ISSN:0021-8979    

DOI:10.1063/1.1984587

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

This paper gives a brief critique, in elementary language, of the principal types of theoretical pictureswhich have been advanced concerning the electronic states of transition metals, especially those of the irongroup. It also calls attention to the possibility that some of the properties of these metals can be correlatedby the use of concepts which have an exact, not just approximate, meaning for a many-electron system.The Fermi surface is probably a concept of this type. Major conclusions are that in the iron group metalsthe 3delectrons ought not to differ radically from those in the free atoms either in number or in spatialdistribution, and that in most, though perhaps not all, of these metals the 3delectrons, magnetic or nonmagnetic,have an itinerant behavior.

ISSN:0021-8979    

DOI:10.1063/1.1984588

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

This paper gives an elementary discussion of the theory ofgvalues and line widths in ferromagneticresonance in certain rare earth garnets. The experimental facts are reviewed briefly.

ISSN:0021-8979    

DOI:10.1063/1.1984589

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

The crystal distortion which arises from the Jahn-Teller effect is discussed in several examples. In thecase of compounds containingCu2+orMn3+at octahedral sites, the lowest orbital level of these ions isdoubly degenerate in the undistorted structure, and there is no spin-orbit coupling in this level. It is shownthat, introducing a fictitious spin to specify the degenerate orbital states, we can discuss the problem byanalogy with the magnetic problems. The “ferromagnetic” and “antiferromagnetic” distortions are discussedin detail. The transition from the distorted to the undistorted structure is of the first kind for theformer and of the second kind for the latter. Higher approximations are discussed briefly. In compounds likeFeO, CoO, andCuCr2O4, the lowest orbital level is triply degenerate, and the spin-orbit coupling is presentin this level. In this case the distortion is dependent on the magnitude of the spin-orbit coupling relativeto the strength of the Jahn-Teller effect term. The distortion at absolute zero temperature and its temperaturedependence are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1984590

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Ferromagnetic interactions of the rare earth elements in dilute solutions or in compounds with non-magneticelements have been discovered and will be described. The Curie points are essentially proportionalto the spin while the saturation moment follows the value for the effective moment. This ferromagneticinteraction, known to take place via the conduction electrons, follows criteria resembling closely those forthe occurrence of superconductivity.It is shown that by suitable combination of similar superconductors and ferromagnets both phenomenacan happen simultaneously in the same crystal.

ISSN:0021-8979    

DOI:10.1063/1.1984591

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

One of the authors (L.N.) describes the mechanism of magnetization of a ferromagnetic single crystal:the elementary domains can be classified in several groups, called “phases” each having a certain direction ofthe spontaneous magnetization. The variation of the total magnetization takes place in various “modes,”corresponding to the number of coexisting phases in a given field. The calculation of the magnetizationcurve in these various modes explains the experimental results for single crystals of iron, pyrrhotine andmagnetoplumbite.The results of measurements at room temperature on this latter compound,6Fe2O3-PbO, are treatedin greater detail. The crystal is uniaxial; for different angles of the external field with thecaxis, the magnetizationfirst increases proportionally to the field, followed by a gradual approach to saturation (exceptfor the angles 0 and 90°). The anisotropy constantK1is first determined (K2is negligible). The two partsof the curve can be interpreted by two “modes,” with respectively, two and one “phases.”The variation of the magnetization of polycrystalline6Fe2O3-BaOsamples can be explained by therotation mechanism only; the crystallites are too small in size to be divided into phases. The experimentalcurves can be explained by taking into account the interactions between crystallites in calculating the lawof approach to saturation.

ISSN:0021-8979    

DOI:10.1063/1.1984592

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Since the discovery of the magnetic yttrium and rare earth iron garnets, a systematic investigation hasbeen made of interactions of magnetic ions and the distribution of both magnetic and non-magnetic ionsin the garnets. The results of substitution of the tetravalent tin forFe3+ion in yttrium-iron garnet (balancedby substitution ofCa2+forY3+ions) have led to the development by Gilleo of a statistical interactiontheory which accounts well for the spontaneous magnetizations and Curie temperatures of the system.This theory has been further strengthened by results from zirconium substituted yttrium-iron garnets andby its successful extension to substituted rare earth iron garnets.In the course of our investigations, many new garnets have been discovered. Several of these have enabledus to observedirectlyinteractions between magnetic ions in dodecahedral and octahedral sites, dodecahedraland tetrahedral sites, octahedral sites only and tetrahedral sites only. The work on the garnets has alsoalready been adequately extensive to lead to the establishment of some rather simple rules pertaining to sitepreference of ions entering the garnets.

ISSN:0021-8979    

DOI:10.1063/1.1984593

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

A crystal sphere of YIG was placed in either a doubly resonant transmission type coaxial cavity or intoa nonresonant transmission line device and biased with a dc magnetic field. A low power microwave signalin the S band frequency range (cw or pulsed) was used to excite resonance. Current pulses of approximately1 &mgr;sec duration and low duty cycle were sent through a low impedance coil that was wound around thecavity or transmission line respectively. These pulses induced pulsed magnetic fields of the order of 1000gauss, which (vectorially) added to the existing dc field.A microwave receiver, attached to the cavity output, detected weak oscillations at higher or lowerfrequencies. Shifts of 280 Mc have been obtained with the cavity, and shifts of 1280 Mc above and 530 Mcbelow the driving frequency have been obtained with the transmission type device.The detected signal is associated with either the rise of the current pulse, or the decay, or two signals mayappear simultaneously corresponding respectively to the rise and decay. This depends upon the angularrelation of the dc field with respect to the cavity or transmission line.

ISSN:0021-8979    

DOI:10.1063/1.1984594

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Measurements of the absorption coefficient of (111) crystal sections of&agr;-Fe2O3were made in the visibleand near infrared. Absorption peaks were observed centered at energies of about12000 cm−1and16000cm1.Reflectivity measurements on the&agr;-Fe2O3crystals were carried out in the30000 cm−1to400 cm−1range. Broad reflectivity peaks are found in the near ultraviolet at24500 cm−1and26000 cm−1, respectively.Very strong reflection peaks resulting from the lattice vibrations are present at energies smaller than700 cm−1.Several quite sharp peaks are found in the absorption spectra of the rare-earth iron garnets of Dy, Ho,Er and Yb in the10000 cm−1–13000 cm−1energy range. The circular dichroism of ytterbium iron garnet isdisplayed by means of absorption curves for the two senses of circularly polarized light.

ISSN:0021-8979    

DOI:10.1063/1.1984595

出版商:AIP    

年代:1960

数据来源: AIP

摘要:

Instabilities in YIG spheres that exist above certain threshold power values of cw microwave field arestudied using samples of about14−12 mmdiam, that have low power line widths of 1 oe or less. Whereas thecharacteristic behavior such as asymmetrical line shape, “jump” effect etc. is somewhat similar to thatreported for disks, the phenomenon is generally different.It has been determined that this instability, which can occur at cw power levels below the threshold forsignificant spin wave growth, is due entirely to the heating effect of resonance absorption upon the anisotropyenergy of the crystal lattice. As a result, the instability is characterized by a threshold curve thatfollows both the extrema and symmetry of the anisotropy curve for a given orientation. A straightforwardtheoretical explanation based on familiar relationships is outlined which fits the instability thresholdvsorientation curve. The temperature instability provides a technique for measuring the “g” factor that isbelieved to be more direct than previous methods.

ISSN:0021-8979    

DOI:10.1063/1.1984596

出版商:AIP    

年代:1960

数据来源: AIP