Stable Magnetic Configurations Research Articles

Domain walls and macroscopic spin-flip-like metamagnetism inGdxCo1−x∕GdyCo1−yexchange-coupled double layers

Amorphous ferrimagnetic ${\mathrm{Gd}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}∕{\mathrm{Gd}}_{y}{\mathrm{Co}}_{1\ensuremath{-}y}$ exchange coupled double layers (ECDL) have been prepared in order to investigate the magnetization reversal processes and the presence of stable magnetic configurations when the chemical compositions of both layers are very similar. The ECDL samples have been grown by co-sputtering from independent pure Co and Gd targets, being $100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ the thickness of each layer. The measurements have been done by the magneto-optical Kerr effect at both sides of the ECDL in the temperature interval $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}--450\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the compositions of the two layers are $x=0.22$ and $y=0.24$. A rich variety of behaviors has been found, being the most relevant of those in the temperature range between the compensation temperatures $({T}_{\mathit{comp}})$ of both layers. Some of these observed behaviors include magnetization reversal by creation/annihilation of a Bloch wall across the ECDL thickness, and a macroscopic spin-flip-like metamagnetic state where the magnetic moments form a double antiferromagnetic state with the presence of an extended Neel wall when the magnetizations of both layers are similar. A temperature phase diagram for the transition fields between different magnetic states has also been deduced, being in very good agreement with the magnetic behavior observed in the whole temperature range.

A fossil origin for the magnetic field in A stars and white dwarfs.

Some main-sequence stars of spectral type A are observed to have a strong (0.03-3 tesla), static, large-scale magnetic field, of a chiefly dipolar shape: they are known as 'Ap stars', such as Alioth, the fifth star in the Big Dipper. Following the discovery of these fields, it was proposed that they are remnants of the star's formation, a 'fossil' field. An alternative suggestion is that they could be generated by a dynamo process in the star's convective core. The dynamo hypothesis, however, has difficulty explaining high field strengths and the observed lack of a correlation with rotation. The weakness of the fossil-field theory has been the absence of field configurations stable enough to survive in a star over its lifetime. Here we report numerical simulations that show that stable magnetic field configurations, with properties agreeing with those observed, can develop through evolution from arbitrary, unstable initial fields. The results are applicable equally to Ap stars, magnetic white dwarfs and some highly magnetized neutron stars known as magnetars. This establishes fossil fields as the natural, unifying explanation for the magnetism of all these stars.

Noncollinear magnetic order in quasicrystals.

Based on Monte Carlo simulations, the stable magnetization configurations of an antiferromagnet on a quasiperiodic tiling are derived theoretically. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that the superposition of geometric frustration with the quasiperiodic ordering leads to a three-dimensional noncollinear antiferromagnetic spin structure. The structure can be divided into several ordered interpenetrating magnetic supertilings of different energy and characteristic wave vector. The number and the symmetry of subtilings depend on the quasiperiodic ordering of atoms.

Ab initiostudy of the magnetic structure of fccFegrown on aCu(001)substrate

First-principle total energy studies of $\mathrm{Fe}$ films on a $\mathrm{Cu}(001)$ substrate are presented. The films are modeled by symmetric $6\mathrm{Fe}∕8\mathrm{Cu}∕6\mathrm{Fe}$ slabs. Both collinear and noncollinear magnetic configurations are considered. The effect of the surface relaxation on the total energy of the system is discussed. It was found that the energy difference between the noncollinear configurations and the favored collinear double-layered antiferromagnetic state is reduced by contraction of the $\mathrm{Fe}$ sublayer but still remains positive. The possibility of stable noncollinear magnetic configurations in the system is discussed.

Magnetic structure of FeMn thin films on (001) FCC substrate

Abstract LEED experiments by Offi et al. (Phys. Rev. B 66 (2002) 064419) have recently revealed that the Fe50Mn50 alloy thin films keeps the FCC structure when growing on Cu (0 0 1) and on Co / Cu (0 0 1) . Stability of magnetic configurations of Fe50Mn50 alloy monolayer film is investigated using ab-initio calculations within density functional theory with gradient corrections. Fe induced magnetic moment of 2.85 μ B with Mn magnetic moment of 3.61 μ B aligned parallel with the ferromagnetic moment of the Co layer have been found as groundstate configuration with the Perdew–Wang functional in agreement with X-ray magnetic circular dichroism results.

Transitions of magnetic configuration at the interface of exchange-coupled bilayers: TbFe/GdFe as a model system

The magnetization profile that forms in the soft part of an exchange-coupled hard/soft bilayer submitted to a rotating in-plane magnetic field has been studied both theoretically and experimentally. The evolution of the stable magnetic configurations as a function of the field orientation has been determined from micromagnetic calculations, for varying strength of the rotating field. The anisotropic magnetoresistance associated with these configurations has been deduced and compared with experimental results obtained on a TbFe/GdFe ferrimag-netic bilayer. A good agreement is obtained between calculated and measured data, without any parameter adjustment. As the applied field is rotated, a twisted magnetization distribution of Bloch type develops in the soft GdFe layer. The simulations show the existence of several possible stable magnetic profiles. These differ either by the amount of magnetization twist only or by the amount of twist and the chirality of the profile. Transport measurements allow one to determine which configuration is actually adopted by the system and in what circumstances the system switches from one configuration to another. Two types of transition are identified. The first one is related to the existence of a hard axis of magnetization in the GdFe layer. The second one corresponds to a change of chirality of the wall.

Decagonal quasiferromagnetic microstructure on the penrose tiling.

The stable magnetization configurations of a ferromagnet on a quasiperiodic tiling have been derived theoretically. The magnetization configuration is investigated as a function of the ratio of the exchange to the dipolar energy. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that for a weak exchange interaction the new structure, the quasiferromagnetic decagonal configuration, corresponds to the minimum of the free energy. The decagonal state represents a new class of frustrated systems where the degenerated ground state is aperiodic and consists of two parts: ordered decagon rings and disordered spin-glass-like phase inside the decagons.

Magnetic domain wall trapping by in-plane surface roughness modulation

Using the influence of surface roughness on the coercivity of thin magnetic films, we have produced an artificially modulated magnetic domain configuration in a chemically homogeneous polycrystalline thin magnetic film. This is achieved by evaporating a thin Fe film on top of a smooth substrate that was previously covered with an array of considerably rougher Ag lines. Magneto-optical Kerr effect measurements revealed the presence of two distinct switching fields in the hysteresis loops. In the intermediate state, a stable magnetic domain configuration of antiparallel aligned domains is achieved. Magnetic force microscopy images indicate that interacting cross-tie domain walls are separating the magnetic domains.

Spontaneous and field-induced magnetic configurations in a Fe/Si/Fe trilayer withferromagnetic interlayer exchange

Magnetization reversal of a Fe/Si/Fe trilayer having a ferromagnetic bilinear andbiquadratic exchange is investigated at 10 and 300 K. The experiments wereperformed using the magneto-optical Kerr effect technique and a SQUIDmagnetometer. The type of stable magnetic configuration is determined for bothtemperatures of the study. The hysteresis loops contain jumps in magnetization,which could be associated with the overcoming of the energy barriers related tothe uniaxial anisotropy hard axes in the two iron layers. The analysis of theexperimental results was performed in the framework of a model in which theferromagnetic bilinear and biquadratic exchange between two magnetic layersseparated by a nonmagnetic spacer and the cubic and uniaxial anisotropywere taken into account. Good qualitative agreement is obtained betweenthe experimental and calculated field dependences of the magnetization.

Magnetization reversal in nanostructured permalloy rings

Three different stable magnetization configurations of soft-type nanostructured permalloy rings are investigated in detail by micromagnetic simulation. During the reversal process, the abrupt change of the hysteresis loop corresponds to the evolution of these states. Beside the quasiuniform and circular states, we find a new pair-vortices state. We compare the magnetic pole density distributions transformed from the simulation patterns with the magnetic force microscopic (MFM) images and find very good agreement between them.

Curvature effects on the dynamics of tearing modes in tokamaks

The curvature effects on the dynamics of magnetic island evolution in tokamaks are investigated both theoretically and numerically. By taking into account perpendicular and parallel heat diffusion, a new dispersion relation is derived for tearing modes that match the linear and nonlinear results. This evolution equation allows a quantitative description over the whole range of island sizes. It predicts a nonlinear instability, i.e., growing magnetic islands in linearly stable magnetic configurations. All these predictions are in excellent agreement with full tridimensional linear and nonlinear magnetohydrodynamic (MHD) computations with the latest version of XTOR [K. Lerbinger and J. F. Luciani, J. Comput. Phys. 97, 444 (1991)]. These results have important consequences on the onset of neoclassical tearing modes because they predict a resistive MHD threshold.

Possibility of Stable Symmetrized Magnetic Configurations with Lower Ellipticity in GAMMA 10

The standard GAMMA 10 configuration is characterized by high ellipticity of the anchor fans, where the thickness of plasma is 2.5cm only. Thin anchor fans are probable places of fast radial transport. Possible way to check this assumption is to diminish the ellipticity. This task is complicated by several requirements to magnetic configuration: symmetrization of magnetic field, MHD stability, ensuring of proper conditions for ICRH and ECRH.In the paper the calculations of GAMMA 10 magnetic field and MHD stability are presented. As a result of calculations the range of currents suitable for stable symmetrized configurations with low ellipticity was determined.

Single crystal study of the one dimensional Ca Co O compound: five stable configurations for the Ising triangular lattice

Single crystals of the one-dimensional phase Ca3Co2O6 of several mm length have been grown. The magnetic study of such a crystal confirms the previous observations on polycrystalline samples: it consists of a triangular lattice of ferromagnetic [Co2O6] chains (\(\) K) antiferromagnetically coupled (\(\) K). The dynamic of these chains array, probed by AC susceptibility, is very slow as shown from the large shift of the freezing temperature from 12 K to 16.5 K as the excitation frequency increases by three orders of magnitude (100 to 103 Hz). The origin of this effect is believed to be the result of different arrangements with close energies for the chain ferromagnetic moments on the triangular lattice. Five stable magnetic configurations have been evidenced by the magnetization as a function of applied field curves registered at 2 K. Their relative magnetizations correspond to m=1/4, 1/2, 1, 2, 3 where m=3 represents the ferromagnetic ordering of three chains on the same triangle, each chain having a m=1 magnetization. A magnetic phase diagram is finally proposed.

Multiple reorientation transition of the magnetization of free surfaces of Fe on Ag(100)

The magnetic properties of $\mathrm{Ag}(100){/\mathrm{F}\mathrm{e}}_{n},$ $nl~16,$ are investigated using the fully relativistic spin-polarized screened Korringa-Kohn-Rostocker method. It is found that (a) ferromagnetic interlayer coupling is the most stable magnetic configuration of free surfaces of Fe on Ag(100) and (b) that in the vicinity of $n=5$ a multiple reorientation transition of the magnetization occurs. The results obtained are in excellent agreement with available experimental data and give an interesting explanation for the occurrence of temperature-dependent effects seen experimentally.

The Magnetic Structuring of the Solar Corona

The magnetic structuring of the solar corona can be seen during a solar eclipse. The X-ray corona has been photographed during recent satelite missions and balloon flights and it is made up of a few main building blocks including (i) coronal loops and (ii) X-ray bright points. These appear to be connected with interacting loops and magnetic cancellation. Much cooler structures, called, quiescent prominences, are also visible in the corona and these are supported against gravity in a stable magnetic configuration.

Saturation fields in Co-Fe/Cu multilayers with giant magnetoresistance: In-plane uniaxial magnetic anisotropy effects

Magnetization processes are investigated by calculation and experiments for Co-Fe/Cu multilayers with giant magnetoresistance, which are prepared on MgO(110) substrates using ion beam sputtering and have in-plane uniaxial magnetic anisotropy (Ku). The experiments show the metamagnetic transition from antiferromagnetic spin structure to ferromagnetic one for an easy axis external field. This is supported by calculation of the magnetic phase diagram for stable magnetization configurations, which shows the metamagnetic transition to occur for (J12/d)/Ku≤1, where J12 is antiferromagnetic layer interaction and d is magnetic layer thickness. The saturation fields from antiferromagnetic to ferromagnetic spin structures are substantially decreased by induction of the in-plane uniaxial anisotropy. The relation Hsh − 2Hse = 2Ku/Ms, found for the saturation fields by the calculation, is confirmed experimentally for the multilayers, where Ms is the saturation magnetization, and Hse and Hsh are the saturation fields for the external field parallel to the easy and hard axes, respectively.

The position angle of Jupiter's linearly polarized synchrotron emission - observations extending over 16 years

Parkes, Owens Valley and Goldstone measurements are presented, showing the variation with central meridian longitude of the position angle of Jupiter's linearly polarized synchrotron emission at wavelengths of 21, 13, 11 and 6 cm; the observations span a total time interval greater than one Jovian orbital period. The form of the position angle versus longitude curve shows a slight dependence on wavelength and epoch. The epoch dependence appears to reflect Jupiter's changing aspect with respect to the earth. The observations yield a Jovian rotation period agreeing to within about 0.02 s with that derived from the decametric burst measurements, implying a highly stable inner magnetic field configuration.

Frequency separation in structure of solar continuum radio bursts

A plot of frequency separation in fine structure in solar continuum radio bursts against emission frequency indicates that the frequency structure cannot represent local proton plasma frequency modulation. However, the observations are consistent with the interpretation of the frequency structure as harmonics of the local electron cyclotron frequency and lead to reasonable estimates of the ratio between magnetic and kinetic pressures in stable coronal magnetic field configurations producing continuum radio sources.

Analysis of Magnetic Thin Film Structure for High Permeability

An analysis is given which predicts that initial permeabilities of the order of a million could be obtained from typical permalloy thin films if these films were in the appropriate type of structure. A three-layered structure, where a permalloy layer is sandwiched between and exchange-coupled to a pair of highly anisotropic magnetic layers, can be arranged so that having the outer layers magnetized in opposite directions is a stable magnetic configuration. Because of the exchange coupling, this configuration creates a domain wall in the plane of the permalloy layer. The methods of micromagnetics are applied in this paper to obtain a mathematical analysis of such domain walls, giving the functional dependences of the wall shape and position on the external field, layer thicknesses and magnetic parameters. Numerical results show that typical permalloy layers of several microns in thickness could exhibit initial permeabilities of a million in external fields of the order of millioersteds. Inductor applications of this structure are briefly discussed, where it is concluded that such inductors would have a maximum operating frequency of 105 to 106 Hz.