The permeability of the magnetic soft underlayer (SUL) was systematically varied by changing the antiferromagnetic exchange coupling, and the effect on recording performance and adjacent track erasure (ATE) and far track erasure (FTE) was investigated. Perpendicular magnetic recording performance highly depends on SUL permeability. Highly permeable SUL can reduce the head erasure field located at far tracks from a stressing track but generates more erasure at adjacent tracks near the side shield. Optimum SUL permeability can improve on-track performance with tolerable ATE or FTE. Finite element method models of a shielded write head and media with different SUL permeabilities were performed. A high-permeability SUL has a larger impact on short-time high FTE fields and potentially affects the erasure at 3-6 tracks from main pole. This shows agreement with experimental results.

The magnetic switching behavior was measured for granular oxide media containing an exchange control layer (ECL). Investigations of the switching behavior show that the initial minor loop slope represents the degree of incoherent switching in the film. Multilayered granular oxide media with different inter-granular coupling in both lateral and vertical directions, is prepared by varying the oxide grain isolation and ECL thickness. The switching behavior of well isolated oxide grains strongly depends on the thermal stability of the switching segments through the ECL layer. The oxide grains with higher thermal stability vertically decouple at thinner ECL, resulting in a stronger incoherency of switching. In addition, the increase of lateral exchange coupling between oxide grains reduces the vertical decoupling at given ECL condition. For media with a smaller cluster size, the ECL magnetization and thickness has a more pronounced effect on the switching, indicating that fabrication of well isolated grains is important to optimize the media switching mode.

We have experimentally and analytically studied the writeability and recording characteristics of perpendicular magnetic recording media in which the anisotropy (Ku) of the granular oxides and the magnetization (Ms) and lateral exchange coupling (Aex) of the cap layer were varied. The analytical results are obtained with a 9-spin model, which consists of 3 grains with 3 layers each, representing dual oxide media with a capping layer. The 9-spin model estimates bit error rate (BER) using an error-pattern-correcting code (EPCC) based iterative channel model. The experimental results qualitatively agree well with the analytical results of the 9-spin model. High Ku gradient oxides and high Ms cap layer improve writeability and BER, whereas high Aex in the cap layer degrades switching field distribution and off-track capability. Media switching field and switching field distribution strongly depend on spatial distribution of the external field. Both experimental and analytical results indicate that an appropriate design of Ku graded media with an optimum cap layer can effectively reduce jitter and switching field distribution for given head fields, resulting in higher recording density.

A systematic investigation of the switching behavior and writeability characteristics of perpendicular magnetic oxide media was performed by varying the number of oxide layers as well as the anisotropy K(u) gradient in multilayered oxides. The media switching behavior highly depends on the magnetic volume of laterally exchange-coupled cap layer as well as the vertical exchange coupling between oxide and cap layers. The media switching field is significantly reduced with incoherent switching mode. The media writeability is further enhanced by employing the multilayered oxides whose anisotropy values vary. Higher K(u) graded oxide media exhibits improved resolution, sharper transition, and higher signal-to-noise ratio. An analytical 9-spin model is used to study the impact of anisotropy grading, intergranular and interlayer exchange coupling on writeability and noise performance. The 9-spin model is found to qualitatively agree with the experimental results. Both model and experiment indicate that optimal tuning of the media anisotropy and exchange to the head design is critical in attaining good recording performance.

We report a systematic study of the switching behavior and writeability characteristics of perpendicular magnetic recording media in which the anisotropy (K u ) of granular oxides, magnetic volume (M s ·t, product of magnetization and thickness) of cap layer, and exchange coupling between granular oxide and cap layer were varied. The media switching behavior highly depends on the M s t of cap layer as well as the oxide-to-cap exchange coupling strength but little on the K u of granular oxides. The media switching field H o at 1-ns scales is strongly affected by switching mode and is greatly reduced with incoherent switching mode. At constant switching mode with fixed M s t of cap layer, the media writeability is primarily determined by the oxide K u value that affects the H o . However, when the switching mode varies with different cap M s t or oxide-to-cap exchange coupling strength, no correlation between H o and writeability is observed. Higher K u oxide media with higher M s t of cap layer exhibits lower H o due to more incoherent switching but substantially lower overwrite (OW) than the lower K u oxide media with lower M s t of cap. Similarly, media with weaker exchange-coupling between oxide and cap layers shows more incoherent switching mode and worse writeability even with lower H o . Our results have shown that oxide-to-cap exchange coupling as well as oxide K u is critical in media design for good writeability at high recording density and can be optimized through selection of appropriate oxide and cap materials.

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.

The effect of interlayer thickness (t IL ) on recording performance of the media showing relatively close to coherent switching behavior is investigated. The range of t IL was widely controlled from 20 to 45 nm by inserting an amorphous CrTa filling layer between NiW(6 nm)/Ru(14 nm) and amorphous FeCoTaZr soft underlayer (SUL). Minor heating effects at a thicker CrTa on a glass substrate are detected, but the change in magnetic properties and microstructure even at t IL = 45 nm is small enough to study the head-to-SUL spacing effect. A thinner IL clearly improves both reverse overwrite (ROW) and signal-to-noise (SoNR 2T ) at 2 T, but it reduces resolution (RES). Less dependence of magnetic core width (MCW) on t IL is shown due to compensation of wider magnetic write width (MWW) and narrower erasure band (EB) at a thinner IL. According to simulation results, a thinner IL enhances field strength underneath the center of the writer and more field penetration into the media but the maximum effective field located near the gap changes very little. These field profiles improve writeability at low frequencies, but they do not affect much at mid-to-high frequencies. This situation can explain better ROW, lower RES, and wider MWW at a thinner IL. Higher crosstrack field gradient at a thinner IL corresponds to narrower EB. The improvement of SoNR 2T at a thinner IL is caused by the increase in signal at low frequency and the further decrease in dc noise. All the recording parameters with increasing t IL are qualitatively consistent with simulation results.

We have experimentally investigated the effects of the interlayer thickness on recording performance, which was measured under various head clearances during writing and reading. In all cases, overwrite and write width always increase as the interlayer thickness is reduced. The signal-to-noise ratio, however, has a more complex dependence on the interlayer thickness: when the interlayer thickness is reduced, SNR is improved if head clearance during writing is large but is degraded if the head clearance during writing is small; SNR's dependence on the interlayer thickness becomes non-monotonic at intermediate head clearance during writing. Our data suggest that this complexity is caused by the interlayer thickness effects on the write field angle and gradient.

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.

We report a systematic study of the switching and recording characteristics of perpendicular magnetic recording media in which the exchange coupling between granular oxide and continuous cap layers was varied. The interfacial exchange coupling strength was controlled by adjusting the magnetization (M s ) and the thickness (t) of the exchange control layer (ECL) between granular oxide and cap layers. The media switching mechanism highly depends on the oxide-to-cap exchange coupling strength as well as the relative moment ratio of cap and oxide layers. Reversal process is coherent for medium with only granular oxide layer and becomes incoherent with incorporation of ECL and continuous cap layers. Optimizing granular oxide-to-cap exchange coupling improves the media writeability as well as the media signal-to-noise ratio (SNR m ). At optimum exchange coupling condition, the switching field is significantly reduced even with higher thermal stability factor (K u V/k B T). However, when the interlayer coupling strength is too weak, independent switching of oxide and cap layers occurs, resulting in poor writeability and high media noise. An optimum design of oxide-to-cap exchange coupling is critical in attaining recording properties for high density recording through selection of appropriate ECL and cap materials.

A new underlayer structure consisting of a magnetic seed was used to reduce the recording layer-to-soft magnetic underlayer (RTS) spacing and its effect on perpendicular recording characteristics was investigated. The RTS spacing is reduced by partially replacing the non-magnetic FCC NiW alloy layer with a magnetic CoNiFe layer that acts as a part of the underlying SUL through magnetic exchange coupling. Magnetic CoNiFe layer promotes predominant FCC (111) planes of NiW layer that enhances epitaxial growth of the subsequent Cr BCC (110), Ru and Co HCP (0002) layers. As a result of improved crystallography with magnetic seed, the Co (0002) c axis dispersion is reduced at lower RTS spacing. The head write-ability becomes stronger at lower RTS spacing and the influence of side fringing fields on the nearest adjacent track erasure is highly dependent on the write head type at different RTS spacing. In addition, the change in RTS spacing also affects the read-back response. The signal-to-media noise (SNRm) is improved at low linear densities regardless of RTS spacing but degrades at higher linear densities when RTS spacing becomes too low. It is important to optimize the RTS spacing and the head-to-media integration to improve overall system performance.