Showing papers on "Magnetization published in 1986"

Layered magnetic structures: Evidence for antiferromagnetic coupling of Fe layers across Cr interlayers.

Abstract: We investigated exchange coupling of Fe layers across Au and Cr interlayers by means of light scattering from spin waves. For Au interlayers we find a continuous decrease of this coupling to zero as the Au thickness is increased from 0 to \ensuremath{\cong}20 \AA{}. For Cr interlayers of proper thickness we find antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field, in analogy to the spin-flop phase of antiferromagnets.

Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals

Abstract: The temperature dependence of the saturation magnetization and the magnetocrystalline anisotropy field have been measured on single‐crystal samples of the R2Fe14B compounds for R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm from 4.2 K to the magnetic ordering temperatures. A spin reorientation transition of the Nd2Fe14B type has been found in Ho2Fe14B at 57.6 K in zero field. Another type of spin reorientation caused by anisotropy compensation between the Fe and the R sublattices exists in Er2Fe14B and Tm2Fe14B. The temperature dependence of the angle of the easy direction of magnetization from the c axis has been measured for R=Nd, Ho, Er, and Tm. The relation between the magnetocrystalline anisotropy and the sublattice magnetization is investigated by employing a simplified two‐sublattice molecular field model.

Transformation of symmetrization order to nuclear-spin magnetization by chemical reaction and nuclear magnetic resonance.

Abstract: A method of obtaining very large nuclear-spin polarizations is proposed and illustrated by density-operator calculations. The prediction is that chemical reaction and rf irradiation can convert the scalar parahydrogen state into polarization of order unity on the nuclear spins of the products of molecular-hydrogen addition reactions. A means of extending the resultant sensitivity enhancement to other spins is proposed in which the transfer of order occurs through population differences not associated with magnetization.

Ordering in ferromagnets with random anisotropy.

Abstract: We summarize and extend our study (using real-space response and correlation functions) of the properties of a continuous-symmetry ferromagnet with random anisotropy, distinguishing between the cases of weak and strong random anisotropy. For the weak-anisotropy case we find three different magnetic regimes, according to the strength of the external magnetic field H. In zero H, the net magnetization is zero, although the ferromagnetic correlation length (FCL) is large. We call a ferromagnet in this first regime a correlated spin glass (CSG). It has a very large magnetic susceptibility, and hence a relatively small coherent anisotropy converts it into a nearly typical ferromagnetic domain structure. Also, a relatively small magnetic field nearly aligns the CSG, producing the second regime, which we call a ferromagnet with wandering axis (FWA). The FWA is a slightly noncollinear structure in which the tipping of the magnetization with respect to the field varies over the system. The tipping angle is correlated over a (field-dependent) correlation length which is smaller than the FCL of the CSG. As the field increases the correlation length in the FWA decreases, until the third regime is reached, wherein the tipping angles (which are smaller than in the FWA) are completely uncorrelated from site to site. We obtain the magnetization or susceptibility (as appropriate) for each of these three regimes. We also show that the temperature dependence of the (single-ion) random anisotropy strength can provide a plausible explanation for certain classes of reentrant phenomena and susceptibility cusps observed in magnetization studies. Neutron scattering studies appear to be consistent with the predicted ${H}^{\mathrm{\ensuremath{-}}1/2}$ dependence of the FCL in the FWA regime, and display the expected rise of the FCL in the CSG regime as the random anisotropy strength decreases with increasing temperature.

Spin anisotropy of ferromagnetic films

Abstract: We calculate the spin anisotropy of ferromagnetic monolayers of Fe, Ni, and V. We find that the easy direction of magnetization is perpendicular to the plane of the monolayer for Fe and V, but in the plane for Ni. The result for Fe explains why spin splitting but no spin polarization is observed in recent photoemission experiments on Fe overlayers.

Magnetic viscosity and thermal activation energy

Abstract: Magnetic viscosity arises when the activation energy E required to produce a change in mangetization is provided thermally. Activation energy functions for single domain particles and materials with ‘‘strong’’ and ‘‘weak’’ domain‐wall pinning are discussed. The magnetic viscosity parameter Sv, given by kT(∂E/∂H)−1T, where kT is the Boltzmann energy and H is the magnetic field, is shown to be proportional to kT/(v Ms), where Ms is the spontaneous magnetization and v is the activation volume swept out as the energy barrier is overcome. For single domain particles and strong domain‐wall pinning the constant of proportionality is one while for weak pinning it is 1/2. The generality of this simple relationship is shown to be independent of the details of the activation model.

A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals

Abstract: Summary. Susceptibility, thermo-remanent magnetization (TRM) and isothermal remanent magnetization (IRM) anisotropy ellipsoids have been determined for several rock samples. The results indicate that the ellipsoid of initial susceptibility is less anisotropic than the TRM and low field IRM ellipsoids which are found experimentally to be of identical shape. This suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non-destructive low field IRM measurements. Such IRM measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured. The results for a series of artificial anisotropic samples containing magnetite particles of different sizes (in the range 0.2–90 μm) were very similar to those for the rocks. In contrast, a comparison of the susceptibility and IRM ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory. Such results imply that susceptibility and IRM ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single-domain particles (magnetization confined to the easy axis) or whether the particles are essentially multidomain.

Amorphization of transition metal Zr alloys by mechanical alloying

Abstract: Amorphous alloy powders of the types NiZr, CoZr, FeZr, and CuZr are produced by mechanical alloying from crystalline elemental powders. The alloying and amorphization process is monitored by microstructural investigations, x‐ray diffraction, and, where applicable, by magnetization measurements. The crystallization temperatures, determined by differential scanning calorimetry, are comparable to those measured for rapidly quenched and solid state reacted amorphous metals of the same compositions.

Long-range incommensurate magnetic order in a Dy-Y multilayer.

Abstract: A multilayer sample of Dy(47 A\r{})/Y(40 A\r{}), produced by molecular-beam-epitaxy techniques, is shown to order magnetically in an incommensurate helix that is coherent over several multilayer periods. The magnetic satellite peak has harmonics resulting from the multilayer modulation. The dc magnetic properties are suggestive of a metamagnet.

Magnetic properties of amorphous Tb-Fe thin films with an artificially layered structure

Abstract: An alternating terbium‐iron (Tb‐Fe) multilayer structure artificially made in amorphous Tb‐Fe thin films gives rise to excellent magnetic properties of large perpendicular uniaxial anisotropy, large saturation magnetization, and large coercivity over a wide range of Tb composition in the films. The films are superior to amorphous Tb‐Fe alloy thin films, especially when they are piled up with a monatomic layer of Tb and several atomic layers of Fe in an alternating fashion. Small‐angle x‐ray diffraction analysis confirmed the layering of monatomic layers of Tb and Fe, where the periodicity of the layers was found to be about 5.9 A. Direct evidence for an artificially layered structure was obtained by transmission electron microscopic and Auger electron spectroscopic observations. Together with magnetic measurements of hysteresis loops and torque curves, it has been concluded that the most important origin of the large magnetic uniaxial anisotropy can be attributed to the Tb‐Fe pairs aligned perpendicular t...

On the Origin of the Curie-Weiss Law of the Magnetic Susceptibility in Itinerant Electron Ferromagnetism

Abstract: The origin of the Curie-Weiss law of the magnetic susceptibility universally observed in ferromagnetic materials is discussed from a point of view that the local spin fluctuation amplitude is almost conserved. A fully consistent treatment is presented which can deal with both the ordered phase and the paramagnetic phase on an equal footing. In the case of weakly ferromagnetic limit, Rhodes-Wohlfarth plot is revised from a new stand point. In the ordered phase, observed systematic trends of magnetization curves are explained in connection with the nature of dynamical spin fluctuation spectra. Some new interesting relations among various magnetic data are derived and compared with experiments. Temperature dependence of magnetic susceptibilities of Fe and Ni is also discussed.

On the critical behavior of the magnetization in high-dimensional Ising models

Abstract: We derive rigorously general results on the critical behavior of the magnetization in Ising models, as a function of the temperature and the external field. For the nearest-neighbor models it is shown that ind⩾4 dimensions the magnetization is continuous atT c and its critical exponents take the classical valuesδ=3 andβ=1/2, with possible logarithmic corrections atd=4. The continuity, and other explicit bounds, formally extend tod>3 1/2. Other systems to which the results apply include long-range models ind=1 dimension, with 1/|x−y| λ couplings, for which 2/(λ−1) replacesd in the above summary. The results are obtained by means of differential inequalities derived here using the random current representation, which is discussed in detail for the case of a nonvanishing magnetic field.

Reduction to the pole as an inverse problem and its application to low-latitude anomalies

Abstract: Traditionally, reduction to the pole has been accomplished either by space- or wavenumber-domain filtering. In the two-dimensional case, this procedure is stable regardless of the latitude, as long as the source strike is not parallel to the horizontal projection of the geomagnetic field. In the three-dimensional case, however, reduction-to-the-pole filtering is stable only at high magnetic latitudes. At latitudes lower than 15 degrees, it is of no practical use due to a sharply increasing instability toward the magnetic equator.The three-dimensional instability of this filtering technique is demonstrated, and the reduction-to-the-pole problem is formulated in the context of a general linear inverse problem. As a result, stable solutions are found by using well-known stabilizing procedures developed for the inverse linear problem. The distribution of magnetization of an equivalent layer of doublets that reproduces the observed data is computed. The magnetic doublets are parallel to the magnetization direction which is assumed constant throughout the sources. The magnetic field reduced to the pole is then obtained by changing the inclinations of the geomagnetic field and the doublets to 90 degrees and recalculating the total field.The usefulness and limitations of the method at low magnetic latitudes are assessed using theoretical data. The effects of noise and anomaly truncation are also investigated for both high and low latitudes. In all cases, application of the proposed method produced meaningful results regardless of the latitude. The method is applied to field data from two different low-latitude anomalies. The first anomaly is due to a seamount in the Gulf of Guinea with reversed magnetization. The geomagnetic field at this location is about -23 degrees. The second anomaly is an intrabasement anomaly from Parnaiba Basin, Brazil, where the magnetization is assumed to be induced by a geomagnetic field with -1.4 degree inclination. The results obtained confirm that the proposed method produces stable, meaningful, reduced-to-the-pole maps.

In-plane magnetic surface anisotropies in Fe(110)

Abstract: In-plane magnetic surface anisotropies have been detected for Fe(110) on W(110) using in situ Conversion Electron Mossbauer Spectroscopy (CEMS). The phenomenon used for the determination of this anisotropy was a switching of the spontaneous magnetizationJ s from [001] to [1¯10] with decreasing thickness. Analysis of the data is performed using a homogeneous magnetization approximation for competing surface and bulk anisotropies, which is justified by a micromagnetic analysis and established experimentally by CEMS. In-plane surface anisotropy constants for the clean Fe(110) surface, the Fe metal-interface and the FeGaAs interface are determined toK s,p FeUHV =0.065 erg·cm−2,K s,p FeMetal =0.040 erg ·cm−2, andK s,p FeGaAs =0.047 erg ·cm−2, all with an estimated accuracy of the order of 10%.

Many-state relaxation model for the Mössbauer spectra of superparamagnets.

Abstract: The stochastic relaxation theory of Anderson and Sack is applied to the M\"ossbauer spectra superparamagnetic particles with uniaxial anisotropy by including all possible values of the component of the magnetization along the quantization axis explicitly in the calculation. It is shown that for particles with a large number of states the M\"ossbauer line shape can be expressed directly in terms of the solution of a differential equation. Explicit solution of this equation in the low-temperature (high anisotropy barrier) approximation leads to effective relaxation rates between the two discrete allowed orientations of the magnetization. These rates are equivalent to those derived by Brown only if the relaxation matrix elements are assumed to have a certain rather arbitrary dependence on the temperature and magnetization.

Magnetization process in rapidly solidified neodymium‐iron‐boron permanent magnet materials

Abstract: Initial magnetization and demagnetization data are reported for three forms of rapidly solidified Nd‐Fe‐B permanent magnet materials: melt‐spun ribbons, hot‐pressed magnets, and die upset magnets. In all three materials, the results are consistent with domain‐wall pinning at Nd2Fe14B grain boundaries as the coercivity mechanism. Thermally demagnetized ribbons are comprised of single domain Nd2Fe14B grains, and both initial magnetization and demagnetization occur by depinning and wall motion. Domain walls move easily in die upset magnets until they become strongly pinned at grain edges. Complete magnetization requires an applied field greater than the coercive field. Hot‐pressed magnets show a mixture of ribbon and die upset behavior.

Time-dependent phenomena in a short-range Ising spin-glass Fe0.5Mn0.5TiO3.

Abstract: Magnetization measurements have been made on an insulating random mixture, ${\mathrm{Fe}}_{0.5}$${\mathrm{Mn}}_{0.5}$Ti${\mathrm{O}}_{3}$. An Ising spin-glass behavior has been observed below ${T}_{g}=21.1$ K. It is shown that the time-dependent phenomena are well characterized by an algebraic function with the exponent described by an exponential function of temperature. The results are compared with the computer-simulation results obtained by Ogielski. In addition, we suggest the existence of a thermodynamic equilibrium state in magnetic fields.

Microstructure, domain walls, and magnetization reversal in hot‐pressed Nd‐Fe‐B magnets

Abstract: Transmission electron microscopy study of hot‐pressed and hot‐deformed Nd0.14(Fe0.94B0.06)0.86 magnets shows that the magnetic alignment is achieved by the formation of a microstructure consisting of platelets of Nd2Fe14B grains with the c axis normal to the platelet surface and parallel to the press direction. Magnetic domain walls, lying parallel to the c axis, run through many grains and are pinned at the intergranular phases that are produced during processing. Magnetization reversal occurs by domain wall motion.

Magnetic domain imaging with a scanning Kerr effect microscope

Abstract: A method for imaging magnetic domains with spatial resolution of less than 0.5 µm is described. The method employs the magnetooptic Kerr effect and is applicable for observing surface domain structures, even if the magnetic surface of interest is covered by a thick transparent overcoat. Polarized laser light, incident at an oblique angle, is focused to a diffraction limited spot onto a surface whose magnetization is modulated at a fixed frequency by a magnetic field. The modulation in the polarization of the light due to the modulation of the magnetization beneath the spot is measured with the use of a lock-in amplifier. An image of the magnetization changes is then constructed by scanning the spot. Domains are revealed because areas of domain wall motion have larger magnetization changes than areas of magnetization rotation within domains. Since this approach minimizes noise and eliminates nonmagnetic signals, the domain image contrast is greatly enhanced. With 0.3 mW of laser power on a spot and a detection bandwidth of 10 Hz, signal to noise ratios of 65 db and a magnetization rotation sensitivity of 0.06° have been obtained in permalloy. Results of domain images in thin-film inductive heads are presented. This technique is ideally suited for the study of the dynamics of wall motion and magnetization rotation in magnetic films.

Magnetic behavior of nonet tetracarbene as a model for one-dimensional organic ferromagnets.

Abstract: Tetracarbene (1) was generated by photolysis of the corresponding tetradiazo compound (2) in a 2-methyltetrahydrofuran glass or a single crystal of benzophenone at cryogenic tempreatures. The temperature dependence of paramagnetic susceptibility revealed its nonet spin multiplicity in the ground state. A behavior suggesting the presence of antiferromagnetic intermolecular interaction was also found when generated in the glass. The field dependence of magnetization of 1 was analyzed in terms of the Brillouin function and the experimental data fit closely to that of the theoretical value for J = */*. This is independent evidence for the nonet spin multiplicity of 1. The characteristic saturation behavior of magnetization in 1 was found to be due to its high spin multiplicity. Thus 1 may be regarded as a molecular superparamagnet. In the polycarbenes, the localized n spins a t carbenic centers are aligned all in parallel through the electron correlation between the mobile K spins, resulting in the intramolecular ferromagnetic spin ordering. Although the mechanism of spin ordering of a spins in polycarbenes is entirely different from that in metallic ferromagnets, the interaction between n and K spins resembles s-d interaction in dilute alloys. The potentiality of polycarbenes as a microdomain in macroscopic ferromagnets will be discussed. The explosive growth in the number of reports on organic conductors and superconductors has arisen from genunie interests in mobility of electrons in organic molecular crystals, components of which are insulators in themse1ves.I On the other hand, molecular design of organic ferromagnets which is becomming the current topic of theoretical interest in organic material science deals with the behavior of electron spins in organic molecules.* One of the strategies toward this goal is to construct organic molecules with high-spin multiplicity as domains in ferromagnets and to introduce ferromagnetic intermolecular (interdomain) interaction among them. High-spin organic molecules are accessible when nonbonding molecular orbitals are present due to the symmetry of the alternant hydrocarbon skeleton. Examples of the above category are m-phenylenebi~(phenylmethylene),~ its higher homologues, 3,6-dimethyleneanthracenediyl1,8-dioxyI$ etc. Recently we have generated tetracarbene, m-phenylenebis((diphenylmethylen-3-y1)methylene) (1) and proved its nonet spin multiplicity in the ground state by analyzing ESR fine structures of the sample oriented in a host single crystal of ben~ophenone.~ Now we have studied the magnetic behavior of this highly important tetracarbene 1 by means of magnetic susceptibility measurements.6 The magnet was composed of a main coil and a pair of reverse Helmholtz coils for the field gradient. The main field gradient, the temperature of a sample, and the weight change were carefully calibrated to enable the absolute magnetic susceptibility measurement. Inhomogeneity of the main magnetic field (HM) was leis than 0.2 T/m at the center of the main coil with the accuracy of lo-’. The accuracy of the field gradient (aHG/az) is better than IO-’ which was calibrated by a GaAs Hall device. The temperature of the cryostat (300-20 K) was regulated by the microcomputer, and data were recorded when the temperature stabilized within O.l%/min. The temperatures between 20 and 4 K were controlled manually by adjusting the flow of liquid helium, and lower temperatures than 4 K were obtained by pumping liquid helium. A platinum-resistance thermometer was used to read temperatures higher than 25 K and a carbon-resistance thermometer for lower temperatures. These temperature readings were calibrated in the whole temperature range with a magnetic thermometer with use of paramagnetic Cr(NH3)I Sculz, H. Adu. Phys. 1982, 31, 299. (b) Torrance, J . B. Arc. Chem. Res. 1979, 12, 79. (2) (a) McConnell, H. M. J . Chem. Phys. 1%3,39, 1910. (b) McConnell, H. M. Proc. Robert A . Welch Found. ConJ Chem. Res. 1967,1I, 144. (c ) Mataga, N. Theor. Chim. Acra 1968,10,372. (d) Ovchinnikov, A. A. Dokl. Akad. Nauk SSSR 1977, 236, 928. (e) Ovchinnikov, A. A. Theor. Chim. Acra 1978,47,297. (f) Buchachenko, A. L. Dokl. Akad. Nauk SSSR 1979, 244, 1146. (8) Breslow, R.; Juan, B.; Kluttz, R. Q.; Xia, C. 2. Terrahedron 1982,38,863. (h) Iwamura, H.; Sugawara, T.; Itoh, K.; Takui, T. Mol. Crysr. Liquid Cryst. 1985, 125, 251. (i) Breslow, R. Ibid. 1985, 125, 261. Q) Ooster, S.; Torrance, J. B.; Scumaker, R. R. The 1984 International Chemical Congress of Pacific Basin Societies, Abstract of Papers 07E28, Honolulu, Hawaii, December 1984. (3) (a) Itoh, K. Chem. Phys. Left. 1967,1, 235. (b) Itoh, K. Pure Appl. Chem. 1978, 50, 1251. (c) Wasserman, E.; Murray, R. W.; Yager, W. A.; Trozzolo, A. M.; Smolinsky, G. J. J . Am. Chem. SOC. 1967, 89, 5076. (4) (a) Seeger, D. E.; Berson, J . A. J . Am. Chem. SOC. 1983, 105, 5144. (b) Seeger, D. E.; Berson, J. A. Ibid. 1983, 105, 5146. (5 ) (a) Teki, Y.; Takui, T.; Itoh, K.; Iwamura, H.; Kobayashi, K. J . Am. Chem. SOC. 1983, 105, 3722. (b) Teki, Y . ; Takui, T.; Itoh, K.; Iwamura, H.; Kobayashi, K., to be published. ( 6 ) (a) Sugawara, T.; Bandow, S.; Kimura, K.; Iwamura, H.; Itoh, K. J . Am. Chem. SOC. 1984, 106, 6449. (b) Kimura, K.; Bandow, S. Solid State Phys. 1984, 20, 467. 0002-7863/86/ 15O8-0368$01.50/0

Holographic interference electron microscopy for determining specimen magnetic structure and thickness distribution

Abstract: A new holographic method utilizing the time-reversal operation of an electron beam has been de- vised to observe separately electric and magnetic phase distributions as interference electron micrographs, which had been unavailable using conventional methods. By this technique, both the magnetization and thickness distributions of three-dimensional cobalt particles have been determined.

Systematics across the UTX series (T = Ru, Co, Ni; X = Al,Ga, Sn) of high-field and low-temperature properties of non-ferromagnetic compounds

Abstract: Results are presented of magnetic and specific-heat studies performed on some equiatomic compounds of the series U TX with a special emphasis on non-ferromagnetic UNiAl, UCoAl, URuA1 and URuGa. The occurrence of magnetism in these compounds is attributed mainly to the 5f electrons of uranium. The systematics of the ground-state properties across the series is discussed with respect to delocalization mechanisms of the 5f electrons. The possible role of spin fluctuations in the low-temperature behaviour of UCoAl, URuAl and URuGa is indicated by magnetic measurements. The compound with the highest γ value, UNiAl (γ = 167 mJ/mol K 2 ), behaves as an itinerant antiferromagnet.

Nonlinear dynamic susceptibilities at the spin-glass transition of Ag:Mn.

Abstract: Mesures des susceptibilites ac non lineaires en fonction de la frequence, de la temperature et du champ magnetique