[P1] Applications

[P1-45] Formation of ferromagnetic clusters affecting the first-order phase transition in off-stochiometric Fe-Rh

*Alex Aubert¹, Konstantin Skokov¹, Andrei Rogalev², Alisa Chirkova¹, Benedikt Beckmann¹, Fernando Maccari¹, Fabrice Wilhelm², Esmaeil Adabifiroozjaei³, Leopoldo Molina-Luna³, Gabriel Gomez⁴, Benedikt Eggert⁴, Katharina Ollefs⁴, Heiko Wende⁴, Oliver Gutfleisch¹ (1. Functional Materials, TU Darmstadt (Germany), 2. ESRF (France), 3. Advanced Electron Microscopy, TU Darmstadt (Germany), 4. CENIDE, University of Duisburg-Essen (Germany))

Keywords:

Magnetocaloric application,First order phase transition

First-order phase transitions (FOPT) materials have a great potential for magnetic refrigeration application, provided a reduction of their thermal and magnetic hysteresis [1,2]. Among the magnetocaloric materials with FOPT, FeRh is usually considered as a reference system because it is a "simple" binary system with CsCl structure exhibiting large adiabatic temperature change [3]. Recently, ab-initio theory predictions have shown that slight changes in the stoichiometry of FeRh has a strong influence on the FOPT characteristics [4]. However, this theoretical prediction was never clearly shown experimentally. Here, we investigate the composition dependence of the transitional hysteresis in FeRh. We compare phase pure bulk samples produced in a similar way in order to neglect the effect of microstructure, phases, synthesis process and strain. It is shown that a Fe excess of only 1 at. % induces a ferromagnetic state in the whole temperature range (from 5 K up to Tc) for a minor portion of the sample (≈10%), while 5 at. % is enough to completely eliminate the FOPT. Element-specific X-ray magnetic circular dichroism (XMCD) measurements suggest that this ferromagnetic contribution arises from residual FeRh ferromagnetic regions. This was observed in party thanks to a novel instrument designed for advanced magnetic study, installed at the ID12 beamline of the European Synchrotron Radiation Facility in Grenoble, France. This instrument offers the unique capability to simultaneously measure element-specific microscopic and macroscopic properties related to the magnetic, electronic and structural characteristics of materials [5]. Using this capability, we probed the FOPT in FeRh, revealing the presence of residual ferromagnetic regions. Mössbauer spectroscopy further suggests that Fe antisite defects are responsible for their formation, as it demonstrates the presence of Fe atoms occupying the 1b (Rh) sites in the CsCl-type structure (see Figure). As a consequence, compared with the equiatomic composition, the slightly Fe-rich sample exhibits completely different FOPT properties, influencing the magnetocaloric performances. Thus, our study sheds light on the origin of the remarkable stoichiometric sensitivity of the FOPT behavior in FeRh. These insights have broader implications for understanding FOPT dynamics and the role of residual ferromagnetic domains [6].

References
[1] J. Liu, et al. Nature materials 11(7) 620 (2012)
[2] O. Gutfleisch et al., Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374 (2074) 20150308 (2016)
[3] M. Fallot, Annales de physique. Vol. 11. No. 10. (1938)
[4] J. B. Staunton, et al. Phys. Rev. B 89 054427 (2014)
[5] A. Aubert, et al. J. Synchrotron Radiation, 32(2) (2025)
[6] A. Aubert, et al. ACS Applied Materials & Interfaces 16 (45), 62358 (2024)

Acknowledgement
This work was supported in part by the German Federal Ministry of Education and Research (BMBF) under Grant BMBF-Projekt05K2019 and in part by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Project 405553726-TRR270.

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