摘要:

We have developed a new infinite‐order perturbation approach for treating strongly anisotropic magnets. This formalism is based on a canonical transformation of a given system into a new system with an effective two‐ion anisotropy which, for example, can be treated by conventional spin‐wave techniques. An expression for the spin‐wave energy for the general conical magnetic moment configuration with arbitrary single‐ion anisotropy has been derived using this formalism. It was found that an analysis of the transverse spin‐wave spectra can at most result in a determination of a renormalized exchange integral and two renormalized anisotropy constants. A comparison of the theoretical predictions of this formalism with experimentally determined spin‐wave spectra indicate that a substantial part of the large two‐ion anisotropy, which had been previously introduced to describe the spin‐wave spectra in the heavy rare earth series, actually resulted from the fitting procedure which was used and not from any physical source.

ISSN:0021-8979    

DOI:10.1063/1.324763

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

We describe a multicomponent coherent potential approximation for the diluted ferromagnet, which includes the effect of the environment on the Ising diagonal energy levels of a site as does previous work, and which also takes account of short‐range order, i.e., the correlation between environments of neighboring sites. Results are presented for simple and body centered cubic cases, and compare well with computer simulation results. They show little difference from the single site CPA theory, a simplified version of which is presented. We expect to find greater differences for the diluted antiferromagnet.

ISSN:0021-8979    

DOI:10.1063/1.324710

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

It is possible to understand low temperature spin waves in classical spin systems in terms of a phenomenological ’’fixed‐length’’ hydrodynamic theory. The particular hydrodynamic theory appropriate for magnets with an underlyingXYsymmetry is exactly soluble, and seems to give a good description of equilibrium order parameter dynamics in one, two, and three dimensions. Here, we exploit this exact solubility to study spin wave dynamics far from equilibrium. Spin waves dominate the nonequilibrium equal‐time correlation functions. In two dimensions, for example, two point equal‐time correlations decay from a state of complete alignment by propagating a cusp‐like singularity from short distances toward large. The cusp travels at twice the spin wave velocity, and leads tok‐space oscillations in the Fourier‐transformed spectrum, ? (k,t). If these oscillations could be detected, they would show up as a time‐dependent bull’s‐eye pattern surrounding Bragg peaks in a neutron diffraction experiment.

ISSN:0021-8979    

DOI:10.1063/1.324711

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

In order to investigate the nature of the spin arrangement in the surface of a semi‐infinite helical magnet, we calculate both the surface spin‐wave frequency spectra and the classical spin‐ordering energies. We present detailed calculations for a (100) surface of a body‐centered tetragonal lattice with nearest neighbor and next nearest neighbor exchange interactions (J1andJ2, which are both negative). The possibility that the exchange interactions in the surface (Js2) may differ from those in the bulk (J2) is taken into account. We find the following: In the case 0≳Js2≳J2, the surface spin‐wave frequencies are real and positive, and hence the uniform helical spin arrangement is stable. On the other hand, in the caseJs2<J2, there exist soft surface spin‐wave modes, which indicate that the uniform helical spin arrangement is not stable. In the latter case, it is shown that a nonuniform spin arrangement, characterized by a rearrangement of spins in the surface, has a lower classical spin‐ordering energy than the uniform state and hence is stable.

ISSN:0021-8979    

DOI:10.1063/1.324712

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Previous theoretical studies of the temperature dependence of the magnon dispersion curves of rare earth metals have usually considered only the renormalization due to spin‐wave interactions and the effect of temperature variations of the magnetocrystalline anisotropy. An important consideration which has not been included in these studies is the possible temperature dependence of the exchange coupling between the localized spins. Taking into account only the shorter conduction electron mean free path and the resulting decrease in the 4f‐4fcoupling asTincreases we can qualitatively understand the general features of the measured magnon dispersion curves. Our results are consistent with recent measurements on Gd alloys which relate the temperature dependence of the magnetic ordering temperature to the conduction electron mean free path.

ISSN:0021-8979    

DOI:10.1063/1.324713

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The neutron scattering by isolated clusters of Mn2+ions in a single‐crystal specimen of KMn0.18Zn0.82F3has been studied as a function of wave‐vector transfer Q at 4.3 K and as a function of temperatureTat Q=2&pgr;a−1(0.55,0.55,0.55). At lowTwe observe peaks corresponding to the ground‐state transitions of isolated pairs and triplets and to two excited‐state transitions of pairs. The frequencies &ugr; of these peaks lead directly to accurate values for the exchange interactionJ, e.g., 0.100±0.004 THz and 0.098±0.002 THz from the ground‐state transitions of pairs (&ugr;=2J) and triplets (&ugr;=7J) respectively. Exact calculations of the neutron scattering by isolated pairs, triplets and quartets of Mn2+ions interacting via nearest‐neighbor antiferromagnetic exchange give a reasonably good description of the principal features of the measurements. There are, however, discrepancies that are probably attributable to the scattering by larger clusters.

ISSN:0021-8979    

DOI:10.1063/1.324714

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

We report a study of the mean spin on finite clusters with 2, 3, 4, 5 and 6 spins in a linearized spin wave model for the dilute antiferromagnet on a square lattice due to Thorpe and Alben. For 2 and 3 spin clusters, the results are compared to the exact results and to the exact solution of the linearized Green’s function equations. We find that, as expected physically, the model treats this property of the small clusters in a manner which is neither self‐consistent nor in accord with exact results. We discuss briefly how these results affect the validity of the use of the Thorpe and Alben model for other more global quantities.

ISSN:0021-8979    

DOI:10.1063/1.324715

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Despite common misconceptions, domain walls are too thick to ’’scatter’’ electrons appreciably. However, electrons crossing a wall apply a torque to it, which tends to cant the wall spins. This could be used to measure the conduction electron spin polarization. Most of the low‐field resistive anomalies observed in pure Fe, Ni and Co at low temperature are caused by the Lorentz force associated with the internal fieldB=Mspresent inside each domain. The existence of low‐resistivity paths extending over many domains accounts for still unexplained magnetoresistance data in iron whiskers. In uniaxial materials, a d.c. eddy‐current loop caused by the Hall effect runs around each wall. The fieldHzgenerated by these loops tends to ’’drag’’ the whole domain structure in the direction of the carrier drift velocity. Also, the Joule dissipation of the eddy currents manifests itself as an excess Ohmic resistance. As predicted, this excess resistance decreases as the square of the field, in amorphous Gd25Co75films, in MnBi films, and in pure bulk cobalt, when the walls are removed by an external field. The excess resistance can also be changed by reorienting the walls.

ISSN:0021-8979    

DOI:10.1063/1.324716

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

We have measured the electrical resistivity between 4.2 K and 300 K and longitudinal and transverse magnetoresistance at 4.2 K of several Ni, Co and Fe based alloys containing variouss,pimpurities such as Sn and Sb. The deviations from Matthiessen’s Rule (DMR) derived from the resistivity data have been fitted to the two current models of Campbelletal. and values for &agr; (the ratio of spin up to spin down resistivities at the impurity site), &mgr; (the same ratio at host sites) andD(the coefficient of spin mixing scattering events) estimated. These data yield values of &agr; close to one, as expected from a simple non‐magnetics,pscreening state around such impurities. The magnetoresistance data also yield values of &agr; close to 1, but there are consistent differences between these data and the two current model predictions with &agr;=1, especially for the Ni and Co based systems.

ISSN:0021-8979    

DOI:10.1063/1.324717

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The spin dependent residual resistivities for a large number of solute atoms in Fe and Ni based ternary alloys are analyzed as arising from three main sources. 1. Coulomb scattering from the impurity charge (or volume) perturbation. This amplitude is found to be similar to that for Cu based alloys. 2. Scattering from a localized moment perturbation at the solute atom. Both a potential and exchange amplitude are found to be spin‐dependent and about the same per &mgr;Bfor Fe and Ni hosts. 3. A more widespread moment perturbation in the host surrounding the solute atom. The shape of this perturbation depends upon whether direct Coulomb exchange or hybridization effects dominate. In Ni, usually hybridization effects dominate and give predominately negative host perturbations. In Fe, Coulomb exchange dominates and the shape of the host perturbation depends upon the number of itinerantd‐like electrons connected with the solute atom.

ISSN:0021-8979    

DOI:10.1063/1.324718

出版商:AIP    

年代:1978

数据来源: AIP