Magnetic phase diagrams and helical magnetic phases in MxNi 1−xBr 2 ( M =Fe, Mn): A neutron diffraction and magneto-optical study

Abstract

Below T N = 44K, NiBr 2 orders as an easy-plane antiferromagnet, but at T IC ∼ 23K it undergoes a transition to a helimagnetic phase whose propagation vector, τ, increases smoothly to a limiting value [0.027,0.027,0] at 4.2 K. The magnetic order in NiBr 2 and M x Ni 1−xBr 2 ( M =Fe, Mn) is investigated by single-crystal neutron diffraction. In Fe x Ni 1− x Br 2 τ changes from the [110] to [100] direction at 3.7(17)% Fe. Its magnitude, and T IC, remain constant up to 9.9(14)% Fe, but above this concentration a collinear, easy-axis structure is stable at all temperatures below T N, which decreases towards the value of 11 K in FeBr 2. Doping 2.9% Mn 2+ into NiBr 2 decreases T IC slightly, in the manner previously observed for diamagnetic dopants. Refinement of powder X-ray diffraction profiles show the lattice distortions in the doped materials to be small, while the structures of some Fe x Ni 1− x Br 2 samples are confirmed by refinement of the powder neutron diffraction profiles. The temperature and field dependence of hot and cold exciton-magnon combination bands in the optical absorption spectra of Fe 2+- and Mn 2+-doped crystals give additional information about magnetic order. The magnetic behaviour is interpreted in terms of perturbations of the exchange constants J 1,2,3, and J′ and the anisotropy constant D.