Magnetic Behavior of Some Orthoferrites in the Anisotropy Change Region

In a number of canted-spin rare earth (RE) orthoferrites, the easy axis for the spontaneous magnetization changes from the a to the c axis with increasing temperature. We have studied the magnetic behavior in the transition region using both torque and vibrating sample magnetometers with applied fields (Ha) from about 0.1 to 6.0 kG. The measurements were made with Ha in the ab, bc, and ac planes. The use of small values of Ha makes it possible to separate the effects of the weak ferromagnetism from the paramagnetism of the RE lattice. In Sm orthoferrite the reorientation occurs over about 20°, centered at 175°C (Tr). For this material in the ac plane the iron lattice is well described by the energy expression E = +½Ku cos2θ+kb cos4θ. Substantially below Tr, Ku≫Kb and the magnetization has a strong uniaxially preferred axis along a. As the temperature increases, Ku decreases and the easy axis splits symmetrically in two, one on each side of a, and these rotate toward c with increasing T. At 175°C, Ku∼0 and the torque curves have only a fourfold symmetry. For T increasing above Tr, Ku becomes increasingly negative, which leads to a uniaxially preferred axis along c. Analogous behavior has been observed for Tm orthoferrite. Here the reorientation occurs over about 12° centered at 86°K. However, for this material Kb is found to depend on the applied field. Furthermore the paramagnetic RE lattice has c as the easy axis for the whole temperature range. A result is the minimum total energy changes from Ha ∥ a to Ha ∥ c as Ha is increased for T < Tr.