ИСТИНА

Peculiar magnetic properties of nearly equiatomic Fe-Rh alloys have fascinated a lot of scientists for decades following the pioneering work of Fallot and Hocart [1], leading to a large number of published experimental and theoretical studies. Chemically ordered FeRh alloys undergo a conventional secondorder transition from the paramagnetic to the ferromagnetic state at Tc ≈ 650 K on cooling, and then a first-order ferromagnetic (FM) – antiferromagnetic (AFM) phase transition at about TF−AF = 350 K with a considerable volume expansion reaching ΔV/V of ∼ 1% during the latter. The magnetostructural FM ↔ AFM transition is accompanied by dramatic changes of physical properties, in particular FeRh exhibits giant magnetoresistive [2] and magnetocaloric [3] effects, and also giant magnetostriction [4]. Among materials with AFM - FM transitions near room temperature, FeRh represents an interesting model for basic research relevant to solid state refrigeration [5]. In recent years the study of FeRh intensified due to its potential applications in spintronics [6]. While it is well-known that properties of FeRh alloys vary substantially with heat treatment, complete understanding of the related phenomena is lacking. We report x-ray magnetic circular dichroism (XMCD) measurements [7] performed at the Rh L2,3 absorption edges to get an insight about both the magnitude and nature of the magnetic moment on Rh atoms in Fe0.49Rh0.51 alloys and the effect of the heat treatment. XMCD spectra were measured at the ESRF beamline ID12 at room temperature and under magnetic fields of up to 17 Tesla. Two arcmelted samples were annealed in vacuum in a sealed quartz ampoule at 1273 K during 48 hours and then one sample was iced-water quenched and another was cooled to room temperature at a rate of 2 K/min. Analysis of x-ray absorption spectra reveals that while the cooling rate does not affect the local electronic structure of Rh, which remains the same in both samples, it alters the Rh magnetic properties. Estimates of Rh magnetic moments have been obtained from the analysis of the XMCD data using magneto-optical sum rules. In the slowly cooled sample, the total magnetic moment of Rh is about 0.91 μB under 17 Tesla field. This result is in a good agreement with 0.9 μB deduced from neutron scattering experiments on a ferromagnetic FeRh single crystal [8]. The main advantage of XMCD over neutron scattering is the possibility to resolve the spin and the orbital magnetic moments carried by the absorbing atom. The ratio of the orbital (0.03 μB) to spin (0.88 μB) moments of Rh in the slow-cooled sample is 0.035. Magnetic properties of Rh in the rapidly quenched sample are slightly but notably different, although the total moment of 0.93 μB carried by the Rh atoms is only marginally larger. Here, the spin moment lowers to 0.84 μB, but the orbital moment is greatly enhanced, reaching 0.09 μB. This value is three times larger than in the slow-cooled sample and the deduced orbital-to-spin moment ratio becomes 0.11. The Rh magnetic moment of both samples varies as a function of applied magnetic field exhibiting first-order metamagnetism (see Figure 1). The element selective magnetization curves, measured by monitoring XMCD signal at the Rh L2-edge as a function of applied field follow the macroscopic magnetization behaviour in agreement with the hypothesis that the Rh magnetic moments arise from the exchange field at Rh atoms induced from neighbouring Fe moments.