The peculiarity of FeRh alloys: a thorough TEM investigation

Abstract text (incl. figure legends and references)

Introduction: Fe50Rh50 alloys are known to have a B2 BCC structure with an antiferromagnetic (AFM) to ferromagnetic (FM) transition at near room temperature. The transition is isostructural with about 1% change in volume and is accompanied by giant magnetoresistive and magnetocaloric effects. Therefore, there has been a lot of interest on the Fe50Rh50 alloy as a model system [1]. Interestingly, the majority of the previously reported work do not provide extensive local atomic structure investigations (mostly thin films). Thus, the structural characteristics of the alloy at the AFM or FM states remains controversial [2]. Since this alloy is among the betta alloy series (CdAu, TiNi, Fe-C, etc…), it is expected to present a pre-martensite structure followed by a martensite structure upon cooling at cryogenic temperature[3]. The martensite was also predicted by extensive first principal calculations[2,4]. However, so far, no evidence has been given regarding the formation of either pre-martensite or martensite structures in the Fe50Rh50 alloy.

Objective: To use various TEM techniques to investigate the FeRh 50/50 alloy and demonstrate that at the AFM state, the alloy is in the pre-martensite state.

Materials and methods: Fe50Rh50 alloys were prepared at the German Electron Synchrotron (DESY) using an electromagnetic levitation facility developed and constructed at the IFW Dresden. The Fe50Rh50 material was alloyed by arc-melting of the appropriate amounts of the pure Fe (99.995%, Alfa Aesar) and Rh (99.9%, Alfa Aesar). After casting, the alloys were heat-treated at 1150°C for 9 days and quenched in water. During the heat-treatment the alloys were sealed in quartz tube with Ar atmosphere. Afterwards, the bulk alloys were cut by wire cutter and thin slices were polished to thickness of ~50 µm. Then, the slices were milled using precision ion polishing system (PIPS) with energy of 3 KeV and angles of 4 degrees. The prepared thin sections were investigated using a variety of TEM techniques including CTEM, HRTEM, STEM (HAADF), and EDS.

Results: Fig. 1 shows the HRTEM and corresponding Fast Fourier Transformed (FFT) images of the FeRh 50/50 alloy in three principal directions ([001], [-110], and [111]). As seen, although the structure of the alloy match perfectly with the B2 BCC structure, there is systematic modulation along certain reflexes (100 and 110). This was also confirmed by HAADF-STEM imaging, the results of which are given as Fig. 2. In the corresponding Z-contrast images, the modulation can only be easily seen in the [111] zone axis.

Conclusions: Our results show that the B2 BCC alloy of FeRh 50/50 at the AFM state possess a pre-martensite structure.

We acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within the CRC/TRR 270 (Project-ID 405553726).

Figure 1. Representative nanostructure of FeRh alloys in three principal directions: a, b and c are HRTEM images in [001], [-110], and [111] zone axes, while d, e, and f are FFT images of a, b, and c. Modulation in HRTEM images are shown as parallel pale green lines, while in FFT images related superlattice reflexes are distinguished by red circles.

Figure 2. Representation of arrangement of Fe (small) and Rh (large) atoms in three principal zone axes of alloy. Modulated structure (dashed parallelograms) is visible in [111] zone axis. For comparison, ideal arrangement of atoms exported from B2-BCC structure are imposed on nanostructures. In imposed images, green and yellow atoms stand for Fe and Rh, respectively.