Uniform Magnetization State Research Articles

Modulated magnetic structure of a nonuniformly stressed FeBO3 single crystal

The magnetization of a nonuniformly stressed FeBO3 crystal along any of the two specific directions in the basal plane (the easy plane) at a temperature of T < 140 K in a magnetic field exceeding the threshold value H0 is found to lead to a transition of the crystal from the uniform magnetic state to the spatially modulated one. The modulated magnetic phase arising under these conditions exists in a certain temperature-dependent field range H0 ≤ H ≤ Hc and is representable in the form of a static spin wave that is linearly polarized in the easy plane of the crystal and has a wave vector k oriented at an angle of ∼30° to the magnetization axis. The field, temperature, and orientation dependences of k are investigated. A physical mechanism is proposed to explain the modulation of the magnetic order parameter of the crystal under study. The results obtained are discussed in terms of the magnetic ripple theory.

Domain Wall Creation in Nanostructures Driven by a Spin-Polarized Current

We report on current-driven magnetization reversal in nanopillars with elements having perpendicular magnetic anisotropy. Whereas only the two uniform magnetization states are available under the action of a magnetic field, we observed current-induced Bloch domain walls in pillars as small as 50 x 100 nm(2). This domain wall state can be further controlled by current to restore the uniform states. The ability to nucleate and manipulate domain walls by a current gives insight into the reversal mechanisms of small nanoelements and provides new prospects for ultrahigh density spintronic devices.

The fluxgate ring-core demagnetization field

The local demagnetization factor for the ring-core flux gate is derived analytically, based on a tangential magnetization model. The results are in good agreement with experimental data for a wide range of ring shape parameters. Approximate expressions in the limit of a narrow, thin ring are obtained, and indicate that the local demagnetization factor scales with the ratio of the cross-sectional area to the total area of the ring. Analytical modeling of the demagnetization factors for a uniform magnetization state results in an underestimate of the local cross-section averaged demagnetization factors by 50% or more.

Continuous spin-reorientation phase transition in the surface region of an inhomogeneous magnetic film

A continuous spin-reorientation transition from a uniform magnetic state with the in-plane orientation of the moments of all atomic layers to a nonuniform canted state in the surface region is considered. This transition was discovered in experiments on the divergence of magnetic susceptibility in a perpendicular magnetic field at a temperature of about 240 K, which is lower than the Curie point of gadolinium, equal to 292.5 K. These experiments were carried out on an ultrathin iron magnetic film deposited on the (0001) surface of a thin gadolinium film. It is shown that, in the vicinity of the spin-reorientation transition, the thermodynamic potential has a form characteristic of the Landau theory of second-order phase transitions. The orientation angle of the moment of the surface atomic layer with respect to the plane of the film, which is chosen as an order parameter, exhibits anomalous behavior and increases with temperature. Expressions are derived for the magnetic susceptibility of each atomic layer. It is shown that, in the vicinity of the transition, the irregular part of the magnetic susceptibility of each atomic layer exhibits behavior characteristic of the susceptibility in the Landau theory: it is less by a factor of two in the low-symmetry phase and diverges at the transition point. The regular part of the magnetic susceptibility of each atomic layer makes an additional contribution to the asymmetry of the total susceptibility in the vicinity of the transition point; this result follows from the fact that the inhomogeneous magnetic system considered is semi-infinite.

Influence of a uniform current on collective magnetization dynamics in a ferromagnetic metal

We discuss the influence of a uniform current $\stackrel{\ensuremath{\rightarrow}}{j}$ on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energy $\ensuremath{\epsilon}(\stackrel{\ensuremath{\rightarrow}}{q})$ has a current-induced contribution proportional to $\stackrel{\ensuremath{\rightarrow}}{q}\ensuremath{\cdot}\mathcal{J}\ensuremath{\rightarrow},$ where $\mathcal{J}\ensuremath{\rightarrow}$ is the spin current, and predict that collective dynamics will be more strongly damped at finite $\stackrel{\ensuremath{\rightarrow}}{j}.$ We obtain similar results for models with and without local moment participation in the magnetic order. For transition metal ferromagnets, we estimate that the uniform magnetic state will be destabilized for $j\ensuremath{\gtrsim}{10}^{9}{\mathrm{A}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}.$ We discuss the relationship of this effect to the spin-torque effects that alter magnetization dynamics in inhomogeneous magnetic systems.

Critical phenomena in magnetic vortex formation probed by noise spectroscopy

Transition between a vortex magnetic state and a uniform magnetic state in a Ni81Fe19 mesoscopic ring has been investigated in terms of resistive-noise spectroscopy. The observed low-frequency noise exhibits critical enhancement around the magnetization saturation. This noise enhancement can be argued from the viewpoint of the critical phenomena due to the chiral-symmetry breakdown of mesoscopic magnetic-structure, which can present a typical mechanism of symmetry transition of magnetic structure in mesoscopic ferromagnets.

Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory

We present here a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM). In order to achieve high-density MRAM, magnetic tunnel junction devices with a small area of low aspect ratio (length/width) is required. However, all MRAMs reported so far consist of in-plane magnetization films, which require an aspect ratio of 2 or more in order to reduce magnetization curling at the edge. Meanwhile, a perpendicular magnetic tunnel junction (pMTJ) can achieve an aspect ratio=1 because the low saturation magnetization does not cause magnetization curling. Magnetic-force microscope shows that stable and uniform magnetization states were observed in 0.3 μm×0.3 μm perpendicular magnetization film fabricated by focused-ion beam. In contrast, in-plane magnetization films clearly show the presence of magnetization vortices at 0.5 μm×0.5 μm, which show the impossibility of information storage. The PMTJ shows a magnetoresistive (MR) ratio larger than 50% with a squareness ratio of 1 and no degradation of MR ratio at 103 Ω μm2 ordered junction resistance.

Induced magnetic superstructure in the FeBO3: Mg weak ferromagnet

The domain structure of a FeBO3: Mg single crystal was studied with a polarizing microscope. It was found that application of a magnetic field along the hard axis in the basal plane of this weak ferromagnet gives rise, within a certain field-strength interval, to a magnetic superstructure observed against the background of the macrodomain structure of the sample. The magnetic superstructure is visually represented as a quasi-periodic system of bands oriented perpendicular to the applied field, with an alternating magneto-optic image contrast along an axis coinciding with the magnetic-field direction. The absence of sharp changes in the contrast of the magnetic superstructure image along this axis is explained as being due to the smooth variation of the sublattice magnetic-moment azimuth with spatial coordinates. The results obtained are discussed within the parameters of the instability of a uniform magnetic state of a system in the random field induced by a magnetic field.

Microscopic magnetization reversal in perpendicular anisotropy CoCr thin films

Magnetic force microscopy was used to observe the magnetization reversal in a CoCr thin film on a grain/subgrain scale. The combination of high resolution topographic and magnetic images were used to relate microscopic magnetization changes to the microstructure of the sample. Both uniformly and partially magnetized grains and both uniform and partial magnetization reversal were observed. Statistically, the uniform magnetic state was more prevalent. In addition, there was a visualization of the flux closure between grains.

Micromagnetic properties and magnetization switching of single domain Co dots studied by magnetic force microscopy

Magnetic force microscopy was applied to study the magnetic properties of Co dot microstructures. The high density magnetic dot arrays were fabricated using nanolithographic techniques on GaAs substrates. The ferromagnetic Co dots were found to be in a single domain state for Co film thicknesses of 7 nm and 17 nm. The magnetization of the as-prepared Co dot array was found to be in a non-uniform state. After applying a magnetic field the Co dots are in a uniform magnetization state. Induced switching of the magnetization of single Co dots by the stray field of the probing tip using an additionally applied in-situ magnetic field has been demonstrated.

Physical and magnetic properties of submicron lithographically patterned magnetic islands

We have developed a procedure for high-resolution patterning of magnetic thin films using direct-write electron beam lithography and a multistep sputter etching process [New et al., J. Vac. Sci. Technol. B 12, 3196 (1994)]. Using this patterning procedure we have defined large arrays of 0.2 by 0.4 μm magnetic islands out of a 200-Å-thick polycrystalline film of cobalt. The physical and magnetic structure of these islands has been examined using atomic and magnetic force microscopy, as well as transmission electron microscopy. Hysteresis loop measurements have been taken from large arrays of weakly interacting islands. The particles are not single domain and show considerable nonuniformity of magnetization when they are completely demagnetized. However, after application and removal of a large field along the long axes of the islands, most of the particles do relax into an almost uniform magnetization state.

Upper bound to a single-domain behavior of a ferromagnetic cylinder

For a right circular, finite ferromagnetic cylinder with a radius R, a parameter, Rc1, is calculated so that for R&amp;gt;Rc1 there is always a nonuniform magnetization state whose free energy is smaller than that of the uniform magnetization state. Numerical computations for the cases of permalloy and of magnetite give Rc1, which is quite close to the recently calculated Rc0 in a cylinder of these materials. The latter is defined, as in the fundamental theorem of Brown on spherical particles, so that the lowest free-energy state is that of a uniform magnetization, if R&amp;lt;Rc0. The closeness of Rc0 and Rc1 makes them a sufficiently good estimate for the critical size at which the particle changes from uniform to nonuniform magnetization state, without going through the complicated numerical computation of the exact value.

Magnetization fluctuations in uniformly magnetized thin-film recording media

The noise power spectrum of uniformly magnetized media is measured. The measured spectra are corrected for playback effects (gap loss, spacing loss, head differentiation, and amplifier gain) to yield the spectrum of the medium magnetization fluctuations. The resulting spectral shape approximately fits a simple Lorentzian form. Fluctuation correlation length and variance are easily determined for all states of medium magnetization. Noise power data are compared to the results of in-situ remanence measurements. DC current applied to the recording head produced the uniform magnetization states for which the noise was analyzed. The remanent magnetization corresponding to each current was determined by recording long-wavelength square waves utilizing the same current amplitude that produced the uniformly magnetized state.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spectrum of magnetization oscillations in a uniaxial ferromagnetic plate

AbstractA theoretical investigation is made for the ground statè of a ferromagnetic plate in the vicinity of the Curie temperature in a normal magnetic field. It is shown that there can exist either a uniform magnetic state, a strip domain structure, or a hexagonal domain lattice depending on the temperature and the magnetic field. The spectrum of magnetization oscillations in each of the possible states is calculated and the stability regions of different phases are determined. The results are generalized for orientational phase transitions in the uniaxial ferromagnetic plate in a oblique magnetic field.