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 Δ_H, which is reported here for the first time, is determined to be 2 cal mole⁻¹.

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 fieldH_ex acting on Sm³⁺ ion is taken to be proportional to the sublattice magnetization of the magnetic element, while in compounds with nonmagnetic elementsH_ex is taken to be proportional to the spin average of the Sm³⁺ ion and is determined self-consistently. In both types of compoundsH_ex 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 (L−S)ion. The crystal fields may have important bearing on the performance of samarium compounds as permanent magnet materials.

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 (ΔM_I=±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.

Compounds of the formula RPd₃B_x (R=rare earth with 0⩽x⩽1) and RPd₃Si_x (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 CePd₃B_x and EuPd₃B_x compared to that in other RPd₃B_x 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, μ_eff=2.54 μ_B) and Eu²⁺ (J=7/2, μ_eff=7.94 μ_B) in the two alloy systems CePd₃B_x and EuPd₃B_x 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 EuPd₃Si_x alloys, susceptibility and Mössbauer studies indicate that in the limiting single phase alloy EuPd₃Si_0.25 the europium ions are on the verge of valence instability. Susceptibility results on CeRh₃B_x alloys are also presented.

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 CeRu_xSi₂, CeRu_2−xOs_xSi₂, CeRu₂Si_2−xGe_x and YbPd₂Si_2−xGe_x. 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, CeRu₂Si_2−xGe_x exhibits larger susceptibility at low temperature, goes to an antiferromagnetic state and finally becomes ferromagnetic. In YbPd₂Si_2−xGe_x, increasing Ge concentration drives Yb atoms to more divalent state. Electronic effects are more pronounced in CeRu_2−xOs_xSi₂ though CeRu₂Si₂ and CeOs₂Si₂ have very nearly the same lattice parameters. It is conjectured that CeRu₂Si_2−xGe_x may be the first Ce-based heavy fermion having a magnetic ground state.

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 (T_sf) in each case is estimated from the measured data. For EuNi₂P₂ the values ofT_sf above 77 K qualitatively agree with those obtained from Mössbauer and susceptibility data employing ionic interconfigurational fluctuation model, but disagree at lower temperatures.

We report here the results of our heat capacityC_p measurements on a monophasic material HoBa₂Cu₃O_7−y. ΔC_p/T_c, the jump inC_p at the superconducting transition temperature (=91 K) of the material is measured to be 31 mJ/mol-K².

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.

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 (−H_max to +H_max) and reverse (+H_max to −H_max) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asH_peak,H_I,H_II 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 (M_rem(H)−M_FC(H)+M_ZFC(H)) being equal to, less than or greater than zero, whereM_rem(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.