摘要:

The characteristic equation for a TE102or TE104rectangular cavity loaded by a ferrite slab in the middle has been derived on the basis of electromagnetic theory. This characteristic equation is used in a new measurement technique with improved precision, theoretical inference, and sample loading. A lot of experimental research and computational analysis have been done for the characteristic equation method. Microwave magnetic spectrum researches have been carried out for a series of polycrystalline microwave ferrites.<UFMMM>

ISSN:0021-8979    

DOI:10.1063/1.348806

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The purpose of this work is to elaborate an excitation theory of spin waves (SW) in anisotropic ferromagnetic films (FF). The theory takes into account both dipole–dipole and exchange interactions and also crystallographic anisotropy of a FF. An arbitrary value and type of anisotropy are considered in terms based on effective demagnetizing factors of anisotropy. Two cases of exchange boundary conditions, namely pinned and unpinned surface spins, are analyzed. Using earlier elaborated normal mode technique1spin‐wave Green functions described linear response of spin‐system of a anisotropic FF to the inhomogeneous microwave magnetic field are obtained. Owing to the explicit Green functions these forms are convenient for use in various problems of radiation and scattering of SW. As an example, we use these Green functions to calculate radiation resistance (RR) of a microstrip transducer. Analysis shows that ‘‘anisotropic’’ features of RR are connected with the peculiarities of dipole‐exchange SW spectrum of anisotropic films. Normal uniaxial and cubic anisotropy in perpendicularly magnetized films [for (111)and (100)‐oriented films] causes the shift of RR frequency dependences only. In all other cases dependence of RR on propagation direction of SW takes place. Normal uniaxial and cubic anisotropy in the tangentially magnetized films [e.g., for (111)and (100)‐oriented films] causes unusual behavior of RR as frequency function owing to the new ‘‘anisotropic’’ branches of SW spectrum. It should be noted that expressions obtained for RR are convenient for practical calculations of characteristics of magnetostatic wave devices based on FF with large anisotropy.

ISSN:0021-8979    

DOI:10.1063/1.348807

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Recently, the finline ferrite devices have attracted people’s attention. Beyeretal. have done theoretical and experimental researches for a finline isolator.1,2In order to make it convenient for theoretical design and experimental adjustment of this device, we have developed a synthetical theory and have carried out the experimental research successfully.3In this paper, we have done the further theoretical analysis and experimental researches for the finline ferrite isolator, such as the impedance matching, the effects of device structure on performances, and the transplantation of a waveguide isolator to a finline isolator problem. Good agreement between design and experiment is obtained. The performances of aX‐band finline isolator are as followsA+<1.5 dB,A−≳18 dB, VSWR<1.5 in a 8% bandwidth, and the bia‐field is lower (about 1000 Oe).<ks>

ISSN:0021-8979    

DOI:10.1063/1.348808

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Numerical results are presented for the damping of the phonon excitations in a Heisenberg ferromagnet for temperatures up to half the Curie temperature and across the whole Brillouin zone. The results are based on the diagrammatic theory of the influence of spin‐phonon interactions in general spin Heisenberg magnets, recently reported by the authors. The results identify four different situations that can be classified according to the ratio &xgr; =cj/(J+a), wherecjis the phonon velocity,J+the transverse exchange interaction, andaa lattice spacing. These are the high‐J+region with &xgr;≪1; the equal‐J+region, 1≲&xgr;≲2.5; the mid‐J+region, 2.5≲&xgr;<2&sqrt;3, and the low‐J+region with &xgr; ≳2&sqrt;3. A number of features of the phonon damping are observed. The most interesting being the sharp peak in the equal‐J+region arising from the scattering by acoustic spin waves, the rich directional dependence in the mid‐J+region, and the sharp spike just above the onset of the phonon damping in the low‐J+region.

ISSN:0021-8979    

DOI:10.1063/1.348809

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Calculations are presented for the influence of isolated magnetic impurities or defects on the spin‐wave spectrum of semi‐infinite ferromagnets and antiferromagnets. A Green’s‐function theory and a Heisenberg model are used to study the spin‐wave excitations associated with a defect at or near a magnetic surface, showing that there may be localized ‘‘defect modes’’ at energies outside the regions of bulk spin waves and surface spin waves. Applications are made to impurities in simple cubic ferromagnets and rutile‐structure antiferromagnets.

ISSN:0021-8979    

DOI:10.1063/1.348810

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

This study is an extension of previous work dealing with single crystals of YIG/GGG/YIG grown in (110) layers. Ferromagnetic resonance (FMR) measurements for fields along the ⟨111⟩ axis were performed. The microwave measurements reveal hysteretic behavior as the FMR frequency is measured for positive and negative magnetic fieldsH. ForHin which the magnetic moment is zero (spin flop) the FMR frequency goes through a discrete and measurable jump or change. This discontinuity in the FMR frequency at the spin‐flop condition is attributed to the magnetostatic interaction between the two YIG layers.

ISSN:0021-8979    

DOI:10.1063/1.348811

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The aim of this work is to develop a theory of nonlinear spin waves (SW) in ferromagnetic films (FF) appropriate for description of rich nonlinear SW dynamics observed in Refs. 1 and 2. We use classical Hamiltonian formalism developed in Ref. 3 for unbounded ferromagnets in combination with the theory of dipole‐exchange SW spectrum for FF.4The main distinctions in theoretical description of nonlinear SW phenomena between the case of unbounded ferromagnet3and the case of FF can be formulated as follows: (1) Local character of SW excitation in FF leads to the increase of threshold values for SW parametric decay processes because the parametrically excited travelling SW carry out energy from the region of pumping localization; (2) additional selection rules should be taken into account in the analysis of SW parametric processes in FF. These selection rules are determined by conservation law for the transverse component of the SW wave number. In particular, in three‐wave decay of surface SW with the increase of its amplitude after degenerate decay into SW pairs having equal numbers (n,n) the nondegenerate decay into SW modes having numbers (n,n+1) takes place. This effect has been recently observed in Ref. 2; (3) three‐wave and four‐wave coefficients of the SW Hamiltonian function in FF turn out to be analytical functions of their arguments even in the longwave spectral regionkL≪1 where the long‐range dipole–dipole interaction is dominant. The analytical properties of these coefficients enable us to derive nonlinear Schrodinger equation which is used for theoretical description of SW envelope solitons formation, SW modulational instability, and formation of SW strange attractor observed in Ref. 1.

ISSN:0021-8979    

DOI:10.1063/1.348812

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The influence of thermal magnons on evolution of a spin‐wave packet is studied. For a short packet, if its length is shorter than the time of phase mixing the damping is small and proportional to the length. If the length of the packet is greater than the time of phase mixing the magnon damping can be obtained from the kinetic equation. The evolution equation for a packet with an arbitrary length is derived. In particular, if the packet arises instantly, one should replace &dgr;(&Dgr;&ohgr;k) →sin(&Dgr;&ohgr;kt)/&Dgr;&ohgr;kin the energy conservation condition in the kinetic equation to allow for the time dependence of the damping. The characteristic time of phase mixing &tgr;phis of the order of 1/&Dgr;&ohgr;k. As a rule &Dgr;&ohgr;k ∼ &ohgr;k, so the time &tgr;phis of the order of the spin‐wave period. If the thermal magnon dispersion is small, the damping grows slowly. This occurs when a nuclear spin wave with a large wave vector is excited. A slow growth of damping can occur in a quasi‐one‐dimensional antiferromagnet also.

ISSN:0021-8979    

DOI:10.1063/1.348813

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The dispersion relation of the spin‐wave excitations in bcc iron has been studied by neutron inelastic scattering at the spallation neutron source ISIS. Magnetic intensity was followed up to 550 meV along the [100] direction. The general form of the dispersion curve is in qualitative agreement with that calculated from a spin‐polarized band model, and in particular the prediction of propagating modes extending above 300 meV has been confirmed.

ISSN:0021-8979    

DOI:10.1063/1.348814

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The Fermi contact contribution to the hyperfine fields on Fe nuclei in Y2Fe17, Y6Fe23, YFe3, YFe2, and Y2Fe14B has been calculated by means of first principles self‐consistent band structure calculations. The calculated and experimental hyperfine fields are strongly correlated, but, similar to results reported earlier for elemental Fe, the absolute values of all calculated hyperfine fields are systematically too small. The 4selectron contribution to the hyperfine fields is shown to result in significant deviations from a simple proportionality relation between the hyperfine fields and the local Fe moments.

ISSN:0021-8979    

DOI:10.1063/1.348815

出版商:AIP    

年代:1991

数据来源: AIP