摘要:

The dominant source of the effective magnetic field at thenuclei in ferromagnets apparently comes from the Fermi contactcontribution of theselectrons in the core. This contribution wasinvestigated for transition element atoms and ions by means of aseries of spin (or exchange) polarized Hartree-Fock calculations(calculations where one-electron orbitals of differingmsareallowed different radial dependence). Calculations were done forboth free-atom and crude crystalline environments. The sensitivityof the3selectrons to3dbehavior was found to contributeto large differences in calculated effective fields. The differencesbetween core polarizations in metals and ions are discussed withthe use of a model for metallic iron (i.e., a3d8configuration)which is more consistent with neutron diffraction and energyband results than the free atom3d6state (the spin polarized3d8calculation shows an expanded3dcharge density but an almostunchanged spin density relative to the3d6results). The resultant(negative) increase in the effective field is not, however, largeenough to overcome the positive contributions of the outerelectrons (estimated a`, la Marshall) so that although a net negativefield is found it is not large enough in magnitude to agreewith the Mo¨ssbauer measurements. A parallel calculation byGoodings and Heine, based on an artificially expanded3dchargeandspin densities, is also found to be similarly deficient. Theirmodel is discussed and shown to be incompatible with neutronexperiments and energy band calculations.

ISSN:0021-8979    

DOI:10.1063/1.2000370

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The nuclear magnetic resonance ofFe57has been observed in iron specimens in the form of powders ofvarious sizes, foils, and whiskers. Our results are consistent with previously reported work of Gossard,Portis, and Sandle on enriched iron, and with the work of Robert and Winter on natural iron. We find thatthe resonant frequency varies slightly among nominally pure iron specimens taken from different sources,suggesting that it is somewhat sensitive to impurity content. Prestrain of the specimen has a marked effect,the resonance in cold-rolled foil being very broad and weak compared to that observed in annealed foils.Observations of nuclear magnetic resonance in iron whiskers oriented both axially and transversely in therf coil offer interesting confimation of the domain wall enhancement mechanism put forth by Gossard andPortis. The temperature dependence of theFe57resonance has been investigated in the range 77°–785°K.

ISSN:0021-8979    

DOI:10.1063/1.2000371

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A number of nuclear resonances in ferromagnetic cobalt are reported and identified as originating inhcp crystallographic structure and in associated fault structure. A tentative assignment of these lineshas been made to possible stacking fault symmetries by correlating the observed resonances with metallurgicalsample treatments.

ISSN:0021-8979    

DOI:10.1063/1.2000372

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Free precession signals and spin echoes have been observed fromFe57,Co59, andNi61nuclei in finely divided multidomain iron,cobalt, and nickel, respectively. Spin-lattice relaxation has beenstudied in all three metals from temperatures in the liquid heliumrange to room temperature or above. Although the relaxation is notstrictly exponential, the rate of recovery of the magnetizationappears to be proportional to the absolute temperature. Spin-spinrelaxation has been studied in natural cobalt and in both naturaland enriched samples of iron and nickel. The spin-spin coupling isvery much stronger in cobalt than in iron or nickel, as expected,because of the very much larger nuclear magnetic moment ofCo59.Spin diffusion through the frequency spectrum of cobalt has beeninvestigated at 4.2 and 77°K by the stimulated echo technique.The results are consistent with a theory of one-dimensional diffusionwith exchange rate and exchange distance of the order of1/T2. The spin-echo technique has also been used in all threemetals to study the effect of domain wall motion on the nuclearresonance. A dc pulse, which displaces the domain walls by acontrollable amount, is placed between the first rf pulse and theecho. This study confirms that the induction signals arise fromnuclei in domain walls and further suggests a connection betweenline broadening and domain wall processes. A weak free-precessionsignal, arising from domain rotation, has been observed fromcobalt in high magnetic fields. The sign of the precession has beendetermined, confirming that the hyperfine field is directed oppositeto the magnetization.

ISSN:0021-8979    

DOI:10.1063/1.2000373

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Two additional nuclear resonances of theF19nuclei are observed in the antiferromagnetic state ofNiF2.These resonances can be observed in zero magnetic field at&ngr; = 26.6 Mc/secand&ngr; = −51.2 Mc/sec. They arebroad, asymmetric, and easily saturated, with an apparent value ofT1∼8 sec, at 20.30°K. They correspondto internal fields along [110] and [1¯1¯0] directions respectively forH0along [110]. These compare quite wellwith the theoretical values of +18.7 Mc/sec and −54.4 Mc/sec calculated from the measured interactionparameters for a spin alignment along [110] and [1¯1¯0]. These two resonances arise fromF19nuclei in thedomain walls.

ISSN:0021-8979    

DOI:10.1063/1.2000374

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

An integrated summary is given of some of the results and physical ideas contained in 16 experimentaland theoretical papers on ferromagnetic resonance contributed to the 1960 Conference on Magnetism andMagnetic Materials. The topics discussed include the uniform mode and anisotropy, nonuniform magnetostaticand electromagnetic modes, spin-wave resonance, sublattice effects, paramagnetic “impurities,”spin-wave scattering, rapidly relaxing ions, determination of spin-wave transition probabilities, thermalrelaxation, and the theory and use of the “parallel pump” method for measuring relaxation and exchangeparameters. Although a complete review of ferromagnetic resonance is not intended, the emphasis is on anorganization of this field and on the introductory and explanatory material presented with each topic.

ISSN:0021-8979    

DOI:10.1063/1.2000375

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The results of microwave resonance measurements of thegfactor and the first- and second-order anisotropy fieldsK1/MandK2/Mare summarized for crystals with the nominal compositionCoxMn1−xFe2O4, where0 ≤ x ≤ 0.10. In the region150°K<T<300°K &Dgr;(K1/M) = Ax,&Dgr;(K2/M) = −Bxandg = a+bx.&Dgr;(K1/M)is the cobalt contribution toK1/M; similarly for&Dgr;(K2/M). The results forK1/Mare in agreement with publishedvalues determined statically on the same crystals. ForT = 290°K, 260°K, 230°K, 200°K, 170°K, and 140°K,Bis 510,1790, 4600, 10 300, 22 400, and 50 000, respectively. At 260°K thevalues foraandbare 2.003 and 0.41 respectively, at 9400 Mc;at 17 000 Mc the values are 1.992 and 0.42. For the lower frequencyat 170°K the values are 2.004 and 0.64. The two microwavefrequencies were used only above 77°K; in this range thegvalues are isotropic to better than 0.5%. At 4.2°K the effect ofthe cobalt upon the variation of resonance field with direction in a(110) plane cannot be accurately described byK1/MandK2/M.The results are compared with the successful theory for the cobaltion substituted intoFe3O4. It appears that modification of theoriginal model is necessary to take into account the differingenvironment of the cobalt ion when substituted intoMnFe2O4.

ISSN:0021-8979    

DOI:10.1063/1.2000377

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A single crystal of tetragonally distorted manganese-iron spinel of compositionMn1.85Fe1.15O4has beenstudied by microwave resonance in the temperature range 77–390°K. The anisotropy has tetragonal symmetry,with thecaxis hard and the tetragonalaaxis (corresponding to the [110] direction in the undistortedcubic structure) easy. The data are analyzed in terms of an anisotropy energy of the form:EA = K1&agr;32+K2&agr;34+K3&agr;14+&agr;24,wherein&agr;1and&agr;2are direction cosines of the magnetization referred to the tetragonalaaxes, and&agr;3is thedirection cosine of the magnetization referred to thecaxis. The second-order axial componentK1/Misapproximately 3 kgauss at room temperature and remains relatively constant as the temperature is lowered to160°K, below which it increases rapidly. The quantityK3/M, which measures the anisotropy in the basalplane, is about −25 gauss at room temperature and increases to a few hundred gauss at 77°K. A temperatureindependentgeff = 2.01adequately describes the measurements. Linewidths vary from 250 gauss in thetetragonal [110] direction to 400 gauss in thecdirection at room temperature and increase with decreasingtemperature.

ISSN:0021-8979    

DOI:10.1063/1.2000378

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Microwave resonance measurements at 55 kMc and lower frequencies onGa2−xFexO3single crystalswhich are both ferromagnetic and piezoelectric have been made from room temperature to the Curie point.The crystals exhibit a large orthorhombic anisotropy with easy, intermediate, and hard directions along theorthorhombicc,a, andbaxes respectively(c<a<b), Linewidths are approximately 1 koe and anisotropyfields approximately 17 koe at room temperature. The origin and representation of the anisotropy is discussed.

ISSN:0021-8979    

DOI:10.1063/1.2000379

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The displacement current due to the interaction of the relatively high dielectric constant of a ferrite withthe electric field in a waveguide generates a magnetic field which may be comparable to the transversemagnetic field in the unperturbed waveguide. Hence, a thin ferrite rod located at anEfield maximum(Hfield minimum) may experience a strong ferromagnetic resonance absorption even though the rf magneticfield at the rod, due to the unperturbed waveguide modes, is negligible. A formula is given for the ratio ofenergy absorbed when the rod is at anEfield max as against being at anHfield max. A formula is also givenfor the biasing field required for resonance when the rod is at anEfield max. Test results agree well withtheory.

ISSN:0021-8979    

DOI:10.1063/1.2000380

出版商:AIP    

年代:1961

数据来源: AIP