Showing papers on "Magnetization published in 1978"

Random anisotropy in amorphous ferromagnets

Abstract: Random anisotropy is present in all amorphous magnetic materials, and depending on its strength, it can dramatically affect the magnetic behavior. We describe the effect of strong random anisotropy in materials such as amorphous Tb‐Fe and Dy‐Fe at low temperature and also examine the role which weak random anisotropy might play in even ideally homogeneous soft materials. Much of our analysis is based on the simple model proposed by Harris, Plischke and Zuckerman for a ferromagnet with random‐axis uniaxial anisotropy. We describe computer simulation results for this model and then develop scale length arguments which allow us to describe fluctuations in the magnetization direction. For a perfectly isotropic distribution of anisotropy axis we find that the conventional ferromagnetic ground state is unstable. The new ground state has large frozen in fluctuations but probably has a considerable moment and is, therefore, not spin glass‐like. This system does not support domain walls of the conventional type. For a system with both a macroscopic anisotropy axis and random anisotropy, we can have domain walls. We present a theory for the intrinsic coercivity which gives values of about 10−6 Oe for Fe‐metalloids and 0.2 Oe for Gd‐Co‐Mo. This indicates that inhomogeneities of larger than atomic scale are limiting the behavior of present materials. A model illustrating aspects of magnetic resonance behavior is also described.

Measurements of particle size distribution parameters in ferrofluids

Abstract: A method is presented by means of which (for a ferrofluid with a lognormal distribution of particle size) it is possible to determine the standard deviation (σ) and median particle diameter (D v ) from the room temperature magnetisation curve. The method has been applied to several commercial ferrofluids, containing ferrite particles, and to fluids consisting of cobalt particles in toluene prepared by the method of Hess and Parker. The particle size distributions have also been measured by using an electron microscope. It has been found that the magnetic data gives estimates of the 'magnetic size distribution' parameters D vm and σ m , which differed from the 'physical size distribution' parameters D vp and σ p obtained from electron microscope data. It is found that D vp > D vm , and \sigma_{p} . These observations are consistent with the results of Kaiser and Miskolczy and Granqvist and Buhrman.

Amorphous magnetic order

Abstract: A review is given of the collinear and random magnetic structures which may be found in magnetically‐concentrated amorphous solids General consequences of a non‐crystalline lattice on the atomic moments, exchange interactions and single‐ion anisotropy are presented Magnetic structures with one and two magnetic subnetworks are then described, taking examples from the literature of each type Some discussion is also given of the low energy excitations and the behavior at the spin freezing or ordering temperature What happens to the familiar forms of magnetic order—ferromagnetism, ferrimagnetism, antiferromagnetism—in the absence of a crystalline atomic lattice? This article sets out to give an overview of magnetic order in non‐crystalline solids in an attempt to answer the question The magnetism of two classes of disordered metals have received particular attention during the past five years; amorphous ferromagnets, notably the Metglas‐type alloys, and spin glasses, typically dilute solutions of transition‐metal impurities in a non‐magnetic crystalline matrix The subject‐matter here will be magnetically‐concentrated amorphous materials, with the accent on non‐collinear spin structures A coherent picture of amorphous magnetism will be presented, but it must be admitted that not all the magnetic structures discussed have been equally well established Some of them are directly analogous to those found in crystals Others are peculiar to amorphous (or disordered) solids, whereas one crystalline magnetic structure seems impossible in a noncrystalline lattice

Iron‐boron metallic glasses

Abstract: By rapid quenching from the liquid, binary Fe100−xBx glasses are produced over the range 12–28 at.% B. Magnetization measurements in these glasses indicate a compositional dependence of the saturation magnetic moment μs which is consistent with the compositional dependence of the density of the material and reflects electron charge transfer from boron to iron. The extrapolated value of μs for noncrystalline Fe metal is found to be close to that for pure crystalline iron (μs=2.2μB); this is lower than the value of 2.4μB previously estimated for amorphous Fe. Based on x‐ray and thermomagnetization data, it is argued that the local atomic arrangements resemble those of bcc iron for the Fe‐rich alloys (x<16 at.%) and are similar to those of orthorhombic Fe3B in the vicinity of x=20. For x≳25, the local atomic structure seems like that of tetragonal Fe2B. These structural changes can explain the unexpected decrease of the Curie temperature with decreasing boron content. The decrease of the crystallization temp...

Magnetic cross-relaxation among protons in protein solutions.

Abstract: The magnetic spin-lattice relaxation rates of solvent water nuclei are known to increase upon addition of diamagnetic solute protein. This enhancement of the relaxation rate is a function of magnetic field, and the orientational relaxation time of the protein molecules can be deduced from analysis of the field-dependent relaxation rates. Although the nature of the interactions that convey information about the dynamics of protein motion to the solvent molecules is not established, it is known that there is a contribution to the relaxation rates of solvent protons that plays no role in the relaxation of solvent deuterons and 17O nuclei. We show here that the additional interaction arises from a cross-relaxation process between solvent and solute protons. We introduce a heuristic three-parameter model in which protein protons and solvent protons are considered as two separate thermodynamic systems that interact across the protein-solvent interface. The three parameters are the intrinsic relaxation rates of each system and a cross-relaxation term. The sign of the latter term must always be positive, for all values of magnetic field, in order for magnetization energy to flow from the hotter to the cooler system. We find that the magnetic field-dependence of the cross-relaxation contribution is much like that of the remaining solvent proton relaxation, i.e., about the same as the deuteron relaxation field dependence. This finding is not compatible with the predictions of expressions for the cross-relaxation that have been used by other authors, but not applied to data over a wide range of magnetic field strength. The model predicts that the relaxation behavior of both the protein protons and the solvent protons is the sum of two exponentials, the relative contributions of which would vary with protein concentration and solvent isotopic composition in a fashion suggestive of the presence of two classes of protein protons, when there is in reality only one. This finding has immediate implications for the interpretation of published proton relaxation rates in complex systems such as tissues; these data should be reexamined with cross-relaxation taken into account.

The role of finite magnetic clusters in Au-Fe alloys near the percolation concentration

Abstract: Au-Fe alloys have been studied by magnetic measurements of various types in the composition range where ferromagnetism evolves from spin-glass character. The percolation limit identified from the results and other data is 15.5 + 0.3 at.% Fe. Between this composition and ∼28% Fe, finite clusters co-exist in significant concentrations with the infinite cluster, and a fairly well-defined temperature observed in the magnetic properties is ascribed to a freezing of these clusters that parallels that of finite clusters in the pre-percolation alloys. Magnetic resonance is observed in alloys with more than about 13% Fe (i.e. in alloys containing large nearest-neighbour clusters) and the temperature dependence of the linewidth correlates with the interpretation given of the magnetization data.

Magnetic properties of amorphous alloy films of Fe with Gd, Tb, Dy, Ho, or Er

Abstract: Amorphous rare‐earth RE(Gd, Tb, Dy, Ho, Er) ‐Fe films prepared by cosputtering were studied. The compositional analysis was obtained from known deposition profiles, x‐ray microanalysis, and the stripe‐width measurements. The structural variation with composition change was investigated by electron diffraction and dark‐field microscopy. The Curie temperature Tc, the compensation temperature Tcomp, the coercive force Hc, the uniaxial anisotropy energy Ku and the static domain properties such as the stripe width Ws, the wall energy σw, and the exchange stiffness constant A were investigated. The systematic variation of Tc and Tcomp associated with a variation of composition and RE species could be described by the Heiman et al. model. The static domain properties could be interpreted in terms of the wall energy model and the mean field approximation of the exchange stiffness constant A.

Magnetic Structure of CsCoCl 3

Abstract: Neutron diffraction measurements were carried out on single crystals of CsCoCl 3 , a quasi-one-dimensional antiferromagnet of Ising-like spins, in the long range ordered region. The determined magnetic structure at 4.2 K is composed of antiferromagnetic stacking of ferrimagnetic c -plane with three sublattices as determined by Melamud et al. The spins are aligned along c -axis and were evaluated to be 3.11 µ B at 0 K by extrapolation. Unusual temperature dependence of magnetic reflection intensity was observed and interpreted as a partial disordering transition in which one of the three sublattices in a c -plane loses its magnetization above 13.5 K resulting in antiferromagnetic c -planes. This transition is discussed in connection with the antiferro-ferrimagnetic transition in the triangular Ising lattice. The critical exponent of the magnetization was found to be 0.34 around the Neel temperature 20.82 K.

Passive magnetic bearings with permanent magnets

Abstract: The various geometrical dispositions which allow to realize passive magnetic bearings with permanent magnets, have been Known for several decades. Devices running by attraction were the first to be built. When new magnets, which can be used in repulsion appeared on the market (SmCo 5 ), new types of passive magnetic bearings, running by repulsion, started to be made. Up to now, to our knowledge, most of the devices realized have been made with radially magnetized rings. We proved that for identically shaped bearings, strengths and stiffnesses were exactly the same in both magnetic configurations : radial and axial magnetization. The proof is only based on the rigidity of the magnetization. These results are still true for axial or radial passive bearings, and can also be used for devices in attraction. Ring shaped magnets with radial magnetization are usually made of several sectors stuck together. Axial magnetization is much easier to manufacture because large sized rings are already industrially provided.

Magnetic properties of amorphous neodymium-transition-metal films

Abstract: Amorphous thin films of NdxCo1−x and NdxFe1−x alloys were prepared over the compositional range 0.08⩽x⩽0.71 by e‐beam evaporation. Magnetization and anisotropy of the samples were studied over a wide temperature range with the aid of a force balance magnetometer, Hall effect measurements, and Mossbauer spectroscopy. It was found that the magnetization of the alloys could not be accounted for by a completely collinear alignment (ferromagnetic) of the Nd and transition‐metal subnetworks. Mean field analysis of the magnetization data showed a large reduction of the Nd–transition‐metal exchange coupling as compared to their Gd analogs. A model was developed which requires that Nd be dispersed in a cone whose axis is parallel to that of the transition‐metal subnetwork by strong coupling to randomly oriented local crystal field axes. This dispersion reduces the Nd net moment to 77% of its free‐ion moment in NdxCo1−x alloys and to 25% of its free‐ion moment in NdxFe1−x alloys. There is evidence that some dispers...

Effect of exchange on interband magneto-absorption in zero gap Hg 1-kMnkTe mixed crystals

Abstract: Magnetoabsorption experiments have been performed at 4.2 and 2 K in the spectral region 220-370 meV, on zero gap Hg1-kMn kTe alloys. The temperature and the composition dependence of the Γ6 → Γ 8 spectrum provides evidence of striking modifications in the Γ 8 and Γ6 spin sublevels due to exchange interactions between localized d electrons and mobile carriers. The theoretical interpretation of the magnetooptical spectra of alloys of low Mn composition is carried out, using the virtual crystal and the molecular field approximations, by including the s-d and p-d exchange interactions within the framework of the Pidgeon and Brown model. By a fitting procedure of the Γ6 → Γ8 transitions energies, the magnetization is determined at 4.2 and 2 K as a function of the magnetic field and the exchange parameters for the Γ 6 and Γ8 levels are evaluated. The Landau level energy spectrum established, at kH = 0, for alloys of composition up to 1.5 % exhibits a SM → SC transition induced by the magnetic field.

Correlation of chemical shifts of protons and carbon-13

Abstract: A 2-dimensional Fourier-transform technique, involving the transfer of nuclear magnetization between protons and 13C, was used to correlate the NMR chem shifts for directly-bonded protons and 13C Illustrative spectra are presented for the Me-group regions of menthone and camphor with natural-abundance 13C The 2-dimensional spectra of geminal di-Me groups show a characteristic pattern involving addnl magnetization transfer through the long-range coupling J(C-C-Me) [on SciFinder (R)]

The magnetic structure of Bi2Fe4O9 – analysis of neutron diffraction measurements

Abstract: The compound Bi2Fe4O9 belongs to the space group Pbam ( D92h), with two formula units per unit cell. Neutron diffraction measurements showed that it is paramagnetic at room temperature and undergoes a transition to an antiferromagnetic state at TN = (264 ± 3) K in agreement with previous susceptibility and Mossbauer measurements. Analysis of the 80 K neutron diffraction pattern yielded a magnetic structure with the following features: (a) The basic translations ao, bo, co of the chemical lattice change into antitranslations in the magnetic lattice. (b) The spins are perpendicular to co. (c) The magnetic structure belongs to the PC2/m space group and is a basis vector to an irreducible space under the Pbam irreducible representations, in accord with Landau's theory of second-order phase transition. The position parameters of the Fe3+ ions in the unit cell were refined. The magnetic moment of the compound was found to be (4.95 ± 0.08) μB, compared with the value of 5 μB for the Fe3+ free ion. The temperature dependence of the { 131 } magnetic reflection peak intensity was measured and found to be in agreement with the sublattice magnetization predicted by the molecular field approximation.

Transition from Pauli Paramagnetism to Band Ferromagnetism in Very Thin Ni Films

Abstract: Ni, Co, and Fe films of a few atomic layers are condensed in UHV at 10 K on metallic substrates. Anomalous-Hall-effect measurements are used to determine the magnetization and the susceptibility of the films. Ni films with a thickness smaller than two atomic layers possess no magnetic moment but show an enhanced susceptibility. Between two and three atomic layers of Ni the susceptibility diverges and thicker films possess a magnetic moment. For Co and Fe, the first monolayers already shows a magnetic moment.

Cooperative Jahn-Teller effect in TmZn

Abstract: Specific-heat, resistivity, neutron-spectroscopy, magnetization, and magnetostriction experiments on TmZn are reported. This equiatomic metallic compound crystallizes with the cubic CsCl structure and shows a first-order Jahn-Teller transition to a tetragonal structure at ${T}_{Q}=8.55$ K. The cell distortion reaches $\frac{c}{a}\ensuremath{-}1=(\ensuremath{-}9\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. The magnetic moment orders along a fourfold axis at ${T}_{c}=8.12$ K, its value being largely field dependent. Due to the vicinity of the transitions, they are mixed and strongly raised in temperature by (even low) applied fields as observed by the magnetization and magnetostriction experiments in the paramagnetic state. These results and the elastic constants previously published are analyzed taking into account the quadrupole-quadrupole exchange and the quadrupole-lattice term beside the Heisenberg exchange and the cubic crystal electric field (CEF) terms; the CEF parameters are ${A}_{4}〈{r}^{4}〉=\ensuremath{-}38\ifmmode\pm\else\textpm\fi{}5$ K and ${A}_{6}〈{r}_{6}〉=\ensuremath{-}19.6\ifmmode\pm\else\textpm\fi{}2$ K/atom. The ground state is the magnetic triplet ${\ensuremath{\Gamma}}_{5}^{(1)}$ in the cubic paramagnetic state. The theoretical study of the dependence of ${T}_{c}$ and ${T}_{Q}$ on the bilinear and biquadratic exchange terms reveals a complex phase diagram. In the case of TmZn, the obtained second-order coefficients allow a good description of all the physical properties now investigated.

The magnetic properties of some DSDP basalts from the North Pacific and inferences for Pacific plate tectonics

Abstract: One hundred seven basalt samples from seven DSDP sites in the North Pacific studied for magnetic properties were found to have a mean magnetization of 0.005 emu/cm3, a mean susceptibility of 0.001, and a mean Koenigsberger ratio of 10. The mean remanence coercivity as measured by the mean demagnetizing field is 90 Oe. The titanomagnetite in most of these old (13–115 m.y.) and highly fractured basalts has been largely oxidized to titanomaghemite during sea floor weathering. The titanomaghemitization is manifest in an increase in Curie temperature from about 160°C to 300°–350°C and a threefold to fourfold decrease in the saturation magnetization. Although a 50% decrease in remanence intensity and susceptibility can be attributed to oxidation at one site, the relation between degree of oxidation and these two properties is obscured at most sites by magnetic grain size differences. A decrease in remanence intensity no doubt occurs but was not observed because the most highly oxidized basalt, that in thin flows and in the margins of thicker flows, is also the finest grained and therefore had a higher initial remanence intensity. Remanence coercivity, while mainly dependent on magnetic grain size, increases about 50% with oxidation. The remanence direction does not appear to be affected by oxidation. Although the remanance inclinations differ considerably from the present geomagnetic field inclination at most sites, the expected range of secular variation is such that the remanence, with the exception ofthat in Meiji Seamount, could have been acquired by the basalts at the present site latitudes. But the site paleolatitudes calculated from the remanence inclinations correspond more closely to the paleolatitudes predicted by assuming that the Pacific plate has moved northward with the motions deduced by Morgan and others from linear volcanic chains. In particular, the coincidence of the magnetic paleolatitude, 19°, measured in Meiji Seamount at the northern end of the Emperor Seamount chain, with the latitude of the island of Hawaii, supports the fundamental assumption of the Morgan model of Pacific plate motion, wherein the volcanic locus or ‘hot spot’ forming the Emperor-Hawaiian chain has remained essentially stationary with respect to the earth's axis.

Mössbauer effect study of the magnetic properties of S=1 ferrous tetraphenylporphyrin

Abstract: Mossbauer spectra of the polycrystalline form of the square planar compound α,β,γ,δ‐tetraphenylporphinato–iron (II) have been observed at a variety of temperatures. Analysis of the resulting spectra yield a positive electric field gradient interaction with quadrupole splitting ΔE=1.51 mm/s (4.2 K) and 1.52 mm/s (300 K). At low temperature, an external magnetic field induces a negligible internal field along the symmetry axis of the molecule. In the transverse direction, it gives rise to a positive internal field with little temperature dependence up to 30 K. The data can be interpreted in terms of a crystal field model involving a low‐lying 3A2g state which is split by spin–orbit coupling into a ground singlet and a doublet lying 80 cm−1 above it.

Magnetization and magnetic anisotropy of TbFe 2 , DyFe 2 , Tb 0.27 Dy 0.73 Fe 2 and TmFe 2

Abstract: Saturation magnetizations were measured on single crystals of TbFe 2 , DyFe 2 , Tb 027 Dy 073 Fe 2 and TmFe 2 Over most of the temperature rang from 4 K to 300 K, these values are substantially larger than those measured earlier on polycrystals The intrinsic magnetic anisotropies, K 1 (0)'s, as determined from magnetic fields required for saturation, are huge ( \sim5 \times 10^{8} erg/ cm3) For these highly magnetostrictive compounds, the magnetoelastic contribution to the anisotropy -\frac{1}{2}c_{44}\lambda\min{111}\max{2} is a significant fraction of the total anisotropy

Superconducting and magnetic properties of ErRh4B4

Abstract: The thermal expansion, magnetization, magnetostriction, and ac electrical resistance of the compound ErRh/sub 4/B/sub 4/ have been measured at low temperatures. The influence of magnetic fields on the superconducting and magnetic properties of this compound is emphasized, and the qualitative behavior of the upper critical field as a function of temperature has been determined from the experiments.

Effect of H2 absorption on the magnetic properties of rare‐earth transition metal compounds

Abstract: Rare‐earth transition metal compounds are known to be able to (reversibly) absorb large quantities of hydrogen gas at moderate pressures. This absorption of H2 gas leads to substantial changes of the magnetic properties. These include hydrogen induced transitions from Pauli paramagnetism to ferromagnetism. Examples also exist of the reverse effect, i.e., where in ferromagnetic compounds the 3d moment disappears upon hydrogen absorption. The changes in magnetic properties, together with experimental results of X‐ray diffraction and Mossbauer‐effect spectroscopy are discussed in terms of charge transfer and changes in interatomic distances. The metastable character of the ternary hydrides often leads to a loss of the long range atomic order upon hydrogen absorption. This affects not only the absorption capacity (after repeated cycling) but also influences the magnetic behavior. Examples are shown where the atomic disorder has led to pronounced thermomagnetic history effects.

Energization of charged particles to high energies by an induced substorm electric field within the magnetotail

Abstract: Observations have shown that electrons and protons are energized up to at least 1 MeV in the magnetotail during substorms. This magnitude cannot be explained by the cross-tail electrostatic field, which typically has a modest potential difference of only 50 kV. A rotational electric field induced by a time-dependent magnetic field does not have such a limitation. In order to evaluate its capabilities a simple model of a localized, growing disturbance in the neutral sheet current was used to calculate perturbation magnetic and electric fields; the model includes the formation of X and O type neutral lines. Plasma sheet test particles were followed in these time-dependent fields by using the full relativistic equation of motion. The most efficient energizing mechanism is a two-step process, with an initial linear acceleration along a neutral line up to moderate energies, followed by betatron acceleration. The former imparts a large magnetic moment ..mu..=W/sub perpendicular//B to the particle as it begins to gyrate and drift in the magnetic field, W/sub perpendicular/ being the transverse kinetic energy. During this drift into regions of stronger magnetic field there is a large increase in W/sub perpendicular/, the relativistic invariant ..gamma mu.. remaining constant. Because of the rotational propertymore » of the induced electric field it is possible for a particle to gain energy even when the drift motion by itself would cause a loss. The model also includes acceleration along magnetic field lines in the plasma sheet, with moderate energy gain. A cyclic pattern of electron and proton precipitation is predicted, such as is observed during auroral breakup.« less

The Magnetic Field of Mercury

Abstract: The USA Mariner 10 spacecraft encountered Mercury three times in 1974–1975. The 1st and 3rd encounters provided detailed observations of a well developed, detached bow shock wave which results from the interaction of the solar wind. The planet possesses a global magnetic field, and modest magnetosphere, which deflects the solar wind. The field is approximately dipolar, with orientation in the same sense as Earth, tilted 12° from the rotation axis. The magnetic moment, 5×1022 Gauss−cm3, corresponds to an undistorted equatorial field intensity of 350γ, approximately 1% of Earth’s. The origin of the field, while unequivocally intrinsic to the planet, is uncertain.It may be due to remanent magnetization acquired from an extinct dynamo or a primordial magnetic field or due to a presently active dynamo. Among these possibilities, the latter appears more plausible at present. In any case, the existence of the magnetic field provides very strong evidence of a mature, differentiated planetary interior with a large core, Rc≈0.7RM, and a record of the history of planetary formation in the magnetization of the crustal rocks.

Magnetic phase transitions of CeSb. I. Zero applied magnetic field

Abstract: The magnetic ordering of the anomalous antiferromagnet CeSb, which has a NaCl crystal structure, was determined in zero applied magnetic field by means of neutron diffraction investigations of single crystals and powder. Below the Neel temperature TN of (16.1+or-0.1)K, there exist six partially disordered magnetic phases of antiphase domain type ((100) superstructures) with (100) orientation of the magnetic moments. At 4.4K, the ordered magnetic moment equals (2.10+or-0.04) mu B, which corresponds to the free-ion value of 2.14 mu mB for Ce3+. The temperature dependence of the ordered moment shows a first-order phase transition at TN. At approximately TN/2 there is a first-order phase transition to a FCC type IA low-temperature configuration. The unusual magnetic properties of CeSb, which result from anisotropic exchange and crystalline electric field effects, resemble those of certain actinide NaCl-type compounds.

Interplanetary magnetic holes: Theory

Abstract: Magnetic holes in the interplanetary medium are explained as stationary, non-propagating, equilibrium structures in which there are field-aligned enhancements of the plasma density and/or temperature. Magnetic anti-holes are considered to be associated with depressions in the plasma pressure. In this model, the observed changes in the magnetic field intensity and direction are due to diamagnetic currents that are carried by ions which drift in a sheath as the result of gradients in the magnetic field and in the plasma pressure within the sheath. The thickness of the sheaths considered is approximately a few ion Larmor radii. An electric field is normal to the magnetic field in the sheath. Solutions of Vlasov's equation and Maxwell's equations are presented which account for several types of magnetic holes, including null-sheets, that were observed.

Selective resputtering‐induced anisotropy in amorphous films

Abstract: A model will be presented to explain the bias voltage dependence of magnetic anisotropy in sputtered amorphous films of gadolinium cobalt alloys. It has been found that the magnetic anisotropy increases with bias voltage up to a critical bias voltage (∠150 V depending on the sputter gas) then decreases precipitously at higher voltages. The model used to explain this behavior is based on selective resputtering of atoms in different surface sites causing an anisotropic distribution of Gd and Co atoms with respect to the growth direction. For example, a Gd adatom in contact with three surface atom neighbors can have 0, 1, 2, or 3 cobalt neighbors. The sputtering threshold for each type of Gd adatom will be different so those with a low threshold will be selectively removed giving a structural anisotropy. This anisotropic atomic distributions can cause different kinds of anisotropy with respect to the growth direction depending on the type of amorphous film. For example, both easy‐axis (GdCo) and hard‐axis (G...