Carl E. Patton

Magnetic Properties of Lithium Ferrite Microwave Materials

Abstract This report presents a comprehensive review of the magnetic properties of lithium ferrite materials. The fundamental properties are considered first. These include crystal structure, magnetization, magnetocrystalline anisotropy, and magnetostriction. The extrinsic magnetic properties, which are related to microwave applications, are then examined. These include coercive force, remanence, and microwave loss. Finally, a systematic review of the effects of substituents, including titanium, zinc, manganese, cobalt, and bismuth, is presented.

Effect of Grain Size on the Microwave Properties of Polycrystalline Yttrium Iron Garnet

Data on the parallel pump and subsidiary absorption threshold at 9.1 GHz and effective linewidth ΔHeff at 9.9 GHz have been obtained for polycrystalline YIG materials with an average grain diameter a0 from 1 to 30 μ. The fine‐grain samples (1–10 μ) were made by hot pressing. The parallel pump spin‐wave line‐widths extrapolated to zero wavenumber, ΔHk→0, vary as a0−1, with ΔHk→0=25 Oe at a0=1 μ. The ΔHk for the fine‐grain samples (a0 ≈ 1 μ) show an inverse k‐dependence. A simple model in which the spin‐wave transit time across individual grains limits ΔHk appears to explain the a0−1 dependence but not the observed k‐dependences for ΔHk. The high‐field ΔHeff is 5.5 Oe for a0=1 μ and varies as a0−1/2. The low‐field values are slightly larger, due to a small two‐magnon contribution arising from coupling to degenerate short‐wavelength spin waves. Even though the high‐field ΔHeff contains no two‐magnon contribution, it is significantly larger than linewidths in single crystals. A recently proposed magnetostrict...

Frequency dependence of the parallel and perpendicular ferromagnetic resonance linewidth in Permalloy films, 2‐36 GHz

Ferromagnetic resonance linewidth data for 75% Ni‐25% Fe evaporated thin films at 2–36 GHz for both parallel and perpendicular orientations show that the two linewidths are the same and increase linearly with frequency above 10 GHz. At lower frequencies the parallel linewidth is in accord with the high‐frequency behavior, but the perpendicular linewidth levels off at 10–30 Oe. These data suggest that the high‐frequency relaxation is characteristic of a Landau‐Lifshitz λ‐type process but that the low‐frequency losses are characteristic of a process intermediate between λ‐type and 1/τ‐type relaxation.

Theory for the First‐Order Spin‐Wave Instability Threshold in Ferromagnetic Insulators of Ellipsoidal Shape with an Arbitrary Pumping Configuration

The spin‐wave instability theory of nonlinear effects in ferromagnetic insulators developed by Suhl for perpendicular pumping and by Schlomann for parallel pumping has been extended to include pumping with an arbitrary microwave field configuration. The theory is applicable to samples of ellipsoidal shape, magnetized to saturation along one principal axis, but not necessarily an axis of rotational symmetry as in the Suhl theory. The theory, in its present form, is quite versatile and applicable to a large variety of situations which are not accessible on the basis of the earlier calculations. Equations are developed specifically for first‐order processes. Several examples are considered to demonstrate the versatility of the theory: (1) Perpendicular pumping with a linearly polarized microwave field; (2) oblique pumping with a linearly polarized field at an angle between 0° and 90° to the static field; (3) perpendicular pumping with a circularly polarized field rotating in the anti‐Larmor sense.

Ultrasensitive Technique for Microwave Susceptibility Determination Down to 10−5

An ultrasensitive technique has been developed which allows the measurement of the microwave susceptibility components down to levels on the order of 10−5 on small spherical samples of ferrite materials. The novel feature is a feedback scheme to lock the signal klystron onto the 3 dB points of the high‐Q cylindrical activity containing the sample. By beating the signal klystron against a crystal‐oscillator‐stabilized reference klystron, the frequency width of the cavity at half‐power can be measured with an error less than a few hundred hertz, leading to the extremely high resolution. The most useful application of the technique to date is the study of microwave relaxation in the form of an effective linewidth several thousand oersteds away from ferromagnetic resonance.

Exchange‐anisotropy field in disordered nickel‐manganese alloys

Magnetization data for disordered and weakly ordered Ni‐Mn alloys from 22.1 to 32.1 at.% Mn have been obtained upon warming from 4.2 to 300 K, for measuring fields up to 100 kOe, after first cooling from 300 to 4.2 K in zero field or in 100 kOe. The measurements show that there is a minimum measuring field for which the data for zero‐field cooling match the data for field cooling. This field, for the disordered alloy, increases from 10 kOe at 22.1 at.% Mn to 90 kOe at 32.1 at.% Mn. Partial atomic ordering reduces this coupling field. The coupling field is smaller than expected from previous studies. The data are qualitatively explained by a recently proposed local‐environment model which considers explicitly the nearest‐neighbor environment of Mn atoms in the alloy.

A review of microwave relaxation in polycrystalline ferrites

The microwave losses in polycrystalline ferrites can be separated into two parts: those associated with resonance (inhomogeneous linebroadening and two magnon processes) and intrinsic losses. In recent years, both types have been studied in great detail. The near-resonance relaxation can be quantitatively explained by anisotropy and/or porosity scattering if the secondary scattering among spin-waves and spin-wave relaxation are taken into account. The off-resonance relaxation is found to be quite sensitive to the polycrystal microstructure. Several mechanisms, including transit time limited relaxation and magnetostrictive phonon scattering, appear to be important, but the understanding of the basic loss processes is far from complete.

Classical theory of spin-wave dispersion for ferromagnetic metals

The properties of the four spin-wave dispersion branches for a planar ferromagnetic metal with TEM microwave excitation at normal incidence have been examined to derive physical insight into (a) their dependence on material parameters and orientation, (b) the interaction between branches, and (c) the polarization character associated with the related ecxitations. The Larmor electromagnetic and spin-wave branches are found to exhibit a peculiar crossover effect at a critical damping value (α ∼ 0·014 for permalloy). The polarizations vary from circular for the static field normal to the surface to elliptical for an in-plane static field. For the in-plane case, the major polarization axes are parallel and perpendicular to the surface for the Larmor and antilarmor branches, respectively. The well known Ament-Rado and Vittoria formulations have also been modified to include the effect of exchange in the damping term of the Landau-Liftshitz equation. The secular determinant is simplified considerably and the form of the dispersion equations is changed significantly. The only large effect on the branches themselves is to increase the attenuating component of the propagation parameterk for large ¦k¦.

Microwave Properties of Partially Magnetized Ferrites

With the surge of design activity of ferrite components for phased array RADARs, a knowledge of the microwave characteristics of partially magnetized ferrites is a very useful asset. Phase shifters, circulators, and switches are most economically designed if the biasing field can be kept small. A preferred configuration consists of the microwave ferrite being latched into a major or minor loop (flux drive) remanent state. For such operation the internal dc magnetic field is quite small (the order of the coercive force). Anisotropy and demagnetizing effects due to sample shape and local inhomogeneities such as pores and second phase prevent the material from being completely magnetized, and hence the sample is in a partially magnetized state.

Threshold Microwave‐Field Amplitude for the Unstable Growth of Spin Waves under Oblique Pumping

The threshold microwave‐field amplitude hcrit required for the unstable growth of spin waves has been investigated for oblique pumping with a linearly polarized microwave field applied at an arbitrary angle with respect to the static magnetic field. The experiment was performed on YIG (1% Dy) spheres at 9.2 GHz, using a rectangular TE102 cavity and a pulsed magnetron source. Data were obtained at static external fields of 400, 600, 1100, and 1500 Oe. In general, hcrit increases with the pump angle. Theoretical expressions for the oblique pumping hcrit have been derived for ferromagnetic insulators of ellipsoidal shape and with axial symmetry about the direction of the static magnetic field sufficient to saturate the sample. The threshold field is determined by maximizing 1/h=cosψ/h∥±sinψ/h⊥ with respect to the angle between the internal field and the spin‐wave propagation direction. Here, h∥ and h⊥ are the parallel and perpendicular pump threshold fields of Schlomann and Suhl. For external fields sufficie...