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.

[1] M. Fallot, Ann. Phys. (Paris) 10, 291 (1938); M. Fallot and R. Hocart, Rev. Sci. 77, 498 (1939)

[2] M. R. Ibarra and P. A. Algarabel, Phys. Rev. B 50, 4196 (1994)

[3] J. U. Thiele, S. Maat, E.E. Fullerton, Appl. Phys. Lett. 82, 2859 (2003),

[4] J. U. Thiele, S. Maat, J. L. Robertson, E.E. Fullerton, IEEE Trans Mag.,40, 2537(2004)

[5] R. O. Cherifi et al., Nature Materials 13, 345 (2014).

[6] X. Marti et al., Nature Materials 13, 367 (2014). 

Figures:

a) Amplitude image of the studied area (630 nm x 190 nm). b) Integrated induction as a function of the temperature for a complete loop. The values are obtained without any application of an external magnetic field from the slope of the phase shift averaged on the whole FeRh layer.

Evolution of the AF->F transition (heating) as a function of the position within the FeRh layer. The intensity corresponds to the integrated induction and shows the effects of interfaces.

Fig. 3. Magnetic phase images of the area presented in Fig.1a) at different temperatures. The graph displays the phase profile along the white arrow (20°C) for all images showing the regular alternation of F domains inside the AF matrix.

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EMC Abstracts - https://emc-proceedings.com/abstract/inside-a-ferh-layer-during-the-ferromagneticantiferromagnetic-transition-a-quantitative-study-by-off-axis-electron-holography/