Showing papers on "Magnetization published in 1968"

Influence of Crystallite Size on the Magnetic Properties of Acicular γ‐Fe2O3 Particles

Abstract: Magnetic properties have been measured for a number of samples of acicular γ‐Fe2O3 particles of the type used in magnetic recording tapes. The average particle size and shape were approximately the same for all samples. However, the average crystallite sizes of the samples, as determined from x‐ray line broadening, ranged from 50 to 700 A. All magnetic properties measured showed a strong dependence on average crystallite size. Saturation magnetization at room temperature, σ, decreased sharply with decreasing crystallite size. An excellent fit to the σ vs crystallite size data was made by assuming that the crystallites were separated by a nonmagnetic grain boundary on the order of 6 A wide. Room‐temperature coercive force decreased with decreasing crystallite size, and the ratio Hc(83°K)/Hc(293°K) increased sharply with decreasing crystallite size.The data support a particle model in which the constituent crystallites interact magnetostatically across nonmagnetic grain boundaries. A critical crystallite diameter of order 400 A may be inferred. Below this crystallite size, superparamagnetic behavior is observed; above this size, noncoherent magnetization reversals in the crystallites are suggested. This model is quite consistent with the ``fanning'' mode of reversal based on a ``chain‐of‐spheres'' model which has been considered by several authors to be most compatible with their data on γ‐Fe2O3 particle assemblies.

Magnetic Properties of SrRuO3 and CaRuO3

Abstract: Magnetization measurements of SrRuO3 and CaRuO3 confirm that SrRuO3 is ferromagnetic with TC = (160±5) °K and that CaRuO3 has antiferromagnetic exchange interactions dominant. They further identify in CaRuO3 a Neel temperature TN = (110±10) °K and parasitic ferromagnetism below TN having a σ0 = (3.2±0.4) × 10−2 emu/g at 4.2°K. High‐field (to 125 kOe) magnetization and neutron‐diffraction data for SrRuO3 are consistent with a reduced spontaneous ferromagnetic moment μ0 = (1.4±0.4) μB and no ordered antiferromagnetic component. Resistivity measurements confirm a low resistivity (ρ<10−3Ω·cm) at room temperature having a metallic temperature coefficient. It is concluded that both compounds represent spontaneous collective‐electron magnetism and that the reduced moment of SrRuO3 is due to holes in both the up‐spin and down‐spin bands. Antiferromagnetic CaRuO3 is predicted to have narrower ``4d'' bands.

Effects of electric and magnetic fields on the structure of cholesteric liquid crystals

Abstract: Electric or magnetic fields acting on the anisotropy of the electric or magnetic susceptibility exert torques within a liquid crystal which may compete with the elastic torques determining its internal structure. A general method is presented for calculating the structural changes thus produced. Two examples are given, and the field strength for the effects is estimated to be 105 V/cm or 105 G. Two experiments are suggested to test the theory.

New Determinations of the Saturation Magnetization of Nickel and Iron

Abstract: Numerous measurements of saturation magnetization of Fe and Ni are reported. Saturation magnetization is determined by consideration of all contributions in the change of magnetization in high magnetic fields, i.e., internal demagnetizing fields, homogeneous rotations, and paramagnetic terms. The value found for the absolute saturation of iron is σ0,∞ = 221.71±0.08 emu/g, corresponding to M = 2.216 μB, in good agreement with previously accepted values. For Ni, the absolute saturation value obtained is σ0,∞ = 58.57±0.03 emu/g, corresponding to M = 0.616 μB, in good agreement with other previous data but 1.6% larger than the Weiss and Forrer's value of M = 0.606 μB.

Theory of magnetization ripple

Abstract: A review is given about the assumptions the development, and the results of the ripple theory. The limits of the linear theory are pointed out. The result of this theory shows a very strong coupling of the magnetization dispersion perpendicular to the mean magnetization, which is caused by the internal demagnetizing field. The exchange constant of this magnetostatic coupling exceeds the quantum mechanical exchange constant A by orders of magnitude. The nonlinear ripple theory includes the component of the internal demagnetizing field parallel to the mean magnetization. This field becomes important when the rms values of the magnetization dispersion are \gapprox 2\deg . The nonlinear theory explains all deviations from a single domain behavior without introducing any special model.

Magnetic Isotherms in the Band Model of Ferromagnetism

Abstract: Calculations are given for the dependence on temperature and magnetic field strength H of the magnetization M of ferromagnetic metals treated on the basis of the itinerant electron model. The spin wave contribution to the low temperature magnetization is considered in the limits of strong and very weak ferromagnetism. For the first limit, problems related to the well-known divergence of the zero field differential susceptibility are briefly discussed. The main part of the paper is concerned with the single particle contributions to the magnetization and associated differential susceptibility for very weak ferromagnetism. A simple equation for the magnetic isotherms is obtained in this limit and shown to be valid over a wide temperature range including 0 °K and the Curie temperature. This equation implies that plots of M 2 against H / M at various temperatures in this range give a series of parallel straight lines. Recently measured isotherms for the material ZrZn 2 are analysed on the basis of the theory, and several characteristic properties of this material are obtained from the analysis.

The Fundamental Theorem of Fine-Ferromagnetic-Particle Theory

Abstract: The theory of fine‐particle magnets is based on the theorem that the state of lowest free energy of a ferromagnetic particle is one of uniform magnetization for particles of less than a certain critical size and one of nonuniform magnetization for larger particles. The theorem is inferred from several approximate calculations and has not been proved rigorously. Rigorous statements can be made if one is content to replace equalities by inequalities and exact values of critical radii by upper and lower bounds. For a sphere of radius a with uniaxial anisotropy, it can be shown that the lowest‐free‐energy state is one of uniform magnetization if a ac1 (for low anisotropy) or ac2 (for high anisotropy), where ac0, ac1 (>ac0), and ac2(>ac0) are determined by the exchange and anisotropy constants and the spontaneous magnetization. These bounds locate the critical radius to within about 12% at low anisotropy but only to within an order of magnitude or more at high.

Néel Ferrimagnets in Large Magnetic Fields

Abstract: In this paper we analyze the high‐field magnetization of Neel ferrimagnets and apply this analysis to our measurements of YbIG and DyIG. We show that in some ferrimagnetic systems, as the temperature is increased at fixed field strength, the magnetization can decrease in one temperature interval, be temperature independent in another, and rise or fall in a third. Conversely, at fixed temperature, as the field is increased, there can be intervals of small and large susceptibility with abrupt discontinuities between them, even in the absence of anisotropy. This behavior is ascribed to the formation of angles between the various sublattice moments and the external field. The angled region can be described by a phase diagram in the H‐T plane. There are second‐order phase transitions at all boundaries, along which the order parameter (the angles) undergoes critical fluctuations. Two features appear technologically promising for some ferrimagnets: (a) a temperature range over which the moment of the ferrimagnet...

Theory of Magnetization Ripple in Ferromagnetic Films

Abstract: In a polycrystalline ferromagnetic film, the magnetization direction is not uniform but exhibits small wave‐like fluctuations known as magnetization ripple. In this paper, a general theory of ripple is developed, extending previous treatments by the present author, Hoffmann, and others. The theory is applicable to almost any magnetic film without gross domain structure, containing randomly oriented local anisotropies arising from inhomogeneities on any scale and of any physical origin. The magnetization direction is assumed to fluctuate in the plane of the film only. In addition to local anisotropy fields, the theory includes the effects of magnetostatic and exchange interactions, and a uniform field that consists of uniform uniaxial anisotropy and external fields. Nonlinear magnetostatic and uniform fields, which in previous treatments have been either neglected or treated as small perturbations, are taken fully into account through third‐order torque terms. The ripple spectrum is derived, and from it a ...

Pinning of flux vortices in type-ii superconductors.

Abstract: The superconducting properties of Pb-Bi alloys are reported. Measurements were made on the epsilon phase (a reversible material with K ∼ 12) and the eutectic, which is this epsilon phase with pure Bi precipitates. It is shown that the irreversible properties of the eutectic are consistent with the pinning of vortices in a reversible matrix of epsilon phase. The critical state is shown to be the same in measurements of magnetization, transverse critical currents and zero field critical currents, and it is extended to cover a composite ferromagnetic material. Experimentally it is found that the variation of critical current with flux density, microstructure and temperature is given by the expression J c = 33S v M(rev.)/B1/2 amps/sq.cm. (S v is the phase boundary area per unit volume and M (rev.) the magnetization of the matrix.) There is strong evidence that the irreversibility is due to pinning at the phase boundaries of precipitates and it is found that the surface currents observed in the epsilo...

Crystal Magnetic Anisotropy and Magnetization of MnSb

Abstract: Single crystals of MnSb with various Mn compositions in e phase were prepared successfully with Bridgeman method. Magnetization and torque measurements were made on these crystals in temperatures ranging from 4.2°K to 800°K. The temperature dependence of saturation magnetization on the sample with 48.2 at. % Mn was in good agreement with Brillouin function of J =3/2∼2, but the samples with Mn rich composition exhibited slight discrepancies from this function. The saturation magnetization per Mn atom at 0°K in Bohr magneton decreases with increasing Mn composition, and these values are well explained quantitatively as excess Mn atoms with 2.2µ B occupy the B site and these spins are arranged antiparallel to those of A site Mn with 3.5µ B . The temperature dependences of the first order crystal magnetic anisotropy of various Mn compositions exhibit analogous function with those of cobalt and nickel derived from Carr's theory.

Mössbauer studies of the cubic Laves iron-rare-earth intermetallic compounds

Abstract: The Mossbauer effect in 57Fe has been used to study the hyperfine fields at the iron sites in polycrystalline samples of the cubic Laves AFe2 intermetallics where A is Ce, Sm, Gd, Tb, Dy, Ho, Er, Y or Zr. The 57Fe spectra in the Tb, Er, Y and Zr intermetallics can be analysed in terms of two distinct six-line 57Fe patterns, whereas those in Dy and Ho exhibit only one. This is shown to be consistent with [111] and [001] directions of magnetization respectively. The cases of Sm, Gd and Ce are found to be more complicated. An analysis of the results is attempted on the basis of a simple model in which the direction of magnetization is determined by the crystalline electric torques acting on the rare earth ions. An estimate is made of the magnetic dipole contributions to the local internal fields. It is found that this leads to a magnetic interaction which is dependent on the iron site and on the direction of magnetization. The hyperfine fields at the 57Fe and 161Dy sites in DyFe2 have also been followed as a function of temperature. This parallels other work done on TmFe2 and allows a comparison of the rare-earth-iron exchange in these intermentallics.

Nature of Spin-Reorientation Transitions

Abstract: The change in the easy axis of magnetization observed in some ferrimagnets has been studied using a phenomenological description. It is found that two second-order phase transitions can occur.

Dynamical interpretation of magnetic recording process

Abstract: The demagnetization effect in magnetic recording must be evaluated, not as static self-demagnetization, but as a dynamic demagnetizing field at an instant when the head field is applied to the medium. From this fact it becomes necessary to obtain a self-consistent magnetization distribution in the medium. A method of calculation and its results are described. The relation between the longitudinal and the vector magnetization is clarified. The experimental results of the recording demagnetization in sinusoidal recording and the pulse width and the peak shift in digital recording are interpreted as the new phenomena that is related to the dynamical behavior of the demagnetizing field in the recording process.

Variation of Palaeomagnetic Stability and Other Parameters in a Vertical Traverse of a Single Icelandic Lava

Abstract: An intensive study has been made of 30 samples from within, and two bakes samples from below a single Tertiary basalt in Eastern Iceland The samples are spaced along a measured vertical traverse from bottom to top The lava is 16·8 m thick, and is reversely magnetized The quantities measured for each core were: 1 Oxidation state as revealed by microscope studies in polished sections of the iron-titanium oxide 2 Oxidation state of the whole rock as determined by FeO and Fe2O3 analyses 3 Titanomagnetite and separate ilmenite grain sizes 4 Magnetic susceptibility 5 Saturation magnetization 6 Curie points and total heating curve in a strong magnetic field 7 Natural magnetization after demagnetization in several alternating fields 8 Koenigsberger's Q factor, slightly modified 9 A new measure of the resistance of the specimen to alternating field demagnetization, the ‘stability factor’ 10 The position of the core above the base of the lava The above quantities vary greatly with position in the lava The results of these measurements show the relationships of these variables to each other; in particular how the palaeomagnetic stability depends on oxidation state of the iron-titanium oxides Stability is high in the lower two-thirds of this lava, and not just at the chilled margins Two anomalous results are (1) that the Curie point is virtually constant regardless of oxidation state (unlike some other lavas), and (2) that the whole rock oxidation state measured by Fe2O3/(2FeO + Fe2O3) does not correlate simply with the oxidation state of the iron-titanium oxides as seen in the microscope

Point Singularities in Micromagnetism

Abstract: Solutions of the micromagnetic equations for the equilibrium distribution of magnetization may have singular points, at which no spontaneous magnetization exists, because the limit of the magnetization vector for an approach to this point on different rays has different values. The boundary conditions at these points are considered. The exchange‐energy density goes to infinity at such a point, but the condition that its integral should be a minimum leads to no definite boundary condition. For a special class of such singularities, namely those at which every direction of the magnetization is accepted once on a small sphere around it, the conditions for a minimum of the field energy are derived.

The theory of the electric and magnetic properties of molecules

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Torque Measurements of the Anisotropy of Energy and Magnetization of Nickel

Abstract: An analysis of experimental methods and the design of an appropriate torquemeter made it possible to improve by several orders of magnitude the accuracy of anisotropy measurements. The experiments performed on nickel between 100°K and the Curie point allowed, for instance, the determination of the eighth‐order anisotropy constant K3 within a few percent. The anisotropy constants K1, K2, and K3 are found to be negative in this whole temperature range. An explanation is given of the main discrepancies in published data between the numerous determinations of anisotropy constants by static methods. The influences of both anisotropy of energy and anisotropy of magnetization are separated and measured as functions of applied magnetic field and temperature in the above‐mentioned range. This interpretation of torque measurements has been confirmed by a direct determination of anisotropic magnetization of nickel at room temperature, and an absolute measurement of the saturation magnetization of nickel at 20°C has ...

Magnetization Curves of Fe–Ni (50–50) Single Crystals Ordered by Neutron Irradiation with an Applied Magnetic Field

Abstract: Spherical Fe–Ni (50–50) single crystals were ordered by neutron irradiation at 295°C in the presence of a magnetic field. Magnetization curves and magnetic anisotropy energy measurements demonstrate the very important directional effect of the irradiation magnetic field on the bulk anisotropy of the single crystal. When the magnetic field is applied along a fourfold axis, the bulk magnetic anisotropy is tetragonal with a very large value for K3. When the magnetic field is applied along a three fold axis, the bulk magnetic anisotropy is cubic and the threefold axis becomes an easy axis of magnetization; the magnetization measured along the easy direction reaches saturation with a certain law of approach, while the magnetization measured along the [100] direction reaches saturation immediately. The model proposed by Neel, in which three types of quadratic ordered domains are assumed to be strongly coupled, fits well the experimental results. An electron microscopy image shows the different type of ordered d...

Magnetic Properties of Rare Earth Metals

Abstract: Publisher Summary This chapter discusses the theory of the magnetic behavior of the lattice of localized moments corresponding to the unfilled 4f shells, and to a description of the relevant experimental information. It begins by describing the equilibrium magnetic arrangements, the basic model for magnetic behavior of the heavy rate earth metals, and the way in which one obtains transitions between various equilibrium magnetic arrangements within this model. The emphasis is on the theory for the excited magnetic states—that is, spin-wave-like states, and the experimental manifestations of these excited magnetic states. These topics are treated in this chapter. It discusses the theory of spin wave behavior when the equilibrium magnetic arrangement is ferromagnetic. That Part includes a discussion of the temperature dependence, as well as discussion of applied field and magnetoelastic effects. The excited magnetic state behavior for periodic moment arrangements is discussed here.

Magnetic and Electrical Properties of Ferromagnetic Alloys Near the Critical Concentration

Abstract: A simple molecular field theory is developed to explain at least qualitatively the behaviour of weakly ferromagnetic and strongly paramagnetic alloys near their critical concentration c F . The free energy of a weakly ferromagnetic alloy can be expanded in powers of magnetization ζ and a modified Landau theory is used to calculate the concentration dependence of the magnetization, specific heat and the susceptibility near c F . It is believed that such a theory can work better in this case than does the usual Landau theory as fluctuations in ζ are relatively small for low temperatures. A change in low temperature resistivity around c F is also expected as a result of ciritcal scattering of conduction electrons. This change was first calculated by Rice (1967) for strongly paramagnetic alloys giving a large T 2 term. However, this calculation is not applicable for c very close to c F . It is shown that for c → c F the paramagnetic resistivity remains finite and has a maximum for c ═ c F , ρ Para c ═ c F ∝ T 5/3 . It is also found that the ferromagnetic resistivity ρ ferro has a similar dependence on concentration and temperature as ρ Para and ρ ferro c ═ c F ═ ρ Para c ═ c F , i.e. the resisitivity is symmetrical around c F .

Origin and Uses of the Faraday Rotation in Magnetic Crystals

Abstract: Light transmitted by a magnetic crystal interacts with the magnetization. In particular, the axis of linear polarization undergoes a rotation proportional to the fractional projection of M on the direction of propagation. This magneto‐optical rotation, the Faraday rotation in magnetic materials, has been studied for many years. The advent of insulating magnetic materials has led to renewed interest of two sorts:(1) The rotational dispersion coupled with that of the absorption coefficient in some cases makes possible the spectroscopic assignment of optical transitions which show magnetic effects. Thus we may use the rotation to extract information on the magnetic behavior of energy levels well above the ground state.(2) Any experimental or technological use of the rotation makes it necessary to take into account the attendant absorption of light. Some insulating magnetic materials have very high values of the rotation per unit attenuation. The magneto‐optical rotation may then be used to ``see'' both dc (domain structure) and rf (magnetostatic spin waves) magnetization distributions within the crystal. Furthermore, the low rotation attenuation is crucial in a number of magneto‐optical devices: rotators, wide‐band modulators, phase shifters, memories, the YIG laser, etc.

Spectrum of the potential field due to randomly distributed sources

Abstract: Covariance and spectral density functions of all three components of random potential fields are mutually related. The spectra of any one component on two different levels are related through an exponential function of the separation of the levels. If, starting from the earth’s surface, we continue the observed random field downward, we find that at a certain level the field becomes unstable. Such a limiting depth can be estimated from the spectrum of the observed field. The spectrum of the random field is related to the spectrum of random density or intensity of magnetization. The random sources are assumed to be confined to a thin sheet or a thick, infinite sheet, or to a semi‐infinite medium. In all these cases, a relation connecting the spectrum of the random density or the intensity of magnetization and the spectrum of the random field has been established.

SPIN-LATTICE INTERACTION IN UO$sub 2$. II. THEORY OF THE FIRST-ORDER PHASE TRANSITION.

Abstract: A theory of the first-order phase transition in U${\mathrm{O}}_{2}$ is presented and discussed in the molecular-field approximation. An isotropic nearest-neighbor exchange and local quadrupole-lattice interaction are taken as the basic interactions in the model. Interesting behavior is obtained due to the two distinct ways in which the collective ground-state degeneracy can be removed at $T=0\ifmmode^\circ\else\textdegree\fi{}$K: a cooperative Jahn-Teller distortion or a polarization of the sublattice magnetization by the exchange field. Depending on the relative gain in free energy obtained by these two mechanisms, one obtains four different types of behavior near the critical point: (1) a second-order transition to a distorted state with no magnetic ordering; (2) a second-order transition to a distorted state followed by a second-order magnetic transition; (3) a first-order transition yielding a discontinuous change in lattice distortion and sublattice magnetization; (4) a second-order magnetic transition accompanied by a weak distortion. The temperature dependence of the elastic constant ${C}_{44}$ is also derived. The parameters required to give a first-order transition in agreement with the measured discontinuity in sublattice magnetization and the correct behavior for ${C}_{44}$ are found to be consistent with the parameters obtained from the measured spin-wave excitations.

Neutron elastic diffuse scattering from mixed magnetic systems

Abstract: In a mixed magnetic system, such as a ferromagnetic alloy, there is a spatial inhomogeneity in the distribution of magnetization and this gives rise to a diffuse scattering of neutrons which is strictly elastic. The observation of this diffuse scattering is capable of giving information about the magnetization distribution on an atomic scale. In practice it is necessary to assume suitable models for the way in which each atom affects the magnetization in its immediate environment. Various models are described and it is shown that, even for concentrated alloys where a number of effects must be allowed for, the cross-section formulae are simple. It follows that neutron experiments on mixed magnetic systems can be interpreted in terms of functions of physical significance.

Magnetic properties of ultramafic rocks from Red Mountain, California

Abstract: A representative ultramafic intrusion at Red Mountain southeast of San Francisco, California was sampled for the purpose of studying the remanent magnetism and susceptibility of ultramafic rocks as functions of serpentinization. Due to the dependence of grain density upon the degree of serpentinization, the former was taken as a measure of the latter. Grain densities were accurately determined from bulk densities by taking into account the porosity and amount of adsorbed water contained in these rocks. In general, the porosity was found to increase with serpentinization. The magnetic data suggest that the remanent magnetization is mainly a chemical remanent magnetization acquired during the process of serpentinization whereby iron atoms, released from the silicate structure of the paramagnetic olivine and pyroxene, are oxidized to form ferrimagnetic magnetite. The intensity of both remanent and induced magnetization increases exponentially with serpentinization. However, the remanent magnetization becomes...

Direct Evidence for Magnetic Domains in Silver

Abstract: Pulsed NMR studies of $^{109}\mathrm{Ag}$ nuclei in a single-crystal specimen of Ag metal at helium temperatures with an applied field ${H}_{0}\ensuremath{\cong}90$ kOe reveal the presence of magnetic domains due to de Haas-van Alphen magnetization through the associated splitting of the $^{109}\mathrm{Ag}$ NMR line. Domain formation is found to have the expected de Haas-van Alphen periodicity, and the domains behave in generally good accord with theory.

Magnetic Properties of an Antiferromagnetic Orthoferrite

Abstract: The orthoferrite DyFeO3, weakly ferromagnetic at room temperature, was found to become antiferromagnetic at 36°K. Symmetry considerations suggest that at this transition point the iron spin‐axis rotates out of the plane perpendicular to the Moriya‐Dzyaloshinsky vector, becoming parallel to it. On this basis the magnetic point group mmm is assigned to DyFeO3 below the transition. Further evidence for the correctness of this assignment has been obtained by the observation of a linear dependence of the susceptibility on the magnetic field χxy=CHz, and cyclic permutations as allowed by the mmm symmetry. Another transition, which we associate with an antiferromagnetic ordering of the Dy3+ moment, is found at TN=3.7°K. The magnetization curves measured below 4.2°K at fields up to 40 kG are interpretable, to a good approximation, in terms of |±15/2〉 Kramers' doublet whose axis lies in the mirror plane of the crystal, making an angle of approximately 30° with the [010] axis. Below TN metamagnetic transitions were...

S = 1 Spin State of Divalent Iron. II. A Mössbauer‐Effect Study of Phthalocyanine Iron (II)

Abstract: Measurements of the 57Fe Mossbauer spectrum of phthalocyanine iron(II) made at 4.2° and 100°K, with the sample in applied magnetic fields of 30 kOe, show that the electric field gradient at the iron nucleus is of positive sign and that there is no large asymmetry in the electric‐field‐gradient tensor. The major axis of the field‐gradient tensor does not appear to be colinear with the magnetic symmetry axis, indicating that there is no orbital degeneracy in the molecule. The hyperfine field at the iron nucleus, measured in the plane of maximum magnetization, is estimated to be + 270 kOe.