摘要:

Investigation of photoluminescence induced by a focused laser beam is shown to be a valuable method for studying composition changes at heterojunctions. The method is applied to Ga1‐xAlxAs&sngbnd;GaAs heterojunctions grown by liquid phase epitaxy. The He&sngbnd;Ne laser used in these experiments excites near‐band‐edge recombination radiation, whose spectral distribution is analyzed. From the peak wavelength the Al concentration can be deduced. When the laser spot is moved in regions of the sample where the Al concentration changes with position, the spectral distribution of the photoluminescence is shifted in a very pronounced way. This results in a geometrical resolution much better than would be inferred from the laser spot diameter alone. The effect has been analyzed starting from a simple model of the process. It is shown that the Al concentration could be obtained with an accuracy of 1–2% and an effective geometrical resolution of a few microns all depending on the concentration gradient at the measuring point.

ISSN:0021-8979    

DOI:10.1063/1.1662191

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

By considering the electric field induced by flux motion, the requirement for stabilization of a hard superconductor against flux jumps has been found to beJc2 < k2CsT0[1+(Dt/Dm)]&mgr;0−1, whereCsis the heat capacity per unit volume of the superconductor,T0is given by ‐Jc(∂Jc/∂T)−1,DtandDmare the thermal and magnetic diffusivities of the superconductor, respectively, andk2is the smallest number satisfying the following time‐independent temperature equation: &Dgr;2(&Dgr;T′) = −k2&Dgr;T′. Using the above criterion, we derive the stability requirements in both adiabatic and dynamic stability limits. Then we discuss the stability of commercial superconductors covered by normal metal. The stability criteria can be obtained by replacingDmin the inequality with the average magnetic diffusivity of the normal metal and superconductor.

ISSN:0021-8979    

DOI:10.1063/1.1662192

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Superconducting properties and electrical resistivities of NbN thin films have been studied as a function of sputtering parameters, film structure, and composition. A good correlation exists between superconductingTc's and film structure and composition regardless of the sputtering conditions used to achieve this structure. The most important parameters affectingTcare lattice parameter, crystal structure, island structure, and impurity content. Substrate temperature, Ar/N2ratio, and impurity content are the principal sputtering parameters affecting film structure, and thusTc.

ISSN:0021-8979    

DOI:10.1063/1.1662193

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The effect of birefringence and dichroism in materials for magneto‐optic memory applications is analyzed. The optical constants of MnAlGe and MnGaGe have been measured. The birefringence and dichroism are approximately equal and are an order of magnitude larger than the magneto‐optic effect. A statistical analysis of the effect of orientation and grain size on noise due to optical anisotropy is also carried out.

ISSN:0021-8979    

DOI:10.1063/1.1662194

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Changes occurring in the chemical state, and the crystallographic and electronic structure of LaCoO3in a reducing atmosphere were determined. These determinations involved the use of thermogravimetric and calorimetric techniques, x‐ray diffraction, and magnetic property measurements. It was shown that the reduction process proceeded through the formation of a series of oxygen‐deficient structures. In the 0–3.25 wt% weight‐loss region, corresponding to the general formula LaCo1−xO3−y, where 0.0<x≤0.08 and 0.0<y≤0.5, the structure is monoclinic. In this structure, the cobalt ion exists as either high‐spin Co2+or low‐spin CoIII. At a weight loss of approximately 3.25 wt% the structure reverted to the original rhombohedral symmetry with a loss of lanthanum and cobalt. The chemical formula was again LaCoO3. Samples reduced to greater than 3.25 wt% possessed a different crystal structure than was previously observed.

ISSN:0021-8979    

DOI:10.1063/1.1662195

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

In connection with the large Faraday and Kerr rotations found previously in the compounds Bi3−2xCa2xFe5−xVxO12(CaVBIG), three series of related garnets containing bismuth have been prepared and the polar Kerr rotation spectra of polycrystalline samples of these materials have been studied. Three strong rotation peaks were observed near 2.7, 4, and 5 eV. In the first series, Y2.2−2xBi0.8Ca2xFe5−xVxO12withx= 0−1.1, the influence of the V5+and tetrahedral Fe3+concentration on the three transitions was studied, in the second series, Y3−xBixFe5O12(x= 0−1.0), the influence of Bi3+concentration, and in the third series, Y2.0−xBi1.0CaxFe5−xZrxO12, the influence of octahedral Fe3+. The transitions near 2.7 and 4 eV are strongly enhanced in proportion to the Bi3+content. The transition near 2.7 eV, which is responsible for the high Faraday rotation in the visible, is proportional to the tetrahedral Fe3+content. The effect of a small mixing of the oxygen −2p, tetrahedral iron 3d, and bismuth 6pelectronic wave functions is discussed in order to explain the results. The large value of the Kerr rotations (1°) is of interest for magneto‐optic applications.

ISSN:0021-8979    

DOI:10.1063/1.1662196

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Superconducting tunneling experiments on niobium single crystals with indium and lead‐bismuth counterelectrodes show an isotropic energy‐gap value of 3.12±0.05 meV, or 3.93kTc, for 26 junction orientations. Self‐consistent analysis criteria have been developed for the determination of gap values. These criteria are not met in the existing literature on transition‐metal tunneling.

ISSN:0021-8979    

DOI:10.1063/1.1662197

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Magnetization and susceptibility measurements on pseudobinary alloys Tb0.25Fe0.75−xAlxand Tb0.105Fe0.895−xAlxare reported for temperatures from 1.6 K to about room temperature. The magnetization behavior in the ordered regime indicates that the coupling scheme between the rare‐earth and the transition‐metal moments stays an antiferromagnetic one on Al substitution for the latter. An unusual magnetization behavior is observed at the lowest temperatures (1.6–10 K). Here strongS‐type saturation develops partly with spontaneous increases of magnetization at critical fields of the order of 20 kOe. This is a result of highly energetic domain walls connected with high anisotropy and disorder on crystallographic sites. Coercive forces up to about 14 kOe are observed in the pseudobinary region at 4.2 K.

ISSN:0021-8979    

DOI:10.1063/1.1662198

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The irradiation‐temperature dependence of the uniaxial anisotropy energyKuhas been examined for a series of well‐stabilized large‐grain([inverted lazy s]0.2 &mgr;)iron‐nickel alloy thin films having compositions between 44% Ni and 90% Ni. The uniaxial anisotropy was developed in these films by irradiating at temperatures of 125, 225, 275, 300, 325, and 400 °C, in the presence of a 180‐Oe magnetic field applied parallel to the film's easy direction of magnetization. Room‐temperature measurements ofKufollowing the irradiations indicate the development of three types of order: (i) iron‐iron pair short‐range directional order which is present for all compositions, (ii) NiFe long‐range order for films with compositions between 44% Ni and 70% Ni which were irradiated below 400 °C, and (iii) Ni3Fe long‐range order for film compositions around 75% Ni. The uniaxial anisotropy energy induced in films irradiated at 400 °C proved to be almost entirely the result of iron‐iron pair ordering as shown by the good agreement obtained between the experimental values ofKuand those calculated using the pair‐ordering theory. At temperatures below 400 °C, the induced anisotropy was primarily the result of NiFe long‐range order for film compositions between 44% Ni and 70% Ni. The results of anisotropy energy measurements for these films indicate the following: (i) the critical temperature for NiFe orderTcis composition dependent and agrees with the values obtained for neutron bulk‐irradiated samples; (ii) the NiFe order mechanism may be governed primarily by single vacancy diffusion; and (iii) the maximum value ofKuobtaind belowTcprobably depends more on the irradiation rate (flux) than fluence.

ISSN:0021-8979    

DOI:10.1063/1.1662199

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The critical currentIcin a plastically deformed Nb single crystal is shown to vary with the method of formation of the flux line lattice (FLL). The largest critical currents are observed when the FLL is formed by cooling the sample from aboveTc(H). Passing a current through the sample during the cooling process, in the same direction as it is subsequently swept, increasesIcby up to 60%. By slowly cycling the current at zero voltage, decreases inIccan be induced with theV‐Icharacteristic asymptotically approaching the characteristic after flux flow. The size of the decrease is found to depend on the number of cycles, and especially for negative current excursions, on the amplitude of these excursions. It is proposed that optimum pinning of the FLL to the underlying pin structure occurs, for thermodynamic reasons, at the moment the FLL is formed. Subsequent changes in the local value of the FLL parameter, either due to changes in magnetic induction or changes in FLL defect structure, lead to decreases in the number of effective pins, and thus to decreases in the critical current.

ISSN:0021-8979    

DOI:10.1063/1.1662200

出版商:AIP    

年代:1973

数据来源: AIP