Inside a FeRh layer during the ferromagnetic/antiferromagnetic transition: a quantitative study by off‐axis electron holography
Authors: Gatel Christophe , Warot‐Fonrose Bénédicte , Rodriguez Luis‐Alfredo , Reyes David , Biziere Nicolas , Cours Robin , Casanove Marie‐José 
Published: December 2016

The ordered FeRh alloy presents very intriguing magnetic properties, among which a remarkable magnetic phase transition from an antiferromagnetic (AF) state at low temperature to a ferromagnetic (F) state just above room temperature accompanied by a 1% volume expansion upon entering the FM state [1‐2]. In recent years, this alloy has encountered a huge regain of interest for its strong potential for future applications: its properties can be usefully exploited in new devices for microelectronics, heat‐assisted magnetic recording [3‐5] or magnetic random access memories based on AFM spintronics [6].
Many studies are devoted to the understanding of the ferromagnetic/antiferromagnetic transition mechanism in FeRh and several models have been proposed. In most of these studies, magnetic properties are measured through macroscopic measurements on the whole film. A local magnetic study could provide new insights regarding the transition process, by clarifying phenomena happening at a nanometer scale.

We propose to bring new details on the F‐AF transition in a FeRh layer by acquiring quantitative magnetic mapping at the nanoscale on a cross section. The experimental technique that combines high sensitivity to the electromagnetic field up to nanometer resolution on a cross sectional sample with an
in situ
temperature control is the electron holography (EH) in a TEM. We have then achieved to get magnetic mapping of a 50 nm FeRh layer grown on a MgO substrate through the F‐AF transition (Fig. 1a).

The evolution of the induction as a function of temperature has been recorded at a local scale and shows similar features than the one obtained by macroscopic measurements (Fig. 1b). However we observed heterogeneity of the transition in the film thickness: near the interfaces, the magnetic transition from the AF state to the F state starts much earlier and is spread over a wider range of temperature than in the middle of the layer (Fig. 2). The interfaces not only lower the transition temperature, but make this transition more difficult to achieve over a long distance. The presence of structural defects (dislocations, …) at the interface with the substrate but also the breaking of symmetry significantly locally modifies the transition F / AF.
Various schemes of the transition have also been evidenced (Fig. 3). For instance, in the heating process (AF‐>F), “homogeneous” transition to the F state at interfaces starts first, following by a F domain nucleation in layer that begins even if the transition at interfaces is not completed, growth of the F domains within the AF matrix and then coalescence until the complete disappearance of the AF state. One of the most remarkable results during the F domain growth is the constant period of about 100 nm reflecting the regular alternation of the areas F and AF (Fig. 3). Note that the value of this period is comparable to the distance between dislocations to get a complete relaxation of a FeRh layer on MgO (80 to 100 nm) and could explain the nucleation F domain pinned by structural defects such as dislocations.