Determination of Critical Volumes in Recording Media

Abstract

In this overview paper the concept of thermal activation of magnetisation reversal is reviewed in terms of the Wohlfarth-Gaunt formalism. This formalism gave rise to the concept of an activation volume of reversal. Other techniques have been developed for the determination of activation or critical volumes of reversal and these are reviewed. It is found that these methods give the same approximate value for the activation volume but the only method based on micromagnetism is the waiting time experiment which is consistent with Gaunt. Factors which affect the activation volume such as incoherent reversal and intergranular coupling are discussed together with measures that may be required in order to generate ultrahigh density recording media that are relatively free from thermal loss of signal.

Time Dependence in Perpendicular Media With a Soft Underlayer

“…For sample A, we are able to use the approximation of Street and Woolley , as the behavior of this sample is quasi-linear in log time. For sample B with significantly larger time dependence, this is no longer applicable and a second-order quadratic fit was applied to the data consistent with (2). The variation of (or ) with is shown in Fig.…”

“…A number of factors affect the level of thermal loss such as the grain size, fluctuations in anisotropy, orientation, etc. Generally, these will be distributed, which leads to an energy barrier distribution [2]. The energy barrier distribution leads to a nonexponential variation of magnetization with time which is often observed to be logarithmic and specifically will be nonlinear when there is a narrow energy barrier distribution [4].…”

“…is of considerable importance as it determines the smallest entity that can reverse and hence determines the intrinsic limit to recording density [2]. Hence, the ability to obtain accurate measurements of is critical to the characterization of all types of media.…”

Magnetic properties of FePt nanoparticles annealed with NaCl

“…In particular, the switching field distribution (SFD) is an important parameter in the sphere of magnetic recording because it represents a measurement of the distribution of energy barriers in a material and affects the transition width [5]. The SFD can be determined Digital Object Identifier 10.1109/TMAG.2006.880153 from measurements of the irreversible susceptibility .…”

“…is a measure of the volume of material that reverses during a single coherent reversal event, i.e., the minimum bit size that could be written to a disk [5] (2) II. EXPERIMENTAL PROCEDURES Two samples of fct FePt nanoparticle powders with varying particle diameter, 4 and 15 nm, have been studied.…”