Spin-Flop Transition and a Tilted Canted Spin Structure in a Coupled Antiferromagnet

We study a uniaxial coupled Heisenberg antiferromagnet that consists of two subsystems of classical spins with small and large lengths and spin-flop transitions in a magnetic field parallel to the magnetic easy axis. It is proved that the anisotropy of inter-subsystem coupling stabilizes an asymmetric canted antiferromagnetic phase with a tilted direction of antiferromagnetism that is not perpendicular to the magnetic field. In contrast to the conventional first-order spin-flop transition, the spin-flop transition from the Neel phase to such a tilted canted antiferromagnetic (TCAF) phase is of the second order in the absence of simple anisotropic energies in the subsystems. The transition from the TCAF phase to the high-field saturated spin phase is of the second order in the strong coupling limit of the exchange interactions J(1) between the small spins, whereas when J1 is finite, it becomes first-order. Therefore, in the former case, the TCAF phase converts the Neel phase continuously into the saturated phase. The transitions to the TCAF phase are accompanied by additional spontaneous symmetry breaking, causing the uniform magnetization to have a nonzero component perpendicular to the magnetic field.