Magnetic and structural phase transitions in BaMnF4

Abstract

Elastic and inelastic neutron scattering experiments were performed on single and polycrystalline samples of BaMn${\mathrm{F}}_4$. The system orders magnetically below $T_N\approx 26$ K and the magnetic structure is shown to be similar to other members of the $\mathrm{Ba}M{\mathrm{F}}_4$ family with $M=\mathrm{Fe}\mathrm{and}\mathrm{Ni}$, 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 ${\mathrm{Mn}}^{2+}$ moment is 4.8${\mathrm{\mu }}_{\mathrm{B}}$. The spin-wave dispersion curves were measured and the exchange constants and anisotropy determined. The structural phase transition occurring at $T_c=247\mathrm{}$ K was studied in detail and shown to be incommensurate, with superlattice peaks below $T_c$ at ${\overrightarrow{\mathrm{q}}}_1=(0.392,0.5,0.5)\mathrm{}$ and multiples of this: ${\overrightarrow{\mathrm{q}}}_2=2\mathrm{}{\overrightarrow{\mathrm{q}}}_1$ and ${\overrightarrow{\mathrm{q}}}_3=3\mathrm{}{\overrightarrow{\mathrm{q}}}_1$. These wave vectors do not change with temperature. The phase transition appeared to be continuous with $\beta =0.31\mathrm{}\pm 0.05$. The dynamical studies of this transition show a soft mode which becomes overdamped at ${\overrightarrow{\mathrm{q}}}_1$ but becomes underdamped as $\overrightarrow{\mathrm{q}}$ increases away from ${\overrightarrow{\mathrm{q}}}_1$ and as $T-T_c$ 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 $T_c$, corresponding to fluctuations of the amplitude and phase of the order parameter. We have as yet been unable to detect these different modes.