S. Esakki Muthu

Influence of Ni/Mn concentration on the structural, magnetic and magnetocaloric properties in Ni50−xMn37+xSn13 Heusler alloys

We report the structure, magnetism and magnetic entropy change in a Mn-rich Ni50−xMn37+xSn13 Heusler alloy system in the composition range 0 ≤ x ≤ 4. An excess Mn content stabilizes the cubic austenite phase at room temperature. Martensitic transition decreases from 305 to 100 K with increasing Mn concentration (x: 0 → 4) and also it was found to shift to a lower temperature with the application of a higher magnetic field. The exchange bias blocking temperature was found to decrease drastically from 149 to 9 K with increasing Mn concentration. A large magnetic entropy change (ΔSM) of 32 J kg−1 K−1 has been achieved for a field change of 5 T in the x = 3 alloy.

Effect of Ni/Mn concentration on exchange bias properties in bulk Ni50−xMn37+xSn13 Heusler alloys

We report the effect of Ni/Mn variation on the exchange bias properties in the bulk Mn-rich Ni50−xMn37+xSn13 (0 ≤ x ≤ 4) Heusler alloys. The excess Mn content was found to increase the exchange bias field while it decreases the exchange bias blocking temperature (TEB) from 149 to 9 K. A maximum shift in the hysteresis loop of 377 Oe is observed for the Ni46Mn41Sn13 alloy. As compared to Mn/Sn variation, Ni/Mn variation strongly influences the exchange bias properties in Ni-Mn-Sn alloys. We observed that if the Mn content is above 37 at. % in Ni-Mn-Sn alloys, the TEB value would show a decreasing trend either by varying the Ni or Sn content.

Role of point defects on the enhancement of room temperature ferromagnetism in ZnO nanorods

Vertically aligned undoped ZnO nanorod arrays (NRAs) have been fabricated on silicon (111) substrates by radio frequency magnetron sputtering. The structural studies illustrate a hexagonal wurtzite structure of the ZnO NRAs with compressive stress. Raman analysis of the E2high phonon mode corroborates the partial relaxation of stress in NRAs by the post growth treatment under oxygen and vacuum atmospheres. The anomalous Raman modes have been attributed to the local vibrations and it corresponds to the silent modes of wurtzite ZnO. The appearance of forbidden modes illustrates the breakdown of the Raman selection rules. The role of point defects on the ferromagnetic behaviour of NRAs was analyzed by optical transitions and correlated with the magnetic properties. Post growth treatment of NRAs under oxygen and vacuum atmospheres significantly suppresses the point defects owing to the enhancement of the crystalline quality. The temperature dependent zero-field cooled and field cooled magnetizations reveal the coexistence of antiferromagnetism and ferromagnetism below 7 K. However, the ferromagnetism is dominant and stable between 7 K and room temperature. The decrease of ferromagnetism in NRAs is directly associated with the compensation of point defects such as zinc and oxygen vacancies as substantiated by the radiative transition between shallow donor and acceptor energy levels. These results confirm that point defects play an important role in enhancing the room temperature ferromagnetism in ZnO NRAs.

Hydrostatic pressure effect on the martensitic transition, magnetic, and magnetocaloric properties in Ni50-xMn37+xSn13 Heusler alloys

We report upon the effect of hydrostatic pressure on the martensitic transition, magnetic properties, and magnetic entropy change in Ni50-xMn37 + xSn13 (x = 2, 3) Heusler alloys. The application of pressure has significantly shifted the martensitic transition temperature to higher values. A large rate of change of the martensitic transition with a pressure of ∼3.1 K/kbar has been obtained for the x = 2 alloy, whereas the Curie temperature changes marginally with pressure (∼0.3 K/kbar). Magnetization of both the austenite and martensite phases decreases with an increase of pressure. The maximum magnetic entropy change of 34 J kg−1K−1 at ambient pressure and 22.5 J kg−1K−1 at 8 kbar was observed around the martensitic transition temperature for the x = 3 alloy.

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.

Inverse magnetocaloric effect in Mn2NiGa and Mn1.75Ni1.25Ga magnetic shape memory alloys

Inverse magnetocaloric effect is demonstrated in Mn2NiGa and Mn1.75Ni1.25Ga magnetic shape memory alloys. The entropy change at the martensite transition is larger in Mn1.75Ni1.25Ga, and it increases linearly with magnetic field in both the specimens. Existence of inverse magnetocaloric effect is consistent with the observation that magnetization in the martensite phase is smaller than the austenite phase. Although the Mn content is smaller in Mn1.75Ni1.25Ga, from neutron diffraction, we show that the origin of inverse magnetocaloric effect is the antiferromagnetic interaction between the Mn atoms occupying inequivalent sites.

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-magnetic field induced phase transformation in Ni46Mn41In13 Heusler alloy

The effect of hydrostatic pressure and magnetic field on the magnetic properties and phase transformation in Ni46Mn41In13 Heusler alloy was investigated. Pressure (P)-magnetic field (H)-temperature (T) phase diagram has been constructed from experimental results. In the P–T contour of the phase diagram, the slope of the austenite-martensite phase boundary line appears positive (dT/dP > 0), while it appears negative (dT/dH < 0) in the H–T contour. The results revealed that pressure and magnetic field have opposite effect on phase stabilization. The combined effect of pressure and magnetic field on martensitic transition has led to two important findings: (i) pressure dependent shift of austenite start temperature (As) is higher when larger field is applied, and (ii) field dependent shift of As is lowered when a higher pressure is applied. The pressure and magnetic field dependent shift observed in the martensitic transformation has been explained on the basis of thermodynamic calculations. Curie temperatur...