摘要:

We have measured the noncubic magnetic anisotropy in seven different compositions of rare‐earth iron garnets, using bulk single‐crystal samples grown from a flux. Several of these compositions supported magnetic bubble domains. The noncubic anisotropy is growth induced, and for each composition we investigated samples from both types of growth facets which occur in bulk garnet growth. Anisotropy energies as large as 7×104erg/cm3were found, and three distinct anisotropy orientations were resolved. The results have been analyzed within the framework of a two‐parameter model for growth‐induced anisotropy, and we find general agreement although there are some discrepancies. For one composition, Y2Gd1Al0.8Fe4.2O12, we have measured the dependence of the noncubic anisotropy on temperature over the range 4.2–300°K. Only part of the observed temperature dependence is in good agreement with a microscopic calculation done by Sturge. Finally, we report on the annealing characteristics of the noncubic anisotropy. Typically tens of hours near 1200°C are required to anneal it away.

ISSN:0021-8979    

DOI:10.1063/1.1661900

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

We have measured the growth‐induced noncubic magnetic anisotropy under (110) facets of flux‐grown garnet crystals at 300°K. The garnets studied have the general formulaRM2Fe5O12, whereRrepresents Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, andMrepresents Y, Lu, or Gd. Measurements were made at room temperature in the (110) plane. The contribution of Gd and of all ions heavier than Tb is found to be small. The growth‐induced anisotropy of all garnets containing Pr, Sm, Eu, or Tb is consistent with a model in which site preferences on the rare‐earth sublattice are smooth functions of the ionic radius mismatch, and the single‐ion magnetic anisotropy is due to anisotropic exchange. The anisotropy of the exchange appears to have the same sign and order of magnitude for all four ions. The behavior of Nd is anomalous and can be attributed to crystal field effects.

ISSN:0021-8979    

DOI:10.1063/1.1661901

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

In an effort to understand more completely the operation of field‐access bubble‐domain propagation circuits, the analysis of Copeland for Permalloy thin‐film elements is generalized to include the effects of a uniform in‐plane drive field and applied to the problem of bubble motion along a simple bar. Results are presented for the magnetization distributions in a bar for various bubble positions and drive field situations along with the corresponding magnetostatic energy wells. The structural and spacing (Permalloy to garnet) dependence of the drive fields and the choice of garnet thickness are examined with an eye toward device optimization.

ISSN:0021-8979    

DOI:10.1063/1.1661902

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The temperature dependence of the anisotropy fieldHkand the domain‐wall coercivityHchave been measured for three different epitaxial films of mixed rare‐earth iron garnets with the nominal compositions Er2Eu1Ga0.7Fe4.3O12, Y1Eu1.85Yb0.15Al1.1Fe3.9O12, and Eu1.7Er1.3Al0.7Ga0.8Fe3.5O12. Each of these garnets supports bubble domains. The anisotropy field was measured using an optical magnetometer, and analytical expressions are developed which relate the optical magnetometer parameters to the anisotropy field. These measurements show that the anisotropy field decreases with increasing temperature at about −1%/°C at 20°C for all three samples. The coercivity, again measured by an optical technique, was also found to decrease with increasing temperature. The coefficient ranged from −1.3%/°C to −3.0%/°C at 20°C depending on the sample. The largest total change inHcfor one of the samples was a factor of 310 over the temperature range −106°C to +80°C. The temperature variation of the saturation magnetization 4&pgr;Msand the material lengthlwere also measured. The temperature coefficients of 4&pgr;Msandlat 20°C varied from ∼0.03%/°C to −1.1%/°C, and −0.9%/°C to 0.5%/°C, respectively. No simple correlation is evident among the temperature dependences ofHk, Hc, and 4&pgr;Msalthoughlvaries predictably with 4&pgr;MsandHk. The large variations in coercivity are not consistent with simple conventional models.

ISSN:0021-8979    

DOI:10.1063/1.1661903

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Behaviors of flux flow states under thermal or magnetic disturbances have been studied using a Pb&sngbnd;In&sngbnd;Sn superconductor with the property ∂JC/∂T>0. Existence of the previously reported stable flux flow states has been confirmed clearly. When heating the sample the flux flow voltages decrease stably to zero (recovery of superconducting state). Also, flux flow states are found to be stable against magnetic field sweeping. They further indicate that magnetization will increase with increasing field sweep rates.

ISSN:0021-8979    

DOI:10.1063/1.1661904

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The distributions of the magnetic field inside and around a superconducting flux exclusion tube due to currents induced in the material by an external magnetic field are examined. A simple macroscopic model for the superconductor is assumed, based on the concept of a field‐dependent critical current density, and London's equations are used to calculate the magnetic potentials in the three regions of interest. The magnetic field inside the tube was mapped using an electron beam, and the results compared with the calculations. It is concluded that the major contribution to the magnetic field present in the tube can be ascribed to the induced dipole and sextupole current distributions in the superconductor. By contrast, the effect of direct field penetration through the narrow slits in the structure is very small.

ISSN:0021-8979    

DOI:10.1063/1.1661905

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The threshold magnetic field for flux jumping in a bulk irreversible type‐II superconductor is known to be independent of its bulk critical current density. The present analysis shows that the flux jump field should be enhanced in a laminated superconductor in which the surface layer has a lower critical current density than the underlayer. For very large critical current density ratios, the limiting enhancement is 41%; for a 1:4 ratio the improvement is 32%. Experiments on laminated samples of Nb&sngbnd;48 wt% Ti alloys demonstrate an improvement in stability which is in good agreement with the theory. The technique may be useful in increasing the overload current rating of an ac superconducting power cable.

ISSN:0021-8979    

DOI:10.1063/1.1661906

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Electronic noise measurements in carbon resistors, germanium, cadmium sulfide, and thermionic devices over the frequency range 2 Hz to 2 kHz are presented, using a real‐time computer‐controlled instrumentation system. The reproducibility and resolution of this system were sufficient to identify small but significant statistical differences between 1/f‐ and Nyquist‐type noise sources which have not been observed in previous investigations.

ISSN:0021-8979    

DOI:10.1063/1.1661907

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The correlation between successive zero‐crossing intervals and their probability densities is experimentally studied for Gaussian processes having power spectra of Butterworth type. The assumption that successive zero‐crossing intervals from a Markov chain in the wide sense is found valid only for a process having a narrow‐band spectrum. The correlation coefficients between the lengths of several successive zero‐crossing intervals of a Gaussian process having a broad‐band spectrum decay slowly and reveal a peculiar oscillatory behavior as the number of intervals is increased. These results are interpreted by introducing a model. Results for Gaussian processes having power spectra of type (f/f0)2m[1+ (f/f0)2]−nare also given. The correlation of intervals is also determined by means of a single interval counter making efficient use of the so‐called bias effect, and its results agree closely with those from ordinary measurements using two interval counters.

ISSN:0021-8979    

DOI:10.1063/1.1661908

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The annealing of copper precipitate colonies in silicon single crystals has been followedin situin the electron microscope hot stage. The colony dissolution was found to be a two‐stage process; the first stage involved the enveloping dislocations and the precipitates attached to these only. The onset of the second stage was associated with a phase transformation and the generation of dislocations from all of the colony precipitates.

ISSN:0021-8979    

DOI:10.1063/1.1661910

出版商:AIP    

年代:1973

数据来源: AIP