Paul M. Jones

Nonmagnetic shell in surfactant-coated FePt nanoparticles

Self-assemblies of chemically-synthesized 3 nm FePt nanoparticles show a dramatic increase of the saturation magnetization from about 210 emu/cm3 in the as-prepared state to about 850 emu/cm3, when annealed above 400 °C for 30 min. This increase correlates directly with the decomposition of the surfactant coating above 400 °C. The low magnetization state is attributed to the formation of a nonmagnetic shell (dead layer) due to oxygen bonds of the polar end group surfactant of oleic acid/oleylamine with Fe. Magnetic analysis in the superparamagnetic regime suggests a 2.7-nm-diameter FePt particle size with core–shell structure of 1.7-nm-diameter FePt magnetic core and ∼0.5 nm nonmagnetic shell.

Thermal stability of Fomblin Z and Fomblin Zdol thin films on amorphous hydrogenated carbon

Thermal desorption spectroscopy has been used to monitor the decomposition kinetics of Fomblin Zdol and Fomblin Z lubricant films adsorbed to the amorphous carbon overcoats of hard disk media. Comparisons have been made between Fomblin Z and Zdol with vastly different molecular weights (MW = 4000 and 50000), and films of Fomblin Z with different thickness (20 and 60 Å). Several species have been observed desorbing from the surface during heating. In all cases decomposition occurs over roughly the same temperature range of 600–750 K. This suggests that the desorption process is the result of decomposition and that the end groups of the Fomblin lubricants are not involved in determining the kinetics of this decomposition reaction. The activation barrier to the decomposition process has been estimated at 114±6 kJ/mol.

Probing the role of carbon microstructure on the thermal stability and performance of ultrathin (<2 nm) overcoats on L10 FePt media for heat-assisted magnetic recording.

An understanding of the factors influencing the thermal stability of ultrathin carbon overcoats (COCs) is crucial for their application in heat-assisted magnetic recording (HAMR) at densities ≥ 1 Tb/in(2). Two types of non-hydrogenated ultrathin (∼1.5 nm) COCs were investigated after being subjected to laser-induced localized heating (at temperatures > 700 K) as envisaged in HAMR. Filtered cathodic vacuum arc (FCVA)-processed carbon with tuned C(+) ion energies of 350 eV followed by 90 eV provides significantly higher sp(3) C-C hybridization than magnetron sputter deposition even at very low thicknesses of ∼1.5 nm. As a result, the FCVA-deposited ultrathin carbon overcoats displayed excellent thermal stability along with improved wear and corrosion resistance. On the other hand, the sputtered carbon exhibited carbon loss and topographical and structural changes after laser irradiation owing to lower sp(3) hybridization. Therefore, this study highlights the pivotal role of carbon microstructure, primarily sp(3) hybridization, in non-hydrogenated carbon overcoats to maintain excellent thermal stability during the recurring high-temperature cycles in a HAMR process.

Quasi-equilibrium AFM measurement of disjoining pressure in lubricant nanofilms II: Effect of substrate materials.

Atomic force microscopy (AFM) was used to measure the disjoining pressures of perfluoropolyether lubricant films (0.8-4.3 nm of Fomblin Z03) on both silicon wafers and hard drive disks coated with a diamondlike carbon overcoat. Differences in the disjoining pressure between the two systems were expected to be due to variations in the strength of van der Waals interactions. Lifshitz theory calculations suggest that this substrate switch will lead to relatively small changes in disjoining pressure as compared to the more pronounced effects reported due to changes in lubricant chemistry. We demonstrate the sensitivity of our AFM method by distinguishing between these similar systems.

van der Waals force calculation between laminated media, pertinent to the magnetic storage head-disk interface

Lifshitz theory of van der Waals interactions is applied to the geometry and materials of the head-disk interface (HDI). A simplified two-substrate model with intervening air gap is used to illustrate the effects of retardation. The calculation of the Hamaker functions for increasingly complex layered structures is then presented to illustrate the importance of the multilayered nature of the HDI on the van der Waals (vdW) interaction. The full Lifshitz-multilayer calculation is then compared to approximations of the vdW force and relative errors are displayed. The results indicate the necessity of performing the full Lifshitz calculation on a realistic layered model of the HDI for accurate force modeling.

Effect of fly height and refractive index on the transmission efficiency of near-field optical transducers

Heat-assisted magnetic recording is a potential remedy to extend the limits of magnetic recording. A high temperature with a steep gradient is used to reduce the local coercivity of the magnetic medium. To achieve such a thermal profile, an intense optical spot well below the diffraction limit is necessary. Transmission efficiency of a near-field optical transducer is affected by various factors at the head-medium interface. Effect of fly height and presence of high refractive index material at the head-medium interface is investigated. Favorable conditions are identified.

Head–Disk Lubricant Transfer and Deposition During Heat-Assisted Magnetic Recording Write Operations

Lubricant accumulation was found on the media surface the instant when the laser is turned OFF during heat-assisted magnetic recording write operations. By changing the write cycles, laser ON/OFF duration, media, and head temperatures, we find that this lubricant accumulation is related to the change in head-media temperatures. The observed lubricant deposition process is restricted to a short time window (1-2 μs) after the laser is turned OFF. An equilibrium model of thermal displacement due to evaporation and condensation processes is presented and used to discuss the effect of the head-media temperature changes on the lubricant accumulation. Possible solutions to minimize the lubricant transfer and deposition are discussed.

Effect of van der Waals forces on molecularly thin lubricant in the magnetic storage head-disk interface

The disjoining pressure of lubricant in the magnetic storage device head-disk interface (HDI) was obtained using Lifshitz theory [Sov. Phys. JETP 2, 73 (1956)] and multilayer film stack model. For this purpose the normal-air-gap-centered calculation of the disjoining pressure was replaced by the lubricant-layer-centered system. In the air-centered system the attractive force between the head and media was found to increase as the lubricant layer was thinned. By switching to the lubricant-centered system the disjoining pressure in a thin film of lubricant in the HDI was found to be negative at small head-to-media separation. By equating the free lubricant disjoining pressure with the lubricant in the head-disk interface, a thinning of the lubricant in the interface due to the presence of the head was calculated. A detailed thermodynamic analysis of the disjoining pressure of the lubricant in the interface revealed that the calculated negative disjoining pressure was due to the dominance of the air-gap-cent...

Studies of the self-ordering process in FePt self-ordered magnetic array media

A model of self-organization in FePt particulate thin films is described. The model is based on Monte Carlo methods and describes the self-organization process from the colloidal state as a result of the evaporation of the solvent and the consequent compression of the film due to surface tension forces. The model predicts the self-ordered state for systems with very narrow particle size distributions (standard deviation of the lognormal distribution σ<0.05). Various forms of interparticle potentials were considered. A comparison of the model with experimental data shows that the interparticle potential must contain an attractive term, in addition to the repulsive term normally expected to arise from the surfactant. Quantum chemistry calculations show that an attractive potential does indeed exist between overlapping oleic acid molecules. Although the ordering predicted by the model is predominantly hexagonal, the conditions for the formation of a square lattice are investigated, and an explanation for the...

Write-Induced Head Contamination in Heat-Assisted Magnetic Recording

One detrimental by-product of heat-assisted magnetic recording writing is the creation of head contamination. Here, we present the current understanding of the driving forces, growth mechanisms, and growth rates of write-induced head contamination. The combination of an evaporation and condensation model with shear forces suggests a flow of lubricant on the head may precipitate contamination. The contamination is observed to grow in the head–media gap until it contacts the media surface, at which point an additional material pickup mechanism can be activated. Evidence of contact-induced transfer and a chemical reaction of the contamination is presented, and the impacts of contamination on head temperatures and thermal gradient is presented. Depending on the contamination properties, head temperatures may be increased substantially, leading to increased reliability risk. Consistent with previous analyses, we find that contamination may increase media thermal gradient.