Showing papers on "Single domain published in 1992"

Rock magnetism and the interpretation of anisotropy of magnetic susceptibility

Abstract: The conventional rules, derived from empirical and theoretical considerations, for the interpretation of anisotropy of magnetic susceptibility (AMS) in terms of microstructure and deformation are subject to numerous exceptions as a result of particular rock magnetic effects. Unusual relationships between structural and magnetic axes (so-called inverse or intermediate magnetic fabrics) can occur because of the presence of certain magnetic minerals, either single domain magnetite or various paramagnetic minerals. When more than one mineral is responsible for magnetic susceptibility, various problems appear, in particular the impossibility of using anisotropy to make quantitative inferences on the intensity of the preferred orientation and consequently on strain. In ferromagnetic grains, AMS may also be influenced by the magnetic memory of the grains (including natural remanence). The effect of alternating field or thermal demagnetization on AMS is briefly discussed. As discussed in this article, various rock magnetic techniques, specific to AMS interpretation, have to be developed for a better assessment of the geological significance of AMS data. These techniques mainly rely on measurements of susceptibility versus magnetic field and temperature, together with anisotropy of remanence. 93 refs., 11 figs., 1 tab.

Energy barriers for thermal reversal of interacting single domain particles

Abstract: An analytic solution to the energy barriers of two interacting single domain particles is presented. Identical volumes and uniaxial anisotropies are assumed with easy axes parallel to an external magnetic field. The locations and heights of the system energy barriers are analytically determined when the line joining the two particles is either parallel or perpendicular to the easy axes and the external field. The lowest energy barriers are saddle points of the energy surface and correspond to reversal modes under thermal agitation. When dipole coupling is not strong, the mode of thermal switching is asymmetric fanning or asymmetric coherent rotation, depending upon the bond angle, rather than symmetric fanning or coherent rotation that occurs at the nucleation field. An effective volume that describes the cooperative effect is calculated and good agreement with the numerical results using the Fokker–Planck equation is obtained. The energy barriers can be used to calculate the superparamagnetic relaxation ...

A method to study alterations of magnetic minerals during thermal demagnetization applied to a fine-grained marine marl (Trubi formation, Sicily)

Abstract: SUMMARY The method presented is based on monitoring changes in the coercivity spectrum of an isothermal remanent magnetization (IRM) during stepwise thermal demagnetization. The procedure allows one to determine alteration temperatures and the coercivities and blocking temperatures of the magnetic minerals involved in the alterations. It distinguishes between thermal decay due to unblocking of the remanence and demagnetization due to alterations of magnetic minerals. As a result, the initial magnetic mineralogy can also be determined. The method is demonstrated with samples of fine-grained marine marls from the Pliocene Trubi formation in Sicily (Italy). Single domain (SD) magnetite is the dominant remanence carrier in this sediment. A first alteration during heating to only 145°C is accompanied by a considerable reduction of coercivities higher than 0.1 T. This alteration is thought to be due to the reduction of stress in superficially maghemitized SD magnetite grains. Between 390 and 480°C grains with magnetite-like properties are produced, most likely by oxidation of pyrite in the sediment. At about 560°C the breakdown of iron-bearing silicates also produces magnetic grains. The magnetic grains formed during both alteration processes have low blocking temperatures. Demagnetization above 510°C is not only due to unblocking of the IRM. Alteration of magnetite to haematite proves to be an important demagnetization mechanism.

Preparation of Fe nanocrystalline in SiO2 by ion implantation

Abstract: We present a novel method, by using an ion implantation and subsequent heat treatment, to prepare Fe nanocrystalline in SiO2. The formation process of Fe granules was monitored by means of conversion electron Mossbauer spectroscopy and the size of which was determined by transmission electron microscopy. The magnetic coercivity of 100 and 70 Oe have been observed at room temperature and 80 K, respectively. A decrease of the coercivity at 80 K is probably caused by the appearance of a multidomain structure in the granules due to reducing the critical radius of single domain.

Magnetic force microscopy imaging of domain-walls in magnetite

Abstract: SUMMARY A magnetic force microscope was used to obtain high-resolution images of the stray magnetic field on the polished surface of a natural crystal of magnetite. These images allow us to model the nature of the magnetic domain boundaries which control the stability of magnetic remanence in multidomain grains. Analytic models of the stray field of domain walls are used to determine the magnetic fine structure of the wall. Two possible geometries of domain wall structure are considered, first that of a Bloch wall which does not lie parallel to the direction of magnetization in the adjacent domains, and secondly that of a Bloch wall with a Neel wall ‘cap’. The experimental data favours the former wall geometry, although it is possible that bending of the wall can occur (forming a Neel wall cap) within about 20 nm of the grain surface.

Single domain high Tc La2−xSrxCuO4 films with tilted CuO2 planes

Abstract: The two fold degeneracy inherent to epitaxial growth of high Tc films on (101) perovskite substrates has been removed successfully. This is demonstrated by single domain, (103) oriented La2−xSrxCuO4 films of 0.04≤x≤0.34 on vicinal (101) SrTiO3 substrates using 90° off‐axis sputtering. (101) substrates that have the surface normal rotated about [010] by 0.5°–3.5° produced essentially single crystal films with the c‐axis direction determined by the sense of the miscut. Misoriented antidomains (103)’ have been eliminated effectively to a percentage less than 1 part in 104. The mechanism for symmetry breaking is understood on the basis of a surface step model in which the energetics promoting single domain growth is derived from nucleation and epitaxy on the (001) face found at surface steps of vicinal substrates. Furthermore, the incommensuration of the c axis interplanar spacings with the a, b, plane lattice parameters in the La2−xSrxCuO4 structure provides a natural selection of (103) domains over (103)’ d...

Aerosol synthesis of gadolinium iron garnet particles

Abstract: Particles of gadolinium iron garnet, Gd 3 Fe 5 O 12 , were produced primarily by an aerosol spray pyrolysis technique starting with solutions of gadolinium and iron nitrates. The as-prepared particles were polydisperse solid spheres. Average diameters in the range 0.05 to 0.8 μ could be obtained by variation of the initial solution concentration. Larger particles to 2 μ were created by direct, non-aerosol, pyrolysis of the solutions. Heat treatment caused sintering and particle coalescence and yielded ∼95% garnet phase. The reaction time to create the garnet phase scaled with the square of the particle diameter, the smallest particles transforming the quickest. Magnetic measurements showed bulk behavior for the saturation magnetization, but the coercivity could be varied with particle size with a maximum near the single domain size.

The effect of mechanical stress on the magnetization curves of Ni- and FeSi-single crystals at strong fields

Abstract: The spatial distribution of the total energy is used to determine the magnetic behavior of ferromagnetic single crystals. For a single ferromagnetic domain there exists only a homogeneous magnetization the direction of which must correspond to a minimum energy direction. Magnetization curves are calculated by observing the stable positions of the magnetization under the application of strong magnetic fields. These magnetization curves show significant variations under different states of mechanical stress for FeSi and Ni crystals. >

Rock mag n etism an d the interpretation of an isotropy of mag n etic suscepti bi lity

Abstract: The conventional rules, derived from empirical and theoretical considerations, for the interpretation of anisotropy of magnetic susceptibility (AMS) in terms of microstructure and deformation are subject to numerous exceptions as a result of particular rock magnetic effects. Unusual relation- ships between structural and magnetic axes (so-called inverse or intermediate magnetic fabrics) can occur because of the presence of certain magnetic minerals, either single domain magnetite or various paramagnetic minerals. When more than one mineral is responsible for magnetic susceptibility, various problems appear, in particular the impossibility of using an- isotropy to make quantitative inferences on the intensity of the preferred orientation and consequently on strain. In fer- romagnetic grains, AMS may also be influenced by the mag- netic memory of the grains (including natural remanence). The effect of alternating field or thermal demagnetization on AMS is briefly discussed. Various rock magnetic techniques, specific to AMS interpretation, have to be developed for a better assessment of the geological significance of AMS data. These techniques mainly rely on measurements of suscep- tibility versus magnetic field and temperature, together with anisotropy of remanence.

Magnetic materials with single-domain and multidomain crystallites and a method of preparation

Abstract: The disclosure relates to a method of forming magnetic materials having tunable magnetic properties and the magnetic materials formed thereby. The magnetic materials contain both single-domain and multidomain particles and have high initial permeability while maintaining coercivity and remanence in the material.

Quantum tunneling of magnetization in metallic FeC ferrofluids

Abstract: We have achieved the observance of the quantum tunneling of magnetization in a ferrofluid system constituted by small single domain particles of FeC in a nonmagnetic carrier fluid. The constancy of the coercive field below 1 K indicates that this temperature corresponds to the crossover from the thermal to the quantum regime.

Observation of Domain Structures in Tetragonal Pb(ZrxTi1-x)O3 Single Crystals by Chemical Etching Method

Abstract: Domain structures in tetragonal Pb(ZrxTi1-x)O3 (PZT) single crystals etched by HF+HCl solution were observed by means of a scanning electron microscope. The relationship between the etch patterns of pseudocubic {100} faces and spontaneous polarization vectors was examined using micro-Raman spectroscopy and electron channeling patterns. Both the a-face and the positive c-face are quickly etched, but the former results in a rough surface while the latter results in a smooth one. The negative c-face is rather slowly etched. Another type of etch pattern with a wedge-shaped section was observed; this is usually observed in PZT ceramics but has not been reported in BaTiO3. This pattern can be explained by the difference in etch rates between various {100} faces and the peculiar domain structures which consist of a single domain region with narrow or wedge-shaped 90° domains. It is assumed that such a domain structure is, although unstable under electrical stress, stable under mechanical stress.

High field magnetism of UPt2Si2 single crystals

Abstract: Magnetization measurements on Upt2Si2 single crystals have been performed in pulsed high magnetic fields up to 60 T A gradual metamagnetic behavior has been observed for fields along both tetragonal axes, accompanied with multistep-like fine structures The anisotropy of the critical fields and the whole aspect of the magnetization curves are qualitatively consistent with a mean-field model with crystalline electric field for this singlet ground state compound

Minor loops in magnetization-dependent Preisach models

Abstract: It is noted that the moving model and the product model have different variations in the height of minor loops and thus a means of differentiating between the models. Measurements show that, for particulate magnetic recording media, the moving model yields more realistic results and that previous reversible magnetization models are inadequate. Two new models for the reversible magnetization that are compatible with the moving model are analyzed. Both models give a symmetrical reversible susceptibility variation along the M-axis, which is not observed experimentally. >

Energy considerations in hysteresis models

Abstract: The energy dissipated by a magnetic material is computed for both a single particle and an assembly of particles represented by a generalized Preisach model. For an isolated particle, the two components of the energy loss are due to sudden change in the irreversible magnetization and due to the change in the ability of the magnetization to store energy because of the change in state. The former loss is equal to the product of the permeability, the applied field, and the discontinuous magnetization change. It is noted that the size of the discontinuous change in magnetization is generally not equal to the difference of the two remanent magnetizations. These energy loss components carry over into the computation for an assembly of particles. >

Magnetostriction in pulsed high magnetic fields of RE2Fe17 single crystals

Abstract: Magnetostriction on single crystals of Y2Fe17 and Er2Fe17 has been measured in pulsed high magnetic fields up to 15 T in the temperature range between 5 and 300 K. We have observed large jumps in the magnetostriction on Er2Fe17, related to first order magnetization processes induced when the magnetic field is applied along the hard magnetic c-axis.

Atomic clustering and transverse fluctuations of magnetic moments in single-domain antiferromagnetic γ-Mn 90 Cu 10

Abstract: Polarization analysis of the diffuse neutron scattering from a single-domain crystal of Mn--10 at. % Cu has been used to separate the scattering from a nonrandom distribution of Cu and Mn atoms and the magnetic scattering from perturbations of Mn magnetic moments. The technique has also allowed unambiguous discrimination between the longitudinal and transverse components of the magnetic defect scattering. Measurements were performed in extended regions of both the (100) and (11\ifmmode\bar\else\textasciimacron\fi{}0) planes at 5 K. The transverse component of the Mn magnetic defect is found to be long ranged and the Mn and Cu atoms strongly cluster on the face-centered-tetragonal lattice.

New category of magnetic materials, their production and use

Abstract: A new category of ferromagnetic materials consists of two phases, namely a hard magnetic phase and a soft magnetic phase, which are structured by interchange coupling of the spins of the two phases. Preferably the weak magnetic phase is of the cubic lattice type and the orientation of the hard magnetic phase is distributed statistically with respect to the principal direction of the soft magnetic phase. The magnetic material is characterized by high reversibility of the remanence and by an isotropic ratio of remanence to saturation magnetization greater than 0.6. A preferred embodiment of the material of the invention has the composition RExFeyBzSiuTv, where RE = rare earth and/or Y, Zr, Hf and T = Cr, Nb, Mo, V. The materials are suitable for permanent magnets, broadband microwave absorbers and magnetic recording media.

Growing thin magnetic films with a mask: Distinguishing between magnetic and instrumental asymmetries

Abstract: A simple technique is presented which allows a very small spin polarization to be distinguished from a vanishing one in spin polarized scanning electron microscopy (spin‐SEM). Ferromagnetic thin films are evaporated through a mask onto a nonmagnetic substrate, thus producing a patterned area. Spin polarization and the element specificity of the secondary electron yield are combined to determine a local ‘‘asymmetry zero’’ where the spin polarization vanishes. With this technique it is possible to distinguish between a single domain thin film and a nonmagnetic one in spin‐SEM. This is exemplified by monolayer Co/Cu (100) films.

Magnetoresistance effect reproduction head

Abstract: A magnetoresistance effect reproduction head which comprises a magnetoresistive layer having a central sensing region and an end domain control region and formed into a thin film of a ferromagnetic material, a hard magnetic layer overlapping with the end domain control region in direct contact therewith and formed into a thin film of a hard magnetic material, a magnetic field and a longitudinal magnetic bias field by ferromagnetic exchange coupling is generated for maintaining the central sensing region of the magnetoresistive layer in single domain state.