Recording physics, design considerations, and fabrication of bit-patterned magnetic medium for next generation data storage systems is presented. (Co/Pd)N magnetic multilayers are evaluated as candidates for bit-patterned medium recording layer materials for their high and easily tunable magnetic anisotropy. The optimized patterned multilayers used in this study had coercivities in excess of 12-14 kOe. Bit patterning was accomplished using ion-beam proximity printing, a high-throughput direct write lithography where a large array of ion beamlets shaped by a stencil mask is used to write an arbitrary device pattern. It is found that the nature of magnetization reversal strongly depends on bit edge imperfections and is likely to contribute to switching field distribution.

In this work, the physics of magnetization reversal in patterned high anisotropy (Co∕Pd)n magnetic multilayer arrays is investigated where the magnetic island size, pitch, recording layer thickness, and the underlying multilayer magnetic properties are varied. Magnetization reversal was studied using magneto-optical Kerr effect magnetometry and magnetic force microscopy and supported by micromagnetic modeling. It is found that magnetic island dimension and/or pitch cannot alone explain the variations in the switching behavior of the patterned arrays and the observed values of switching field distribution (SFD). It is found that the ratio of switched magnetic islands to the total number of islands for a giving reversing field depends strongly on the magnetic island geometry. Stray fields from neighboring magnetic islands result in relatively minor influence on the switching characteristics. Micromagnetic modeling was used to further understand the magnetization reversal in patterned arrays. It is found that the bit-edge imperfections such as tapering contribute significantly to the SFD.

The correlation between the magnetic properties and the microstructural and chemical composition modifications of Co∕Pd magnetic multilayers upon annealing in ultrahigh vacuum at 250°C is presented. Magnetic characterization using magnetic sample magnetometer shows the vertical magnetic anisotropy increase and the switching field distribution decrease in the annealed samples. The larger values of magnetic anisotropy in the annealed samples are further shown using the magnetic force microscopy of the ac demagnetized states in the Co∕Pd multilayer films. X-ray diffraction rocking curves show an improvement in the texture and the initial magnetization curve slopes indicate the decreases in defect densities. Overall, vacuum annealing under optimal conditions improves the magnetic properties of Co∕Pd multilayers for applications in ultrahigh density magnetic recording.

The physics of magnetic signal propagation in one-dimensional antiferromagnetically coupled nanomagnetic arrays is studied using micromagnetic modeling. The results are used to develop the design guidelines such as the criteria for the interelement spacing for efficient operation and the error suppression due to the magnetization misalignment in individual elements in the array. The propagation speed is found to decay significantly as the damping is increased. The external “clocking” field is applied to improve the data channel characteristics. However, premature relaxation of the end elements inhibits the proper operation of longer channels. A proposed solution is a zone-by-zone propagation scheme, which is compatible with the pipelining approach. Simulation results demonstrate a possibility of successful signal propagation at 2 GHz clocking field frequency with no limitation on the length of the channel.

We present the results of micromagnetic studies on stability and bit propagation in the patterned nanodot chains. In particular, we discuss periodic results of long-array switching using a clocking field.

(Co/Pd)N multilayers exhibit high vertical magnetic anisotropy and have been extensively explored as recording medium candidates for high density magnetic recording applications. In this work, (Co/Pd)N multilayers are deposited by magnetron sputtering and patterned into large periodic arrays of 200 nm islands to enable controlled domain wall injection for quantitative comparison of magnetic anisotropy energies. Magnetic properties are correlated with x-ray photoelectron spectroscopy data, an approach commonly used to probe the binding energies and valence band positions. Confirming theoretical predictions, it is demonstrated that the degree of d-shell hybridization at Co/Pd interfaces directly correlated with the magnitude of magnetic anisotropy.

Design considerations and fabrication of bit-patterned magnetic recording media are presented. The application of ion-beam proximity printing, a high-throughput direct-write lithography, to media patterning is evaluated. Ultra-high magnetic anisotropy (Co/Pd)(N) magnetic multilayers are analyzed as candidates for patterned recording layers. Following patterning, optimized multilayers are shown to exhibit coercivity values well in excess of 14kOe. It is found that the magnetization reversal in patterned bits takes place via domain wall nucleation and propagation. The nucleation field and the location of the nucleation site strongly depend on the bit edge imperfections and contribute to finite switching field distribution. Playback off a bit-patterned media using various magnetic reader designs is analyzed using reciprocity theory.

Domain wall dynamics in magnetic nanodots is critical to the understanding of the magnetization reversal mechanisms in bit-patterned arrays, the issues of writeablility, data rate maximization, and bit stability. In this work, micromagnetic simulations were carried out to investigate the dynamics of domain walls in disk-shaped nanostructures with large built-in perpendicular anisotropy. Due to the strong demagnetizing effect, the domain wall motion falls into the supercritical regime. A 90 degrees phase shift of the wall velocity is developed due to the finite thicknesses. The mean value of the wall velocity increases as the domain wall propagates away from the center. This induced asymmetry causes the frequency of the wall oscillations to be halved. At large diameters, the wall acceleration deceases and the periodicity is lost. The in-plane magnetization configuration shows that multiple spin wave modes are present. The absence of the coherency in the magnetization orientations causes phase canceling. The out-of-phase motion of neighboring segments reduces the wall acceleration.

Ion irradiation of continuous and patterned (Co∕Pd)n magnetic multilayer films has been studied as a mean to control magnetic anisotropy as well as to evaluate possible ion irradiation damage involved in ion-beam proximity lithography patterning. The coercivity of patterned medium was found to decrease from 11kOe for as patterned samples to 0.3kOe for samples with 800μC∕cm2 ion irradiation. Remnant squareness of the patterned samples remained essentially unchanged. As the number of bilayers increases in the sample, the effects vary, suggesting that several mechanisms of damage occur. Significantly, for typical irradiation doses used in ion-beam proximity lithography, no measurable alteration of magnetic properties was observed.

(Co/Pd) N multilayers with Co and Pd layer thicknesses of only a few monolayers exhibit high vertical magnetic anisotropy and have been extensively explored as recording medium candidates for high density magnetic recording applications. In the work reported here, the magnetic properties of (Co/Pd) N multilayers deposited by magnetron sputtering and designed for bit-patterned medium applications are correlated with X-Ray Photoelectron Spectroscopy (XPS) data – an approach commonly used to probe the binding energies and valence band positions. Although the XPS probing depth is limited to ˜2–3 nm, it is sufficient for the evaluation of the 1–2 topmost bilayers in a multilayer stack, and allows us to infer the relevant details of the bandstructure of the entire film. Confirming theoretical predictions, we demonstrate that the degree of d-shell hybridization at Co/Pd interfaces directly correlates with the magnitude of the magnetic anisotropy. Significantly, the highest hybridization of Pd atoms is observed for about one monolayer thick Co layers in the bilayer stack. Variation of the deposition conditions (e.g., deposition pressure) shows a measurable influence on d-electron hybridization, multilayer microstructure, and magnetic anisotropy.

This work presents an annealing study of high-anisotropy (Co/Pd) N magnetic multilayers designed for bit-patterned medium recording applications. Magnetic multilayers were deposited by magnetron sputtering at 2.5mT argon pressure at room temperature and annealed at different temperatures (up to 250 °C) for up to 2 hours in atmosphere and in vacuum. Depending on the annealing time, the samples annealed in atmosphere exhibited two distinct modes of magnetization reversal. In samples annealed for a time shorter than some critical time, t c , where t c is a function of the annealing temperature, the magnetization reversal occurs by domain wall injection and propagation. In samples annealed for times longer then t c , the magnetization reversal mode switches to magnetization rotation. Using XPS, it is found that the transition is accompanied by the formation of oxidizes at the grain boundaries leading to exchange decoupling of the grains. In samples annealed at higher temperatures, the increases of the coercivity of as high as 30 times the coercivity of as prepared samples are observed. Significantly, annealing in vacuum showed only small modification of magnetic properties as manifested by relatively minor modifications of vertical M-H loops and unchanged morphology of domain patterns in AC demagnetized state.

We present the results of micromagnetic studies on realistic patterning defects in perpendicularly oriented magnetic thin films. Both undercut and line edge roughness are investigated systematically with simulations using simple test structures to see the effect of the side wall angle, the roughness amplitude on a nanostructureiquests switching field, and the roughness period on mathematically tractable figures. We then run simulations of hysteresis loops of actual 200 nm diameter nanostructures using AFM images to define the structure boundary and compare the results to MFM images of DC demagnetized dots.

We describe the lithographic structuring of large-area patterned medium samples with sub-50nm features using ion beam proximity lithography (IBPL). The quality of the patterns formed in IBPL system is primarily limited by the quality of the stencil masks. Hence, the emphasis of this work has been to develop a reliable mask fabrication process that can achieve a size uniformity that is suitable for patterned media. We have developed a mask fabrication approach that incorporates palladium as a hard mask for transferring the lithography pattern through a silicon nitride membrane. A conformal gold coating allows for further reduction of the mask features without a significant increase in the feature size variation. An average standard deviation of 3nm and 5nm was measured during various steps of the stencil mask fabrication and after printing using IBPL in PMMA resist, respectively. Patterned medium prototypes with features ranging from 40nm to 300nm have been fabricated and magnetic properties measured. A 6-12 fold increase in coercivity was measured for multilayer samples after patterning. Ion irradiation of patterned multilayer samples was also studied as a means to control magnetic anisotropy as well as to evaluate possible ion irradiation damage involved in ion-beam proximity lithography patterning. Patterned multilayer samples show a decrease in coercivity from 11kOe for as-patterned to 0.3kOe for 800 mu C/cm(2) and suggests that ion irradiation can be an integral part of bit patterned medium fabrication for anisotropy control.

Magnetoresistive playback off bit patterned magnetic recording media is studied. Three playback configurations are compared: not shielded sensors, shielded sensors commonly used in magnetic recording systems, and shielded differential sensors. Influence of patterned medium parameters including bit-filling ratio, bit shape, bit position jitter, bit-size variation on the playback signal is studied. Playback signal amplitude, D50, and signal-to-noise ratio are used to compare different playback sensor configurations. It is found that in general, shielded differential readers offer superior performance as compared to both shielded single sensor readers and not shielded readers.

Selected aspects of the recording physics of data storage systems based on patterned magnetic recording medium are discussed. The micromagnetic study of the magnetization reversal in patterned magnetic recording medium is presented. The effects of bit size, head/medium misregistration, bit-to-bit spacing, soft underlayer (SUL) patterning, and side wall roughness on magnetization reversal are explored. The results offer the guidelines for the design and optimization of patterned magnetic recording medium.

In this work, a combinatorial approach to the synthesis of magnetic multilayers is explored. Combinatorial libraries of Co/Pd multilayer thin films were prepared using off-axis magnetron sputtering to enable thickness gradients across the wafer-magnetic properties of the multilayers are controlled by the thicknesses of Co and Pd layers in the repeated bi-layer stack. Polar magneto-optical Kerr effect (MOKE) was used to map magnetic properties of the combinatorial libraries. Multivariate regression analysis and back-propagation neural network (neural networks are known to better handle nonlinear approximations) were used to analyze the combinatorial data and to enable predictive capabilities. In the multivariate analysis, the relationship between the descriptive and output variables was approximated by a second order polynomial of Co and Pd thickness.The neural network model was utilized inversely to design a multilayer with pre-determined magnetic properties.

An approach to fabrication of a patterned magnetic recording medium for next generation data storage systems is presented. (Co/Pd)(n) magnetic multilayers are evaluated as candidates for patterned medium materials for their high and easily controllable magnetic anisotropy. The multilayer films deposited on a Ta seed layer enable high intergranular exchange coupling - an essential feature of a patterned magnetic recording medium. The quality of (Co/Pd)(n) superlattices was optimized via deposition conditions and monitored using low-angle x-ray diffraction. An estimated in-plane (hard-axis) magnetization saturation field in excess of 40 000 Oe was observed. Vertical (easy-axis) hysteresis loops for as-deposited continuous magnetic multilayers exhibited a low coercivity of 930 Oe, indicating highly uniform ( magnetically) films with weak domain wall pinning. Ion-beam proximity lithography was used to pattern magnetic multilayers into 43 nm islands on a 135 nm pitch. Following patterning, easy-axis coercivity increased nearly 15-fold to 12.7 kOe.

Selected aspects of the recording physics of magnetic data storage systems based on patterned medium are discussed. Considerations for the choice of a recording layer material are outlined. A micromagnetic study of magnetization reversal in patterned magnetic recording medium is presented. The effects of bit geometry, medium thickness, head/medium magnetic spacing, air-bearing surface geometry, write pole material, and write misregistration on magnetization reversal are explored. The influence of a recording layer design on playback resolution is evaluated. The results offer the guidelines for the design and optimization of patterned magnetic recording medium.