MAGNETIC AND STRUCTURAL PHASE-TRANSITIONS IN BAMNF4

Elastic and inelastic neutron scattering experiments were performed on single and polycrystalline samples of BaMnF4. The system orders magnetically below TN26 K and the magnetic structure is shown to be similar to other members of the BaMF4 family with M=Fe and Ni, i.e., an antiferromagnetic ordering along both the b and c directions with a two-dimensional magnetic character perpendicular to b. The antiferromagnetic axis is rotated ∼9°from the orthorhombic b axis and the ordered Mn2+ moment is 4.8μB. The spin-wave dispersion curves were measured and the exchange constants and anisotropy determined. The structural phase transition occurring at Tc=247 K was studied in detail and shown to be incommensurate, with superlattice peaks below Tc at q→1=(0.392,0.5,0.5) and multiples of this: q→2=2q→1 and q→3=3q→1. These wave vectors do not change with temperature. The phase transition appeared to be continuous with β=0.31±0.05. The dynamical studies of this transition show a soft mode which becomes overdamped at q→1 but becomes underdamped as q→ increases away from q→1 and as T-Tc increases. High-resolution studies also show the existence of a diverging central peak. A theoretical description of the phase transition suggests the existence of four modes below Tc, corresponding to fluctuations of the amplitude and phase of the order parameter. We have as yet been unable to detect these different modes. © 1979 The American Physical Society.