摘要:

The spectrum of modes of oscillation of a ferromagnetic spheroid situated in a uniform dc magnetic field is discussed. It is shown that, for the size of sample usually used in microwave experiments, a part of the spectrum consists of magnetostatic modes for which exchange and propagation may be ignored. The behavior of these modes is discussed at some length and it is shown that the analysis gives a satisfactory description of the observed absorption curves.

ISSN:0021-8979    

DOI:10.1063/1.1723117

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

In order to interpret correctly the experimental results obtained from ferrimagnetic resonance experiments, it is necessary to understand completely the multiplicity of lines that can be observed. Most of these lines have been shown [R. L. White and I. H. Solt, Jr., Phys. Rev.104, 56 (1956); J. E. Mercereau and R. P. Feynman, Phys. Rev.104, 63 (1956); L. R. Walker, Phys. Rev.105, 390 (1957)] to be higher order precessional modes which are quite well understood. There still exists, however, some uncertainty connected with the uniform precessional mode. The uniform mode is of most importance since it is for this mode that values of line widths,gvalues, and anisotropy constants are usually quoted in the literature. It can be distinguished experimentally from the higher order modes since (a) its intensity of absorption is proportional to the uniform component of the rf magnetic field over the sample and (b) its intensity decreases much less rapidly with diminishing sphere size than for the higher order modes. We have observed, however, that there are several lines possessing these characteristics of the uniform mode. The relative intensities of these lines are strong functions of both temperature and frequency, they have different anisotropy constants, and their separation decreases with sphere radius. Data exhibiting these properties will be presented for experiments on spherical single crystals of MnZn ferrites performed in the microwave frequency range of 8000–12 000 Mc. A discussion of several possible mechanisms that might give rise to this phenomena will be given.

ISSN:0021-8979    

DOI:10.1063/1.1723118

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

A study of ferromagnetic resonance at 9300 mc and 3000 mc in single crystals of yttrium iron garnet is described. Cavity perturbation techniques are used with polished spherical samples 9 to 11 mils in diameter. The line widths along the hard axis [100] are found to be 2.3 and 5.7 oe, respectively, at 9300 mc and 3000 mc. It is believed the 2.3 oe is the narrowest resonance line yet observed on spheres of ferromagnetic material at room temperature. Because of the narrow line widths, the nonlinear behavior of these crystals is of particular interest. At 9300 mc the rf field,hcrit, for saturation of the main resonance line is 46 millioersteds. Using Suhl's spin wave theory of nonlinear effects in ferromagnets, the 46 millioersteds (associated with az‐directed spin wave) predicts a spin‐lattice relaxation timeT1of 7.4×10−8sec, in good agreement with the 7.0×10−8sec measured by other independent methods. ThisT1then predicts an ``intrinsic'' line width of 1.54 oersted. Thus the observed line width of these crystals at 9300 mc is within a factor 1.5 of its theoretical limiting value. At 3000 mc, where the subsidiary resonance coincides with the main resonance,hcritis 5.0 millioersteds (associated with spin waves notzdirected). As the temperature is increased toward 340°K, an interesting effect is observed. Thehcritrises quite abruptly; in fact it changes by 10.5 db between 335°K and 340°K. This and other observations are compared with Suhl's nonlinear theory with good results.

ISSN:0021-8979    

DOI:10.1063/1.1723119

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

The observation of spin‐wave resonance in Permalloy samples of extremely low anisotropy has been extended to liquid nitrogen temperature. Measurements at both room and liquid nitrogen temperature have been carried out at 3000 and at 4000 Mc/sec. At room temperature the complete experimental resonance curve can be explained at both frequencies on the basis of the spin‐wave resonance theory of Ament and Rado by using an average exchange stiffness constant,A, of (3.3±0.5)×10−6erg/cm and agfactor of 2.06±0.01. The agreement between experiment and theory would be destroyed if a Landau‐Lifshitz or Bloch damping term were included. The theory of Ament and Rado predicts that as the temperature is lowered the exchange broadening of the resonance line increases and the resonance field decreases. Both effects have been observed qualitatively but neither is as large as would be expected from the theory. It has been found that at a given temperature the plot of the imaginaryversusreal part of the equivalent permeability at 3000 Mc/sec differs only by a scale factor from the corresponding plot at 4000 Mc/sec. At room temperature the above frequency increase causes a decrease in the permeabilities of 30% from the corresponding values at 3000 Mc/sec, as compared to a predicted 15% decrease. The discrepancy between theory and experiment at the low temperature is about the same at one frequency as at the other. Since the corresponding resonance fields differ by a factor of two, any effects due to incomplete saturation seem to be ruled out.

ISSN:0021-8979    

DOI:10.1063/1.1723120

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

It is suggested that under certain conditions the interpretation of ferromagnetic resonance phenomena in metals should not make use of Ohm's law because the electronic mean free path may not be negligible in comparison to an appropriately defined effective skin depth. Such conditions can exist even at moderately low temperatures provided the equivalent permeability is sufficiently large. The anomalous effects due to the mean free path are shown to be particularly important in spin‐wave resonance, i.e., in a modified ferromagnetic resonance which is appreciably influenced by the exchange interactions existing in the skin depth. A suitable perturbation method is developed for calculating these anomalous effects on the basis of Maxwell's equations and the spin‐wave equation. Using a relation proposed by Reuter and Sondheimer, as imple first‐order result is then obtained for the surface impedance and for the equivalent permeability derived therefrom. The limitations of the underlying physical model are pointed out, and the applicability of the theory to existing experimental results is briefly discussed.

ISSN:0021-8979    

DOI:10.1063/1.1723121

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

A dynamical equation, describing the response of the magnetization to an applied field, is proposed. The analysis is based on a spin‐wave picture. The dynamical equation contains three parameters, defined in terms of particular magnon‐magnon and magnon‐phonon transition probabilities. A more detailed theory for one of these parameters is developed. The resultant size and shape dependence of the magnetic loss is suggestive of experimental observations.

ISSN:0021-8979    

DOI:10.1063/1.1723122

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

An inhomogeneously broadened magnetic resonance line is narrowed by dipole fields. The theory of this effect is developed in terms of the classical equations of motion of a magnetized medium by expanding the magnetization and fields into Fourier series. An expression is obtained for the imaginary parts of the susceptibility tensor. In the applicable range of the theory, the line width decreases inversely as the magnetization, and is strongly shape dependent.

ISSN:0021-8979    

DOI:10.1063/1.1723123

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

Ferromagnetic resonance line widths in dense polycrystalline and single crystal specimens, the magnetic moment and the anisotropy field (K1/Ms) have been measured as a function of temperature. Between room temperature and the Curie point the excess resonance line width, which is given by &Dgr;H(excess)‐&Dgr;H(polycrystal)—&Dgr;H(single crystal), is found to have the same temperature dependence as the saturation magnetization. This result is obtained even though the polycrystalline density (>98%) is sufficiently high to be in the range where the excess line width has usually been attributed to anisotropy broadening. In the region below room temperature &Dgr;H(excess) becomes dependent on the anisotropy field. An approximate criterion, that is suggested by the data, to determine whether the anisotropy field or the magnetic moment is responsible for the broadening is the ratioK1/Ms2≷0.25. The compositions measured were Mn1.24Fe1.76O4and Mn1.49Fe1.51O4. The polycrystallinegfactor (gp) is greater than the single crystalgfactor (gs) and bothgfactors are functions of the temperature. The shift in the polycrystalline resonance peak is discussed with reference to the Standley Stevens theory and limited agreement is found. The anisotropy constant,K1, has been found to vary as the fifth power ofMsfor these ferrite compositions.

ISSN:0021-8979    

DOI:10.1063/1.1723124

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

A theoretical determination is made of the resonance curve of a nonconducting polycrystalline ferromagnetic material with uniaxial crystalline anisotropy. A finite intrinsic line width is assumed for the single crystal, and effects of grain boundaries, impurities, inhomogenieties, and volume and surface divergence of the magnetization are ignored. The results are compared qualitatively with experiment.

ISSN:0021-8979    

DOI:10.1063/1.1723125

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

Line broadening because of crystalline anisotropy and the random orientation of the crystallites is investigated for the case in which the anisotropy field is much larger than the saturation magnetization. The theory predicts an absorption line with shoulders on each side which correspond to grains in which a hard or an easy direction is aligned with the dc field. With increasing anisotropy the low‐field side of the line becomes more prominent and for&ggr;Ha/&ohgr;≳·5a secondary absorption peak occurs on the low‐field side of the main resonance. Measurements in nickel ferrite aluminates at X band agree reasonably well with the predictions, if it is assumed that the first order anisotropy constant is negative and that all higher order anisotropy constants are zero.

ISSN:0021-8979    

DOI:10.1063/1.1723126

出版商:AIP    

年代:1958

数据来源: AIP