The metamagnetic transition in equiatomic FeRh alloys has been known over considerable time period; it is almost 80 years since the antiferromagnetic to ferromagnetic (AF-FM) transition at room temperature in these materials was first reported . Although more than 300 papers have been published, at least quarter of which is theoretical work, there is no clear agreement about the origin of the metamagnetic transition in FeRh, so this system still attracts the attention of researchers all over the world. Over time FeRh has been suggested for diverse applications such as heat assisted magnetic recording and low-power spintronics [2,3], solid-state cooling using magnetic refrigerants , and recently in medicine for hyperthermia treatment .
Figure 1: Temperature dependence of magnetization of Fe49Rh51 in magnetic fields 0.5T and 2T.
The AF-FM transition in FeRh is of first order, which implies it is accompanied by a hysteresis (Fig. 1). The crystal structure remains of B2 (CsCl) type during the transition but a large expansion (1 %) occurs in the volume of the unit cell. The Rh atoms only carry a magnetic moment in the FM state whereas the moment of the Fe atoms remains practically the same in both AF and FM states. The metamagnetic transition is accompanied by a giant magnetocaloric effect (MCE) . Among the materials showing a large MCE, FeRh holds the top place with the adiabatic temperature change of 12.9 K . For more information on the activities on magnetocaloric materials in IFW, please see the page on magnetocalorics.
Although it is not likely that FeRh will be used in bulk form in magnetic refrigerators due to raw materials costs, it is interesting as a model system for understanding fundamental spin-lattice interactions at first order transitions. Furthermore, the temperature of the metamagnetic transition in FeRh can be easily tuned over a broad temperature range by the application of a magnetic field, pressure and/or by changing the composition. Doping FeRh with other d-block elements influences, aside from the transition temperature, the effects observed at the AF-FM transition (resistivity drop, for instance ). Fig. 2 shows the effect of substituting Fe with Co on the estimated magnetic entropy change of (Fe,Co)Rh alloys during the transition. Recently an empirical model has been proposed to describe the dependence of FeRh on modification by other transition models .
The magnetic structure in AF and FM states is shown. Figure 2: Estimated magnetic entropy change of (Fe1-xCox)0.49Rh0.51 in the field change of 2 T.
Priority aspects of FeRh investigation at IFW:
- Structural and microstructural characterization, phase formation processes.
- Elucidating the various contributions to the metamagnetic in FeRh modified by other transition metals.
- Thin films of FeRh modified by other transition metals as controlled model systems.
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