Stoner-Wohlfarth Particles Research Articles

Soft-adjacent-layer self-biased magnetoresistive heads in high-density recording

Various biasing techniques have been proposed for vertical magnetoresistive stripe detectors to improve their linearity. In the soft-adjacent- layer design, a high-permeability film (SAL) is located close to a magnetoresistive stripe (MR). The sense current in the MR magnetizes the SAL, which in turn generates a magnetostatic bias field at the MR. Thin-film heads based on the SAL concept have been fabricated on sapphire, using conventional methods, and tested. A theoretical description in which the SAL and the MR are treated as coupled Stoner-Wohlfarth particles has been generalized to include misorientation of the anisotropy axes and to include all applied field directions in the plane of the device. This provides a convenient description of the magnetic characteristics in uniform fields. For track widths of 39 μm, the 3.1 kHz slot RMS-signal-to-RMS-noise ratio at 80 KFCI and 19.1 cm/sec is 45 dB and is tape-noise limited. Barkhausen instability is evident at lower bit densities where the fields from the transitions are large, but is not a limiting factor at 80 KFCI. A comparison of the signal spectra with that of an externally biased head demonstrates that the bias angle across the element height is more uniform with SAL biasing.

Demagnetizing influence on a Stoner-Wohlfarth particle array

The effect of sheet demagnetizing on the classic Stoner-Wohlfarth particle is calculated. The model used is a two dimensional array of the Stoner-Wohlfarth particles with easy axis normal to the array plane. The particles are assumed to be decoupled except for sheet demagnetizing fields. Particles are assumed to be close enough together that volume average moment density can be used to compute the demagnetizing fields. Assuming there are two sub-ensembles of particles (one switched and one unswitched), an energy expression is found and both it and its derivatives are evaluated to determine the equilibrium magnetization orientations. The results, from an iterative calculation, are the sheared hysteresis loops. The sheared hysteresis loops are calculated for various angles of applied fields and for varying strengths of demagnetizing. In addition, an estimate of the rotational hysteresis parameter R H is made as a function of the strength of the demagnetizing fields.

Switching behaviour of interacting magnetic particles

The magnetostatic properties of particulate magnetic media depend strongly on the packing density because of the magnetic interactions of the particles. To investigate the influence of these interactions we considered two models. One was a dipole model consisting of anisotropic dipoles and dealing with only one component of the magnetic moment. The other model consisted of two interacting Stoner-Wohlfarth particles, where the magnetic moments might rotate. We observed that in the latter model the coercive force decreases under practical conditions with the strength of the interaction, while in the first model it increases.

Magnetization of a two-component Stoner-Wohlfarth particle

The free energy of an arbitrary two-component Stoner-Wohlfarth particle is derived and general criteria for stable states of magnetization are established. Hysteretic properties of coaxial prolate spheroids composed of γ-Fe 2 O 3 coated iron particles are calculated numerically and the specific extension to epitaxial cobalt ferrite γ-Fe 2 O 3 particles amongst other two-component systems is indicated.

Theoretical aspects of demagnetization tensors

A magnetometric demagnetization tensor is defined for uniformly-magnetized samples of arbitrary geometry. It is shown that this is a real symmetric tensor with non-negative diagonal elements and unit trace. Diagonalization of the magnetometric demagnetization tensor yields the Brown-Morrish equivalent ellipsoid and the conclusion that Stoner-Wohlfarth particles of any surface geometry have only two easy directions of magnetization. Transformation of the tensor, subject to periodicity constraints, leads to the conclusion that uniformly-magnetized samples with rotational periodicity, of order n \geq 3 about a single axis, have isotropic behavior perpendicular to that axis. It is also shown that if a sample has spherical periodicity with four or more vertices it does not exhibit shape anisotropy. Various methods for determining the numerical values of the tensor elements are presented.

Rotational hysteresis loss in grain-oriented silicon-iron

An improved torque magnetometer has been used to measure accurately the rotational hysteresis loss in grain-oriented silicon-iron for various percentage orientations and thicknesses. The experimental results are discussed in terms of multi-domain concepts and of the Stoner-Wohlfarth single-domain particle, of which the rotational-hysteresis behaviour has been evaluated. The possible relevance of this theory and its extensions to the experimental results are considered.