摘要:

Variational calculations of the magnetization distribution &psgr; (r, &thgr;) about spherical inclusions (magnetic and nonmagnetic) in ferromagnets have been made using the trial function&psgr;=n=13m=1∞Rmr−man(m)P2n1(cos&thgr;),where thean(m) are coefficients. Numerical results are presented for the approach to saturation for an ensemble of noninteracting hard particles in a soft matrix as a function of inclusion radiusR, magnetizationI0, and exchange constantAof the matrix. Particular emphasis has been paid to the convergence of the series involved and to values of applied fieldHfor which the work is applicable. Increasing the number of terms in &psgr; beyondm∼10 can lead to results which violate the assumption of &psgr; small (the second‐order description); such a criterion has been ignored in past work. It is further shown that for &bgr;(=2&pgr;R2I02/A)>102the approximations are again violated forH<4&pgr;I0; e.g., in Fe withR=200 Å, applicability is limited toH>2×104Oe and in YIG withR=3×103Å,H>1.8×103Oe.

ISSN:0021-8979    

DOI:10.1063/1.1657539

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Optical observations of the interaction of domains with atomic scale imperfections are presented. Thin transparent {100} orientation wafers of GdIG are prepared from 20 mil‐thick slabs by chemical polishing, and in this process etch pits develop which reveal line imperfections in the crystal. These may be individual dislocation lines or other minor structural defects. Domains are both pinned and nucleated at these lines. Interesting cooperative effects occur when a number of lines can be identified in the same sample. It is often observed, for example, that nucleation and initial domain growth take place on the same side of all the lines. In some cases a small residual domain near a line imperfection appears to influence the domain structure throughout the sample, and has the effect of favoring one saturated remanent state over the other. When the residual domain collapses, the effect disappears. Magnetostrictive stress between the domain and the rest of the sample may be responsible for this behavior.

ISSN:0021-8979    

DOI:10.1063/1.1657540

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

According to theoretical predictions, the entropy of spin fluctuations in a paramagnetic metal with Stoner susceptibility enhancement factorSis (omitting numerical coefficients) &Dgr;S/S0≃lnS+S3T2ln (T/Ts), where S0is the entropy of the electrons without spin fluctuations andTs=TFermi/S. The first term gives rise to an enhancement of the specific heat and the thermal expansion at absolute zero. The second term should cause a low‐temperature anomaly in both properties. The enhancement of the specific heat has been reported in NiRh and Pd:Ni alloys, and the NiRh system also shows low‐temperature specific‐heat anomalies. The thermal expansion &agr; is more sensitive than the specific heatCto each of these contributions, approximately in the ratio &Dgr;&agr;/&agr;0/&Dgr;Cv/Cv0≃1/ &ggr;0∂lnS/∂lnV, where &ggr;0is the electronic Gruneisen parameter without spin fluctuations. Our magnetostriction data for Pd:Ni and NiRh alloys determines ∂ lnS/∂ lnV, which is proportional toSin accordance with the Stoner expression for the susceptibility. The thermal expansion of the Pd:Ni alloys as temperature approaches zero shows the expected enhancement, and the most strongly enhanced sample shows a low‐temperature anomaly. The low‐temperature thermal expansion of the NiRh alloys in general increases with Ni concentration, but the behavior is complicated by clustering effects. The ordered intermetallic compound Ni3Ga is found to have a large magnetostriction corresponding to its strong exchange enhancement, but as for the disordered alloys, the low‐temperature anomaly in its thermal expansion depends upon the annealing treatment.

ISSN:0021-8979    

DOI:10.1063/1.1657541

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The specific heat of Cu&sngbnd;Al and Cu&sngbnd;Al:Fe alloys containing 0, 5, and 10 at.% Al and 0.07 at.% Fe has been measured in the temperature range 1.2° –14° K. The excess specific heat due to the Fe impurities is decreased with increasing Al concentration. The data are compared with various predictions based on the Kondo model for dilute magnetic alloys. The decrease in the excess specific heat is consistent with the corresponding decrease in the magnetic moment of the Fe with increasing Al concentration determined by high‐temperature susceptibility data. A further systematic change in the temperature dependence of the excess specific heat is attributed to an increase in the Kondo parameterTKwith increasing Al. The measurements strongly support the hypothesis that the excess specific heat is associated with the Kondo effect, and that interactions among impurities is not the mechanism.

ISSN:0021-8979    

DOI:10.1063/1.1657542

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

We use an equation of motion approach to the problem of a spin ½ localized impurity in a metal in a magnetic field. The symmetries of the zero field are no longer present, and we find it necessary to define seven Green's functions. At‐matrix formalism is developed after truncating the equations of motion. We find that six of the Green's functions are expressible in terms of fourt's that satisfy coupled nonlinear singular integral equations. Our treatment takes account of the difference ingvalues of the conduction electrons and impurity electrons, and determines the electron magnetization self‐consistently. The corresponding Hilbert boundary value problem cannot be solved by the procedures which were useful in zero field. Instead, we numerically solve our system of finite‐field integral equations by an iterative procedure starting from the exact zero‐field results. We obtain results which are qualitatively similar to experiment and exhibit the freezing out of the spin‐flip scattering process and a subsequent suppression of the compensated state as the magnetic field increases.

ISSN:0021-8979    

DOI:10.1063/1.1657543

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A new Green's function decoupling scheme has been applied to the problem of how correlation affects the occurrence of local moments in metals. Using Anderson's one orbital model, the decoupling level is above Hartree but presumably too low to include the Kondo effect. The scheme is similar to Hewson's but includes certain energy shifts which make magnetic states more likely to occur. For very weak transfer interaction between the impurity and conduction electrons, both magnetic and nonmagnetic solutions are found. The nonmagnetic state usually has lower energy in agreement with Schrieffer and Mattis and others, but when the intra‐atomic Coulomb energy is sufficiently large, the magnetic solution has the lower energy. For strong enough transfer interaction the magnetic solution, of course, disappears.

ISSN:0021-8979    

DOI:10.1063/1.1657544

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

We present evidence, based on resistivity and susceptibility measurements, that the Kondo temperatureTKof Mo&sngbnd;Nb alloys containing Co is essentially independent of the host Mo&sngbnd;Nb composition. Since the density of statesN(0) of these alloys is known to change by about 20%, the above data indicates thatTKdoes not vary as exp[−1/N(0)J], or thatN(0)Jmust remain constant.

ISSN:0021-8979    

DOI:10.1063/1.1657545

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The diffuse scattering of neutrons from Ni&sngbnd;Cu alloys near the critical composition (∼44 at.% Ni) was measured at 4.2°K. For the ferromagnetic alloys of 50−46 at.% Ni (with Curie temperatures of 40°−9°K and spontaneous moments &mgr;0of 0.06−0.02 &mgr;B/at.) the results give a magnetic‐scattering cross section (d&sgr;/d&OHgr;) that is unexpectedly large at low scattering vectors (&Kgr;) and decreases rapidly with increasing &Kgr;. If the ferromagnetic moment were distributed equally among all the Ni atoms, the predictedd&sgr;/d&OHgr; would be much smaller and essentially constant over the same range of &Kgr; (∼0.2–0.9 Å−1). Instead, our Ni&sngbnd;Cu results closely resemble those previously reported1for the ferromagnetic polarization about an Fe impurity atom in Pd and can be interpreted analogously in terms of polarization clouds extending over a few hundred atoms. From the measured values ofd&sgr;/d&OHgr; and &mgr;0it is deduced that the average moment per polarization cloud is ∼8 &mgr;Bwhile the concentration of these clouds varies from ∼0.7% (for the 50% Ni alloy) to ∼0.2% (for 46% Ni). Some chemical clustering is also detected in these alloys but of much shorter range. Significant neutron scattering by critical fluctuations is observed for the paramagnetic alloys closest to the critical composition. A fuller report on this work will appear elsewhere.

ISSN:0021-8979    

DOI:10.1063/1.1657547

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Diagrammatic perturbation theory is used to obtain corrections of second order in the particle density to the effective electron‐electron interaction appearing in Edwards' theory of spin waves in ferromagnetic metals. A contact interaction model involving a single tight‐bindings‐like band is employed. When account is taken of changes in the electron self‐energy, it is found that there is a high degree of cancellation of the possible corrections. The result is to confirm the validity of the simpletmatrix approach in predicting stability of the ferromagnetic state against spin‐wave excitations, and in calculating the energy of long‐wavelength spin waves. An expression is also obtained for the effective interaction in the low‐density limit for a many‐band system.

ISSN:0021-8979    

DOI:10.1063/1.1657548

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The theory of local spin excitations in a ferromagnetic metal described by the simple one‐band, strong‐correlation model is briefly discussed. Localized spin‐wave impurity states of the type found in magnetic insulators are identified with poles or peaks in the local (transverse) reduced‐susceptibility function. The connection between local‐spin canting in the impurity ground state and low energy local magnons is established. Local ``polar'' spin excitations involving fluctuations in the magnitude of the impurity moment give rise to structure in the local longitudinal susceptibility function. Low‐energy polar modes are expected when the impurity ground state is nearly unstable to the formation of a localized moment which differs from that of the ferromagnetic host. The role of the localized electronic state is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.1657549

出版商:AIP    

年代:1969

数据来源: AIP