In this paper, some issues concerning the reliability of heat-assisted magnetic recording (HAMR) media are highlighted. The large roughness of the grain structure originates from a surface energy mismatch between the FePt grains and the graphene-like segregant material. A simple roughness model, based on interfacial energies, is proposed that can quantitatively predict media grain structure and roughness. The thermal behavior of the disk lubricant is reviewed both experimentally as well as theoretically using molecular dynamics (MD) and density functional theory (DFT). The lubricant film can be subjected to evaporation and oxidation, both of which follow an Arrhenius reaction rate. MD also predicts that the disk carbon overcoat can undergo structural changes under thermal transient exposure in the nanosecond time frame, and Raman imaging performed on a disk zone that was HAMR written shows small but unequivocal changes, consistent with an increase in carbon sp(2) cluster size.

We systematically vary the strength of the vertical interlayer exchange coupling (VIEC) between the magnetic layers (MLs) of an advanced exchange coupled composite media and experimentally investigate, through magnetic viscosity and dynamic coercivity measurements, the impact on the thermal stability. The strength of the VIEC is controlled through exchange coupling layers of varied thicknesses, whereas MLs are unchanged. We estimate the thermal energy barrier ΔEo as a function of the VIEC within the standard Arrhenius-Neel relaxation and Sharrock formalisms and characterize the effect of the VIEC on the degree of incoherency in the grain reversal mechanism. The analysis shows that while ΔEo drops for weaker VIEC, the ratio of ΔEo to MsHo energy per activation volume is maintained near 2 and weakly optimizes for intermediate VIEC.

We analyze the magnetic design for different generations of perpendicular magnetic recording (PMR) media using resonant soft x-ray small angle x-ray scattering. This technique allows us to simultaneously extract in a single experiment the key structural and magnetic parameters, i.e., lateral structural grain and magnetic cluster sizes as well as their distributions. We find that earlier PMR media generations relied on an initial reduction in the magnetic cluster size down to the grain level of the high anisotropy granular base layer, while very recent media designs introduce more exchange decoupling also within the softer laterally continuous cap layer. We highlight that this recent development allows optimizing magnetic cluster size and magnetic cluster size distribution within the composite media system for maximum achievable area density, while keeping the structural grain size roughly constant.

We show that thermal-stability and the associated switching field in well segregated, nanoscale granular materials is explained by grain boundary and interface effects. Grain boundaries pose a fundamental limit on scaling rooted in their chemical and morphological structure, while exchange interactions across interfaces cause the switching to deviate from the expected coherent Stoner-Wohlfarth behaviour. Measurements and simulations of CoCrPt-systems show a clear shift in applied field angle behaviour, arising from exchange-coupling between magnetic-phases, while the quantitative switching field can only be explained by a transition layer surrounding the grains. These results are potentially significant for Heat-Assisted-Magnetic Recording and Bit-Patterned-Media Recording.

The volume of a magnetic grain, together with its anisotropy, determines the probability of thermally activated reversal. Thus for grain volume distributions where the median volume is close to the superparamagnetic limit there will be a sub-set of grains which are either superparamagnetic on the time scale of a typical magnetic measurement (10s), or the reverse due to magnetostatic fields from surrounding grains. We use this effect to probe exchange coupling in segregated granular materials, using CoCrPt-SiOx granular recording media as model systems. As the film thickness is reduced below 10 nm, the remanent magnetization of these films decreases, due to thermal activation and magnetostatic reversal. Varying film thickness and temperature allows us to thermally select a population of grains that contribute to the measurement. Exchange coupling is characterized by the angle dependence of remanent coercivity where we associate a breaking of symmetry from the Stoner-Wohlfarth model towards the Kondorsky model as a measure of the incoherency of reversal. Combining these models allows an estimate to be made of the volume fraction of grains that are exchange coupled and we find that, for well segregated CoCrPt-SiOx media, approximately 8% of the magnetic volume undergoes some degree of exchange coupling.

By analyzing the magnetic cluster size of various media, we have investigated the variation of inter-granular exchange coupling with grain size. With the reduction of the grain size below 8.0 nm, higher inter-granular exchange coupled media showed a large increase in magnetic cluster size, while lower exchange media did not show as dramatic a cluster size change. This cluster size “knee” position was consistent with reduction in signal-to-noise ratio. To utilize small grain media for high density recording, it is desirable to push this “knee” position toward smaller grain size.

The effects of thermal fluctuations are characterized in granular perpendicular media of two different grain sizes. We extract short time switching fields Ho and intrinsic switching field distributions and find that after correcting for thermal effects and the demagnetizing fields, the switching field distributions are almost independent of grain size, in contrast to the long time scale behavior with broader distributions observed for the small grain media. In addition, the shape anisotropy fields of the magnetic clusters are shown to decrease the observed Ho and are more significant as grain size is reduced.

We describe a newly developed initial minor loop slope (IMLS) experimental method capable of measuring reversal switching in exchange spring perpendicular media. The slope peak height is centered near the medium's coercivity H c . The magnitude of the peak signal is correlated with the soft top (cap) layer's moment, M s t, and the exchange coupling strength between the cap and hard magnetic oxide layers. The peak position was correlated to the media Hc for media that is thermally stable and migrates to smaller fields in less stable media. The IMLS results are compared with both remanent angular dependence coercivity, H cr , measurements and micro-magnetic simulations. This technique provides a simple method of characterizing incoherent switching behavior in different types of exchange spring media.

This paper demonstrates that electrochemical impedance spectroscopy combined with atomic force microscopy analysis can successfully characterize the coverage ability of an overcoat on perpendicular magnetic recording media. Rougher media, brought about by lower surface energy oxide segregants, can adversely impact the overcoat integrity. The role of the capping layer, and its ability to somewhat planarize the overall structure, is also discussed.

We employed a new field sweep ratio dependence analysis method on the polar Kerr loop of exchange spring media to estimate the short time switching field, H0, and thermal stability factor, KUV/kBT, at various normalized Kerr signal levels, M, using modified Sharrock’s equation. For weakly coupled media, we observed that large switching volume changes around a “kink” in the Kerr loops. We observed a rapid increase in the KUV/kBT values before the kink, which suggests rapid switching of large areas of the soft layer via domain wall propagation. Small interfacial decoupling in exchange spring media between the soft and hard layers showed large degradation in media writability.