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The effects of doping, hydrostatic pressure, and thermal quenching on the phase transitions and magnetocaloric properties of the Mn1−xCoxNiGe system have been investigated. Cobalt doping on the Mn site shifted the martensitic structural transition toward lower temperature until it was ultimately absent, leaving only a magnetic transition from a ferromagnetic (FM) to a paramagnetic (PM) state in the high-temperature hexagonal phase. Co-occurrence of the magnetic and structural transitions to form a first-order magnetostructural transition (MST) from the FM orthorhombic to the PM hexagonal phase was observed in samples with 0.05 < x < 0.20. An additional antiferromagnetic–ferromagnetic-like transition was observed in the martensite phase for 0.05 < x < 0.10, which gradually vanished with increasing Co concentration (x > 0.10) or magnetic field (H > 0.5 T). The application of external hydrostatic pressure shifted the structural transition to lower temperature until an MST was formed in samples with x = 0.03 and 0.05, inducing large magnetic entropy changes up to −80.3 J kg−1 K−1 (x = 0.03) for a 7-T field change under 10.6-kbar pressure. Similar to the effects of the application of hydrostatic pressure, an MST was formed near room temperature in the sample with x = 0.03 by annealing at high temperature (1200 °C) followed by quenching, resulting in a large magnetic entropy change of −56.2 J kg−1 K−1. These experimental results show that the application of pressure and thermal quenching, in addition to compositional variations, are effective methods to create magnetostructural transitions in the MnNiGe system, resulting in large magnetocaloric effects.
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Room temperature magnetocaloric effects in monoclinic Cr3Te4
We report a systematic investigation of anisotropic magnetocaloric effects in a crystalline, monoclinic Cr3Te4 sample grown by the chemical vapor transport (CVT) method. The maximum magnetic entropy change −ΔSmaxM is 3.31 J kg−1 K−1 for the c axis (3.16 J kg−1 K−1 for the ab-plane) and the relative cooling power (RCP) is 340 J kg−1 for the c axis (350 J kg−1 for the ab-plane) near the Curie temperature with a magnetic field (μ0H) change of 9 T. With the scaling analysis of ΔSM, all rescaled ΔSM(T, H) curves collapse onto a single universal curve, indicating a second-order magnetic phase transition in Cr3Te4. Furthermore, −ΔSmaxM follows the power law of Hn with n = 0.656 ± 0.005. The RCP and δTFWHM have Hc and Hb dependence on field, with c = 1.179 ± 0.011 and b = 0.498 ± 0.005, respectively, which led us to estimate the critical exponents of β = 0.359 ± 0.013, γ = 1.646 ± 0.057, and δ = 5.578 ± 0.190.
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- Award ID(s):
- 2039380
- PAR ID:
- 10569212
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 137
- Issue:
- 4
- ISSN:
- 0021-8979
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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