A Comparative Study on the Magnetocaloric Properties of Ni‐Mn‐X(‐Co) Heusler Alloys

Abstract

We present a comprehensive study on three selected Heusler alloy systems. Ni-Mn-X(-Co) systems with X ¼ Al, In, Sn are compared with respect to the relevant magnetocaloric properties of their magnetostructural phase transition, namely martensitic transition temperature as well as its field dependence, magnetization change, and width of the thermal hysteresis. The latter one is strongly determining the reversibility of the magnetocaloric effect. Therefore the understanding of how to tailor it by extrinsic and intrinsic factors is of great importance. Our study of the magnetocaloric properties leads to the conclusion that the width of thermal hysteresis can be correlated to the magnetization change of the phase transition. Consequently, the adiabatic temperature change under cycling can largely vary despite similar values of isothermal entropy change for Ni-Mn-In-Co and Ni-Mn-Sn-Co. This result therefore shows the importance of tailoring sharpness, thermal hysteresis, and field dependence of the phase transition to achieve high values for the isothermal entropy change as well as a large magnetocaloric cooling effect in the different Heusler alloys.

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

“…[186] However, compounds with an increased amounto fC os ubstitution show a smallert hermal hysteresis,w hich leads to ar eversible DT ad of À1.2 Kc ompared to À2.5 K( 50 %r eversibility) for the first field cycle for Ni 45.7 Mn 37.9 Sn 11.5 Co 4.9 . [186] Detailed investigations of the influence of thermal hysteresis on the reversibility of the magnetostructural phase transition predict that magnetic fields of 9-12 Tare necessary to induce af ully reversible MCE for the Ni-Mn-Sn-Co system. [187][188][189] During ac yclic process with lower magnetic fields,t he material cannot overcomet he thermal hysteresis completely and the characteristict emperatured ependence of magnetization, which represents the fractions of austenite and martensite during the transition, is described by minor loops inside the thermal hysteresis area.…”

“…The compounds with little or no amount of Co are characterized by a larger thermal hysteresis and lower field sensitivity of the phase transition compared to the Ni–Mn–In compounds . Therefore, the magnetocaloric signal of the second field cycle is often overlaid by heating due to the conventional MCE near the Curie temperature of the austenite phase, and the material does not show any reversibility by direct Δ T ad measurements for Ni 50 Mn 36 Sn 13 Co 1 , Ni 48.6 Mn 34.9 Sn 16.5 , and Ni 51.2 Mn 35.1 Sn 13.7 . However, compounds with an increased amount of Co substitution show a smaller thermal hysteresis, which leads to a reversible Δ T ad of −1.2 K compared to −2.5 K (50 % reversibility) for the first field cycle for Ni 45.7 Mn 37.9 Sn 11.5 Co 4.9 .…”

“…Therefore, the magnetocaloric signal of the second field cycle is often overlaid by heating due to the conventional MCE near the Curie temperature of the austenite phase, and the material does not show any reversibility by direct Δ T ad measurements for Ni 50 Mn 36 Sn 13 Co 1 , Ni 48.6 Mn 34.9 Sn 16.5 , and Ni 51.2 Mn 35.1 Sn 13.7 . However, compounds with an increased amount of Co substitution show a smaller thermal hysteresis, which leads to a reversible Δ T ad of −1.2 K compared to −2.5 K (50 % reversibility) for the first field cycle for Ni 45.7 Mn 37.9 Sn 11.5 Co 4.9 . Detailed investigations of the influence of thermal hysteresis on the reversibility of the magnetostructural phase transition predict that magnetic fields of 9–12 T are necessary to induce a fully reversible MCE for the Ni–Mn–Sn–Co system …”

Energy Applications of Magnetocaloric Materials

“…The adiabatic temperature change and the isothermal entropy change of the magnetocaloric material should be correctly indicated under the cycling conditions and realistic conditions for any refrigeration or heat pumping system. This important fact actually concerns hysteretic materials and was excellently addressed in a recent publication by Gottschall et al This is also the reason why low‐cost and rare‐earth‐free Heusler alloys, despite having been the focus of intensive research, are currently not being considered for the development of magnetic refrigeration and heat pumping systems, except there are relevant new findings …”

Probing the Martensitic Microstructure of Magnetocaloric Heusler Films by Synchrotron Diffraction

“…Here, we investigate the Ni−Mn−Ga−Co system as a model system for magnetocaloric Heusler alloys, which also show good magnetocaloric properties with other elements like In, Sn, and Al when used instead of Ga . These alloys transform from a cubic austenite into a tetragonal martensite.…”