Spin-Reorientation and Antiferromagnetic Resonance in the Low Temperature Phase of Perovskite Crystal KMnF3

The temperature dependence of anisotropy energy in the canted antiferromagnet KMnF 3 with Dzyaloshinsky-Moriya interaction has been investigated by analyzing the antiferromagnetic resonance. At the liquid nitrogen temperature and in the absence of the applied field, the antiferromagnetic axis of sublattice magnetizations is parallel to [001] of the tetragonal pseudocell. But, at T R ∼50K, this axis begins to rotate away. This behavior is related with an anomalous maximum of the magnetic susceptibility. The a.c. magnetic susceptibility calculated from the free energy expression on the spin system indicates that \(\chi_{[001]} \equiv -(\frac{\partial^{2}F}{\partial{H}^{2}})_{H=0} = \frac{1}{2J}\frac{H_{\text{D}^{2}}}{\{H_{\text{E}}(H_{\text{K}}-H_{\text{K}'})-2H_{\text{E}}H_{4}\}} \), for T > T R and \(\frac{1}{4J}\frac{H_{\text{D}^{2}}}{\{2H_{\text{E}}H_{4}-H_{\text{E}}(H_{\text{K}}-K_{\text{K}'})\}}+\frac{1}{J} \sin^{2}\theta\), for T < T R , where H 4 and H K - H K' are, respectively, the cubic anisotropy field and the orthorhombic anisotropy field. The experimental result shows that there exists a second order phase transition due to the onset of a gradual rotation of the antiferromagnetic axis.