摘要:

Spin‐wave resonance (SWR) has been applied to investigate the influence of Ni‐ion implantation onto the magnetic properties of Permalloy films. The implantation has been carried out at room temperature with different doses at Ni‐ion energies of 95 keV. In films of 30–60 nm thicknesses, which are thin compared to the implantation range, with increasing dose the uniform mode is shifted to higher resonance fields in the parallel configuration. In the perpendicular configuration one obtains a shift to lower fields. The corresponding variation of the effective magnetization as a function of the implantation dose could be reproduced by assuming a change of the average volume magnetization. In thicker films (150–250 nm) standing spin waves are excited which are only slightly modified by implantation compared to the as‐prepared samples. The observed effects can be attributed to changes of the surface anisotropy.

ISSN:0021-8979    

DOI:10.1063/1.352412

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A theory of nonlinear interactions of spin waves (SW) propagating in thin magnetic films is developed. We use the classical Hamiltonian formalism for SW in an unbounded ferromagnetic medium in combination with the theory of dipole‐exchange SW spectrum for magnetic films to derive a nonlinear Schro¨dinger equation for the SW envelope which gives a good qualitative description of the formation of the envelope soliton experimentally observed in yttrium‐iron garnet (YIG) films.

ISSN:0021-8979    

DOI:10.1063/1.354052

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The EPR linewidths of (C3H7NH3)2Mn1−xCdxCl4have been measured in the paramagnetic region as a function of temperature. An Arrhenius behavior exp(b/T) is observed withbstrongly dependent on Cd concentration. This behavior in the pure compound is characteristic of soliton‐like (skyrmion) excitations withbrelated to the excitation energy. In the doped compound, gapless nonlinear excitations are possible, resulting in a much different temperature‐dependent linewidth. It is concluded that skyrmions dominate the low‐temperature thermodynamics of two‐dimensional magnets.  

ISSN:0021-8979    

DOI:10.1063/1.352413

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

M‐type hexagonal ferrites have been incorporated in a coupled dielectric image guides structure for nonreciprocal microwave circuits design. Two different field configurations have been considered in this paper: for case I thecaxis of the ferrite materials was aligned along the direction of propagation; for case II thecaxis was aligned in the transverse direction. Electromagnetic wave scatteringSparameters of the device were measured for an external biasing magnetic field applied parallel and perpendicular to thecaxis of the ferrite slab. We have observed nonreciprocal microwave effects in the ferromagnetic resonance regime as a result of a nonuniform external biasing magnetic field along the propagation direction. We believe that ferrite devices operating at millimeter‐wavelength frequencies, such as isolators, filters, modulators, switches, phase shifter, etc., can be implemented from our device design.

ISSN:0021-8979    

DOI:10.1063/1.352414

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The magnetostatic dispersion relations for a device consisting of two ferrite slabs of thicknessd, separated by an air gap of widthw, and placed between parallel ground planes is derived. In particular, edge waves and the influence of structure parameters on the pinning conditions of coupled magnetostatic waves to the edge of the ferrite slab are investigated. The obtained theoretical results have been confirmed experimentally.  

ISSN:0021-8979    

DOI:10.1063/1.352415

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Self‐biased millimeter‐wave circulators can be built using hexaferrite materials with high internal anisotropy field, thus avoiding the use of large and heavy permanent magnets. The design principles of these devices are considered thoroughly in the article for both below‐ and above‐resonance circulators. The article treats the classical structure of a half‐wave‐long ferrite cylinder, two dielectric spacers, and two metal transformers. A simple microwave technique for the measurement of the necessary parameters of the hexaferrite material is used. The choice of an adequate material with a proper value of the internal anisotropy field is discussed. An accurate procedure for the dimensioning of the ferrite insert and the spacers that eliminates the tedious cut‐and‐try work is described. A way to find the effective internal magnetic field is shown. Simple relations for the calculation of the matching elements are also given. Results on an above‐resonance 33 GHz self‐biased circulator that support the theory are presented.

ISSN:0021-8979    

DOI:10.1063/1.352416

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Bound states with energy below the two magnon band are proven to exist for any wave vector in the triangular ferromagnet. This statement is in contrast with early conclusions1based on numerical calculations. On the other hand, we stress that the proof of the existence of bound states in 2D lattices is beyond any possible numerical accuracy so that analytic approaches are essential. This is due to logarithmic divergences of some integrals which assure a nonvanishing binding energy for a vanishing wave vector, but which are so weak that they escape any numerical evaluation. Consequently, we confirm that the presence of at least a two magnon bound state for any wave vector seems to be a reliable signal of absence of long range order.

ISSN:0021-8979    

DOI:10.1063/1.352417

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Inelastic neutron scattering has been employed to study the magnetic excitations of Ho3+in a grain‐aligned sample of the high‐Tccompound HoBa2Cu3O7. From these measurements, we derive direct information on the anisotropic nature of the pair coupling within the rare‐earth sublattice. The HoBa2Cu3O7sample was grain‐aligned at high temperatures under the action of an external field. This procedure resulted in an alignment of the grains along thezaxis, whereas thexandyaxes remained randomly oriented. We have particularly chosen HoBa2Cu3O7, since its crystal‐field level scheme is well‐established and characterized by a series of low‐lying states1which can be sufficiently resolved to detect even very small dispersion effects. The dispersive behavior of the &Ggr;3→&Ggr;4crystal‐field excitation is exemplified in Fig. 1 for various values ofQ=(x’,z),x’=&sqrt;x2+y2, atT=1.5 K. The absence of energy dispersion along thezaxis proves the predominantly 2D nature of the magnetic coupling between the Ho3+ions, as actually expected from their large separation along thezaxis. The observed energy spectra correspond to a superposition of transverse and longitudinal components which we have been able to separate through selected scans inQspace. Nearest‐neighbor exchange parameters are estimated on the basis of an effective two‐level model. The coupling of the Ho spins turns out to be extremely anisotropic.2

ISSN:0021-8979    

DOI:10.1063/1.352418

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The spin‐wave approximation in a Heisenberg ferromagnet is valid at low temperatures where the spin‐wave interactions are negligible. At higher temperatures these interactions become highly nonlinear and the spin‐wave approximation should break down. In a recent paper Fishman and Vignale1predicted that this breakdown should occur near the temperatureT¯=0.4zJs, wherezis the number of nearest neighbors,Jis the ferromagnetic coupling constant, andsis the spin of the ferromagnet. In another related paper Fishman and Liu2have calculated that above this temperatureT¯ a second pole appears in the correlation function at an energy close to 2zJs. The origin of this new mode is the strong coupling of transverse and longitudinal fluctuations that forces the spins to precess about their local mean field. When the ‘‘spin‐wave’’ energies are close to the precessional mode energy 2zJsa splitting of the ‘‘spin‐wave’’ branch is predicted. In order to test this prediction we have performed a high‐resolution inelastic neutron scattering experiment to measure the spin waves in EuO, a prototypical Heisenberg ferromagnet, in the vicinity of theq‐Eregion where this splitting should occur. Although our experiment was performed atT=20 K, which is aboveT¯≊12 K, our measurements failed to observe the predicted splitting of the ‘‘spin‐wave’’ branch. Only a single resolution‐limited spin‐wave branch was observed, in agreement with earlier measurements by Passelletal.3

ISSN:0021-8979    

DOI:10.1063/1.354050

出版商:AIP    

年代:1993

数据来源: AIP