Effect of Co and Fe on the inverse magnetocaloric properties of Ni-Mn-Sn

At certain compositions Ni-Mn-$X$ Heusler alloys ($X$: group IIIA-VA elements) undergo martensitic transformations, and many of them exhibit inverse magnetocaloric effects. In alloys where $X$ is Sn, the isothermal entropy change is largest among the Heusler alloys, particularly in Ni$_{50}$Mn$_{37}$Sn$_{13}$ where it reaches a value of 20 Jkg$^{-1}$K$^{-1}$ for a field of 5T. We substitute Ni with Fe and Co in this alloy, each in amounts of 1 at% and 3 at% to perturb the electronic concentration and examine the resulting changes in the magnetocaloric properties. Increasing both Fe and Co concentrations causes the martensitic transition temperature to decrease, whereby the substitution by Co at both compositions or substituting 1 at% Fe leads to a decrease in the magnetocaloric effect. On the other hand, the magnetocaloric effect in the alloy with 3 at% Fe leads to an increase in the value of the entropy change to about 30 Jkg$^{-1}$K$^{-1}$ at 5T.Comment: 5 pages, 7 figures. Accepted for publication in the Journal of Applied Physic

Magnetization easy-axis in martensitic Heusler alloys estimated by strain measurements under magnetic-field

We study the temperature dependence of strain under constant magnetic-fields in Ni-Mn based ferromagnetic Heusler alloys in the form Ni-Mn-$X$ ($X$: Ga, In, Sn, Sb) which undergo a martensitic transformation. We discuss the influence of the applied magnetic-field on the nucleation of ferromagnetic martensite and extract information on the easy-axis of magnetization in the martensitic state.Comment: 3 pages, 3 figures. Accepted for publication in Applied Physics Letter

Reversible and irreversible colossal barocaloric effects in plastic crystals

The extremely large latent heat exchanged in phase transitions involving strong molecular orientational disordering has recently led to the proposal of plastic crystals as a feasible solution for solid-state barocaloric eco-friendly cooling technologies. Here we determine the reversible barocaloric response of four plastic crystals derived from neopentane [C(CH3)4]: (NH2)C(CH2OH)3 (TRIS for short), (NH2)(CH3)C(CH2OH)2 (AMP), (CH3)C(CH2OH)3 (PG) and (CH3)3C(CH2OH) (NPA). All of them display colossal entropy changes at their ordered-plastic phase transition, which is a primal requirement for competitive barocaloric materials. However, we show that it is also important to verify that the large barocaloric effects can be achieved using pressures that, while being moderate, are large enough to overcome the pressure-dependent hysteresis. From this quantity and using the quasi-direct method, we determine the minimum pressure needed to achieve reversible barocaloric effects, prev, for each compound. Specifically, we find a small and moderate prev for PG and NPA, respectively, which therefore display colossal reversible barocaloric effects comparable to harmful fluids used in current refrigerators and thus confirm the potential of plastic crystals as excellent alternatives. Instead, in TRIS and AMP, the obtained prev is excessive to yield reversible barocaloric effects useful for cyclic applications.This work was supported by the MINECO projects MAT2016-75823-R and FIS2017-82625-P, the DGU project 2017SGR-42, the UK EPSRC grant EP/M003752/1, and the ERC Starting grant no. 680032. X. M. is grateful for support from the Royal Society

Temperature and magnetic field dependences of the elastic constants of Ni-Mn-Al magnetic Heusler alloys

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Temperature and magnetic field dependences of the elastic constants of Ni-Mn-Al magnetic Heusler alloys

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In-situ observations of a martensitic transformation in a CuZnAl single crystal driven by stress or strain

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