The only known crystals which have a spontaneous ferromagnetic moment at room temperature and are transparent well into the visible spectrum are ferric borate, FeBO3, and ferric fluoride, FeF3. Each of these rhombohedral materials is a canted antiferromagnet with a small moment lying in the plane perpendicular to its unique axis. The Curie temperatures are 348° and 363°K and the room‐temperature moments are 115 and 40 G, respectively. Both materials contain ferric ions in a slightly distorted octahedral environment. Both are green with a series of absorption bands in the visible, a fundamental absorption edge in the violet (FeBO3) or ultraviolet (FeF3), and a large Faraday rotation per unit absorption at room temperature, as high as 14°/dB at 5250 A for FeBO3 and 16°/dB at 4050 A for FeF3. Recent measurements of the magnetic, optical, magneto‐optical, and microwave resonance properties of these materials are discussed. Potential applications of room temperature transparent ferromagnets include microwave magneto‐optical modulation of visible lasers with very low modulation power per unit bandwidth, optical deflection and isolation, magneto‐optic displays, and holograms. Each of these applications is discussed along with the limitations imposed by the optical absorption and birefringence of the available materials. Birefringence interferes with the Faraday effect in most noncubic crystals. Methods for minimizing this interference are described. Finally, requirements for useful transparent ferromagnets are defined and guidelines for finding new materials are suggested.

Room‐Temperature Ferromagnetic Materials Transparent in the Visible

Ferric fluoride (FeF3) is a transparent green uniaxial weak ferromagnet (canted antiferromagnet) with a Curie temperature of 365 °K. FeF3 is far more transparent in the visible and uv than any other known material which has a spontaneous magnetic moment at room temperature. FeF3 at room temperature has 6 absorption bands between 7000 A and the band edge which is at 2440 A. There is a local absorption minimum of α = 4.4 cm−1 in the green at 5225 A. The Faraday rotation is 180 °/cm at 5225 A and increases to 1100 °/cm at 3000 A. The birefringent phase retardation at 5200 A is about 80 000 °/cm. The two indices of refraction are about 1.54 in the visible spectrum.

MAGNETO‐OPTICAL PROPERTIES OF A GREEN ROOM‐TEMPERATURE FERROMAGNET: FeBO3

Ordered magnetic frustration — V. Antiferromagnetic structure of the hexagonal bronzoid HTBFeF3; Comparison with the non frustrated rhombohedral form

Reversible Sodium and Lithium Insertion in Iron Fluoride Perovskites

Electrical conduction mechanisms in FeF3 thin films

Resonance studies at 70 GHz have been performed in crystals of FeF3 from liquid‐helium temperature to 400°K. Below 367°K the field for resonance is anisotropic and temperature dependent. Because of a high‐temperature cubic‐to‐rhombohedral phase transition, the observed spectra of the twinned samples consist of superpositions of the resonance lines resulting from single crystals with c axes parallel to each of the four 〈111〉 directions of the cubic unit cell. The angular dependence is well described by a two‐sublattice model applied to each of the single‐crystal orientations. The model contains symmetric and antisymmetric exchange interactions and a weak anisotropy field parallel to the c axis. Both high‐ and low‐frequency modes have been observed. The effective fields used in the two‐sublattice model have been obtained as functions of temperature from the resonance data at special applied field orientations. It is found that the c axis is a hard direction of magnetization between the Neel point and 251°K and changes to an easy axis at low temperatures. This effect is interpreted as a result of competing sources of anisotropy. Good agreement is found between the AFMR results and temperature dependences calculated using the molecular‐field approximation.

J. R. Shane

Papers

Antiferromagnetic Resonance in Twinned Crystals of FeF3