We investigated the martensitic transition and the magnetic properties of Mn 1-x CoGe melt-spun ribbons. The as-prepared Mn 1-x CoGe ribbons crystallize in austenite hexagonal phase with a textured structure. The postannealing process promotes the formation of the martensitic phase and homogenization of the alloy, resulting in a first-order magnetostructural transition in the annealed ribbons, and thus a giant magnetocaloric effect. The magnetic entropy change around the transition reaches 19 J/kgK for a magnetic field change of 0-5 T. Furthermore, it is found that the hysteresis loss around the magnetostructural transition is negligible in present annealed ribbons, which would facilitate the application of Mn 1-x CoGe alloys.
MnCoGe-based compounds undergo a giant negative thermal expansion (NTE) during the martensitic structural transition from Ni2In-type hexagonal to TiNiSi-type orthorhombic structure. High-resolution neutron diffraction experiments revealed that the expansion of unit cell volume can be as large as ΔV/V ∼ 3.9%. The optimized compositions with concurrent magnetic and structural transitions have been studied for magnetocaloric effect. However, these materials have not been considered as NTE materials partially due to the limited temperature window of phase transition. The as-prepared MnCoGe-based compounds are quite brittle and naturally collapse into powders. By using a few percents (3-4%) of epoxy to bond the powders, we introduced residual stress in the bonded samples and thus realized the broadening of structural transition by utilizing the specific characteristics of lattice softening enforced by the stress. As a result, giant NTE (not only the linear NTE coefficient α but also the operation-temperature window) has been achieved. For example, the average α̅ as much as -51.5 × 10(-6)/K with an operating temperature window as wide as 210 K from 122 to 332 K has been observed in a bonded MnCo0.98Cr0.02Ge compound. Moreover, in the region between 250 and 305 K near room temperature, the α value (-119 × 10(-6)/K) remains nearly independent of temperature. Such an excellent performance exceeds that of most other materials reported previously, suggesting it can potentially be used as a NTE material, particularly for compensating the materials with large positive thermal expansions.
Abstract The most widespread cooling techniques based on gas compression/expansion encounter environmental problems. Thus, tremendous effort has been dedicated to develop alternative cooling technique and search for solid state materials that show large caloric effects. An application of pressure to a material can cause a change in temperature, which is called the barocaloric effect. Here we report the giant barocaloric effect in a hexagonal Ni 2 In-type MnCoGe 0.99 In 0.01 compound involving magnetostructural transformation, T mstr , which is accompanied with a big difference in the internal energy due to a great negative lattice expansion( ΔV/V ~ 3.9%). High resolution neutron diffraction experiments reveal that the hydrostatic pressure can push the T mstr to a lower temperature at a rate of 7.7 K/kbar, resulting in a giant barocaloric effect. The entropy change under a moderate pressure of 3 kbar reaches 52 Jkg −1 K −1 , which exceeds that of most materials, including the reported giant magnetocaloric effect driven by 5 T magnetic field that is available only by superconducting magnets.
Phase transition and the magnetocaloric effect (MCE) in Nd0.5Sr0.5MnO3 (NSMO) epitaxial thin films were tailored through controlling the lattice-mismatch-induced-strain by depositing on (011)—(La0.18Sr0.82)(Al0.59Ta0.41)O3 and SrTiO3 (STO) single crystalline substrates, respectively. The NSMO film grown on STO, exhibiting uniaxial like tensile strain of 1.3% along the in-plane [100] direction, undergoes a paramagnetic to ferromagnetic transition at ∼210 K followed by a ferromagnetic to A-type antiferromagnetic transition at ∼179 K upon cooling; meanwhile, the film grown on LSAT, exhibiting anisotropic in-plane tensile strains of 0.36% along [100] and 0.50% along [01¯1] directions, undergoes further transition to CE-type antiferromagnetic transition at ∼145 K. NSMO/LSAT with such transitions facilitates a strong MCE over a much wider temperature range from 90 to 170 K, with the magnetic entropy change comparable to the recently reported La0.25Ca0.75MnO3 bulk. These findings suggest that control of strain in manganite films with first-order phase transition is a feasible way to broaden their MCE temperature range.
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