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Transparent magnetic oxides
CURRENT TOPICS
Transparent Magnetic Oxides.--A new class of magnetic oxides, structurally distinct from ferrites, has recently been discovered. These materials, known as rare-earth-iron garnets, are transparent, permitting the internal magnetic domain structure to be seen with a polarizing microscope. The discovery of ferrimagnetism in these garnets was made first at the Laboratoire Electrostatique et de Physique du Metal of the Institute Fourier in Grenoble, France, and independently by S. Geller and M. A. Gilleo of Bell Telephone Laboratories, where the optical and magnetic resonance behavior of the garnets is being studied by J. F. Dillon, Jr., employing single crystals grown by J. W. Nielsen. Most magnetic materials, metals and ferrites alike, are opaque to visible light. Thus, their internal magnetic structure is not visible, and the way magnetic domains are oriented within them has been inferred from reflection of polarized light by their surfaces or by delineating domain boundaries at these surfaces with colloidal magnetic oxides. Yttrium iron garnet, Y3Fe.o(FeO4) ~, is the most completely studied member of this new garnet family. It has a Curie temperature of 545 o K. and a spontaneous magnetization at zero temperature and infinite field of 4.96 Bohr magnetons per molecule, close to the theoretical value of 5.0. This magnetization results from superexchange interactions through the 02- ions between Fe 3+ ions in crysta]lographically different positions in the cubic lattice. The number of these interactions per Fe 3+ ion in yttrium-iron garnet is 3/5 that in a ferrite and, correspondingly, the ob-
[J. F.I.
served Curie temperature of 545°K is 0.64 of that for magnetite (848 ° K). Of great interest is the fact that yttrium-iron garnet contains magnetic ions with but a single valence. Both X-ray and neutron diffraction studies have shown clearly that, unlike the ferrites, the interactions between identical magnetic io.ns wholly occupying two different crystallographic sites are responsible for ferrimagnetism in the garnet structure. In yttrium-iron garnet the width of the ferrimagnetic resonance absorption line at 9300 and 24,000 megacycles per second depends on crystallographic direction. Line widths of only 0.8 oersted were observed along the [100] direction in thin disks at 10,000 megacycles per second, when the temperature was held at 540 ° K. These line widths increased with decreasing temperature and passed through peak values of several hundred oersteds between 65 ° K. and 4.2° K. A polarized light beam passing through the transparent magnetic domains in rare-earth-iron garnets has its plane of polarization rotated in one direction in one domain but in the opposite direction in an adjacent and oppositely magnetized domain. This Faraday rotation is wavelengthdependent and amounts to several degrees per mil of thickness; it makes the domains within the crystals clearly visible. The internal domain structure can therefore be studied over a wide range of temperatures and magnetic field conditions. Thus, for the first time, it is now possible to correlate Faraday rotation in a magnetic material with spectroscopic data over a broad temperature range.
Transparent Magnetic Oxides.--A new class of magnetic oxides, structurally distinct from ferrites, has recently been discovered. These materials, known as rare-earth-iron garnets, are transparent, permitting the internal magnetic domain structure to be seen with a polarizing microscope. The discovery of ferrimagnetism in these garnets was made first at the Laboratoire Electrostatique et de Physique du Metal of the Institute Fourier in Grenoble, France, and independently by S. Geller and M. A. Gilleo of Bell Telephone Laboratories, where the optical and magnetic resonance behavior of the garnets is being studied by J. F. Dillon, Jr., employing single crystals grown by J. W. Nielsen. Most magnetic materials, metals and ferrites alike, are opaque to visible light. Thus, their internal magnetic structure is not visible, and the way magnetic domains are oriented within them has been inferred from reflection of polarized light by their surfaces or by delineating domain boundaries at these surfaces with colloidal magnetic oxides. Yttrium iron garnet, Y3Fe.o(FeO4) ~, is the most completely studied member of this new garnet family. It has a Curie temperature of 545 o K. and a spontaneous magnetization at zero temperature and infinite field of 4.96 Bohr magnetons per molecule, close to the theoretical value of 5.0. This magnetization results from superexchange interactions through the 02- ions between Fe 3+ ions in crysta]lographically different positions in the cubic lattice. The number of these interactions per Fe 3+ ion in yttrium-iron garnet is 3/5 that in a ferrite and, correspondingly, the ob-
[J. F.I.
served Curie temperature of 545°K is 0.64 of that for magnetite (848 ° K). Of great interest is the fact that yttrium-iron garnet contains magnetic ions with but a single valence. Both X-ray and neutron diffraction studies have shown clearly that, unlike the ferrites, the interactions between identical magnetic io.ns wholly occupying two different crystallographic sites are responsible for ferrimagnetism in the garnet structure. In yttrium-iron garnet the width of the ferrimagnetic resonance absorption line at 9300 and 24,000 megacycles per second depends on crystallographic direction. Line widths of only 0.8 oersted were observed along the [100] direction in thin disks at 10,000 megacycles per second, when the temperature was held at 540 ° K. These line widths increased with decreasing temperature and passed through peak values of several hundred oersteds between 65 ° K. and 4.2° K. A polarized light beam passing through the transparent magnetic domains in rare-earth-iron garnets has its plane of polarization rotated in one direction in one domain but in the opposite direction in an adjacent and oppositely magnetized domain. This Faraday rotation is wavelengthdependent and amounts to several degrees per mil of thickness; it makes the domains within the crystals clearly visible. The internal domain structure can therefore be studied over a wide range of temperatures and magnetic field conditions. Thus, for the first time, it is now possible to correlate Faraday rotation in a magnetic material with spectroscopic data over a broad temperature range.