R. Thiyagarajan

Influence of chemical substitution, magnetic field, and hydrostatic pressure effect on martensitic and intermartensitic transition in bulk Ni49-xCuxMn38Sn13 (0.5≤ x ≤ 2) Heusler alloys

In this work, we report the influence of chemical substitution of Ni site by Cu, magnetic field of 50 kOe, and external pressure effect on martensite and intermartensite transformations in Ni49-xCuxMn38Sn13 (0.5 ≤ x ≤ 2) Heusler alloys. The substitution of Cu for Ni decreases the lattice parameter and introduces the intermartensitic transformation in this alloy series. Further, the intermartensitic transformation vanishes for higher Cu content (x = 2). The application of external magnetic field and hydrostatic pressure suppresses the intermartensitic transformation in Ni49-xCuxMn38Sn13 (0.5 ≤ x ≤ 1.5) and Ni49-xCuxMn38Sn13 (x = 0.5) alloys, respectively. Also, the external magnetic field favors the stabilization of austenite phase while the external hydrostatic pressure favors the martensite phase.

Influence of Si substitution on the structure, magnetism, exchange bias and negative magnetoresistance in Ni48Mn39Sn13 Heusler alloys

The effect of partial Si substitution for Sn on the structure, magnetism, magnetic entropy change, exchange bias and negative magnetoresistance in Ni48Mn39Sn13?xSix(1???x???4) Heusler alloy systems is investigated. A small amount of Si in the Sn position influences the characteristic transition temperatures and the magnetocaloric effect. A maximum positive entropy change of 5.13?J?kg?1?K?1 is observed for a field change of 9?T in the x?=?1 alloy. An increase in Si content increases the exchange bias field. The exchange bias field and the coercive field strongly depend on the Si content. For the x?=?4 alloy, an exchange bias field of 354?Oe is observed. In addition, a negative magnetoresistance is observed in this alloy system, which decreases with Si content. The increase in resistivity at the martensitic transition could be attributed to the formation of superzone boundary gaps that changes the density of states near the Fermi surface.

Effect of hydrostatic pressure on magnetic and magnetocaloric properties of Mn-site doped perovskite manganites Pr0.6Ca0.4Mn0.96B0.04O3 (B=Co and Cr)

We have investigated the effect of hydrostatic pressure on magnetization and magnetocaloric properties of Pr0.6Ca0.4Mn0.96B0.04O3 (B = Co and Cr) polycrystals. At ambient pressure, both the systems undergo a first-order paramagnetic insulator to ferromagnetic metallic transition. On increasing the hydrostatic pressure, the ferromagnetic Curie temperature (TC) shifts towards high temperature at a rate of dTC/dP = 34 K/GPa (23.15 K/GPa) and 31.1 K/GPa (20.7 K/GPa) during cooling (warming) for B = Co and Cr, respectively. While the hysteresis in magnetization decreases with increasing pressure, P = 1 GPa is insufficient to completely suppress the first order transition. The magnetic entropy change (ΔSM) was calculated for various pressures up to 1 GPa and under a magnetic field of μ0H = 1-5 T. The application of pressure, as well as the magnetic field, shifts the peak in ΔSM(T) towards a higher temperature. The maximum value of ΔSM (ΔSMmax) is increased by the magnetic field but decreased by the pressure. It...

Effect of hydrostatic pressure on magnetic entropy change and critical behavior of the perovskite manganite La0.4Bi0.3Sr0.3MnO3

We report the effect of magnetic field (H) and hydrostatic pressure (P) on the order of magnetic transition of polycrystalline La0.4Bi0.3Sr0.3MnO3 which undergoes a first-order paramagnetic (PM) to ferromagnetic (FM) transition in La0.7−xBixSr0.3MnO3 series. The ferromagnetic Curie temperature (TC) increases with increasing H (12.01 K/T-cooling and 10.28 K/T-warming) and P (8.1 K/kbar-cooling and 6 K/kbar-warming). The first-order FM transition becomes second-order under the applied magnetic field of 9 T and pressure of 9.1 kbar. We have analyzed the critical behavior associated with the second order PM-FM transition at 9.1 kbar. The estimated critical exponents (β = 0.5217, γ = 1.209, and δ = 3.162) are found to be close to the mean-field model. Pressure suppresses metamagnetic transition in magnetization isotherms observed above TC in ambient pressure and enhances the magnetic entropy change (ΔSm). The ΔSm was found to increase by 50% under hydrostatic pressure of 9.1 kbar at TC = 240 K. This study sugg...

Pressure-induced colossal piezoresistance effect and the collapse of the polaronic state in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7

We have investigated the effect of hydrostatic pressure as a function of temperature on the resistivity of a single crystal of the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn(2)O(7). Whereas a strong insulating behaviour is observed at all temperatures at ambient pressure, a clear transition into a metallic-like behaviour is induced when the sample is subjected to a pressure (P) of ~1.0 GPa at T < 70 K. A huge negative piezoresistance ~10(6) in the low temperature region at moderate pressures is observed. When the pressure is increased further (5.5 GPa), the high temperature polaronic state disappears and a metallic behaviour is observed. The insulator to metal transition temperature exponentially increases with pressure and the distinct peak in the resistivity that is observed at 1.0 GPa almost vanishes for P > 7.0 GPa. A modification in the orbital occupation of the e(g) electron between 3d(x(2)-y(2)) and 3d(z(2)-r(2)) states, as proposed earlier, leading to a ferromagnetic double-exchange phenomenon, can qualitatively account for our data.

Effect of pressure on normal and superconducting state properties of iron based superconductor PrFeAsO0.6F y (y = 0.12, 0.14)

The effect of high pressure (up to 8 GPa) on normal and superconducting state properties of PrFeAsO0.6F0.12, an 1111-type iron based superconductor close to optimal doped region, has been investigated by measuring the temperature dependence of resistivity. Initially, the superconducting transition temperature (Tc) is observed to increase slowly by about 1 K as pressure (P) increases from 0 to 1.3 GPa. With further increase in pressure above 1.3 GPa, Tc decreases at the rate of ~1.5 K/GPa. The normal-state resistivity decreases monotonically up to 8 GPa. We have also measured the pressure dependence of magnetization (M) on the same piece of PrFeAsO0.6F0.12 sample up to 1.1 GPa and observed Tc as well as the size of the Meissner signal to increase with pressure in this low-pressure region. In contrast, for an over-doped PrFeAsO0.6F0.14 sample, magnetization measurements up to 1.06 GPa show that both Tc and the Meissner signal decrease with pressure. The present study clearly reveals two distinct regions in the dome-shaped (Tc-P) phase diagram of PrFeAsO0.6F0.12.