Changes in the magnetic structure of multiferroic BiFe$_{0.80}$Cr$_{0.20}$O$_{3}$ with temperature

The results of a Mössbauer study of the magnetic structure of multiferroic BiFe$_{0.80}$Cr$_{0.20}$O$_{3}$ in the temperature range of 5–550 K are presented. It is found that a collinear antiferromagnetic structure of the $G$ type is present in BiFe$_{0.80}$Cr$_{0.20}$O$_{3}$ at temperatures below 260 K. Above 260 K, an anharmonic spin wave with a magnetic anisotropy of the easy-axis type with a high value of the anharmonicity parameter m arises. With a further increase in the temperature, the m parameter decreases and tends to zero at $T\sim$ 420 K, at which a harmonic spin wave comes into existence. Above a temperature of about 420 K, the m parameter increases again and the spin wave becomes anharmonic with an easy-plane magnetic anisotropy. At the Néel temperature, $T_N$ = 505 $\pm$ 10 K, the sample undergoes a transition from the magnetically ordered to the paramagnetic state. The change in the type of magnetic anisotropy at T 420 K is explained by competing contributions of different signs to the effective magnetic anisotropy constant and their different temperature dependence for the BiFe$_{0.80}$Cr$_{0.20}$O$_{3}$ multiferroic.