Magnetoelastic coupling in the incommensurate antiferromagnetic phase of FeOCl

The antiferromagnetic phase transition of FeOCl has been studied by temperature-dependent x-ray-diffraction experiments, magnetic susceptibility, heat capacity, and dielectric measurements. The magnetic phase transition is found to be accompanied by a monoclinic lattice distortion, affecting the angle $\gamma$ between crystallographic axes parallel to the layers comprising the quasi-two-dimensional magnetic system. The temperature-dependent magnitude of $\gamma$ shows the phase transition to be of second order. Satellite reflections occur in x-ray diffraction with twice the magnetic modulation wave vector. These positions are temperature dependent, providing evidence for an incommensurate character of the magnetic order. The observed Néel temperature is $T_N=81.0\left(2\right)$ K.

Figure 1

(a) Magnetic susceptibility of a crystal of FeOCl from batch 1. Inset: Derivative with respect to temperature of the quantity ${\chi }_{\mathrm{mol}}\times T$. (b) Magnetic susceptibility of a crystal of FeOCl from batch 2. (c) Heat capacities of the same crystals measured in zero external magnetic field. Inset: Enlargement of the temperature range where a $\lambda$-type anomaly is seen. (d) Relative dielectric constant of FeOCl at 1 kHz measured perpendicular to the crystal plate.

Figure 2

The crystal structure of FeOCl.

Figure 3

Diffracted intensity as a function of the scattering angle 2$\theta$ and the crystal orientation $\omega$ for reflection $\left(220\right)$ of crystal A (d) and crystal B (a–c) at selected temperatures. $\Delta$2$\theta$ and $\Delta$$\omega$ indicate the deviation from the center of the scan in units of $0.01$ deg.

Figure 4

Diffracted intensity as a function of the scattering angle $2\theta$ for the reflection $\left(220\right)$ for both crystals at different temperatures. All peaks were fitted by Gauss functions.

Figure 5

Temperature dependence of the monoclinic angle $\gamma$. The (red) solid line represents a fit of Eq. (1) to the five data points at $T>40$ K. The (green) dashed line is a fit to all eight data points.

Figure 6

Incommensurate component $q_b$ of the modulation wave vector ${\mathbf{q}}_X=(0,q_b,0)$, as determined by $\mathbf{q}$ scans along ${\mathbf{b}}^{*}$. The dashed line indicates value $0.550=2\times 0.275$.