摘要:

The reversible magnetization curveM(H) of a polycrystalline ferromagnet has been investigated with the objective of examining the properties of the singularities located atH= −HA, the anisotropy field, along the hard direction. Using a simple model that approximates the system as a continuous assembly of noninteracting particles, we show that the singular point becomes apparent in the successive derivativesdnM/dHnplotted as functions ofH. Indeed, differentiation accentuates more and more the singularity hidden in the magnetization curve, and the minimum ordern*at which it can be detected is the order for which the functiondn*M/dHn*has a discontinuity in the slope at the singular point. The general formula for the dependence ofn*on the symmetry properties of the hard direction is given together with the analytical expression for the shape and amplitude of the singularity in the most important cases. The same sort of phenomenon is shown to be present in the reversible transverse susceptibility, and the general expression fornt*is also given. For all the symmetry cases,nt*turns out to be lower thann*. Experimental tests of the theory have been carried out for binary axes of both uniaxial and cubic materials—namely, BaFe12O19, Fe, and CoFe2O4. The singularity appears ind2M/dH2, as expected, and position and amplitude are in good agreement with the values predicted by theory. The suggestion is made that singular point detection (SPD) could be used as a new technique for measuring the anisotropy in polycrystalline samples.

ISSN:0021-8979    

DOI:10.1063/1.1663823

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Recent measurements reveal significant changes in the magnetic moment and superparamagnetic Curie point during the initial stages of atomic order in Ni3Mn alloys. The rate of increase of the induced moment at 8 kG and 300°K with annealing time at 400°C exhibits a discontinuity at about 1.5 h. The superparamagnetic Curie temperature is also found to increase from 145°K for the disordered alloy to 250°K after an extremely brief (20 min) anneal. These results show that significant atomic rearrangement is actually taking place rather quickly during the initial stages of ordering, contrary to the previously accepted model of homogeneous long‐range‐order development which saturates only after extremely long annealing times (1000 h or more), proposed by Marcinkowski and Brown. The discontinuity at 1.5 h is tentatively associated with the short‐range‐order saturation observed for anneals at higher temperatures, also at about 2 h, and the peak in the field‐induced uniaxial anisotropy after 2 h of annealing.

ISSN:0021-8979    

DOI:10.1063/1.1663824

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Two groups of unidirectionally solidified Co&sngbnd;Co3Nb eutectic samples containing small differences in nickel content were analyzed for their magnetic domain structures using TEM. The two groups showed no observable domain structures in the as‐solidified condition, but the group of samples containing the lower nickel content (<0.01 wt%) developed a strip‐type domain structure when the samples were thermally cycled. When favorable crystallographic and defect structures existed, the thermal agitation of the heat treatment cycles was able to assist in nucleating the magnetic domain structures that were observed.

ISSN:0021-8979    

DOI:10.1063/1.1663825

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The wall dynamics in two samples of a garnet with the composition Y1.7Eu0.65Tm0.65Ga0.7Fe4.3O12are compared. Both samples have an easy axis normal to the plane of the material, but in one case the sample is an epitaxial film and is magnetically isotropic in the plane, and in the other case it is a thin platelet cut from a bulk sample and has in‐plane anisotropy. The dynamic behavior as measured by the bubble‐collapse technique is significantly different in the two samples. The sample with the in‐plane anisotropy has an approximately linear velocity reponse to ∼6000 cm/sec corresponding to a drive field of 60 Oe, whereas the isotropic sample saturates at a velocity of ∼650 cm/sec for a drive field of about 20 Oe. However, both samples have nearly the same dynamic behavior as measured by the bubble‐transport technique. The results are compared with the existing theories. In addition, theAandBparameters describing growth‐induced anisotropy for this composition are determined

ISSN:0021-8979    

DOI:10.1063/1.1663826

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

By empirical methods similar to those outlined previously for the iron garnets, molecular‐field coefficients are reported for the complicated LiTiZn ferrite system. As in the case of the garnets, the intrasublattice coefficients NAAand NBBare linearly dependent on the Ti4+and Zn2+concentrations, but the intersublattice coefficient NABhas a quadratic dependence on the Ti4+substitutions. Where possible, the computed results are compared with experiment, and the agreement is found to be good over the ranges 0≤t≤0.9 and 0≤z≤0.4.

ISSN:0021-8979    

DOI:10.1063/1.1663827

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Magnetization measurementsM(H,T)are made on several commercially drawn Nb&sngbnd;Cu and NbTi&sngbnd;Cu multifilamentary conductors as a function of applied magnetic field and temperature. The samples are distinguished by their filament sizes or the amount of cold work. From these measurements the pinning forceFp(H,T) and the critical current characteristicsJc(H) are deduced for all samples. Using this information Kramer's scaling laws for flux pinning in multifilamentary conductors were tested. It is found that the model with some reservations seems to account for most of the observed behavior in the Nb&sngbnd;Cu and NbTi&sngbnd;Cu systems.

ISSN:0021-8979    

DOI:10.1063/1.1663828

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

We have studied the current‐induced magnetic‐flux structures in type‐I superconducting constrictions using a high‐resolution magneto‐optical method. The samples were Pb films 60–100 &mgr;m thick with a single constriction of 0.5–1.0 mm width, similar to the Anderson‐Dayem bridge geometry. In the resistive state, moving flux‐tube trains are nucleated at both sample edges, at first precisely at the narrowest portion of the constriction. With increasing current, the flux‐tube trains extend more and more into the outlying regions performing a curved path. From the resistive voltage and our magneto‐optical observations the flux‐tube nucleation rate for a flux‐tube train is estimated using the Josephson relation.

ISSN:0021-8979    

DOI:10.1063/1.1663829

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The anisotropy and magnetostriction constants of gallium‐substituted yttrium iron garnet single crystals of composition Y3Fe5−tGatO12with 0≤t≤1.5 have been investigated by means of ferrimagnetic resonance in the temperature range 4.2≤T≤500 K. From the concentration dependence atT=4.2 K the single‐ion contribution of the tetrahedral and octahedral anisotropy was found to beKd= −0.0019 cm−1andKa= −0.012 cm−1, respectively. In the ranget<1 of linear concentration dependence of the magnetostriction constants at T=4.2 K, a variation &Dgr;&lgr;100/t= −1.3×10−6and &Dgr;&lgr;111/t= +3.0×10−6was found.

ISSN:0021-8979    

DOI:10.1063/1.1663830

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Enhancement of the Kerr effect was studied to assess its effectiveness in improving readout characteristics of a magneto‐optical memory utilizing MnBi films. SiO was used to enhance the Kerr effect by coating it on magnetic films. The coating results in a change, not only in the Kerr rotation but also in the ellipticity and reflectivity of the films. Changes of these quantities were calculated as a function of the SiO thickness, using optical constants of both MnBi and SiO, i.e., the refractive indicesn2andn3of SiO and MnBi, respectively, and the off‐diagonal element &egr;′ of the dielectric tensor of MnBi. The optical constants determined in this work weren2=(2.0±0.02)−(0.02±0.02)i,n3=3.1−3.5i, and &egr;′ = −1.2−0.4iat a wavelength of 6328 Å. The Kerr rotation and the reflectivity were measured for various SiO thicknesses and were compared with the results of theoretical calculations. Experimental and theoretical results agreed well with each other. The readout signal‐to‐noise ratio, which is a function of the Kerr rotation, ellipticity, and reflectivity, is expected to be improved by a factor of more than 2, when SiO is coated on the MnBi film surface.

ISSN:0021-8979    

DOI:10.1063/1.1663831

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The problem of the calculation of the electric field distribution of a system of infinite parallel cylindrical conductors in the presence of a grounded plane is reexamined. The exact solution of a class of two‐dimensional harmonic problems in multiply connected domains proposed by Burnside in 1893 is reviewed. This is shown to agree with recent generalizations of the method of images. An improved version of the exact solution which is more convergent than the conventional methods when the conductors are very close to one another is presented. The potential usefulness of the theoretical ideas and the mathematical apparatus used here for technological applications is emphasized.

ISSN:0021-8979    

DOI:10.1063/1.1663832

出版商:AIP    

年代:1974

数据来源: AIP