Magnetostructural phase transitions in NiO and MnO: Neutron diffraction data

Structural and magnetic phase transitions in NiO and MnO antiferromagnets have been studied by high-precision neutron diffraction. The experiments have been performed on a high-resolution Fourier diffractometer (pulsed reactor IBR-2), which has the record resolution for the interplanar distance and a high intensity in the region of large interplanar distances; as a result, the characteristics of both transitions have been determined simultaneously. It has been shown that the structural and magnetic transitions in MnO occur synchronously and their temperatures coincide within the experimental errors: $T_{\mathrm{str}}\approx T_{\mathrm{mag}}\approx(119\pm1)$ K. The measurements for NiO have been performed with powders with different average sizes of crystallites ($\sim1500$ nm and $\sim138$ nm). It has been found that the transition temperatures differ by $\sim50$ K: $T_{\mathrm{str}}=(471\pm3)$ K, $T_{\mathrm{mag}}=(523\pm2)$ K. It has been argued that a unified mechanism of the “unsplit” magnetic and structural phase transition at a temperature of $T_{\mathrm{mag}}$ is implemented in MnO and NiO. Deviation from this scenario in the behavior of NiO is explained by the quantitative difference — a weak coupling between the magnetic and secondary structural order parameters.