Search

Abstract: Strong coupling effects in magnetocaloric materials are the key factor to achieve a large magnetic entropy change. Combining insights from experiments and ab initio calculations, we review relevant coupling phenomena, including atomic coupling, stress coupling, and magnetostatic coupling. For the investigations on atomic coupling, we have used Heusler compounds as a flexible model system. Stress coupling occurs in first‐order magnetocaloric materials, which exhibit a structural transformation or volume change together with the magnetic transition. Magnetostatic coupling has been experimentally demonstrated in magnetocaloric particles and fragment ensembles. Based on the achieved insights, we have demonstrated that the materials properties can be tailored to achieve optimized magnetocaloric performance for cooling applications. [ABSTRACT FROM AUTHOR]

Doping with Co and Fe is a promising strategy to enhance the performance of Ni‐Mn‐based magnetocaloric materials. We investigate here the impact of these two elements on the magnetic properties and the martensitic transformations in Mn‐rich Ni‐Mn‐Al Heusler alloy. Based on ab initio ground‐state energies, we explain the martensitic transformation temperatures in these alloys. The magnetic ground states of the austenite and the martensite phases are discussed on the basis of magnetic exchange parameters. A thorough comparison of the transition temperatures and the change in saturation moments during martensitic transformation brings into light the relative potential of the two substitutional elements for future applications. [ABSTRACT FROM AUTHOR]

The paper discusses the stabilization of the martensite in NiMnGa at finite temperatures that is caused by the substitution of Ni by Pt. For this purpose a recently developed ab initio based formalism employing density functional theory is applied. The free energies of the relevant austenite and martensite phases of NiPtMnGa are determined incorporating quasiharmonic phonons and fixed-spin magnons. In addition the dependence of the transition temperatures on the Pt concentration is investigated. Though our results are in qualitative agreement with estimates based on ground-state energies, they clearly demonstrate that a proper treatment of finite temperature contributions is important to predict the martensitic transition quantitatively. [ABSTRACT FROM AUTHOR]

[Display omitted] • Incipient phase transformations in NiMnGa thin films were probed across temperature. • Stress-induced phase transformations emerge as pop-ins for thinner films. • Non-modulated martensite forms during pop-ins at high transformation stresses. • Thickness constraints increase the phase-transformation stress. Ni-Mn-Ga shape-memory alloys are promising candidates for large strain actuation and magnetocaloric cooling devices. In view of potential small-scale applications, we probe here nanomechanically the stress-induced austenite–martensite transition in single crystalline austenitic thin films as a function of temperature. In 0.5 µm thin films, a marked incipient phase transformation to martensite is observed during nanoindentation, leaving behind pockets of residual martensite after unloading. These nanomechanical instabilities occur irrespective of deformation rate and temperature, are Weibull distributed, and reveal large spatial variations in transformation stress. In contrast, at a larger film thickness of 2 μm fully reversible transformations occur, and mechanical loading remains entirely smooth. Ab-initio simulations demonstrate how an in-plane constraint can considerably increase the martensitic transformation stress, explaining the thickness-dependent nanomechanical behavior. These findings for a shape-memory Heusler alloy give insights into how reduced dimensions and constraints can lead to unexpectedly large transformation stresses that need to be considered in small-scale actuation design. [ABSTRACT FROM AUTHOR]