Alexander Yulievich Dobin

Recording on Bit-Patterned Media at Densities of 1 Tb/in

We present a comprehensive analysis of the areal density potential of a bit-patterned media recording. The recording performance is dominated by written-in errors rather than traditional signal-to-noise considerations. Written-in errors are caused by statistical fluctuations of the magnetic properties and the locations of the individual dots. The highest areal densities are obtained with a combination of a pole head, a soft magnetic underlayer, and a storage medium of the composite type. Areal density scenarios of up to 5 Tb/in2 are analyzedRecording on bit-patterned media, BPM, is one way to postpone the superparamagnetic limit to higher densities. Here we investigate the recording potential of BPM. The fundamental idea of bit-patterned media is that one grain represents one bit so that the entire volume of the bit resists the effect of thermal agitation and higher recording density can be achieved. Previous investigations of a BPM recording system have shown that recording densities greater than 1 Tb/in2 should be possible [2].

^2

Perpendicular recording media consisting of isolated CoPt magnetic grains separated with a nonmagnetic oxide grain boundary can be prepared by sputtering a Ru interlayer and then cosputtering CoPt with an oxide material, with low adatom mobility. The oxide material moves into the grain boundaries and isolates the magnetic grains. An increase in the oxide volume fraction in the magnetic layer does not affect the saturation magnetization but significantly reduces the magnetocrystalline anisotropy of magnetic grains. This may impose a limit on the areal density capability of this media design since media with smaller grains will require a larger oxide volume fraction that will reduce magnetocrystalline anisotropy of the CoPt based magnetic grains.

and Beyond

Using a ldquodragrdquo tester in which the recording head is in direct contact with a perpendicular media disc, we show experimentally that the footprint of the head has a ldquodoughnutrdquo shape, as predicted by theory. A change in the switching properties of the media, from coherent rotation in conventional perpendicular media to incoherent rotation in exchange spring media, can be detected through the transformation of the head footprint, from ldquodoughnutrdquo to a dome-like shape.

Influence of oxide on the structural and magnetic properties of CoPt alloy

A modified slope model is proposed that allows a determination of the transition parameter and the cluster size for perpendicular recording. The presence of the field gradient in the down-track direction causes the clusters to be smaller in the down-track than in the cross-track direction. The results are in good agreement with micromagnetic simulations for a wide range of parameters

Effect of Media Reversal Mode on Head Footprint

XCHANGE spring systems have been proposed recently as a possible solution for increasing write-ability of recording media. The structure allows for the reduction of the switching field, significantly more substantial than the reduction expected by averaging the anisotropy fields of the hard and soft layers. The reversal process in case an external field is applied parallel to the easy axis of the hard layer has been described in [2]: a domain wall develops in the soft phase that subsequently propagates through the interface and assists switching the hard phase. Separating the magnetic properties of the soft and hard layers in the structure is essential in the process of developing the media. Films containing only the hard phase can be used to measure the saturation moment and anisotropy field and , for example by employing the torque method. The torque curve—showing how the component of magnetization perpendicular to the field varies with the angle when the sample is rotated in a constant field—has a sinusoid shape and is easily expressed mathematically if we assume that the hard layer behaves as a single spin. By fitting the experimental data an effective anisotropy is determined, to which the demagnetization term equal to must be added to obtain the crystalline Hk. The difficulty of measuring the properties of the soft phase-M and come from the fact its microstructure changes significantly if the soft layer is grown by itself, compared to the case when it is grown on top of the hard layer. Thus we cannot use films composed of the soft layer only, we must measure directly the exchange spring structure. In analyzing the torque curve of the exchange spring, we must take into account the nonuniformity of magnetization orientation. Partial domain walls develop at the boundary of the two phases, due to the difference in anisotropy [3]. The magnetization of the hard/soft layer combination can be modeled theoretically as a one-dimensional chain of spins, extending from the bottom of the hard layer to the top of the soft layer. We deduced analytical formulas for the dependence of spin orientation on the position in the chain, when the sample is rotated in constant

A Simple Model for Transition Width and Cluster Size in Perpendicular Recording

We demonstrate that antiferromagnetic coupling can be used to suppress the formation of stripe domains in FeCoX layers. Antiferromagnetic coupling forces the magnetization in FeCoX layers to align antiparallel, which increases the magnetostatic energy of the stripe domain state in FeCoX∕Ru∕FeCoX. This increases the minimum critical thickness for which stripe domains are observed in both FeCoX layers of the FeCoX∕Ru∕FeCoX structure. The critical thickness scales with the strength of antiferromagnetic coupling, Jex, and almost doubles for antiferromagnetic couplings larger than about 2mJ∕m2.