摘要:

The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb&sngbnd;Nb point‐contact junctions at 4.2 °K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1, 0.3, and 9.2 GHz. The point contact is in a nonresonant structure and fed by a constant current source: neither a strong rf coupling of the contact with a cavity nor a stabilizing shunt resistance was proved necessary. The direct observation of frequency conversion was performed with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1600 GHz. The experiments with intermediate frequenciesfiof 0.1 and 0.3 GHz indicate that conventional frequency conversion does not occur but that the observed signals are related to noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator‐mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. The results show that low‐frequency current components are induced in the junction, which can give large spurious signals when small i.f. are used.

ISSN:0021-8979    

DOI:10.1063/1.1663650

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

this paper reports details of the magnetic domain configuration during crystal growth and the exciting phenomena during the thermal cycle around the Curie temperature. The experiments suggest the conditions for producing large hexagonal single‐crystal MnBi platelets: (i) Substrates must be clean; (ii) Bi and Mn should be evaporated in this sequence with the atomic ratio of Mn to Bi between 0.5 and 0.7; and (iii) annealing should be done with the thermal cycle through the critical temperaturesTc1(about 360 °C) andTc2(about 350 °C), under high vacuum. On the other hand, the most useful films for Curie‐point writing with high bit density are produced by annealing the double‐layer film, with the ratio of Mn to Bi between 1.5 and 2.0, at a low temperature of 200–300 °C.

ISSN:0021-8979    

DOI:10.1063/1.1663651

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A detailed study has been made of the ac loss in cylindrical samples of Nb3Sn carrying a transport current at 60 Hz. Samples were prepared by a vacuum codeposition process. At low currents, up to 550 A/cm at 4.2 °K in some samples, the loss is dominated by a surface fluxoid pinning mechanism, varies no faster than the square of the current, and in many cases less rapidly, and is temperature independent. At higher currents, the loss is described by the critical‐state model. Formation of the superconductor in cylindrical layers separated by another material provides enhanced bulk fluxoid pinning, thereby increasing the critical current density and decreasing the loss at high currents. The magnitude of the ac loss in this material up to 10–12 °K is comparable to that reported in niobium at 4.2 °K.

ISSN:0021-8979    

DOI:10.1063/1.1663652

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Optical absorption measurements are reported for the damaged surface layers produced by proton and neon bombardment of iron‐garnet liquid‐phase‐epitaxial films. The excess optical absorption is shown to be an absorption tail on O(2p)→Fe(3d) charge‐transfer bands centered near 4 eV with about 10% contributed by higher‐energy optical transitions. The distribution of states for Ne bombardment is different from that for H bombardment, and in the case of Ne the state distribution depends on the dose. Saturation at a photon energy of 2.2 eV occurs at an absorption coefficient of ∼2×104cm−1. The average damage density over the damaged layer thickness is observed to increase with decreasing H energy, in agreement with hard‐bubble‐suppression data and nuclear stopping power considerations. Based on optical absorption and hard‐bubble‐suppression results it is estimated that the dose window for hard bubble suppression corresponds roughly to an average disorder range from ∼3–30% amorphous (amorphous defined optically). Annealing studies show that no simple activation energy description applies to the damaged material. Deep states anneal out more rapidly than shallow states for both H and Ne bombardment.

ISSN:0021-8979    

DOI:10.1063/1.1663653

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A theory of Bloch‐line (BL) dynamics in magnetic bubble materials is proposed and applied to the problem of domain collapse with a small number of BLs. We derive from the Landau‐Lifshitz‐Gilbert equation the dynamic reaction force, composed of gyrotropic and viscous terms, on a single moving BL. From this follows easily the wall mobility for equally spaced BLs. However, we show that BLs tend to cluster statically at distances comparable to the BL width. If the BLs are clustered on one side of a circular‐cylinder domain, then a change of the bias field causes the centers of bubble and cluster to move on spiral trajectories. The predicted displacement of the bubble center is large enough for visual confirmation, but the bound on coervicity required to confirm the spiral shape may be too stringent. The collapse mobility for the clustered case is typically much greater than that for the case of uniform spacing. The theory is supported by collapse‐time data of Malozemoff for a garnet film of medium viscosity (&agr;=0.12). In garnets of low viscous damping of(&agr;≲0.01)we predict that coercivityHcgives rise to nonlinearity of the velocity‐drive relation, with a characteristic ``knee'' veolcity, (2/&pgr;)&ggr;Hcr, for collapse of a domain of radiusr.

ISSN:0021-8979    

DOI:10.1063/1.1663654

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Magnetic domain structures induced by bending (axis of bending along [010]) in (100) Fe&sngbnd;Si single crystals have been investigated by means of the Kerr technique and the Lang method. In the zone of the sample under tension (A), the magnetization is parallel to the surface and the classical 180° wall domain pattern is observed. In part of the zone under compression (B), the easy axis is normal to the surface and two kinds of stray‐field‐free structures (P and N) similar to the Landau and Lifschitz structure are observed. The magnetic configurations in zones A and B are linked by an inner magnetic structure located near the neutral plane which is a (100) 90° wall and a (100) 180° wall for structures P and N, respectively. An hexagonal pattern is observed on x‐ray topographs in the case of both structures P and N. We propose tentatively to relate this pattern to the classical zigzag instability of the 90° walls in the Landau closure domains (structure N), as well as (structure P) to a new kind of instability of the (100) 90° wall, due to the magnetostrictive relaxation of long‐range stresses created by such a wall. The energy of each structure is calculated. Theoretical periodicities compare well with the experimental ones. The volume occupied by the structures in zone B increases when the radius of curvature decreases, up to one‐half of the sample volume. A critical value of the radius of curvature is computed, above which structure P or N disappears.

ISSN:0021-8979    

DOI:10.1063/1.1663655

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The complex magnetic permeability of Nb‐44 at.% Ti has been measured using a modified Hartshorn ac bridge and a lock‐in amplifier. The out‐of‐phase signal can be directly related to the energy losses, and critical current can be calculated using the Bean model. Samples were irradiated in a fission neutron spectrum to a dose of 3.2×1018neutron/cm2. All measurements were takenin situat approximately 4.5 °K in an external magnetic field continuously swept to 6.7 kOe while applying ac magnetic fields at 88 Hz with amplitudes between 8 and 500 Oe. Irradiations and measurements were performed on an annealed and a cold‐worked sample of the alloy. For the annealed sample, no changes were observed inVLand thus inJcafter 8.2×1017neutron/cm2and after an anneal to 320 °K. A decrease of approximately 8% was observed inJcfor the cold‐worked sample after 3.2×1018neutron/cm2. One‐half of this decrease was recovered by 270 °K.

ISSN:0021-8979    

DOI:10.1063/1.1663656

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The saturation magnetization of gallium‐substituted yttrium iron garnet single crystals of composition Y3[Fe2−xGax](Fe3−yGay) O12with 0≤x+y≤1.5 has been investigated in the temperature range 4.2≤T≤Tc. The magnetization values are below most data of previous investigations on this system. From the fit of the molecular field theory to the experimental results atT=4.2 K the fraction of the tetrahedral gallium ionsft=y/(x+y) could be deduced and are found to range between 1 and 0.91 for 0≤x+y≤1.5. The compensation temperature occurring forx+y>1 has been studied as a function of the gallium distribution.

ISSN:0021-8979    

DOI:10.1063/1.1663657

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Results of heat‐capacity and resistivity measurements performed on PrAl3between 1.5 and 300 K are utilized to reveal and characterize the influence of the crystalline electric field on the ground‐state multiplet of Pr+3in PrAl3. Also reported are the results of heat‐capacity measurements on the isostructural nonmagnetic counterpart LaAl3for which there are only vibrational and electronic contributions toCp. Below 180 K, the excess heat capacity of PrAl3over that of LaAl3exhibits of Schottky‐type thermal anomaly peaking at 25 K and is attributable to excitation within the crystal‐field states. Resistivity results indicate a temperature‐dependent spin‐disorder contribution at low temperatures associated with the splitting of the ground‐state multiplet by the crystal field. The higher‐temperature resistivity behavior is observed to be a linear function of temperature and can be readily assigned to phonon contribution. The crystal‐field interaction is analyzed using the crystal‐field Hamiltonian,HCF=W246(1 − |y|)O2/F2+y[xO4/F4+ (1 − |x|)O6/F6], which incorporates a second‐order term to account for the nonideal crystallographicc/aratio exhibited by PrAl3. Best agreement between experimental and calculated heat capacity, resistivity, and susceptibility was obtained using the parametersx=−0.60 andy=0.65 with a positive value of W246. A singlet ground state results with an over‐all splitting of the crystal‐field levels of 132 K. The parametersxandyare in reasonable agreement with calculations based on the point‐charge model.

ISSN:0021-8979    

DOI:10.1063/1.1663658

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Six modes of transmission electron microscopy (TEM) are compared by a numerical simulation of the image formation. The comparison includes five modes of the conventional electron microscope (CEM) (axial bright field, Unwin's phase plate, central stop dark field, tilted beam dark field, conical illumination dark field) and the annular detector mode of the scanning transmission electron microscope (STEM). It is assumed that the illumination is perfectly coherent and that the interaction between electron beam and specimen may be described as an entirely elastic event. Furthermore, the influence of radiation damage and noise is neglected. The restrictions due to these approximations are discussed and shown to be unessential to the conclusions. Recent results from single‐atom scattering theory are used to describe the specimen, a one‐dimensional model of a thin carbon film carrying six osmium atoms. The calculated through‐focus series demonstrates the nonlinearity of most modes of the CEM, which generates interference artifacts and introduces focusing difficulties and the risk of misinterpretation of the micrographs. STEM images, however, are shown not to be subject to these artifacts, which thereby explains the excellent quality of micrographs generated by this new instrument.

ISSN:0021-8979    

DOI:10.1063/1.1663659

出版商:AIP    

年代:1974

数据来源: AIP