D. V. Maheswar Repaka

Magnetocaloric properties and critical behavior of high relative cooling power FeNiB nanoparticles

Low cost magnetocaloric nanomaterials have attracted considerable attention for energy efficient applications. We report a very high relative cooling power (RCP) in a study of the magnetocaloric effect in quenched FeNiB nanoparticles. RCP increases from 89.8 to 640 J kg−1 for a field change of 1 and 5 T, respectively, these values are the largest for rare earth free iron based magnetocaloric nanomaterials. To investigate the magnetocaloric behavior around the Curie temperature (TC), the critical behavior of these quenched nanoparticles was studied. Detailed analysis of the magnetic phase transition using the modified Arrott plot, Kouvel-Fisher method, and critical isotherm plots yields critical exponents of β = 0.364, γ = 1.319, δ = 4.623, and α = −0.055, which are close to the theoretical exponents obtained from the 3D-Heisenberg model. Our results indicate that these FeNiB nanoparticles are potential candidates for magnetocaloric fluid based heat pumps and low grade waste heat recovery.

Giant magnetocaloric effect in magnetoelectric Eu1-xBaxTiO3

We report the magnetic entropy change (ΔSm) in magnetoelectric Eu1-xBaxTiO3 for 0.1 ≤ x ≤ 0.9. We find −ΔSm = 11 (40) J/kg·K in x = 0.1 for a field change of 1 (5) T, respectively, which is the largest value among all Eu-based oxides. ΔSm arises from the field-induced suppression of the spin entropy of Eu2+:4f7 localized moments. While ∣−ΔSm∣ decreases with increasing x, ∣−ΔSm∣ = 6.58 J/kg·K observed in the high spin diluted composition x = 0.9 is larger than that in many manganites. Our results indicate that these magnetoelectrics are potential candidates for cryogenic magnetic refrigeration.

Normal and inverse magnetocaloric effects in ferromagnetic Pr0.58Sr0.42MnO3

We report magnetization, magnetic entropy change (ΔSm), and its correlation with magnetoresistance (MR) in Pr0.58Sr0.42MnO3. It is shown that the magnetization upon field-cooling shows a steplike decrease at TS = 134 K much below the ferromagnetic transition (TC = 300 K). While the low temperature transition is first-order, the high temperature transition is second-order as suggested by the hysteresis behavior in magnetization. In a magnetic field range accessible with an electromagnet, the magnetic entropy decreases at TC (ΔSm = −2.33 J/kg K with a refrigeration capacity of 65.88 J/kg for a magnetic field change of ΔH = 2 T) whereas it increases at TS (ΔSm = +0.7 J/kg K) upon magnetization. The unusual inverse magnetocaloric effect found at TS within ferromagnetic state is ascribed to orthorhombic to monoclinic structural transition. We show that ΔSm versus T curves under different magnetic fields can be collapsed into a single master curve using a scaling method. Importantly, we find that negative MR in...

Giant magnetothermopower in charge ordered Nd0.75Na0.25MnO3

We report magnetization, resistivity, and thermopower in the charge-orbital ordered antiferromagnet Nd0.75Na0.25MnO3. Magnetic-field induced collapse of antiferromagnetism is found to be accompanied by a giant negative magnetothermopower (=80%–100% for a field change of 5 T) over a wide temperature (T = 60–225 K) and giant magnetoresistance. While the field-induced metamagnetic transition in magnetization is reversible upon field-cycling at T > 40 K, it is irreversible at lower temperatures and this has impact on magnetoresistance, magnetothermopower as well as change in the temperature of the sample. Our results indicate high sensitivity of thermopower to changes in the magnetic state of the sample.

Magnetocaloric properties of Eu1−xLaxTiO3 (0.01 ≤ x ≤ 0.2) for cryogenic magnetic cooling

We report magnetic and magnetocaloric (MCE) properties of polycrystalline Eu1−xLaxTiO3 samples over a wide composition range (0.01 ≤ x ≤ 0.20). It is found that the ground state changes from antiferromagnetic for x = 0.01 (TN = 5.2 K) to ferromagnetic for x ≥ 0.03 and the ferromagnetic Curie temperature increases from TC = 5.7 K for x = 0.03 to TC = 7.9 K for x = 0.20. The x = 0.01 sample shows a large reversible isothermal magnetic entropy change of −ΔSm = 23 (41.5) J/kg K and adiabatic temperature change of ΔTad = 9 (17.2) K around 6.7 K for a field change of μ0ΔH = 2 (5) Tesla. Although the peak value of −ΔSm decreases as La content increases, it is impressive in x = 0.2(−ΔSm = 31.41 J/kg K at T = 7.5 K for μ0ΔH = 5 T). The large value of MCE arises from suppression of the spin entropy associated with the localized moment (J = 7/2) of Eu2+:4f7 ions. This large MCE over a wide compositional range suggests that the Eu1−xLaxTiO3 series could be useful for magnetic cooling below 40 K.

Magnetic field dependence of electrical resistivity and thermopower in Ni50Mn37Sn13 ribbons

We report magnetization, magnetoresistance (MR) and magnetothermopower (MTEP) of melt spun Ni50Mn37Sn13 ribbons which exhibit an austentite to martensite phase transition at a temperature (TM) ≈ 294 K. Upon cooling from 400 K, dc-resistivity and thermopower show abrupt changes at TM, indicating a change in the electronic density of states. The thermopower is negative from 400 K down to 10 K. Application of a magnetic field of μ0H = 5 T decreases TM by 5 K and induces large negative MR (-23%) but positive MTEP (9%) near TM. While the MR is appreciable from TM down to 10 K, MTEP is significant only below 60 K (MR = -2.5% and MTEP = +300% at 10 K). The magnetic field dependence of resistivity and thermopower show either reversible or irreversible behavior near TM, depending on whether the sample is zero-field cooled or field-cooled, which indicates that the electronic band structure near TM is magnetic history dependent.

Influence of Ga doping on rare earth moment ordering and ferromagnetic transition in Nd0.7Sr0.3Co1−xGaxO3

We report the impact of dilution of Co sublattice by non-magnetic Ga3+ ion on the magnetic, electrical, and magnetoresistive properties in Nd0.7Sr0.3Co1−xGaxO3 for x = 0–0.12. Field-cooled magnetization of the parent compound (x = 0) shows an anomalous maximum at T* = 54.6 K much below the onset of ferromagnetic transition (TC = 160 K) of the Co sublattice, which is attributed to the polarization of Nd-4f moments antiparallel to the Co-3d sublattice. Both TC and T* shift to low temperature with increasing x and the Nd-4f spin reverses from antiparallel to parallel with increasing strength of the magnetic field. While the value of high field magnetization is not seriously affected by Ga doping, coercive field at 10 K increases dramatically with increasing x. Ga substitution transforms ferromagnetic metallic state into ferromagnetic insulating state for x ≥ 0.03 and decreases the magnitude of magnetoresistance from 6% for x = 0% to 0.5% for x = 0.12.

Pressure dependence of resistivity and magnetoresistance in Pr-doped La0.7Ca0.3MnO3

We report the effects of magnetic field (μ0H = 0–7 T) and hydrostatic pressure (P = 1 bar to 9.53 kbar) on electrical resistivity in the phase separated manganite La0.3Pr0.4Ca0.3MnO3. The resistivity shows a first-order transition from paramagnetic insulating to ferromagnetic metallic state at a temperature T = TIM in ambient pressure and zero magnetic field. The first-order transition becomes second-order with increasing pressure and/or magnetic field. Both hydrostatic pressure and magnetic field decrease the magnitude resistivity and shift the resistivity peak at T = TIM towards high temperature with different rates (dTIM/dH = 13.5 K/T for P = 1 bar, 8.8 K/T for P = 9.53 kbar, and dTIM/dP ∼ 4.42 K/kbar in zero field). However, the magnitude of the magnetoresistance decreases with increasing pressure. Baroresistance in the absence of magnetic field for ΔP = 9.53 kbar reaches nearly 100% around 150 K. Interestingly, while the resistivity at a constant temperature shows irreversible behaviour upon cycling ...

Pressure dependence of resistivity and magnetic properties in a Mn1.9Cr0.1Sb alloy

We report magnetic-field and hydrostatic pressure dependent electrical resistivity and magnetic properties of a Mn1.9Cr0.1Sb alloy. Upon cooling, the magnetization of Mn1.9Cr0.1Sb exhibits a first-order ferrimagnetic to antiferromagnetic transition at the exchange inversion temperature, TS = 261 K under a 0.1 T magnetic field. Our experimental results show that TS decreases with increasing magnetic field but increase with increasing hydrostatic pressure. The pressure induced transition is accompanied by a large positive baro-resistance of 30.5% for a hydrostatic pressure change of 0.69 GPa. These results show that the lattice parameters as well as the bond distance between Mn-Mn atoms play a crucial role in the magnetic and electronic transport properties of Mn1.9Cr0.1Sb. This sample also exhibits a large inverse magnetocaloric effect with a magnetic entropy change of ΔSm = +6.75 J/kg.K and negative magnetoresistance (44.5%) for a field change of 5 T at TS in ambient pressure which may be useful for magne...