Phase Coexistence and Kinetic Arrest in the Magnetostructural Transition of the Ordered Alloy FeRh

Abstract

In materials where two or more ordering degrees of freedom are closely matched in their free energies, coupling between them, or multiferroic behavior can occur. These phenomena can produce a very rich phase behavior, as well as emergent phases that offer useful properties and opportunities to reveal novel phenomena in phase transitions. The ordered alloy FeRh undergoes an antiferromagnetic to ferromagnetic phase transition at ~375 K, which illustrates the interplay between structural and magnetic order mediated by a delicate energy balance between two configurations. We have examined this transition using a combination of high-resolution x-ray structural and magnetic imaging and comprehensive x-ray magnetic circular dichroism spectroscopy. We find that the transition proceeds via a defect-driven domain nucleation and growth mechanism, with significant return point memory in both the structural and magnetic domain configurations. The domains show evidence of inhibited growth after nucleation, resulting in a quasi-2 nd order temperature behavior.Magneto-structural phase transitions offer new ways of accessing multiferroic properties, as well as a probe of the relationships between structure, magnetic and electronic ordering 1 . The phenomenology of these coupled transitions is immensely varied, as they may arise in any condensed matter system where structural, magnetic and electronic energies are closely matched. This can result in large changes of electronic/magnetic order with very subtle structural transitions. Often, materials that display such multiferroic behaviors have complex crystalline structures, with multiple avenues for distortions and symmetry changes that may result in electronic property modifications. While these materials offer much promise for discovery of new phases and applications, systems that exhibit coupled transitions with a relatively simple structural change may enable better control of the transition as well as the opportunity to study the physics of the coupling.The CsCl-structured alloy FeRh experiences a first-order structural phase transition above room temperature characterized by a ~1% isotropic volume expansion coupled with a magnetic transition from a low-temperature antiferromagnetic (AFM) phase to a high-temperature ferromagnetic (FM) phase [2][3][4][5][6][7][8][9][10] . During the magnetic transition, the Fe sublattice switches from antiparallel to parallel exchange, while the Rh develops a magnetic moment of approximately 0.9 µ B 11-14 . The transition temperature can be tuned easily by an applied magnetic field, with a reported effect of −9 K/T 15,16 . In addition, other external stimuli, such as piezoelectric strain has been found to influence the magnetization of the FM phase 9 as well as the magnetocaloric properties 17 near the transition. In epitaxial films, the isotropic expansion is modified by substrate clamping to become an out-of-plane tetragonal distortion 18 . The transition temperature may vary in films with thickness and substrate mismatch playing a role, ho...

Magnetic response of FeRh to static and dynamic disorder

“…By comparing the thermal-driven and the structural disorder-driven phase transition a jump-like behaviour of the remanent magnetization dependence of the hyperfine field can be observed. Based on these findings, it may be possible to induce structural defects by varying the microstructure, for example, due to variations in the volume fraction of the A1-phase 47 and, therefore, utilizing the defect-driven nucleation of ferromagnetic domains, as it was suggested in the work of Keavney et al 32 These findings show a strong correlation between the lattice and the magnetic structure in FeRh thin films and the possibility to tailor the phase transition by defect engineering.…”

Asymmetric magnetic relaxation behavior of domains and domain walls observed through the FeRh first-order metamagnetic phase transition

“…However, due to the presence of latent heat the same behavior is not expected through firstorder phase transitions 1 . As such, the dynamics of firstorder phase transitions are not as well understood and so remain a topic of active investigation [4][5][6][7][8] . Recent breakthroughs in imaging techniques, capable of tracking various material properties, has led to a surge of interest in materials that exhibit first-order phase transitions 4,6,7 .…”

Strain and voltage control of magnetic and electric properties of FeRh films

“…The subsequent growth of each island is apparently restricted by the surrounding material, which implies that material not neighboring a defect has a higher transition temperature. This kinetic arrest of the domain growth is attributed to the competition of energetically favorable strain states [42], and again points to the relevant role of strain on the AFM-FM transition.…”

“…The role of defects as nucleation centers of the FM/AFM domains can be well appreciated in Figure 6(b) were the nano-XRD and XMCD-PEEM data of a given region in the film are compared after different thermal cycling processes [42]. Data clearly show that structural and magnetic domains appear at the same position upon cycling.…”