Showing papers on "Magnetization published in 1981"

Magnetic structure of small NiFe2O4 particles

Abstract: The magnetic structure of small NiFe2O4 particles has been investigated. Samples (in the few hundred angstrom size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mossbauer spectra of the 57Fe nuclei, obtained with a longitudinal magnetic field applied, unambiguously establish that a non‐collinear structure exists. Further, the magnetic moment at low temperatures is appreciably lower than the value reported for bulk material. A model is proposed in which the NiFe2O4 particles consist of a core with the usual spin arrangement and a surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependence of this structure is also reported.

Kinetic effects in coercivity measurements

Abstract: The coercivity of a magnetic material is a crucial parameter in determining its use in recording. The apparent value of this quantity is found to depend upon the time scale of interest. A method of measurement that observes changes of magnetization on a short time scale will yield a larger coercivity than one that measures changes over a longer time scale. The cause of this phenomenon is thermally assisted reversal of magnetic domains. Thus, for a magnetic recording material, the coercivity relevant to a high-speed writing process is greater than that relevant to transition broadening and long-term storage stability. The difference between the short-and long-term coercivities becomes more pronounced as the size of the magnetic switching unit becomes smaller. Therefore, these kinetic effects will become increasingly important in determining the ultimate limitations of future high-density media. Data are presented that illustrate the variation of coercivity with time scale in presently used and experimental recording materials.

Calculation of force-free magnetic field with non-constant α

Abstract: A numerical method is developed for solving the force-free magnetic field equation, ▽ × B = α B, with spatially-varying α. The boundary conditions required are the distribution of B n (viz. normal component of the field on the photosphere) as well as the value of α in the region of positive (or negative) B n . Examples of calculations are presented for a simple model of a solar bipolar magnetic region. It is found that the field configuration and the energy stored in the field depend crucially on the distribution of α. The present method can be applied to a more complex configuration observed on the Sun by making use of actual magnetic field measurements.

Diffraction of X-rays by magnetic materials. I. General formulae and measurements on ferro- and ferrimagnetic compounds

Abstract: The calculation of the amplitude of X-rays scattered by a magnetically ordered substance, carried out in the relativistic quantum theory (i.e. taking the spin into account), is detailed. The effect of the orbital momentum is described in an appendix. The practical formulae dealing with the polarization of the beams are given both in a simple form for the usual experiments and in a complete form, using the Stokes vectors, for the most general case. The experiments show a change in the intensity of the X-rays diffracted by a ferromagnetic (pure iron) or a ferrimagnetic (zinc-substituted magnetite) powder when the magnetization, perpendicular to the diffraction plane, is reversed. The relative values of these intensity changes range from 10-4 to 5 x 10-3 and agree in sign and magnitude with the predictions. They are proportional to the spin-density structure factor multiplied by the imaginary part of the charge- density structure factor; the large anomalous scattering of the Cu Kα radiation in the iron-containing samples is used in the present experiments.

Magnetomechanical coupling and permeability in transversely annealed metglas 2605 alloys

Abstract: We have measured the magnetomechanical coupling factor, relative permeability, and magnetization as a function of annealing temperature and applied field in transversely annealed samples of Metglas 2605 CO (Fe 67 CO 18 B 14 Si 1 ) and Metglas 2605 SC (Fe 81 B 13.5 Si 3.5 C 2 ). In 2605 CO the maximum coupling factor of 0.71 was obtained for ribbons annealed between 360 and 375°C in fields greater than 1 k Oe. Plots of the impedance and admittance circles indicated a very low loss material. In samples with a coupling factor of 0.71, the Q value from the impedance circle was 35, while the Q value from the admittance circle was 120. In the 2605 SC alloy considerably higher coupling factors were calculated (0.9 to 0.96) due to the much lower anisotropy. The magnetization of the samples are mainly rotational in nature.

Magnetization Process of Exchange-Coupled Ferrimagnetic Double-Layered Films

Abstract: Magnetization curves are theoretically calculated for exchange-coupled ferrimagnetic double-layered (ECDL) films, each layer of which has a uniaxial anisotropy with the easy axis perpendicular to the film plane and exhibits a rectangular hysteresis loop. With this model are obtained the conditions for the appearance of anomalous and inverted hysteresis loops observed by the Kerr magneto-optic method. Experiments are made on Gd-Fe/Tb-Fe double-layered films deposited successively in a vacuum better than 10-6 Torr. By comparing the experimental switching field for the Gd-Fe layer in ECDL films with the theory, the energy density of the interface wall is estimated to be 1–2 erg/cm2.

T‐S diagram for gadolinium near the Curie temperature

Abstract: The magnetocaloric effect in polycrystalline Gd was measured at temperatures from 190 to 370 K in applied fields from 1 to 7 T. The magnetocaloric temperature changes were combined with existing zero‐field specific heat data to construct a T‐S diagram for Gd near the Curie point. Experimental values of ΔT were also compared with values calculated from a simple mean field theory, which predicts rather well both the general shape of the magnetocaloric curve and the relative magnitudes of the temperature changes in various measuring fields. The maximum ΔT with applied field was about 14 K at 7 tesla, and maxima in all applied fiels occurred near 292 K. The relatively large magnetocaloric effect in Gd near room temperature is attractive for potential magnetic refrigeration applications, and the experimental T‐S diagram may now be used to refine estimates of the performance of Gd as a solid magnetic refrigerant.

Micromagnetism and microstructure of amorphous alloys (invited)

Abstract: In amorphous ferromagnetic alloys the characteristic properties of magnetization processes and of the hysteresis loop are sensitively influenced by fluctuations of material properties and defect structures. A detailed analysis of experimental results on the basis of the theory of micromagnetism for inhomogeneous systems shows clearly that the ferromagnetic phase transition is mainly influenced by intrinsic exchange fluctuations whereas the characteristic properties of the magnetization curve are governed predominantely by defect structures. The defect structures are identified as agglomerates of the free volume which are formed during the rapid quenching process.

Co energy and magnetization anisotropies in RCo5 intermetallics between 4.2 K and 300 K

Abstract: The magnetocrystalline anisotropy has been known for a long time to be very large in the RCo5 intermetallics. The recent availability of high D.C. magnetic fields up to 200 kOe has enabled us to carry out an extensive experimental study of the energy and magnetization anisotropies in YCo5 from 4.2 K to 300 K. The magnetization anisotropy reaches 4% at 4.2 K. From a polarized neutron study on NdCo5 we find that this anisotropy of magnetization originates mainly from the 2c site. Previous measurements on YCo5 had shown that the orbital contribution to the 2c moment is large. Hence, we conclude that the Co magnetization anisotropy in RCo5 comes from a large orbital contribution to the magnetic moment. Finally, we account for the observed discontinuity in the magnetization at its reorientation in RCo5 ferrimagnets, and for the absence of a similar discontinuity in NdCo5 which is a ferromagnet.

Simple approximation for magnetization curves and hysteresis loops

Abstract: A rational function approximation for magnetization curves and hysteresis loops is presented. This approximation leads to Rayleigh relations and the law of approach to saturation as limit forms. Furthermore, the magnetic permeability and the magnetic energy density is computed. Finally, this analysis is applied to commercial ferrite cores and pure ceramic samples.

Magnetic properties of sheet silicates; 2:1:1 layer minerals

Abstract: Magnetization, susceptibility and M6ssbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of the Tzg triplet of 1,120 K. Predominant exchange interactions in the iron-rich samples are ferromagnetic with J= 1.2 K, as for other trioctahedral ferrous minerals. A peak in the sus- ceptibility of thuringite occurs at Tm = 5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the M6ssbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4~ 1). A factor favouring this random ferromagnetic ground state over the planar antiferromag- netic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illus- trate how long range ferromagnetic order may be destroyed by vortices forming around the Fe 3 +--Fe 3 + pairs. tral in chlorites. Substitution of trivalent ions for Mg in the brucite layers is compensated by replacement of Si by A1 in the tetrahedral sheets of the talc layers (Bailey 1980). The general formula for a trioctahedral chlorite is

1H nuclear magnetic resonance studies of domain structures in polymers

Abstract: The structures of amorphous (noncrystalline) domains in polyethylene, conditioned (at 150 °C for 15 min) and unconditioned polypropylene films and a vitrain portion of Kentucky No. 6 coal have been investigated using the Goldman‐Shen NMR pulse sequence which monitors the transfer of 1H magnetization from the amorphous to the crystalline domains. The diffusion equation is solved with respect to such transfer. The size and shape of the amorphous domain is inferred by fitting the theoretical results to the experimental data. It seems that the layer‐like configuration is a poor description for the polyethylene and polypropylene films unless the spatial orientation of the amorphous domain is isotropic. However the spherical configuration is inadequate for Kentucky No. 6 vitrain. The average width b of the amorphous domain is 90–150 A for polyethylene, 20–30 A for uncontinued polypropylene film, and 30–50 A for the conditioned films. The diffusion of magnetization in the amorphous domain of Kentucky No. 6 is f...

Neutron reflection as a probe of surface magnetism

Abstract: Polarized neutrons reflected from the surface of a ferromagnet are sensitive to the magnetization close to the surface as well as to the bulk magnetization. In a calculation using parameters appropriate to nickel it is shown how the two contributions can be separated. Neutron measurements can determine the first moment of the magnetic disturbances at the surface, with a sensitivity corresponding to one magnetic dead layer.

Magnetic and transport properties of granular cobalt films

Abstract: Simultaneous magnetization and magnetoresistance measurements have yielded the magnetic phase diagram of the granular Co-Si${\mathrm{O}}_{2}$ system. The superparamagnetic-to-ferromagnetic transition temperature changes gradually with composition from \ensuremath{\sim}80 K at 20 vol% Co to \ensuremath{\sim}280 K at 43 vol% Co. The results confirm the theory of spindependent tunneling in granular magnetic metals and yield the value of $JP=(1.0\ifmmode\pm\else\textpm\fi{}0.3)$ meV. Here $P$ is the spin polarization of the tunneling electrons and $J$ their exchange-coupling constant. This quantity has not been measured previously for cobalt. A quantitative comparison shows that the molecular-field theory does not provide an adequate description of the magnetization of granular cobalt.

Faraday rotation measurements of time dependent magnetic phenomena in insulating spin glasses (invited)

Abstract: We report here on a detailed study of the relaxation of the magnetization in an external field and of the remanent magnetization for the insulating spin glass: Eu0.4Sr0.6S. The Faraday rotation has allowed us to extend previous experiments near and just above the spin glass temperature Tfo = 1.55 K on a large time scale (10−6

Detailed magnetization study of an amorphous ferromagnet

Abstract: Exhaustive magnetization measurements have been carried out on the amorphous ferromagnet ${\mathrm{Fe}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{14}$${\mathrm{B}}_{6}$ in the temperature range 4.2 to 700 K in fields up to 16 kOe. Magnetization at low temperatures is found to decrease in accordance with the predictions of an isotropic nearest-neighbor Heisenberg model. It is observed that in this alloy the rms range of exchange interaction equals roughly the mean nearest-neighbor distance. A theoretical interpretation has been provided for the observation that the spin-wave stiffness constant $D$ is smaller in amorphous than in crystalline ferromagnets. Magnetization data taken in the critical region, besides yielding the values for the Curie temperature and critical exponents as ${T}_{C}=519.9\ifmmode\pm\else\textpm\fi{}0.2$ K, $\ensuremath{\beta}=0.38\ifmmode\pm\else\textpm\fi{}0.01$, $\ensuremath{\gamma}=1.31\ifmmode\pm\else\textpm\fi{}0.02$, and $\ensuremath{\delta}=4.46\ifmmode\pm\else\textpm\fi{}0.04$, satisfy the magnetic equation of state characteristic of a second-order phase transition. The observed exponent values not only obey the static scaling relation but also present a reasonable agreement with the theoretical values deduced from a three-dimensional Heisenberg model. They do, however, exhibit a slight but systematic shift towards the molecular-field values. This observation has been shown to imply an increased effective range of the exchange interaction in the critical region. Long-range forces are found to have a negligible influence on the critical fluctuations of magnetization. Critical amplitude values suggest that the elementary moments involved in the magnetic order-disorder transition are those forming a part of the giant superparamagnetic moments which are found to exist for temperatures well above ${T}_{C}$. Above 575 K, the susceptibility obeys the Curie-Weiss law. A careful analysis of the susceptibility data reveals that the spin polarization of conduction electrons gives a negligible contribution to the distribution of the magnetic moment and that the moments on Ni and Fe atoms remain localized even in the paramagnetic state. No evidence has been found for the indirect exchange between spins on magnetic atoms mediated by conduction-electron spin, as conjectured by Kaneyoshi in amorphous ferromagnets, in the present glassy alloy.

Magnetic Properties and Moessbauer Spectra of Several Iron(III) Dicarboxylic Acid Complexes.

Abstract: : Polymeric iron (III) complexes of malonic, succinic, furmaric, and phthalic acids have been prepared and studied by variable-temperature (15-300 K) magnetic susceptibility, 57Fe Mossbauer spectroscopy, and infrared spectroscopy. In addition, properties of iron(III) acetate have been reinvestigated using these same techniques. The magnetic susceptibilities of these complexes have been described by a theoretical model which includes, in addition to intramodular exchange terms, a parameter describing intermolecular spin-exchange effects. As a consequence of this model it was not necessary to assume an isosceles triangular arrangement of the iron(III) ions in these materials in order to explain the magnetic data. Thus, the inclusion of an intertrimer exchange parameter, which varies from 1/2.1 cm for the acetate complex to 1/11.7 cm for the o-phthalate complex allowed for a complete description of the temperature-dependence of the magnetic susceptibilty of these compounds. Mossbauer and infrared spectroscopic studies were utilized to ascertain the correctness of oxidation state and structural assignments. (Author)

Magnetic properties of amorphous and crystallized (Fe0.82B0.18)0.9Tb0.05La0.05

Abstract: The magnetic properties of (Fe0.82B0.18)0.9Tb0.05La0.05 in both the amorphous and crystallized states are presented. When amorphous, the alloy is magnetically soft and has a low coercive force, comparable to the corresponding alloy without rare earths. Upon annealing near the crystallization temperature of 900 K, however, the intrinsic coercive force at 300 K rises to 9 kOe, with a remanent magnetization of slightly less than 5 kG. In the amorphous state this and related alloys appear to have potential for use in devices requiring large, isotropic magnetostriction, while in the crystallized state they appear potentially useful as low cobalt permanent magnets.

Mean field theory for Heisenberg spin glasses

Abstract: The infinite-ranged spin glass model is studied in the general case of m-component spins. Results for the magnetization laws, the shape of the phase diagram, the nature of the transitions are presented.

Magnetization of compositionally modulated CuNi films

Abstract: We report static magnetization measurements on the compositionally modulated ferromagnetic alloy Cu/Ni, which, contrary to earlier ferromagnetic resonance measurements, show the moment per Ni atom is reduced relative to pure Ni. The low‐temperature magnetization is found to vary linearly with modulation amplitude, but, surprisingly, the Curie temperature is found to be almost amplitude independent.

Neutron diffraction study of magnetic ordering in UPd2Si2, UPd2Ge2, URh2Si2 and URh2Ge2

Abstract: Neutron diffraction measurements on polycrystalline samples show that UPd2Si2, UPd2Ge2 and URh2Si2 become antiferromagnetic below temperatures of 150K, 140K and 137K respectively. The magnetic moment ordering in UPd2Si2 and UPd2Ge2 is represented by a static longitudinal wave propagating along the c axis of the tetragonal body-centred crystal lattice. The magnetisation wave is polarised in the direction of the c axis: q=(0, 0, 0.662+or-0.010)c* in UPd2Si2 and q=(0, 0, 0.748+or-0.010)c* in UPd2Ge2. At 40K, UPd2Si2 exhibits a magnetic phase transition: a commensurate magnetisation wave with q=(0, 0, 1)c* represents the growth of a simple antiferromagnetic ordering. URh2Si2 is a collinear antiferromagnet with coinciding magnetic and chemical unit cells. The sequence of ferromagnetic sheets piled up along the tetragonal axis is given by +-. The values of the root mean square of the magnetic moment on the U ion at 4.2K are found to be 1.96 mu B, 1.97 mu B and 1.96 mu B for UPd2Si2, UPd2Ge2 and URh2Si2 respectively. The observed magnetic ordering schemes are discussed in terms of the RKKY exchange interaction. Neutron diffraction measurements performed at zero magnetic field show that URh2Ge2 is paramagnetic at 4.2K.

The magnetic behavior of CeB6: Comparison between elastic and inelastic neutron scattering, initial susceptibility and high-field magnetization

Abstract: We report measurements of the elastic and inelastic neutron scattering, initial susceptibility and high-field magnetization on thoroughly prepared poly- and single crystalline samples of CeB6. Part of these experiments have been performed at temperatures down to 60 mK and magnetic fields up to 70 kOe. Our neutron-diffraction data provide the first proof that CeB6 is an antiferromagnet belowTN≃2K as has been suggested by previous bulk experiments. The reduced value of the low-temperature magnetic moment both below and aboveTN points to the existence of a Kondo effect of theΓ7 crystal-field (CF) ground state of Ce3+. From the low-temperature width of the quasielastic neutron line, the Kondo temperature is inferred to beTK≃3 K. The thermal variation of the initial susceptibility (forT>20K) is semiquantitatively explained invoking, besides the Kondo effect, aΓ7-Γ8 CF splitting of ≃70 K and magnetic interactions, which are about 10 times stronger betweenΓ8 states than those betweenΓ7 states. This largeΓ8-Γ8 exchange interaction is also assumed to account for the most striking result of this work, i.e. the lack of any CF-transition peak up to 44 meV in our inelastic neutron-scattering spectra.