Hall-effect characterization of the metamagnetic transition in FeRh

Abstract

The antiferromagnetic ground state and the metamagnetic transition to the ferromagnetic state of CsCl-ordered FeRh epilayers have been characterized using Hall and magnetoresistance measurements. On cooling into the ground state, the metamagnetic transition is found to coincide with a suppression in carrier density of at least an order of magnitude below the typical metallic level that is shown by the ferromagnetic state. The carrier density in the antiferromagnetic state is limited by intrinsic doping from Fe/Rh substitution defects, with approximately two electrons per pair of atoms swapped, showing that the decrease in carrier density could be even larger in more perfect specimens. The surprisingly large change in carrier density is a clear quantitative indication of the extent of change at the Fermi surface at the metamagnetic transition, confirming that entropy release at the transition is of electronic origin, and hence that an electronic transition underlies the metamagnetic transition. Regarding the nature of this electronic transition, it is suggested that an orbital selective Mott transition, selective to strongly-correlated Fe 3d electrons, could cause the reduction in the Fermi surface and change in sign of the magnetic exchange from FM to AF on cooling. Gesellschaft antiferromagnet (AF) that exhibits a first-order transition to the α phase, become a ferromagnet (FM) around T N = 350 → 400 K [2-4]. On heating, the metamagnetic transition between the two different but both fully ordered magnetic states (type II AF [5] and FM) is accompanied by an isotropic 1% volume expansion [6,7], a large entropy release [8], and a large drop in the resistivity [3]. The metamagnetic transition temperature is highly sensitive to the composition x in Fe x Rh 1−x [9, 10] and chemical doping [11], T N decreases with increased applied magnetic field [4,12] and increases with the application of pressure [13]. Neutron diffraction [5,14] and more recently x-ray magnetic circular dichroism (XMCD) measurements [15] indicate that part of the 3.3µ B magnetic moment centred on the Fe in the AF phase is transferred to the Rh in the FM phase, with µ Fe ∼ 2.2µ B and µ Rh ∼ 0.6µ B . The Curie temperature for the high-temperature FM phase is ∼670 K [3], comparable to the Curie temperature of alloys with x > 0.53 [9].The transition also occurs in epitaxially-grown and polycrystalline FeRh thin films [4, 10,16,17], albeit usually with a wider temperature hysteresis (∼15 K instead of the ∼5 K usual for bulk specimens). Thin films of FeRh have been of much recent interest because of a potential application in heat-assisted high-density magnetic recording, taking advantage of the extremely fast switching (within a few ps) from AF to FM that can be achieved with ultrashort laser pulses [18,19]. Further research on FeRh thin films is now focussed at tuning the transition towards specific applications, by growing epitaxially on a variety of substrates and in heterostructures [4,[20][21][22].There is an ongoing debate about the origin of this ...

Large resistivity modulation in mixed-phase metallic systems

“…literature 33,34 . The different intrinsic resistivities of the AFM and FM phases, namely Dr AFM À FM , are not apparent in Fig.…”

Sputter Growth and Characterization of Metamagnetic B2-ordered FeRh Epilayers

“…The simplest measurement is of the resistivity ρ of the film, since ρ in the FM phase is much less than in the AF phase 35,36,20 . The temperature dependence of ρ for the same 25 nm FeRh epilayer for which X-ray data were shown is plotted in Figure 5, measured using a standard fourpoint probe method: spring-loaded, gold-plated pins were pressed on to the sample surface to make contact to the sample, which was mounted on a heater stage in a small custom vacuum chamber to prevent any sample oxidation when hot.…”

“…The pressure should remain better than 1 x 10 -6 Torr throughout. The effects of varying this anneal temperature can be found in de Vries et al 20 9. Shut off heater power and cool the samples to room temperature.…”

“…Fe and Rh will form alloys at any composition, however a B2-ordered compound is the equilibrium state for stoichiometries in the near-equiatomic range 49-53% atomic Fe 17,18 , a large entropy release 19 , a large drop in the resistivity 14 , and a large increase in the carrier concentration 20 . Neutron diffraction 21,16 and more recently XMCD measurements 22 , and is suppressed by ~8 K/T of applied magnetic field 25,15 .…”

Magnetothermodynamic Properties and Anomalous Magnetic Phase Transition in FeRh Nanowires

“…This phase transition is accompanied by a large change in longitudinal and Hall resistivity [2]- [4], lattice constant [5] and entropy [6], [7]. The transition temperature is tunable around room temperature by a control of chemical composition [8], [9], so FeRh alloys are candidate materials for several applications such as heat-assisted magnetic recording [10], [11], AFM memories [12], [13], and memristor devices [14].…”