Time dependence of switching fields in magnetic recording media (invited)

Abstract

Coercivity and other experimental measures of switching field depend upon the time scale of interest. This time-scale dependence has practical importance in magnetic recording, because the effective time scales of writing and storage are very different. A simple model accounts for the time-scale dependence of coercivity in terms of the thermally assisted crossing of an energy barrier whose height is reduced by the applied field. Fitting this barrier-crossing model to data provides an estimate of the volume that must switch magnetization direction in overcoming the barrier. The assumption of Stoner–Wohlfarth reversal is used to obtain an initial estimate of the dependence of the barrier height on the field. With some adjustment of the resulting energy expression, the model gives good agreement between calculated switching volume and actual particle volume for advanced recording particles of three types: acicular oxide, acicular metal, and barium ferrite platelets. The model can be used to estimate minimum practical particle volumes for use in magnetic recording. Switching due to fields nearly perpendicular to the particle’s preferred axis, sometimes used as a measure of the anisotropy field, also shows the effect of thermal assistance. The switching volume determined from such measurements, like that from coercivity, approximates the particle volume.

A guideline for atomistic design and understanding of ultrahard nanomagnets

“…33-can be related to contributions from off-site spin-orbit scattering of the Fe states, which seem also to be responsible for the quenching of the interfacial orbital magnetic moments of Fe as shown in the upper panel of Figure 2. Thus, the type of the particle's surface termination can be deemed of particular importance if the protective shell consists of heavy elements, which possess a large spin-orbit-coupling parameter.…”

“…Examples at three different temperatures can be seen in Figure 5a. For ensembles of non-interacting nanoparticles with diameter d, uniaxial anisotropy K eff and randomly oriented easy axes of magnetization, the temperature dependence of coercivity can be described by Sharrock's equation 33 :…”

Continuously graded anisotropy in single (Fe53Pt47)100−xCux films

“…The dynamic H C , H C,dyn , is the field required to bring the magnetization to zero after a characteristic time, t o , and functionally obeys Sharrock's equation. 23 In our calculations, an attempt frequency f o =5ϫ 10 9 Hz is used and we can presume n =1. 22,24,25 From this analysis, the thermal stability factor, defined as ␤ ϵ ⌬E o / k B T, and time independent intrinsic H C , H o , can then be extracted.…”

Recording performances in perpendicular magnetic patterned media

“…Sample B being an optimized CoCrPt alloy, its effective anisotropy is of about 9×10 5 J/m3, almost one order of magnitude higher than sample A (see Table 1). The barrier height ΔE, which prevents the magnetization reversal, is thermally activated and depends on the reversal mode [21] [22]. By magnetic after-effect measurements, we have already shown that ΔE for sample B was larger than seen in sample A [13] (see table 1).…”