  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.
