Glassy Magnetic Transitions and Accurate Estimation of Magnetocaloric Effect in Ni-Mn Heusler Alloys.

Abstract

In this work, the structural and magnetic transitions of Heusler alloy Ni50Mn34In14Ga2 have been carefully studied through measurements of heat flow and magnetization under DC and AC magnetic fields. This alloy undergoes the transition sequence of spin-glassy martensite (SPM) → ferromagnetic austenite (FA) → paramagnetic austenite at ∼225 and ∼305 K, respectively, during heating. Splitting of zero-field-cooling (ZFC)/field-cooling (FC) curves in martensite is caused by the slowdown dynamics of spin glass as evidenced by frequency dispersion and aging effects. The development of a spin-glass state is believed to be the result of strain relaxation and interaction of ferroelastic twin walls in the martensite. The magnetocaloric effect (MCE) at the SPM-FA transition was then measured using indirect, quasi-direct, and direct methods. The MCE magnitudes are controlled by the entropy changes associated with the first-order martensite transition and magnetic ordering of austenite under the magnetic field. The existence of a spin-glass state in martensite can also improve the reversibility of the magnetostructural transitions, which is beneficial for the improvement of the reversibility of associated MCE. These results provide an in-depth understanding of the transitions and magnetic properties of the Ni-Mn Heusler alloys and suggest that the MCE at the first-order magnetostructural transitions estimated solely using indirect methods may need some revision.