Co-Cr-(Ga/Al)-Si magnetic shape memory alloys exhibit the unique re-entrant martensitic transitions [1] and the elastocaloric switching effect [2]. Both behaviors result from a competition between lattice instability and ferromagnetism in these alloys, giving rise to coupled phenomena not having close analogs in conventional shape memory alloys.
In this talk, observations of such phenomena at elevated temperatures (> 400 K) using laser-ultrasonic approaches will be reported for single-crystals of the Co-Cr-Ga-Si alloy. Two unusual transitions appear: Firstly, upon heating, the low-temperature cubic phase transforms abruptly into the martensite phase, which is a transition not governed by an elastic instability of the lattice and is, thus, accompanied with a fast and massive release of mechanical energy. Secondly, after the martensite transforms to austenite through a conventional reverse martensitic transition at $\sim$ 700 K, the austenite phase becomes chemically stabilized, but with cooling it undergoes an unusual, magnetism-triggered, transition between mechanically stable and unstable lattices. The latter is observed as a strong mechanical stiffening of the cubic phase over the Curie point at 560 K, and can be probably interpreted as a metal-to-half-metal transition. Both these phenomena open pathways to non-conventional high-temperature applications of Co-Cr-based shape memory alloys.

[1]  X. Xu, T. Omori, M. Nagasako, A. Okubo, R.Y. Umetsu, T. Kanomata, K. Ishida, R. Kainuma, Cooling-induced shape memory effect and inverse temperature dependence of superelastic stress in Co2Cr(Ga,Si) ferromagnetic Heusler alloys, Applied Physics Letters, 103 (16) (2013) art. no. 164104.

[2] T. Odaira, A. Xu, X. Xu, T. Omori, R. Kainuma, Elastocaloric switching effect induced by reentrant martensitic transformation, Applied Physics Reviews, 7 (3) (2020) art. no. 031406.