摘要:

If a spin wave propagates through a region of nonuniform magnetic field in which the effective magnon wavenumber equals the phonon wavenumber, it is partially converted into an elastic wave. The conversion efficiency depends primarily on the field gradient (H′) at the crossover point. Theories have been developed both for the case of weak magnetoelastic coupling and strong magnetoelastic coupling. For weak coupling the magnon‐phonon conversion efficiency &eegr;mpis &eegr;mp=H′crit/|H′| (for |H′|>>H′crit), whereas for strong coupling &eegr;mp= 1−&pgr;2exp(−H′crit/|H′|) (for |H′|<<H′crit). HereH′crit=&pgr;b22&ohgr;/cM&mgr; is a critical field gradient,b2is one of the magnetoelastic constants, &ohgr;/2&pgr; the signal frequency,cthe velocity of (transverse) sound,Mthe saturation magnetization, and &mgr; the shear modulus. It has been assumed that the dc magnetic field is applied along a cube edge of a cubic crystal and that the material is elastically isotropic. For yttrium iron garnet (YIG) at room temperature and a signal frequency of 3×109sec−1,H′crit≃5×104Oe/cm. The field gradients encountered in practice are usually appreciably smaller. Thus the strong coupling theory applies and the magnon‐phonon conversion should be substantially complete. The magnetoelastic interaction also gives rise to a reflected wave which originates in the crossover region. The amplitude of the reflected wave is much smaller than the amplitude of the transmitted wave.

ISSN:0021-8979    

DOI:10.1063/1.1702867

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Reagent grade potassium chloride usually contains sufficient water to hydrolyze some of the KCl so that crystals grown from it contain KOH unless they are treated with either HCl or Cl2. Thus crystals grown in platinum without pretreatment show the well‐known OH band. Crystals grown in silica do not show this OH band but they nevertheless contain a source of hydrogen as shown by the formation of theU2band on exposure to ultraviolet light. It is shown that such crystals have an absorption band at 187 m&mgr; and form bands at 193.5 and 360 m&mgr;, in addition to other bands previously recorded, on exposure to uv light. Various possible models for the centers responsible for these absorption bands are discussed, together with the photochemical reactions involved.

ISSN:0021-8979    

DOI:10.1063/1.1702868

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Data are presented for Young's modulus of (110) [001] oriented Fe‐3.1% Si up to 1200°C for various angles to the rolling direction. These data exhibit a strong decrease in Young's modulus with increasing temperature during the ferromagnetic transformation so that the Young's modulus of the paramagnetic state is lower than for the ferromagnetic state. This effect also exists for Ni and Co, and is attributed to a decrease in interatomic forces accompanying the decrease of the exchange interaction between 3d electrons. This ferromagnetic modulus relaxation leads to a new specific heat termC&Dgr;&thgr;which represents the energy absorbed in decreasing the mean frequency of the vibrational spectrum. The value ofC&Dgr;&thgr;, calculated from Young's modulus data, remains appreciable above the Curie temperature, thus accounting for the apparent excess specific heat of the ferromagnetic metals above that temperature. Separation of the total electronic specific heat suggests that three‐fourths of the low‐temperature electronic specific heat of Fe, Ni, and Co is of magnetic origin and that the magnetic specific heatCmincreases linearly with temperature up to 0.7 Tcand then increases rapidly to a peak at Tc, the Curie temperature. Well above Tc, a residual electronic term of nonmagnetic origin is found. The calculated values of magnetic entropy and energy derived fromCmfor Fe, Ni, and Co are in reasonable agreement with Heisenberg theory usings=½.

ISSN:0021-8979    

DOI:10.1063/1.1702869

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

A theory is given for the spin‐dependent scattering contribution to the resistivity of a ferromagnetic alloy as a function of temperature, composition, and short‐range order. A spin‐dependents‐dexchange interaction of the Zener‐Yosida type is assumed responsible for the major part of the scattering. Steady‐state pair correlations between spins in the ferromagnetic alloy are also considered in deriving the resistivity. On ignoring the effect of short‐range order on the electronic structure, the short‐range order is seen to affect both the temperature‐dependent and temperature‐independent parts of the scattering. For binary alloys of a monvalent metal solvent with transition element solutes, short‐range order tends to increase the resistivity relative to that of a random structure. Clustering tends to decrease it.

ISSN:0021-8979    

DOI:10.1063/1.1702870

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The electrical conductivity of single crystals of Cr2O3has been measured as a function of temperature from 600° to 1400°C and as a function of oxygen partial pressure from 1 to 10−6atm. The results show a high‐temperature ``intrinsic'' conductivity that is independent of oxygen pressure, and a low‐temperature defect‐controlled conductivity that varies with oxygen pressure in a manner which cannot be determined from measurements on single crystals. The very low mobility deduced from the intrinsic conductivity suggests an interpretation in terms of the theory of small polarons. Although the possibility of charge transport in very narrow bands cannot be ruled out on the strength of the experimental data, it seems less likely. The model suggested is that of charge formation and transport in localized 3dlevels of the cations.

ISSN:0021-8979    

DOI:10.1063/1.1702871

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The paramagnetic susceptibility, electrical resistivity, and lattice parameters of nickel‐rich nickel‐tantalum solid solutions (10 compositions) have been studied. The paramagnetic measurements extend to about 1500°K. The resistivities have been obtained at 4.2°, 77.2,° and 297.0°K. The lattice parameters of samples quenched from 1473°K were determined at 298°K. All these data indicate that the influence of tantalum on the physical properties of nickel is very similar to that of niobium. It is demonstrated by means of the magnetic susceptibility measurements that the generally accepted Curie‐Weiss law, with an additive temperature‐independent susceptibility term, is not applicable to the solid solutions of tantalum in nickel. The susceptibility and resistivity data are briefly discussed from the viewpoint of recent theoretical and experimental developments. The lattice parameters are analyzed using the previously suggested atomic volume scheme and elasticity theory. The agreement between the experimental values and the predictions is satisfactory.

ISSN:0021-8979    

DOI:10.1063/1.1702872

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Magnetization per gram of an iron sphere &sgr; as a function of temperatureThas been studied between the ferromagnetic Curie temperatureTFCandTFC+30°K with applied magnetic fields between 180 and 1360 Oe. The temperatureTFC, appearing as an abrupt decrease in the &sgr; vsTcurves, occurs at 1044±2°K. Mass magnetic susceptibility of iron, above the ferromagnetic Curie temperature in small magnetic fields, is proportional to (T‐TFC)−4/3, which is the theoretical prediction for a Heisenberg three‐dimensional ferromagnet.

ISSN:0021-8979    

DOI:10.1063/1.1702873

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

An analysis is presented of the parallel pumping of magnetoelastic waves in ferromagnets. The resulting general threshold expression has as special cases the spin wave and elastic wave limits. The analysis clarifies the nature of the instability near the spin‐wave‐elastic‐wave crossover where the normal modes are magnetoelastic. It is shown that the magnetoelastic notch in the spin‐wave threshold curve (hcritvsHdc) observed in yttrium iron garnet is not the expected behavior for materials with large magnetoelastic coupling constants, small elastic losses, and low saturation magnetization. Under these conditions and for frequencies above the crossover frequency it is shown that the lowest threshold may result from the elastic‐wave instability instead of the spin‐wave instability.

ISSN:0021-8979    

DOI:10.1063/1.1702874

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Electrical and optical properties of gallium arsenide diodes with aP‐P0‐Nstructure were studied between 4.2° and 90°K. The diodes were prepared by first diffusing manganese, and then zinc, inton‐type gallium arsenide, keeping the zinc junction depth much shallower than that of the manganese junction. TheP0region, dominated by the manganese, becomes one of high resistivity at these temperatures because of freezeout of holes on the deep‐lying manganese centers. The low‐resistivityNandPregions on either side serve as electron and hole injecting contacts, respectively. The diodes are electroluminescent and exhibit a negative resistance over a portion of their current‐voltage characteristics when a forward bias is applied. At 77°K, a typical voltage for the onset of the negative resistance is 5 V at a current of 5 mA; at about 2.2 V the dynamic resistance becomes positive again. The spectrum of the emitted light indicates recombination through both zinc and manganese centers. It appears that the mechanism responsible for the negative resistance is not the one suggested by Lampert. Possibly, the absorption by theP0region of light emitted at theP‐P0boundary is responsible for the phenomena observed, as suggested by Dumke. At voltages below the onset of the negative resistance, the current‐voltage relation in theP0region obeys Ohm's law at low injection levels but eventually the current depends on the square of the voltage. This behavior, as well as the spatial origin of the light, is in good agreement with the theory of Ashley and Milnes.

ISSN:0021-8979    

DOI:10.1063/1.1702875

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The electron probe microanalyzer has been used to study the local variations in recombination radiation from gallium arsenide. The intensity of recombination radiation from different specimens varies by several orders of magnitude. Local fluctuations are observed which are attributed to surface deformation and to non‐uniform distribution of impurities. Within experimental error, the wavelength distribution was indistinguishable from the wavelength distribution from GaAs diodes.

ISSN:0021-8979    

DOI:10.1063/1.1702876

出版商:AIP    

年代:1964

数据来源: AIP