Aubert Alex, Skokov Konstantin, Rogalev Andrei, Chirkova Alisa, Beckmann Benedikt, Maccari Fernando, Dilmieva Elvina, Wilhelm Fabrice, Nassif Vivian, Diop Léopold V B, Bruder Enrico, Löfstrand Julia, Primetzhofer Daniel, Sahlberg Martin, Adabifiroozjaei Esmaeil, Molina-Luna Leopoldo, Gomez Gabriel, Eggert Benedikt, Ollefs Katharina, Wende Heiko, Gutfleisch Oliver
Functional Materials, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany.
European Synchrotron Radiation Facility, 38000 Grenoble, France.
ACS Appl Mater Interfaces. 2024 Nov 13;16(45):62358-62370. doi: 10.1021/acsami.4c12432. Epub 2024 Oct 29.
Among the magnetocaloric materials featuring first-order phase transitions (FOPT), FeRh is considered as a reference system to study the FOPT because it is a "simple" binary system with a CsCl structure exhibiting a large adiabatic temperature change. Recently, ab initio theory predicted that changes in the Fe/Rh stoichiometry in the vicinity of equiatomic composition strongly influence the FOPT characteristics. However, this theoretical prediction was not clearly verified experimentally. Here, we investigated the composition dependence of the transitional hysteresis in FeRh. 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 ) 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. We attribute the formation of such domains to Fe antisite defects, as Mössbauer spectroscopy demonstrates the presence of Fe atoms located at the 1b (Rh) sites in the CsCl-type structure. 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.
在具有一级相变（FOPT）的磁热材料中，FeRh被视为研究FOPT的参考体系，因为它是一种具有CsCl结构的“简单”二元体系，表现出较大的绝热温度变化。最近，从头算理论预测，在等原子组成附近Fe/Rh化学计量比的变化会强烈影响FOPT特性。然而，这一理论预测尚未得到实验的明确验证。在此，我们研究了FeRh中转变磁滞的成分依赖性。结果表明，仅1原子%的Fe过量会在样品的一小部分（≈10%）的整个温度范围（从5 K到）内诱导出铁磁态，而5原子%就足以完全消除FOPT。元素特异性X射线磁圆二色性（XMCD）测量表明，这种铁磁贡献来自残余的FeRh铁磁区域。我们将这种畴的形成归因于Fe反位缺陷，因为穆斯堡尔谱证明在CsCl型结构的1b（Rh）位点存在Fe原子。因此，与等原子组成相比，略富Fe的样品表现出完全不同的FOPT特性，影响了磁热性能。因此，我们的研究揭示了FeRh中FOPT行为对化学计量比显著敏感性的起源。这些见解对于理解FOPT动力学和残余铁磁畴的作用具有更广泛的意义。
