FeRh ground state and martensitic transformation

Abstract

Cubic B2 FeRh exhibits a metamagnetic transition [(111) antiferromagnet (AFM) to ferromagnet (FM)] around 353 K and remains structurally stable at higher temperatures. However, the calculated zero-Kelvin phonons of AFM FeRh exhibit imaginary modes at M-points in the Brillouin zone, indicating a premartensitic instability, which is a precursor to a martensitic transformation at low temperatures. Combining electronic-structure calculations with ab initio molecular dynamics, conjugate gradient relaxation, and the solid-state nudged-elastic band (SSNEB) methods, we predict that AFM B2 FeRh becomes unstable at ambient pressure and transforms without a barrier to an AFM(111) orthorhombic (martensitic) groundstate below 90 ± 10 K. We also consider competing structures, in particular, a tetragonal AFM(100) phase that is not the global groundstate, as proposed [Phys. Rev. B 94, 180407(R) (2016)], but a constrained solution.

Solid-state synthesis, magnetic and structural properties of interfacial B2-FeRh(001) layers in Rh/Fe(001) films

“…Our hypothesis is the amorphous phase appears above 100 °C at the Rh/Fe interface, which is then transformed into a nanocrystalline state containing B2 nanograins of martensite variants with lattice parameters close to αʺ martensite. This is consistent with the possible existence of various structural phases of FeRh, predicted by ab initio calculations 54,55 and found in the experimental study 36 . Competition between the α l ʹ and αʺ phases during the partial crystallization suppresses grain growth and stabilizes the nano-grained B2 structures in an amorphous matrix.…”

“…These temperatures are close to or coincide with the reverse martensitic transformation starting temperatures A s (B2-TiNi) ~ 100 °C, A s (B2-NiAl) ~ 180 °C, A s (B2-CdAu) = 67 °C. The reversible α l ʹ ↔ αʺ transition in B2-FeRh possesses all the characteristics of a martensitic transformation (α l ′-austenite, αʺ-martensite), because it can pass at high speeds 52,53 , has isotropic volume changes at the transition 1 , can be www.nature.com/scientificreports/ induced by the application of stress 19,20,24,25,[37][38][39][40] and magnetic field 1,9,10 , has martensitic instabilities 36,54 and the α l ʹ and αʺ lattices have a cube-on-cube orientation relationship. According to the phase diagram, the transition α l ʹ ↔ αʺ has a minimum temperature T k ~ 100 °C among other structural transformations in the Fe-Rh system.…”

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

“…Interestingly, due to the alternation of the Fe‐moment on their simple‐cubic sublattice in the G‐type AF order, FeRh bears some resemblance to the L2 1 Heusler structure, if the differently oriented Fe‐atoms were regarded as independent atomic species. First‐principles studies predicted very recently an unstable mode in the AF phonon dispersion, which could indicate the presence of another stable monoclinic or orthorhombic phase at very low temperatures . At large tetragonal distortions, a competing tetragonal phase has also been predicted .…”

Reliable thermodynamic estimators for screening caloric materials

“…In materials with structural and magnetic instabilities (or, more generally, "dimpled" potential energy surfaces), this assumption is invalid, at least near temperatures, where the crossover between states occur and key associated properties manifest. With the premartensitic instability 80 in AFM(111) B2 FeRh, similar (but smaller) to that in NiTi austenite, 16,17 care must be taken to calculate accurately the lattice entropy.…”

“…The FeRh groundstate and a martensitic transformation in the AFM phase at cryogenic temperatures were recently addressed. 80 Here we focus on estimators 8 to predict thermodynamics at the metamagnetic transformation near room temperature. We find that quantities relevant to calorics can be calculated in a quantitative agreement with measurements (Table I).…”