Transmission electron microscope characterisation of iron-rhodium epilayers

Iron-rhodium (FeRh) alloys exhibit an unusual magnetostructural transition, making them a fascinating topic of research. When the alloy is approximately equiatomic (Fe48Rh52 to Fe56Rh44) it is in a caesium chloride (CsCl) structure. At room temperature it is antiferromagnetic (AFM), making a first-order phase transition to a ferromagnetic (FM) state when heated above ~350K. There is also a 1% increase in unit lattice volume upon heating to the FM phase, as well as an increase in entropy and decrease in resistivity. The transition can be modified by doping, applied strain and applied magnetic field among other methods. Thin film FeRh has potential uses as part of a spin valve system for magnetic data storage and as a suitable choice for a memristor.

The work presented here demonstrates the methods approached to preparing a variety of sputter-deposited thin film FeRh samples for characterisation in a transmission electron microscope (TEM) as well as by techniques such as X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). These include samples capped with epitaxial chromium (Cr) and tungsten (W) to raise and lower the transition temperature respectively. After cross-section or plan-view preparation using either a conventional ion polishing method or a focused ion beam, the samples were characterised using a variety of methods in the TEM. Initial characterisation has examined the crystal structure, layer thickness and interfacial roughness of these films, confirming results from bulk X-ray measurements. It has been found that FIB-prepared samples do not exhibit the phase transition whereas ion polished samples are unaffected. Compositional analysis of the interface FeRh makes with a magnesium oxide (MgO) substrate finds a change in iron-to-rhodium ratio while characterising the FeRh/cap interface finds significant Fe diffusion into Cr-capped samples with no interdiffusion seen in identical W-capped FeRh films. Analysis of the transition dynamics observing changes in strain and ferromagnetic domains via heating experiments have confirmed the nucleation and growth of the phase change at the interfaces. Finally, there is some evidence to suggest that a martensitic change is occurring in FeRh through the transition. The presence of twins in the FeRh as well as extra ordering spots in a FIB-prepared cross-section allude to a more complex transition than previously thought.