A number of garnets including yttrium garnet, Y3Fe5O12, and mixed garnets of the forms (Y1−αSmα)3Fe5O12 and (Y1−βGdβ)3Fe5O12 have been prepared. X-band ferromagnetic resonance and saturation magnetization have been measured as a function of temperature. At room temperature samarium substitution for yttrium produces an approximately linear increase of line width with samarium content with practically no change in magnetization. Gadolinium substitution, on the other hand, has a much smaller effect on line width but produces an approximately linear decrease in saturation magnetization. Possible microwave application of these materials are discussed.
A simple apparatus is described for measuring the magnetic loss as a function of magnetic field for ferrites at a frequency of 10,000 mc. The width of the ferromagnetic resonance curve is as important a factor in designing microwave devices as is the resistivity of these materials. The method of preparing ferrites and the physical properties of polycrystalline nickel ferrites as affected by preparation and firing techniques are discussed. It is shown that pores in the ferrite body substantially increase the width of the ferromagnetic resonance curve. The direct-current resistivity can be made to vary from 10 ohm-cm. to lo9 ohmcm. by changing the firing temperatures. The variation in the relative permeability with the bulk density of the body is discussed with reference to domain rotation and domain wall movement.
A discussion of continuous and discontinuous grain growth in nickel ferrites is presented. Experimental evidence indicates that the presence of polyphase material is favored by excess nickel oxide and that discontinuous grain growth can be accounted for by excess iron oxide. Quantitative measurements of isothermal grain-growth rates in stoichiometric nickel ferrite yielded an energy of activation of 90 kcal. per mole for the process.
A new nickel ferrite has been developed which contains simultaneously aluminum and gallium. Compositions have been prepared according to the formula NiAlyGaxFe2−(x+y)O4. The total substitutions of gallium and aluminum have been varied from 0 to 1.2. Several ratios of aluminum to gallium have been made. The unique feature of this system is that since the gallium substitutes in the tetrahedral sites and aluminum in the octrahedral sites, the compensation point normally associated with nickel aluminum spinels can be shifted to much higher nonmagnetic ion substitutions. There exists a composition for which there is no compensation point. Measurements of saturation magnetization, Curie temperature, linewidth, and geff factor have been made. A C-band circulator-switch using one of these materials has been constructed and successfully tested at 75-kw peak power with a 0.001 duty cycle. This device was also successfully tested at a power level of 475 w cw in an ambient temperature of 85°C.
Significant improvements on high-power handling capability of ferrites in the nickel-aluminum-gallium spinel system (NAGS) have been obtained by hot pressing these materials. The materials investigated were NiAlxGayFe2−(x+y)O4, where x+y was 0.975. The x/y ratio was varied from 2.24:1 to 2.95:1. Cavity measurements were made on these samples at S band to approximately 2 MW peak which is equivalent to approximately 35 rf Oe. Nonlinearity was determined by observing the decrease in the peak of susceptibility χ″ at an applied field required to maximize χ″. Results are compared to samples prepared by conventional ceramic techniques in this system and in the magnesium-manganese-iron system. Conventionally prepared and flame-sprayed materials in the nickel-aluminum-gallium spinel system showed an improvement of approximately ten times in power handling capability over a ferrite in the magnesium manganese system. Samples of nickel-aluminum-gallium spinels that were hot pressed showed a hundred times improvement in the power handling capability compared to the magnesium manganese ferrite. A description of the techniques and procedures required for flame spraying and hot pressing materials in this system is given.
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