Showing papers on "Magnetization published in 1970"

Time Saving in Measurement of NMR and EPR Relaxation Times

Abstract: By producing a train of absorption or dispersion signals (continuous‐wave magnetic resonance) or free induction decays (pulsed magnetic resonance) it is possible to save time in spin‐lattice relaxation measurements due to the fact that it is not necessary to wait for equilibrium magnetization before initiating the train. The relaxation time may be calculated from the train according to a simple rapidly converging iteration.

Convective instability of ferromagnetic fluids

Abstract: Convective instability of a ferromagnetic fluid is predicted for a fluid layer heated from below in the presence of a uniform vertical magnetic field. Convection is caused by a spatial variation in magnetization which is induced when the magnetization of the fluid is a function of temperature and a temperature gradient is established across the layer. A linearized convective instability analysis predicts the critical temperature gradient when only the magnetic mechanism is important, as well as when both the magnetic and buoyancy mechanisms are operative. The magnetic mechanism predominates over the buoyancy mechanism in fluid layers about 1 mm thick. For a fluid layer contained between two free boundaries which are constrained flat, the exact solution is derived for some parameter values and oscillatory instability cannot occur. For rigid boundaries, approximate solutions for stationary instability are derived by the Galerkin method for a wide range of parameter values. It is shown that in this case the Galerkin method yields an eigenvalue which is stationary to small changes in the trial functions, because the Galerkin method is equivalent to an adjoint variational principle.

Characteristics of the Trochoidal Electron Monochromator

Abstract: An electron monochromator for use in an axial magnetic field is described. Electrons are injected parallel to the magnetic field and an electric field is applied in a perpendicular direction. The electrons thus describe trochoids and drift in a direction perpendicular to both the electric and magnetic fields and disperse according to their initial velocities. An electron energy width at half‐maximum of 0.020 eV can be obtained, with a transmitted current of about 10−9 A.

Magnetic Properties of Stable Dispersions of Subdomain Magnetite Particles

Abstract: Stable colloidal dispersions of subdomain‐size magnetic particles have been prepared that retain their liquid characteristics in the presence of a magnetic field. Magnetization versus applied field curves were obtained as a function of particle size and volumetric solids concentration for magnetite dispersions in a wide variety of carrier liquids. The magnetic properties of these colloidal dispersions have been corrolated by superparamagnetic theory in terms of the size distribution of the suspended particles and their volumetric concentration, correcting for the formation of a nonmagnetic surface mantle one‐unit‐cell thick.

The electrodynamics of magneto-electric media

Abstract: A general magneto-electric medium is one in which there is a linear, reciprocal relationship between the magnetic field and the electric polarization, and between the electric field and the magnetic polarization, as well as the more familiar linear relationship between the magnetic field and the magnetic polarization and between the electric field and electric polarization. In this paper the relationship between the fields and polarizations is expressed by means of a single tensor equation relating the field and polarization tensors in the medium. The properties of the rank four susceptibility tensor, required to establish this relationship, are investigated. A general tensor wave equation is obtained for a homogeneous anisotropic medium. The equation to the wave vector surface is obtained as a six by six determinant, which must vanish, and which gives well-known results when applied to simple anisotropic dielectric, or magnetic, crystals. When this equation is applied to an ideal magneto-electri...

Concavity of Magnetization of an Ising Ferromagnet in a Positive External Field

Abstract: An inequality for correlation functions in an Ising model with purely ferromagnetic interactions between pairs of spins is established and used to show that the magnetization in such a model is a concave function of external field H for H > 0. The concavity of magnetization, which holds not only for spin‐½ but also for arbitrary‐spin Ising ferromagnets, provides a basis for certain thermodynamic inequalities near the ferromagnetic critical point, including one involving the ``high temperature'' indices α and γ.

Microwave Behavior of Partially Magnetized Ferrites

Abstract: The high‐frequency permeability of partially magnetized ferrites is calculated for some simple domain configurations, comprising only ``up''‐ and ``down''‐domains. The method used is based upon the magnetostatic approximation and neglects exchange effects, but is otherwise substantially rigorous. The components of the effective permeability tensor (ratio of average induction to average magnetic field) in general depends upon details of the domain configuration in addition to the net dc magnetization. When the dc magnetization is cycled between the two states of complete magnetization the high frequency permeability, considered as a function of the dc magnetization, in general shows hysteresis. Detailed calculations of the high‐frequency permeability have been carried out for the case in which the domain configuration is cylindrically symmetric, i.e., invariant under rotation around the direction of magnetization. For any such domain configuration the two relevant components μeff and κeff of the effective ...

The spontaneous resistivity anisotropy in Ni-based alloys

Abstract: From experiments on a number of Ni based alloys, it is shown that the difference in resistivity for currents perpendicular to and parallel to the magnetization direction is due to spin-orbit induced resistivity transfer from the spin down electron current to the spin up electron current

Magnetic Properties of some Cubic Rare‐Earth‐Iron Compounds of the Type RFe2 and RxY1−xFe2

Abstract: Magnetic measurements in fields up to 30 kOe and for temperatures between 4.2° and 1000°K have been performed for cubic Laves phase compounds RFe2 (R=Gd, Tb, Dy, Ho, Er, and Y) and RxY1−xFe2 (R=Gd, Tb, and Er). The Curie temperatures and saturation moments at 4.2°K are given. A compensation in the temperature dependence of the magnetization is only observed for the various compounds in a certain range of x values. Conditions for the occurrence of a compensation point are derived. For the compounds RxY1−xFe2 the iron moment remains constant for 0≤x≤0.8. Above x=0.8 the iron moment increases strongly. The variation of the Fe moments with x is discussed in terms of a nonlocalized moment associated with the iron ions. The moments shown by Er and Tb in ErFe2 and TbFe2 are both 8.3 μB. For most of the compounds RFe2 the lattice constants are given.

High‐Resolution NMR Study of Relaxation Mechanisms in a Two‐Spin System

Abstract: A description of nuclear spin–spin and spin–lattice relaxation is presented for a system of two coupled nonequivalent spin−12 nuclei subject to mutual dipole–dipole interaction and interactions with external magnetic species Expressions are derived for the six transition probabilities linking the four energy levels, in terms of an internal dipole–dipole relaxation parameter TD, external relaxation parameters TXA for nucleus A and TXB for nucleus B, and the degree of correlation (C) between the interactions of A and B with external magnetic species These molecular relaxation parameters are related to the initial rates of recovery of the NMR signal intensities after a selective pulse has rotated the magnetization vector of a chosen line through π radians in the rotating frame of reference A small general‐purpose computer has been used to control these pulse experiments and to program a synthesizer to provide the required stable audiofrequency signals Measurements have been made on the high‐resolution pr

Metallic alloys and exchange-enhanced paramagnetism: application to rare- earth--cobalt alloys.

Abstract: A phenomenological theory is presented of the magnetic properties of two-component alloys, one of whose components has a permanent magnetic moment, and the other an exchange-enhanced paramagnetic susceptibility. The paramagnetic susceptibility, magnetic-ordering temperature, and magnetization at low temperature are discussed in terms of this theory, and the results obtained are applied to cubic alloys $A{\mathrm{Co}}_{2}$ between rare earth ($A$) and cobalt.

Exchange Striction Effects in MnO and MnS

Abstract: Lattice constants of MnO and MnS have been measured as a function of temperature from 7 to 300\ifmmode^\circ\else\textdegree\fi{}K Below the N\'eel temperature, the fcc lattices undergo a trigonal distortion as well as a volume contraction By subtracting the usual thermal expansion from this volume contraction, the isotropic exchange striction has been studied For MnO below the N\'eel temperature (${T}_{N}\ensuremath{\cong}118\ifmmode^\circ\else\textdegree\fi{}$K), good agreement with the randomphase Green's-function theory previously developed to describe sublattice magnetization is obtained for the trigonal distortion; on the other hand, for the volume contraction, there are no theoretical results available for the region immediately below the N\'eel temperature to compare with the experimental next-nearest-neighbor spin-correlation curve which has been obtained For MnS, below ${T}_{N}(\ensuremath{\cong}147\ifmmode^\circ\else\textdegree\fi{}\mathrm{K})$ the observed trigonal distortion was found to be a contraction along the [111] direction; the magnetization curves suggest, but do not positively prove, the absence of a biquadratic-exchange effect

Electron microscopy on high-coercive-force Co-Cr composite films

Abstract: Electron microscopy was used to analyze the crystallographic and magnetic structures of high-coercive-force Co-Cr composite films. The chromium sublayer appears to give high coercive forces by causing the growth of cobalt in the hexagonal phase with a relatively narrow distribution of grain sizes. Magnetization reversal was examined by Lorentz microscopy of films with coercive fields up to H_{c} = 900 Oe. It proceeds by nucleation and extension of magnetostatically coupled domains. The progressive limitation of these extensions as the coercive force increases illustrates the connection between the magnetostatic coupling, the coercive force, and the squareness of the hysteresis loop. Lorentz microscopy was also used to investigate a recorded transition between NRZ 1 and 0 states and showed a saw-toothed structure, characteristic of a magnetization in the plane of the film.

Magnetization changes induced by stress in a constant applied field

Abstract: Measurements have been made of magnetization curves obtained by applying stresses to various polycrystalline magnetic materials in the presence of a small, constant, magnetic field. The materials examined were nickel, mild steel, and both isotropic and cube textured silicon iron. Residual stray fields were carefully compensated. Both compressive and tensile stresses were applied in the range 0 to 10 kg mm−2. The curves are compared with the theory proposed by Brown in 1949, in which the stress is replaced by an equivalent field, which has been found to be valid at the smaller stresses. There are many features which cannot be reconciled with Brown's analysis, in particular the difference between tension and compression. A tentative explanation is put forward based on discontinuous changes in domain structure.

Paramagnetic Anisotropy, Electronic Structure, and Ferromagnetism in Spin S = 32 Manganese(II) Phthalocyanine

Abstract: Measurements are reported of magnetization and the average magnetic susceptibility of manganese (II) phthalocyanine in the range 1.7–300°K, which confirm that the Mn(II) atom is in an S = 3 / 2 spin state, and that weak ferromagnetic interactions are present in the crystal, presumably between adjacent molecules. Single crystals of MnPc are moderately anisotropic; the principal magnetic moment μ‖ = 4.0 μB remains constant between 80–300°K while μ⊥ increases from 4.4 μB (300°K) to 5.0 μB (90°K). This magnetic anisotropy is consistent with a 4A2g ground state into which the excited term 4Eg is mixed by spin–orbit coupling. Possible pathways for superexchange are considered.

Influence of Electron-Phonon Interactions on the de Haas-van Alphen Effect

Abstract: The thermodynamic potential and magnetization are calculated for a system of electrons interacting with phonons in the presence of a magnetic field. The results are used for a numerical evaluation of the de Haas-van Alphen amplitude of the oscillations of the magnetization as a function of temperature in mercury. The calculated results show that although we are dealing with a strong-coupling system, the experimental observations of the de Haas-van Alphen effect should be very similar to that expected for free particles. We also apply the techniques used in the derivation to the case of nearly ferromagnetic electron systems and calculate the enhancement of the argument of the spin-splitting factor. The analysis for nearly ferromagnetic systems leads to the prediction that the cyclotron frequency which enters the amplitude of the oscillations is reduced by the same mass-enhancement factor as that which enters the specific heat.

The dysprosium-iron system: Structural and magnetic properties of dysprosium-iron compounds

Abstract: The DyFe system has been investigated over the whole concentration range by means of X-ray diffraction, thermoanalysis, and metallography. The constitutional diagram is given. Four intermetallic compounds have been observed: DyFe 2 , DyFe 3 , Dy 6 Fe 23 and Dy 2 Fe 17 ; the structure type and the lattice parameters are given. Measurements of the bulk magnetization have been performed on all DyFe compounds between 4.2 ° and 1200 °K. Their Curie points decrease with increasing iron concentration. The compounds DyFe 3 and Dy 6 Fe 23 show a compensation point.

Spin Ordering in a Spin-Pair System

Abstract: A new type of spin ordering in a spin-pair system is theoretically investigated. In the system, the intra-pair exchange interaction is antiferromagnetic and is much stronger than the inter-pair exchange interaction. In an external magnetic field, the excited triplet of the pair splits, and the lowest component of the triplet crosses the ground singlet at a certain magnitude of the field. In the vicinity of the point of level crossing, even the weak inter-pair exchange interaction causes a considerable amount of mixing between the singlet and the lowest component of the triplet. Due to the mixing, an ordering of the spin component perpendicular to the external field occurs. This ordering explains an anomaly which has been found by Haseda et al. in their experiment of cooling by adiabatic magnetization of Cu(NO 3 ) 2 2.5H 2 O.

Analysis of Saturation Magnetic Recording Based on Arctangent Magnetization Transitions

Abstract: The minimum transition length for the arctangent magnetization transition is calculated. Readback voltage for the arctangent transition, where the magnetization contains components both in and normal to the coating plane, is calculated via the reciprocity theorem and Karlquist's fringe field equations. Several misconceptions currently existing in the literature are discussed.

Crystal structure and magnetic properties of cerium‐iron compounds

Abstract: The crystal structure, the magnetic properties, and the phase relations of cerium-iron compounds have been investigated. A phase diagram is presented. Lattice constants are given for CeFe2, α-Ce2Fe17, and β-Ce2Fe17. The compound CeFe2 is ferromagnetic with Tc = 230° K. Investigations by means of the Mossbauer effect and bulk magnetization indicate an antiferromagnetic–ferromagnetic transition near 70° K and an antiferromagnetic-paramagnetic transition close to 270° K for the compound β-Ce2Fe17. The ordering temperatures and saturation moments for the CeFe compounds are compared with magnetic data presented for the isostructural YFe compounds. Die Kristallstruktur, die magnetischen Eigenschaften und die Phasenbeziehungen von Cerium–Eisen-Legierungen wurden untersucht. Ein Phasendiagramm wird angegeben. Die Gitterkonstanten werden fur CeFe2, α-Ce2Fe17 und β-Ce2Fe17 mitgeteilt. Die Verbindung CeFe2 ist ferromagnetisch mit Tc = 230° K. Untersuchungen mittels Mosbauereffekt und Volumenmagnetisierung zeigen einen antiferromagnetischen–ferromagnetischen Ubergang bei 70° K und einen antiferromagnetischen–paramagnetischen Ubergang bei 270° K fur die Verbindung β-Ce2Fe17. Die Ordnungstemperaturen und Sattigungsmomente fur die CeFe-Verbindungen werden mit magnetischen Daten verglichen, die fur die isostrukturellen YFe-Verbindungen angegeben werden.

Spin Ordering in a Spin-Pair System

Abstract: A new type of spin ordering in a spin-pair system is theoretically investigated. In the system, the intra-pair exchange interaction is antiferromagnetic and is much stronger than the inter-pair exchange interaction. In an external magnetic field, the excited triplet of the pair splits, and the lowest component of the triplet crosses the ground singlet at a certain magnitude of the field. In the vicinity of the point of level crossing, even the weak inter-pair exchange interaction causes a considerable amount of mixing between the singlet and the lowest component of the triplet. Due to the mixing, an ordering of the spin component perpendicular to the external field occurs. This ordering explains an anomaly which has been found by Haseda et al. in their experiment of cooling by adiabatic magnetization of Cu(NO 3 ) 2 2.5H 2 O.

Paramagnetic Anisotropy, Low Temperature Magnetization, and Electronic Structure of Iron(II) Phthalocyanine

Abstract: Magnetic anisotropy, magnetization, and average magnetic susceptibility have been measured for iron (II) phthalocyanine. The anisotropy was measured from the room temperature to about 90°K, the average susceptibility from 296 to 1.57°K, and the magnetization was measured at 1.57°K for field strengths between 1100 and 15 000 Oe. The results are interpreted in terms of a 3B2g ground state with a zero‐field splitting parameter D = 64 cm−1, and g‖ = g⊥ = 2.74. Iron (II) phthalocyanine is an unusual example in which a large magnetic anisotropy arises entirely from the zero‐field splitting. The danger inherent in the existing method of evaluation of ligand‐field parameters from the average magnetic susceptibility data has been highlighted, and the advantages of magnetic anisotropy studies in deducing the ligand‐field parameters and the electronic structures are discussed. A change of about 70° of the “setting angle” between 290 and 90°K is observed in this crystal when measured with b axis vertical. It is shown...

The antiferromagnetic-ferromagnetic transition in iron-rhodium alloys

Abstract: The critical field required to induce this transition in Fe-Rh alloys close to the equiatomic composition has been measured using pulsed magnetic fields up to 280 kOe. Measurements of the thermal expansion and high field magnetization have also been carried out. It is found that application of Kittel's theory of exchange inversion to the results leads to serious numerical discrepancies. A four sublattice model is developed which indicates the importance of the excess entropy of the ferromagnetic over the antiferromagnetic phase in exciting the antiferromagnetic-ferromagnetic transition. The exchange iteration β between iron sublattices is small and negative and, although its lattice parameter dependence is large and positive, there is no suggestion of exchange inversion corresponding to a change in sign of β at the antiferromagnetic-ferromagnetic transition. Thus the ferromagnetic state is stabilized by the excess entropy even in the presence of antiferromagnetic interactions and this stability is maintained right up to the Curie temperature

Demagnetizing Fields in Thin Magnetic Films Due to Surface Roughness

Abstract: The magnetic field generated by the magnetization has been calculated for films with arbitrarily rough surfaces for the case in which the magnetization is uniform throughout the film. Simple results are obtained on the assumption that the film surface is not too ``ragged,'' i.e., for the case in which the inclination of the local film surface relative to the mean surface is fairly small (<45°). A rough estimate of the demagnetizing factor for a direction in the plane of the film is given by π〈p2〉/λd, where 〈p2〉 is the mean square deviation of actual film surface from the mean surface, λ the predominant wavelength of the surface roughness, and d the film thickness. The results are discussed in the context of induced uniaxial anisotropy and ferromagnetic resonance.

Photoelectron Spin Polarization and Ferromagnetism of Crystalline and Amorphous Nickel

Abstract: We observe electron spin polarization in photoemission from nickel, undetected in earlier experimental investigations. The saturation electron spin polarization for films prepared on hot substrates is higher than for films on cold substrates and the preferential direction of the magnetic moment is parallel to the magnetization even for electrons excited from energy states near the Fermi level.

Domain‐Wall Processes in SmCo5 Powders

Abstract: Measurements of minor loops of SmCo5 powders have revealed that wall motion is predominant in the magnetization process. A relation between the reversible susceptibility χ of the minor loops and the coercivity Hc has been found, showing that the coercivity of the investigated material is determined by the pinning of domain walls, rather than nucleation. The relation χHc2 = constant has been found in a wide range of coercivities obtained by aging of the material. The aging process is interpreted as a decrease of the density of pinning sites, each site retaining its pinning force. The influence of hydrogen on Hc, which is reported recently, is interpreted likewise. A hysteresis loop measured on a single particle of a few microns size confirms these interpretations and allows for a refined model of pinned‐wall coercivity of SmCo5 powders. A full account will be submitted as a regular article to this Journal.