X-ray investigation of long-range antiferromagnetic ordering in FeRh

Abstract

The meta-magnetic phase transition of equiatomic FeRh allows the 3 temperature model of phase transitions to be probed as it undergoes: (i) a lattice expansion, (ii) an antiferromagnetic (AF) to ferromagnetic (FM) transition [1], and (iii) a change in electronic structure. A spin canting model has been used to describe a spin re-orientation pathway [2], where a feedback loop with induced Rh moments promotes FM order. To investigate such behaviour, a probe of the AF spins is required [3]. In general, the zero-stray field of anti-ferromagnetic materials means that it is hard to detect and perform such measurements. Here, we demonstrate a probe of long-range AF order using non-resonant x-ray scattering. The G2-type order of FeRh in the AF state provides a probe of AF order using ‘magnetic’ Bragg peaks [4].We demonstrate that these peaks can be measured using x-rays at energies below the Fe K edge. Due to the low efficiency of the magnetic scattering in this material, we describe a grazing incidence geometry to optimise the intensity (see Fig. 1). Based on Scherrer analysis we estimate a coherence length of 40-80 nm, smaller than previous reports in XMLD experiments [5], indicating we are limited by structural defects. The temperature dependent behaviour of the AF order shows an inverse correlation to the emergence of FM moment as seen in Fig. 2. The AF-FM transition as determined by the ordering appears later indicating a mixed phase [6]. Long-range AF order appears to persist when FM moment is already established.Time resolved experiments based on this technique may determine the existence of a meta-stable AF state on time scales where the FM band structure has already formed [7]. It has been shown that the lattice expansion occurs in ≈ 30 ps [8], but it is not known how the long range AF order evolves on such timescales. X-ray diffraction can therefore be used to ascertain if how and if the AF structure is related to lattice expansion occurring at the phase transition.  Dependence of the (3/2,1/2,1/2) peak intensity upon grazing incidence angle (E=4.95 keV)  The inverse correlation between the intensity of the FeRh (1/2, 3/2, 1/2) magnetic peak, and the saturation moment of the FeRh film