Role of disorder when upscaling magnetocaloric Ni-Co-Mn-Al Heusler alloys from thin films to ribbons

B Weise,T Hickel,N Teichert,A Hütten,A Waske,B Dutta

doi:10.1038/s41598-018-27428-8

Abstract

Research in functional magnetic materials often employs thin films as model systems for finding new chemical compositions with promising properties. However, the scale-up of thin films towards bulk-like structures is challenging, since the material synthesis conditions are entirely different for thin films and e.g. rapid quenching methods. As one of the consequences, the type and degree of order in thin films and melt-spun ribbons are usually different, leading to different magnetic properties. In this work, using the example of magnetocaloric Ni-Co-Mn-Al melt-spun ribbons and thin films, we show that the excellent functional properties of the films can be reproduced also in ribbons, if an appropriate heat treatment is applied, that installs the right degree of order in the ribbons. We show that some chemical disorder is needed to get a pronounced and sharp martensitic transition. Increasing the order with annealing improves the magnetic properties only up to a point where selected types of disorder survive, which in turn compromise the magnetic properties. These findings allow us to understand the impact of the type and degree of disorder on the functional properties, paving the way for a faster transfer of combinatorial thin film research towards bulk-like materials for magnetic Heusler alloys.

Highlights

Thin films are very good model systems to study the properties of functional magnetic materials[1] when varying their chemical composition[2] and degree of disorder[3]
We investigate experimentally the impact of chemical disorder present in the melt-spun ribbons of Ni-Co-Mn-Al alloy on the magnetocaloric properties entropy change ΔST and thermal hysteresis
We have shown that it is possible to reproduce the excellent properties of highly ordered magnetocaloric Ni-Co-Mn-Al thin films with melt-spun ribbons if an appropriate heat treatment is applied

Summary

Introduction

Thin films are very good model systems to study the properties of functional magnetic materials[1] when varying their chemical composition[2] and degree of disorder[3]. Magnetocaloric Heusler alloys have gained attention recently due to their versatile properties upon slight variation in the chemical composition[5,6] In this material class, the martensite to austenite transition is associated with large changes in entropy and correspondingly large adiabatic temperature changes, which form the basis of a magnetic cooling cycle[7]. We investigate experimentally the impact of chemical disorder present in the melt-spun ribbons of Ni-Co-Mn-Al alloy on the magnetocaloric properties entropy change ΔST and thermal hysteresis (δThyst). Our observations are supported by first principles calculations, which provide a qualitative picture of the underlying disordered atomic configurations and their subsequent impact on the above mentioned magnetocaloric properties

Methods

Results

Discussion

Conclusion