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Higher order exchange interactions leading to metamagnetism in FeRh

The origin of the metamagnetic antiferromagnetic-ferromagnetic phase transition of FeRh is a subject of much debate. Competing explanations invoke magnetovolume effects and purely ther- modynamic transitions within the spin system. It is experimentally difficult to observe the changes in the magnetic system and the lattice simultaneously, leading to differing conclusions over which mechanism is responsible for the phase transition. A non-collinear electronic structure study by Mryasov [O.N. Mryasov, Phase Transitions 78, 197 (2005)] showed that non-linear behavior of the Rh moment leads to higher order exchange terms in FeRh. Using atomistic spin dynamics (ASD) we demonstrate that the phase transition can occur due to the competition between bilinear and the higher order four spin exchange terms in an effective spin Hamiltonian. The phase transition we see is of first order and shows thermal hysteresis in agreement with experimental observations. Simulating sub-picosecond laser heating we show an agreement with pump-probe experiments with a ferromagnetic response on a picosecond timescale.