Atomistic modelling of metamagnetic transition in FeRh with four-spin exchange

The metamagnetic transformation of FeRh from antiferromagnetic (AFM) to ferromagnetic (FM) ordering makes it suitable for a wide scope of applications, ranging from magnetic recording media to antiferromagnetic spintronics and magnetic refrigeration. Exchange spring systems of FeRh coupled with a hard magnetic layer (FePt) are a promising approach for heat-assisted magnetic recording technologies that assure high density of stored information.
It has been shown that different temperature scalings of the exchange interactions can lead to a first-order phase transition in FeRh systems (Barker, J., & Chantrell, R. W. (2015). Higher-order exchange interactions leading to metamagnetism in FeRh. Physical Review B, 92(9), 094402). This model assumes the presence of a higher-order exchange term in the form of four-spin exchange, that arises from four consecutive hops of electrons from one spin configuration to the spin-flipped one, the higher order four-spin interaction being mediated in FeRh by the Rh atoms. At small temperatures the four-spin exchange is responsible for the AFM ordering, while, at higher temperatures the FM ordering is given by the bilinear exchange since the four-spin term decreases more rapidly with temperature than the bilinear term.
In this work, the first-order phase transition that appears in FeRh is systematically studied via the four-spin parametric model that was previously given in literature. A degeneracy in the ground-state of the four-spin exchange system is found. The effect of the parameters entering into the spin Hamiltonian was systematically analysed. The model has been implemented for FeRh and then developed in order to consider other materials with different crystal structures. As nanoscale applications of the FeRh systems are more practical due to the high cost of Rh, the finite size effects of FeRh grains and thin films are systematically investigated. As a further test of the model, ultrafast simulations have been performed. In accordance to the literature, it is found that, by laser-heating the FeRh system, the ferromagnetic ordering is generated in picosecond time-scales. Additionally the dynamical and equilibrium properties of FePt/FeRh bilayers have been systematically investigated, as this system is of particular interest for recording media applications.