex, increasing Ge concentration drives Yb atoms to more divalent state. Electronic effects are more pronounced in CeRu2−xOsxSi2 though CeRu2Si2 and CeOs2Si2 have very nearly the same lattice parameters. It is conjectured that CeRu2Si2−xGex may be the first Ce-based heavy fermion having a magnetic ground state.

    • Nuclear magnetic resonance studies in rare earth ternary phosphides

      R Vijayaraghavan

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      The results of31P Knight shift (KS) and spin-lattice relaxation time (T1) measurements in the temperature interval 4.2–300 K are reported for the compounds RENi2P2(RE=Ce, Eu, Yb) in order to understand the nature of the ground state in these compounds. KS results conclusively establish that all these compounds exhibit non-magnetic ground states. The temperature dependence of spin-fluctuation temperature (Tsf) in each case is estimated from the measured data. For EuNi2P2 the values ofTsf above 77 K qualitatively agree with those obtained from Mössbauer and susceptibility data employing ionic interconfigurational fluctuation model, but disagree at lower temperatures.

    • Specific heat anomaly in the 90 K superconductor HoBa2Cu3O7−y

      B K Basu S M Pattalwar R N Dixit S Y Shete A K Rajarajan L C Gupta R Vijayaraghavan

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      We report here the results of our heat capacityCp measurements on a monophasic material HoBa2Cu3O7−y. ΔCp/Tc, the jump inCp at the superconducting transition temperature (=91 K) of the material is measured to be 31 mJ/mol-K2.

    • Indications of superconductivity in the 200–300K range in the Bi-Ca-Sr-Cu-O system

      C N R Rao R A Mohan Ram L Ganapathi R Vijayaraghavan

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      Some of the Bi-Ca-Sr-Cu-O compositions show indications of onset of superconductivity in the 200–300 K region, possibly due to the intergrowth of different layered sequences.

    • Magnetization studies in niobium and YBa2Cu3O7 samples

      B V B Sarkissian A K Grover G Balakrishnan Ravi Kumar P L Paulose R Vijayaraghavan V Sankaranarayanan C K Subramanian

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      The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−Hmax to +Hmax) and reverse (+Hmax to −Hmax) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asHpeak,HI,HII etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (Mrem(H)−MFC(H)+MZFC(H)) being equal to, less than or greater than zero, whereMrem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.

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