First order spin flop transition in yttrium iron garnet (YIG)

We have studied the field dependence of the sublattice magnetization of ferrimagnetic yttrium iron garnet (YIG) using neutron scattering. In contrast to the macroscopic spontaneous magnetization that shows the normal field dependence of a soft ferromagnet (sudden saturation at the demagnetization field and no hysteresis) in neutron scattering a field induced first order spin flop transition with considerable hysteresis is observed at a critical field of H c ∼ 580  G (external field). Considering that with neutron scattering the antiferromagnetic component of ∼4/5 of the total moment is detected preferentially while in the macroscopic magnetization samples the ferromagnetic component of ∼1/5 exclusively it becomes clear that ferromagnetic and antiferromagnetic component have a completely independent field (and temperature) dependence. This indicates that the two magnetic structures have to be viewed as two weakly coupled order parameters. In the zero field ground state the moment orientations of the two ordering structures are orthogonal. Only for fields H 0 > H c a nearly collinear ferrimagnetic order is established by the field.