M. W. Muller

Spatial noise phenomena of longitudinal magnetic recording media

The authors have studied the spatial noise characteristics of uniformly magnetized media and implemented a synchronous magnetic pulse detection technique to measure waveforms from precise radial and azimuthal locations on a magnetically recorded disk. They introduce a correlation analysis which includes measuring noise waveforms from the same position on the disk under different remanent magnetic states. Results from this analysis demonstrate that at remanence only minor changes occur in the output waveform and its (presumed) corresponding magnetic structure from write to write, while at DC demagnetization large variations are seen for successive writes. This sensitive correlation technique is used to analyze the random and deterministic components of media and head noise. >

Spin-stand measurements of time and temperature dependence of magnetic recordings

Spin-stand experiments were performed to study the effect of temperature and demagnetizing fields on the stability of magnetic recordings. Tracks of varying linear bit densities were written on a low Mrδ medium that was heated in situ to different temperatures. The readback amplitude, which reflects the changes in the magnetization of the recordings, was observed from 32 ms to 12 h after the tracks were written. The readback amplitude was found to decay with time. The rate of decay increased at higher densities due to higher demagnetizing fields. A further increase in the decay rate was observed at higher temperatures. This increase in decay rate exceeded that predicted by proportional temperature scaling.

Domain Formation in a Ferromagnetic Plate

The nonlinear micromagnetic equations are used to obtain an approximate analytical solution for the distribution of the magnetization vector in the central plane of a ferromagnetic plate with uniaxial magnetocrystalline anisotropy. It is shown that the micromagnetic nucleation mode derived from the linearized equations develops into a strip domain pattern with well‐defined Bloch walls in a rather narrow range of applied magnetic field near the saturation value. The stability of the domain pattern is discussed qualitatively.

Evidence for non-cubic magnetostriction in epitaxial bubble garnets☆

Abstract Epitaxial bubble garnet films grown on non-magnetic garnet substrates exhibit a dominant growth or stress induced uniaxial anisotropy, which is responsible for the stripe and bubble domain structures, and the intrinsic cubic magnetocrystalline anisotropy which can affect bubble device performance. The anisotropy constants have been deduced from measurements of stripe domain nucleation in the garnet films. We extend this measurement technique and its interpretation so that it also yields values of the magnetoelastic interactions. The measurement is based on observing the details of the topography of the nucleating domain structure, specifically the orientation of the nucleating stripe domains as a function of the orientation and magnitude of the applied magnetic field. The interpretation is based on a micromagnetic analysis of the conditions for homogeneous second order stripe domain nucleation. The contributions to the phenomena of the cubic anisotropy and of the magnetostriction are included in the analysis as perturbations. The theory produces predictions which are compatible with qualitative earlier experiments reported in the literature. It provides a satisfactory quantitative account of systematic new observations we have made on a GdTmY bubble garnet film with the specific objective of measuring magnetostriction. Analysis of the experimental data yields strong evidence for a non-cubic component of the magnetostriction possibly associated with the same growth-kinetic mechanism that gives rise to the non-cubic anisotropy. The sign and magnitude of the macroscopic non-cubic magnetoelastic constant is estimated from the experimental results.

Stripe domain statics in garnet films

Abstract Magnetic garnet films grown epitaxially on nonmagnetic garnet substrates exhibit a growth or stress-induced uniaxial anisotropy in addition to the cubic magnetocrystalline anisotropy associated with their crystal symmetry. When the uniaxial anisotropy is dominant over the cubic, such films exhibit stripe or bubble domain structures; even a small cubic anisotropy component can have a decisive effect on the behavior of the domains in applied fields. We report an experimental study of the quadistatic behavior of domains in fields applied to a (111) film in the film plane along (11 2 ) and ( 1 10). The experimental results are interpreted by a new theory that gives good agreement with the observed behavior, and yields an accurate measurement of the cubic and uniaxial anisotropy constants. The main qualitative features of the results are: In a ( 1 10) field, the walls are Neel walls perpendicular to the field. In a (11 2 ) field the walls are Bloch walls parallel to the field, the domain magnetization in adjacent stripes is not symmetrical about the film plane, and adjacent stripes are not of equal width; the domain period first shrinks and then expands with increasing field; and even though the applied field has no component perpendicular to the film plane, the film develops a net perpendicular magnetic moment.

Texture induced spatial variation in switching fields

A method for mapping the microscopic spatial variation of switching fields in magnetic recording media using a magnetic force microscope is described. The method is applied to samples with different substrate textures to examine the effect of texture on magnetic switching behavior. Reversals are found to occur in chains along the texture direction for fields coincident with the texturing. For fields applied transverse to the texture, no long range order is seen in the reversal process. Untextured media also show some tendency for reversal to progress along the field direction.

MFM observation of localized demagnetization in magnetic recordings

The micromagnetic details of magnetization reversal were studied using Magnetic Force Microscopy (MFM). We developed a correlation technique using MFM images to measure changes in magnetization. These correlations are related to the changes in magnetization, as measured in a Vibrating Sample Magnetometer (VSM). This technique was used to examine the onset of demagnetization near a transition.

Irregular grain structure in micromagnetic simulation

A randomized microstructure based on the Voronoi diagram is proposed for micromagnetic models. Simulations illustrate variability of extrinsic magnetic properties with microstructure, medium noise dependence on medium properties, and jitter dependence on trackwidth.

Submillisecond spin-stand measurements of thermal decay in magnetic recordings

The time dependence of magnetic recordings is determined on a spin stand by observing the amplitude of different spectral components of the readback signal from about 200 /spl mu/s to 11 s after writing. Measurement times shorter than one revolution period of the disk are obtained by placing separate write and read heads on the same track. An increased decay rate in the fundamental signal amplitude is observed at higher linear recording densities. The decay deviates from ln (t) at short times. Comparison of the decay rates of the spectral components of different density recordings indicates transition broadening over time.

Williams-Comstock type model for sawtooth transitions in thin film media

We describe a model of the write process of saw-tooth transitions on longitudinal thin film media. The width of the transitions is determined by the combined action of the head and dipolar fields against the coercivity of the film. We use the model to predict the dependence of transition width and location of the transition center on flying height, gap width, and write current for a Karlqvist head field. The transition width approaches the minimum permitted by the transitions dipolar field for low flying height. Unlike models of the write process on particulate media, it predicts that the transition does not relax after the film leaves the write head.