Cubic-Magnetic and Optical Behavior of Orthorhombic RbFeF3

On cooling through 87°K, RbFeF3 undergoes a first-order transition from a tetragonal antiferromagnetic phase to an orthorhombic ferromagnetic phase where c/a>1.003. In this magnetic phase the magnetic energy obtained from torque and magnetization measurements is describable by a cubic-anisotropy expression. The easy axes are 〈100〉 (referred to the high-temperature cubic phase) and the effective anisotropy field is greater than 105 Oe. Strain-gauge measurements at 77°K indicate that the crystal structure does not deform such that the c axis always follows the magnetization. The measured magnetostriction Δl/l∼ 50×50−6. Optically the crystal appears uniaxial with the optic axis along the magnetization vector 〈100〉. This is just the behavior expected of a cubic crystal distorted by magnetostriction. At 40°K the crystal undergoes another phase transition. The magnetic behavior is still cubic; the easy axes merely changing from 〈100〉 to 〈110〉. To our knowledge this is the first example of a material whose low symmetry as determined by x rays is not reflected in its magnetic and optical properties.