Polycrystalline Yttrium Iron Garnet Research Articles

Magnetic Annealing Effects in Silicon-Doped Garnets

Many properties of polycrystalline yttrium iron garnet (YIG) and reduced single-crystal YIG have been attributed to the presence of Fe2+ ions which provide mobile electrons that can diffuse locally through the lattice. It was the purpose of this research to study the modification to the low-temperature anisotropy caused by the presence of Fe2+ ions in silicon-doped YIG, Y3Fe5−δSiδO12 with 0<δ<0.18. Silicon-doped YIG demonstrates a time-dependent induced anisotropy proportional to doping when magnetically annealed to low temperatures. The kinetics and temperature dependence of this induced anisotropy have been studied. It is further found that these doped garnets show a constant, high-field component of rotational hysteresis which is both frequency- and temperature-dependent. The intrinsic, unannealed anisotropy behaves in a novel way. At room temperature the anisotropy coincides with normal YIG, while between 150°K and 300°K | K1 | increases above that of pure YIG Below about 150°K the curves develop asymmetry and appear to be no longer purely cubic. At 4.2°K the easy axis has shifted to the [100] direction (>3% doping) indicative of a change in sign in K1. Silicon-doped lutetium iron garnet and polycrystalline lutetium iron garnet were found to behave in a similar manner. The results are interpreted in terms of an electron-diffusion model which enables the Fe2+ ion to effectively wander among four nonequivalent octahedral sites. A simple kinetic rate theory is at least partially successful in explaining: (1) the time dependence of the torque curves, (2) the rotational hysteresis, and (3) some of the apparent low-temperature deviations from cubic symmetry.

Slow Waves in Ferrites and Their Interactions with Electron Streams

The slow waves that propagate in longitudinally magnetized ferrite rods enclosed in cylindrical wave-guides are investigated theoretically. The backward, azimuthally symmetric volume waves that propagate in a lossy ferrite rod are studied with the quasistatic analysis and the lossy permeability tensor derived from the Bloch-Bloembergen phenomenological formulation. The attenuation constant is proportional to the linewidth, and for polycrystalline yttrium iron garnet (YIG) (ΔH ≈ 50 Oe), the loss is approximately 30 dB/cm. The interaction of these waves with an electron stream which passes longitudinally through a hole in the center of the ferrite rod is studied; for normal values of beam perveance (∼10−6 A/V3/2), backward-wave oscillations should occur only for rods having a linewidth less than several oersteds and a length greater than a few centimeters. When the ratio of rod radius to beam hole radius is 1.5, the interaction parameter C is reduced by about 25% of the value when this ratio approaches infinity. Brillouin diagrams are shown for the azimuthally dependent volume and surface waves in a lossless ferrite. Also, the results of a dynamic analysis are shown demonstrating the relationship between the magnetostatic modes and the waveguide modes. The quasistatic analysis is shown to give a close approximation to the correct dispersion relation even in the limit of fast waves, provided the radius is sufficiently small.

Oxygen Ion Diffusion in Single‐Crystal and PoIycrystaIIine Yttrium Iron Garnet

The oxygen ion self‐diffusion coefficient was determined for single‐crystal and polycrystalline yttrium iron garnet (Y3Fe5O12). The rate of exchange between oxygen gas enriched with the stable isotope 18O and solid yttrium iron garnet was measured. Oxygen ion diffusion rates were found to be the same in single‐crystal and 8μ polycrystalline Y3Fe5O12 between 1100° and 1400°C. This is in contrast to previous measurements of anion diffusion in several alkali halides and in AI2O3 where a strong dependence of diffusion rates on the presence of grain boundaries was found. Enhancing oxidation rates in dense, reduced yttrium iron garnet at low temperature by minimizing the final fired grain size in the sintering process does not appear to be possible on the basis of the results obtained in this investigation. The temperature dependence of the diffusion coefficient of oxygen measured at 100 torr can be represented by D= 0.40 exp (‐65.4/RT) cm2 per second.

Natural Spectrum of Polycrystalline YIG as a Function of Density

The initial permeability of polycrystalline yttrium iron garnet has been measured as a function of density at frequencies from 5 kc/sec to 7.5 Gc/sec. The samples were prepared by grinding very pure single crystals into a powder and then pressing and sintering. The samples of highest density have two absorption peaks that occur at approximately 1 Mc/sec and 200 Mc/sec. This is in very good agreement with observations on polycrystalline samples prepared with coprecipated powders but not with those prepared with raw oxides. The separation between the two absorption peaks decreases as the sample density decreases, until ultimately only one peak is observed in a sample with 72% density. The results are interpreted in terms of the changing sample porosity.

Dynamic Squareness in Polycrystalline Garnets

Certain samples of polycrystalline yttrium iron garnet and lutetium iron garnet have previously been found to acquire a square 60-cps hysteresis loop when cooled to 78°K in a magnetic field. It is found that these are not normal square-loop materials, for at sufficiently slow excitation (ca. 0.5 cps for yttrium iron garnet, one cycle per hour for lutetium iron garnet) they revert to normal, but strongly frequency-dependent loops. At 78°K, for proper amplitude step excitation, the response of these cores is a delayed pulse. For lutetium iron garnet this delay can be several hundred μsec in duration. The behavior of these materials can be qualitatively explained by domain wall instability produced by a diffusion after-effect mechanism.

Uniaxial Anisotropy in Polycrystalline Garnets

In samples of polycrystalline yttrium and lutetium iron garnet which exhibit domain-wall relaxation effects, it is possible to establish uniaxial anisotropy by cooling to low temperature in a magnetic field. This magnetic anneal leads to the development of rectangular hysteresis loops. In lutetium iron garnet,where a particularly strong anneal is found, a uniaxial anisotropy energy of 4500 erg/cc is obtained. After cooling, the easy axis can be shifted in direction by altering the orientation of the applied field; the shift in axis occurs with a finite time constant (order of 15 sec for LuIG).

Effect of Indium Substitution in Yttrium Iron Garnet. High Permeability Garnets

Initial studies on the effect of small substitutions of indium for iron in polycrystalline yttrium iron garnet showed a substantial increase in the initial permeability over that of YIG. Since previous substitution of other ions has produced either little change or a decrease in the permeability, we have studied the effect of indium substitution on the garnet structure in more detail. Due to the size increase of In3+ radii over that of Fe3+,the substitution of In3+ into the garnet structure is limited to the larger of the available Fe3+ sites. The general formula can be written Y3(Fe2−xInx)Fe3O12 where x may take on values up to 2.0. X-ray determination of the lattice parameters for this series shows a linear dependence on x with an apparent change in slope for x>0.6. For x<0.65 there is no “detectable” trace of a second phase and the lattice constants vary from 12.376±0.005 A (x = 0) to 12.445±0.005 A (x = 0.6). For x = 0.65 a small perovskitephase is present, which becomes more apparent with larger x. Since the lattice constant of the garnet phase in the two-phased samples does increase with x, it appears that x will enter the garnet structure for x>0.65. However, repeated firings at various temperatures have so far failed to remove the perovskite phase. The relative initial complex permeability μ* was measured over a frequency range from 103 to 2×109 cps on high density toroids. Increases in μ0′, the low-frequency value of μ′, over that of YIG were found for all of the single-phased garnets in the series. A plot of μ0′ as a function of x shows an essentially linear increase from μ0′ = 112 for x = 0 (YIG) to μ0′ = 518 for x = 0.6. Measurements of μ0′ as a function of temperature up to the Curie point for this series are also presented. Magnetic moment measurements were made from −196°C to the Curie temperature for this series. As in the case of other substitutions for Fe3+, the substitution of In3+ resulted in a decided decrease in the Curie temperature from 278±2°C for YIG to Tc = 148±2°C for x = 0.6. The room temperature moment is only slightly higher than that of YIG (6.8μB). On the basis of comparison of their thermal magnetic properties, it could be expected that Sc3+ substitution in small amounts would also produce a rise in μ0′. The increase in μ0′ is attributed to the lowering of the Curie temperature while the magnetic moment is held nearly constant or slightly increased.

Magnetostriction of Yttrium Iron Garnet

An adnesive strain gage was used to measure the magnetostriction of polycrystalline yttrium iron garnet. The relative change in length was plotted against the applied magnetic field. From this curve the saturation magnetostriction constant Λs = --(3.7 plus or minus 0.5) x 10-7 was obtained. This small value appeared to be explained by dipolar interaction.

Saturation Effects in Ferrimagnetic Resonance

Saturation curves of susceptibility as a function of rf magnetic field have been measured for a number of samples of polycrystalline yttrium iron garnet of varying linewidth. The curves exhibit a region of susceptibility linear in 1/hrf at high rf powers. The region of the initial decline in susceptibility is linear in hrf2,which is predicted both by a small signal relation derived from Callen's ferromagnetic dynamical equation and by a calculation of Schlömann which includes inhomogeneity interactions. A new method of determining the critical field for spinwave build-up is discussed. This method takes account of the fact that the initial decline in susceptibility is not simply due to the Suhl mechanism for saturation in ferrites.

Line Widths in Polycrystalline Yttrium Iron Garnet

The line widths at the half-power point for ferromagnetic resonance in a number of polycrystalline yttrium iron garnet samples have been measured at 16 kmc. It has been determined that, starting with mixtures of Y2O3 and Fe2O3, sound parts with small grain size and having line widths as low as 35 to 40 oersteds can be obtained on the iron-rich side of stoichiometry in Y3Fe5O12. However, narrower line widths (∼20 oersteds) in large grained materials are obtained by using less than the stoichiometric amount of iron and firing at temperatures higher than 1450°C. This is due to minimizing the disrupting influences of oxygen loss in iron-rich areas at high temperatures. Line widths do not appear to be broadened by the addition of small amounts of nickel, cobalt, or manganese to convert excess iron to high resistance forms of spinel in the iron-rich compositions.