Showing papers on "Magnetization published in 1983"

Excitation phenomena and line intensities in high-resolution NMR powder spectra of half-integer quadrupolar nuclei

Abstract: A theoretical analysis of the general excitation phenomena of half-integer $Ig\frac{1}{2}$ nuclear spins in a strong radio-frequency field shows that coherence between the affected spin states leads to complicated time behavior of the total nuclear magnetization. Very short excitation pulses lead to quantitatively useful high-resolution NMR spectra of quadrupolar nuclei in powder samples spun at the magic angle, while selective spin-state time development during longer rf pulses permits the use of two-dimensional Fourier-transform NMR techniques with simultaneous measurement of the isotropic chemical shifts and the corresponding quadrupole interaction parameters.

Observation of macroscopic collective behavior and new texture in magnetically-doped liquid crystals

Abstract: The observation of macroscopic collective behavior in a nematic liquid crystal doped with magnetic grains is reported. This behavior is manifested as a uniform molecular orientational distortion of the entire matrix upon the application of external magnetic fields as low as <1 G. The dependence of the distortion on field strength, dopant concentration, and sample thickness is presented, and a theory which quantitatively accounts for the results is given. At high field intensities, the doped nematic exhibits a heretofore unreported texture of complex cellular topology.

Variations in magnetization intensity and low-temperature titanomagnetite oxidation of ocean floor basalts

Abstract: The remanent magnetization of the oceanic crust exhibits a systematic long-term variation which correlates with the amplitudes of marine magnetic anomalies. After a sharp initial decrease of natural remanent magnetization intensity, a minimum is reached at ∼20Myr, followed by a gradual increase up to ages of 120 Myr. The progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour.

Generalized Slater-Pauling curve for transition-metal magnets

Abstract: An informative new way of plotting magnetization data for transition-metal alloys is described. The plot is used to distinguish a variety of generic alloy types including: 1) a large class for which the magnetization data fall near a common straight line, 2) magnetically strong and weak amorphous alloys, and 3) Co alloys with metalloids, such as Si and P, exhibit regular, easily interpreted behavior. Self-consistent spin-polarized energy-band calculations are used to explain the trends revealed by the plotting construction.

Magnetostatic spin-wave modes of a ferromagnetic multilayer

Abstract: The spin-wave spectrum of a ferromagnetic multilayer is derived in the dipolar-magnetostatic limit. Restrictions are imposed with respect to in-plane magnetization ${M}_{s}$ and propagation of the spin waves in plane and perpendicular to ${M}_{s}$. A formula is derived from which the spin-wave branches of a multilayer with a finite number $N$ of magnetic layers can easily be calculated. In the limit of infinitely large $N$ the solutions form a band whose edges and density of states are given by simple expressions. The existence of an additional singular solution depends on the ratio of the thicknesses of magnetic and nonmagnetic layers. We show that these modes can be classified by an integer number $j$, where $j=1,\dots{}$, $N\ensuremath{-}1$, and derive a relation between $j$ and the amplitude distribution. As an experimental test of these theoretical results we have also observed the multilayer modes by means of light scattering, and we find good agreement between theory and experiment.

Magnetic properties and structure of cobalt-platinum thin films

Abstract: The magnetic properties of RF sputtered Co-Pt alloy thin films were studied as a function of Pt content from 0 to 80 at%. At room temperature, ferromagnetic films were obtained in the range 0-32 and 40-80 at% Pt. For Pt contents between 32 and 40 at%, discontinuities in the magnetization, magnetostriction, and coercivity versus Pt content were observed; however no discontinuity was observed in the resistivity. The structure of films containing about 25 at% Pt is a mixture of hexagonal and face-centered cubic (FCC) phases. At this composition the magnetostriction is small, but coercivities are large-700 to 2000 Oe-and dependent upon film thickness. The coercivities of these films do not change with heat treatment up to temperatures of 600°C but decrease markedly at 700°C. The properties of equiatomic Co-Pt film s are similar to those of bulk alloys. In particular the large coercivity observed in films after heal treatment at 500° to 700°C is due to the formation of an ordered tetragonal phase within the face-centered cubic matrix. The structure of films of about 75 at% Pt is initially a disordered face-centered cubic phase and with heat treatment beginning at 500°C an ordered face-centered cubic phase forms. The coercivity of these films (∼200 Oe) does not change with annealing at 500°C. It decreases slightly upon further annealing at 600°C to 700°C. Electron microscope observations were used to correlate the magnetic properties with film structure.

NMR Study on the Spin Structure of CeB 6

Abstract: 11 B NMR measurement has been made on a typical dense Kondo system CeB 6 mainly in the intermediate temperature phase II by using a single crystal. From the analysis of the angular dependence of the NMR spectra, we propose a quite unusual spin structure in phase II in which the external field induces not only the uniform magnetization but the antiferromagnetic moment which has a triple- q structure.

Magnetization reversal in cobalt-phosphorus films

Abstract: A brief review is given of preparation, structure, and magnetization reversal in Co–P thin films for digital recording. From this, a magnetization reversal theory is developed which correctly predicts a number of observed facts about Co–P. Among these are &mA0.1 (conventional theory predicts 0.64 and 0.51, respectively), avalanche and cluster reversal mechanisms, transverse ripple, and sawtooth structure in recorded bits. Limitations to high bit and track density recording are discussed. It is suggested that lower Mr/Ms remanent squareness values may alleviate these density limiting mechanisms.

The dependence of magnetic domain structure upon magnetization state with emphasis upon nucleation as a mechanism for pseudo-single-domain behavior

Abstract: The stable carriers of the paleomagnetic record in rocks are commonly fine pseudo-single-domain (PSD) particles between about 1 and 20 μm in diameter. However, the underlying mechanism which determines PSD behavior has previously remained in debate. To study this problem, magnetic domain patterns have been observed with the Bitter method on particles of natural pyrrhotite and intermediate titanomagnetite in various magnetization states, including natural remanent magnetization (NRM), after alternating field demagnetization of the NRM and during high-field hysteresis. From these observations it is evident that two primary mechanisms govern high-field hysteresis behavior: (1) ‘bulk’ pinning of fully developed domain walls by intervening energy barriers, and (2) nucleation, the creation and expansion of walls into the particle volume to produce a fully developed domain structure. These observations demonstrate that nucleation can explain several important aspects of PSD behavior. Typically, multidomain particles are entirely governed by bulk wall pinning, since they readily develop walls whose dimensions are comparable to the particle diameter prior to or upon removal of a strong field. For these grains the average domain wall spacing is relatively insensitive to the particular magnetization state, being approximately proportional to r (r being the grain diameter), in accordance with equilibrium calculations. In contrast, particles that are at least partially controlled by nucleation fail to develop a discernable domain wall structure at saturation remanence. However, these same grains can easily accommodate walls in other magnetization states. It is found that nucleation can involve two physically distinct but sometimes experimentally inseparable processes: (1) initial creation of walls at surface imperfections, and (2) unpinning of minute wall fragments from strong potential energy ‘traps’ near the grain surface. When either process fails, a grain remains locked in a metastable, single-domain-like state at saturation remanence and requires application of a reverse field Hn, called the nucleation field, before walls appear. In pyrrhotite, the probability for nucleation failure rises with decreasing grain size and has been determined through observation to be given by f(w = 0) = 1.2 exp (−0.46 r)r being in micrometers). Such single-domain-like particles can thus contribute substantially to the saturation remanence in the PSD range. The nucleation field determined experimentally for pyrrhotite is given by Hn ≃ 1200/ r Oe. In many fine pyrrhotites, Hn is sufficiently strong to reverse completely the magnetization through a single Barkhausen jump of the nucleated wall. These observations thereby demonstrate that nucleation becomes increasingly more dominant as the particles become smaller, a manifestation of the random distribution of active nucleation sites. Nucleation may therefore account for much of the magnitude and grain size dependence of hysteresis parameters in the PSD range as well as resulting in a gradual transition between multidomain and PSD behavior. Fine particles completely controlled by nucleation during hysteresis behave in a strikingly parallel manner to classical single domains and are therefore quite appropriately described as being pseudo-single-domain.

Viscous magnetization of 0.04–100 μm magnetites

Abstract: Summary Acquisition and decay of small-field viscous magnetization have been measured as a function of temperature (9-500°C) for four synthetic magnetite dispersions ranging in mean particle size from 0.04 to 0.22 pand at room temperature only for an additional six samples (2-100 μm). Room-temperature viscous magnetization is pronounced in the single-domain range, slight between 0.1 and 5 μm, and again substantial in the intermediate multidomain range (10-15 μm). In all experiments, the acquisition rate exceeded the decay rate, resulting in residual undecayed viscous remanent magnetization (VRM) after a zero-field decay time equal to the time of exposure to a field. Viscous magnetization is enhanced at elevated temperatures but not in direct proportion to absolute temperature. Instead, there appears to be a low-temperature (< 20°C) threshold below which viscous effects are negligible and an approach to saturation at high temperature. VRM produced isothermally at a given temperature is significantly more difficult to erase by heating to a higher temperature than most thermal activation theories predict. Furthermore, even in quite brief experiments, the viscosity coefficient S= |σJ|σlog t| increased with time, the increase being more pronounced at high temperature. The implications of these observations are that viscous overprinting of natural remanent magnetization in magnetite-bearing rocks, particularly during burial or intrusive reheating, may be more extensive and more difficult to erase than previously believed.

Critical measurements in the spin glass CuMn

Abstract: We present experimental data on the magnetization M(H) of a CuMn 1 at. % in the range Tc≤ T ≤ 4 Tc, 0 < H < 7 teslas. We took special precautions in order to eliminate systematic errors and improve the reliability of the data. It was then possible to study the temperature dependence of the first coefficients A 1, A3, A5 in the expansion of the low field magnetization data in terms of odd powers of [FORMULA], in the range 1.1 Tc ≤ T ≤ 4 Tc. From the divergence of A3 and A5 (which vary over 3 and 6 orders of magnitude respectively in this range), we derive two exponents (γ = 3.25, β = 0.75 ± 0.25) which allow the rescaling of all our data points onto a universal function. The success of the scaling argument is strong evidence in favour of the existence of a phase transition in three dimensions for RKKY spin glasses.

Theory of Agglomeration of Ferromagnetic Particles in Magnetic Fluids

Abstract: For some magnetic fluids, large agglomerates of magnetic particles are formed when a weak magnetic field is applied. In this paper we regard this phenomenon as a kind of gas-liquid phase transition induced by the magnetic field, and study the condition for the formation of the agglomerates by a simple mean field theory. It is found that if the particle is small enough, no phase separation occurs even if the magnetic field is infinitely large, while if the particle is large, the liquid phase appears at certain magnetic field. The van der Waals attraction between the particles enhances the agglomeration.

The evolution of magnetic order in CrFe alloys. II. Onset of ferromagnetism

Abstract: For pt.I see ibid., vol.13, p.441 (1983). The magnetic phase diagram for CrFe alloys was determined by neutron scattering and low-field magnetisation. Two critical concentrations were found as the state of magnetic order evolved from itinerant antiferromagnetism for Fe concentrations less than cA=16.0+or-0.5% to ferromagnetism for Fe concentrations greater than cF=19.0+or-0.5%. Spin-glass behaviour was observed between the two critical concentrations. The onset of ferromagnetism was studied by neutron small-angle scattering. Analysis of the concentration and temperature dependence of the magnetic correlation range below cF suggested that a geometrical element is involved in the development of ferromagnetic long-range order; these results were in semi-quantitative agreement with models for the percolation multi-critical point. The bulk magnetic properties of alloys between the two critical concentrations were consistent with a simple fine magnetic particle model of percolation clusters of Fe moments fluctuating against barriers arising from magnetostatic shape anisotropy.

Effect of thermodynamic fluctuations of magnetization on the bound magnetic polaron in dilute magnetic semiconductors

Abstract: We consider the magnetic field and temperature dependences of the spin splitting for an electron localized on a shallow donor, taking into account the $s\ensuremath{-}d$ coupling with the surrounding magnetic ions. We calculate the probability distribution of the spin splitting $\ensuremath{\Delta}$ with the thermodynamic fluctuations of magnetization included. They are responsible for persistence of the spin splitting at ambient temperature in the absence of the field. The probability distribution of the spin splitting is the main factor determining the energy and the shape of the optical transition line between the spin-split states. We compare our results with recent experimental results on spin-flip Raman scattering in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}$. Our theory provides a satisfactory quantitative description of the donor electron accompanied by a nonuniform cloud of magnetization in that material.

The physical basis

Abstract: A magnetic field is produced by the movement of an electrical charge. So, at the lowest level, the movement of an electron results in the creation of a magnetic field. Electrons will normally spin about their axes and also orbit their nucleus and therefore have two types of motion that can produce magnetic fields. All substances can thus be regarded as being magnetic at an atomic level and can be classified into two types. In diamagnetic substances the electron shells are full and the precession of electron orbits when placed in a magnetic field results in the creation of a magnetic field in the opposite direction to the applied field. The magnetization acquired per unit field applied, the magnetic susceptibility, is small, less than 10−5 SI. In substances in which the electron shells are incomplete, paramagnetic materials, each atom has a magnetic moment due to the uncompensated electron spins. When placed in a magnetic field, the electron orbits precess but the magnetic moment is aligned in the same direction as the applied field and is generally stronger than that of diamagnetic substances, having susceptibilities of the order of 10−3 to 10−5 SI.

Magnetic Phase Transitions in Itinerant Electron Magnets Hf 1− x Ta x Fe 2

Abstract: Magnetic phase transitions in itinerant electron magnets Hf 1- x Ta x Fe 2 have been studied from measurements of Mossbauer effect, magnetization and X-ray diffraction The first order magnetic phase transition from ferromagnetic to antiferromagnetic state has been observed with increasing temperature in the Ta concentration range 01< x <03 It is concluded that the magnetic phase transitions in Hf 1- x Ta x Fe 2 can be interpreted in terms of the recent theoretical result by Moriya and Usami on magnetic phase transitions in itinerant electron systems

Alternating linear-chain antiferromagnetism in copper nitrate Cu ( N O 3 ) 2 .2.5 H 2 O

Abstract: Current interest in the behavior of Heisenberg alternating antiferromagnetic quantum chains has been stimulated by the discovery of an unusual class of magnetoelastic spin-Peierls systems. Copper nitrate, Cu${(\mathrm{N}{\mathrm{O}}_{3})}_{2}$.2.5${\mathrm{H}}_{2}$O, does not display a spin-Peierls transition, but its dominant magnetic behavior is that of a strongly alternating antiferromagnetic chain with temperature-independent alternation. A remarkable, simultaneous fit is demonstrated between theoretical studies and a wide variety of zero- (low-) field experimental measurements, including susceptibility, magnetization, and specific heat. The fitting parameters are $\ensuremath{\alpha}(\mathrm{degree}\mathrm{of}\mathrm{alternation})=0.27$, $\frac{{J}_{1}}{k}=2.58$ K, ${g}_{b}=2.31$, and ${g}_{\ensuremath{\perp}}=2.11$. Slight systematic discrepancies are attributed to weak interchain coupling. Theoretical studies also predict a rich variety of behavior in high fields, particularly in the region involving the lower and upper critical fields, ${H}_{{c}_{1}}=28$ kOe and ${H}_{{c}_{2}}=44$ kOe. Experimental specific-heat measurements at $H=28.2 \mathrm{and} 35.7$ kOe show quantitative agreement with theory in this interesting parameter region. The fitting parameters are the same as for zero field and, again, small discrepancies between theory and experiment may be attributed to interchain coupling. The exceptional magnetic characterization of copper nitrate suggests its use for further experimental study in the vicinity of the high-field ordering region.

Photomagnetic recording medium

Abstract: PURPOSE:To obtain a photomagnetic recording medium having large coercive force and a large angle of Kerr rotation and requiring a small quantity of recording energy by forming a thin film of an amorphous alloy contg a heavy rare earth element and a light rare earth element on a substrate so that the film has an axis of easy magnetization in a direction perpendicular to the surface of a magnetic film CONSTITUTION:A thin film of an amorphous alloy represented by formula I or II(where R is a light rare earth element such as La, Ce, Pr, Nd, Pm, Sm or Eu, each of A and B is a heavy rare earth element such as Gd, Tb or Dy, A and B are different from each other, 015<=x<=035, 000

Paramagnetic impurity effects in NMR determinations of hydrogen diffusion and electronic structure in metal hydrides. Gd 3 + in Y H 2 and La H 2.25

Abstract: Measurements are reported of the temperature dependence of the proton spin-lattice and spin-spin relaxation times ${T}_{1}$ and ${T}_{2}$ in yttrium and lanthanum dihydrides containing controlled levels of gadolinium as low as 50 ppm. The results demonstrate unambiguously that paramagnetic ions in concentrations so low as to have heretofore been regarded as insignificant have marked effects on the magnitude, frequency dependence, and temperature dependence of ${T}_{1}$ and to a lesser extent on ${T}_{2}$, and on the electronic structure and hydrogen diffusion parameters derived therefrom. The ${\mathrm{Gd}}^{3+}$ ion contributes an additional spin-lattice relaxation rate ${T}_{1p}$, which in these hydrides arises entirely from the dipolar coupling between impurity and proton moments. Proton magnetization is transported to the relaxation centers by spin diffusion at low temperatures and by hydrogen-atom diffusion at intermediate and high temperatures. The rate ${R}_{1p}$ is directly proportional to Gd-ion concentration at both low and high temperatures, but in the atom diffusion regime ${R}_{1p}$ is 20-25 times greater than for spin diffusion. The impurity-induced relaxation is shown to have profound effects on the apparent nuclear-nuclear dipolar relaxation rate ${R}_{1d}$ associated with hydrogen diffusion. At impurity levels as low as 10 ppm Gd, a secondary minimum appears in the temperature dependence of ${T}_{1}$ which may be readily misinterpreted in terms of a second motional process with lower activation energy. Even lower impurity levels yield a characteristic "slope-change" effect, which may be construed as indicating a change in the activation energy for hydrogen diffusion. At low temperatures ${R}_{1p}$ interferes with the determination of the conduction-electron contribution ${R}_{1e}$ and the Korringa product ${T}_{1e}T$. Separation of ${R}_{1e}$ and ${R}_{1p}$ is complicated by the fact the ${R}_{1p}$ is not temperature independent as has typically been assumed. Methods of achieving this separation are discussed, and it is shown experimentally that this difficulty can be circumvented by replacing the major part of the hydrogen with deuterium, thereby inhibiting spin diffusion. Measurement of ${T}_{1}$ as a function of resonance frequency and of ${T}_{2}$ can also be of value in separating the various sources of relaxation.

Thermodynamic Critical Fluctuation Theory

Abstract: The conventional thermodynamic fluctuation theory does not include local correlations and, hence, fails at volumes of the order of the correlation volume ${\ensuremath{\xi}}^{d}$. In this paper, an attempt is made to include local correlations in a thermodynamic theory by considering fluctuations in a sequence of concentric systems of decreasing volume. Tests are made on fluctuations in the magnetization within the one-dimensional Ising model. The results are a clear improvement over those of the conventional theory.

Magnetic separation using chelated magnetic ions

Abstract: Disclosed is a new method for magnetic separation from a carrier fluid of biological cells or other organic or inorganic particles having a negative surface charge. A paramagnetic salt is mixed with an aqueous solution of a chelating agent, for example EDTA. The cells or particles to be separated are mixed with this paramagnetic carrier solution, and the resulting mixture is exposed to a high gradient magnetic field. The cells or particles attract the chelated paramagnetic cations preferentially to the carrier solution such that the particles develop a magnetic susceptibility greater than that of the carrier liquid. The method achieves highly repeatable separations at lower magnetization fields than existing high gradient magnetic separation methods. Feasibility has been shown for erbium chloride and dysprosium chloride, with whole blood as the sample. In this instance, single-pass magnetic separation efficiencies significantly greater than those attainable with paramagnetic erythrocytic hemoglobin are readily available.