Review of Recent Work on the Magnetic and Spectroscopic Properties of the Rare-Earth Orthoferrites

The rare-earth orthoferrites are a family of canted antiferromagnets which show an unusual variety of magnetic properties. This article begins with a brief review of the ``allowed'' spin configurations compatible with the crystallographic symmetry of the orthoferrite structure and a summary of the experimentally observed spin configurations, spin reorientation temperatures, compensation temperatures, etc. We then review recent research on these materials, grouping most of the recent work into four major categories; studies of the spin reorientation transition, studies of the rare-earth spin ordering, spectroscopic studies directed at determining magnetic interaction parameters, and studies of magnetic domains and domain walls. The spin reorientation process has been established to occur over a finite temperature range as a fourth-order anisotropy, generally small, comes to dominate the orientational behavior in the temperature region where the usually domainant second-order anisotropy passes through zero and changes sign. The spin reorientation has been monitored in a number of RE orthoferrites by several techniques; magnetic torque, microwave absorption, neutron diffraction, and optical spectroscopy. A few of the rare-earth orthoferrites show RE-RE interaction strong enough to cause an ordering of the rare-earth ions at temperatures on the order of 2°–6°K; this ordering process has been documented by all the above techniques plus Mössbauer-effect and magnetic-susceptibility measurements. Spectroscopic measurements, showing the increase in RE ground-state exchange splittings for the configurations stable at the lower temperatures, give a qualitative understanding of the mechanisms causing the spin reorientation and spin ordering processes, and promise detailed quantiative understanding of the unique magnetic behaviors seen in the orthoferrites. The low moment and high anisotropy of the orthoferrites make possible the fabrication of thin plates magnetized normal to the plate. Such plates are semitransparent, and the Faraday rotation through them can be used to study domain structures or can be used as a readout mechanism for memory and logic devices utilizing single small stable domains as the active element. Recent studies of such domains and of domain wall energy and mobility in the orthoferrites are accordingly reviewed. Finally, several investigations are summarized which utilize the orthoferrites as a vehicle for examining general properties of magnetic systems, such as critical-point phenomena.