• S K Malik

      Articles written in Pramana – Journal of Physics

    • 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.

    • 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.

    • Consolidation of YBaCuO powders by shock loading

      N C Soni Ram Prasad Ashok Mohan M D Vora S Banerjee C V Tomy S K Malik H S Yadav K R K Rao

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      Superconducting oxide-copper monoliths have been fabricated by shock-wave loading using cylindrical and plane geometries. Bulk densities up to 96% T.D. have been obtained by varying detonation parameters. The superconducting properties, interface bonding and microstructure of the compacts have been evaluated.

    • Comparison of irreversibility temperatures determined via DC and AC magnetization techniques in conventional superconductors

      A K Grover S Ramakrishnan Ravi Kumar P L Paulose S K Malik P Chaddah

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      A comparison has been made of irreversibility temperature determined by four different methods in few specimens of lead (type-I) and niobium (type-II). The merger ofMZFC(T) andMFC(T) curves giveTr(H) values lower than those evident from vanishing the hysteresis in isothermal DC magnetization. The identification of peak temperature inxH(T) data withTr(H) is appropriate only if the contribution from changes in the normal state electrodynamics can be isolated and the peak is narrow. The appearance of differential paramagnetic effect inxH(T) data is adequate to imply reversibility, however, its efficacy to precisely locate irreversibility line remains to be established.

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