FeRh and MnAs alloys present a magnetic transition as a function of temperature. The mechanisms of this transition were mainly studied with surface investigation techniques without visualization of the magnetic reorganization in volume. For the first time, the use of in situ electronic holography on thin films demonstrates the effects of discontinuities and defects on the magnetic transition at the nanometer scale.
The control of a magnetic state by thermal or electrical activation is essential for the development of new magnetic devices, for example for magnetic recording. Compounds such as FeRh or MnAs, which undergoes a magnetic transition from a ferromagnetic state to a state of zero magnetization (antiferromagnetic or paramagnetic) close to room temperature are expected for such applications. However, the mechanisms involved in the transition are still under debate as only surface techniques in previous studies have been carried on, while the transition is a three dimensional phenomenon.
In this study, in situ heating / cooling electron holography is used to quantitatively map the magnetization of FeRh and MnAs thin films from the cross-section view through the magnetic transition at the nanoscale. We demonstrate the possibility of measuring very locally a magnetization vs temperature loop. This approach reveals for both alloys an inhomogeneous spatial distribution of the transition temperature but also a variation of the temperature range required to complete this transition in the whole layers. These results show the effects of the surface and the interface with the substrate. For FeRh alloy, we evidenced an unexpected transition mechanism with first the appearance of a periodic spacing of nucleated ferromagnetic domains followed by a spatial extension during transition monitoring. For MnAs thin films, we demonstrate the appearance of the types of ferromagnetic domains during the transition with different magnetic anisotropy and spatial extension. Such ferromagnetic domains play an important role in the stabilization of the magnetic walls.
Beyond these results on the fundamental transition mechanisms, our work brings a new illustration of the development of experiments of electronic holography under solicitation, here by the control of the temperature. The study under an electric field would also provide information on the magnetoelectric coupling in multiferroic compounds.
This project was supported by the ANR EMMa project (ANR12 BS10 013 01).
C. Gatel, B. Warot-Fonrose, N. Biziere, L.-A. Rodriguez, D. Reyes, R. Cours, M. Castiella, M.-J. Casanove, “ Inhomogeneous spatial distribution of the magnetic transition in an iron-rhodium thin film ”, Nature Communications (2017), doi:10.1038/ncomms15703.
C. Gatel, X. Fu, V. Serin, M. Eddrief, V. Etgens and B. Warot-Fonrose “ In depth spatially inhomogeneous phase transition in epitaxial MnAs film on GaAs(001) ”, Nano Letters 17, pp 2460-2466 (2017)
Christophe GATEL, CEMES (CNRS)
gatel at cemes.fr, 05 67 52 43 49
Bénédicte WAROT, CEMES (CNRS)
warot at cemes.fr, 05 67 52 43 48