T. Thomson

Characterization of the magnetic modification of Co/Pt multilayer films by He+, Ar+, and Ga+ ion irradiation

Co/Pt multilayers can exhibit large perpendicular anisotropies and coercivities that are very sensitive to the quality of the Co/Pt interfaces. We have characterized the dependence of coercivity of Co/Pt multilayers on irradiation with various ion species (He+, Ar+, and Ga+), energies (20 keV–2 MeV), and doses (1011–1017u2009ions/cm2), in order to understand better the nature of the structural changes responsible for the magnetic changes. We find that, in general, the system is much more sensitive to irradiation than expected on the basis of a nearest-neighbor coupling model and simple ballistic ion-beam mixing.

Magnetic characterization and recording properties of patterned Co/sub 70/Cr/sub 18/Pt/sub 12/ perpendicular media

A lithographically patterned magnetic medium is one of the proposed routes to magnetic recording at a density beyond that thought to be possible using conventional recording media due to thermal instability caused by superparamagnetism. Using a focused ion beam to pattern a granular Co/sub 70/Cr/sub 18/Pt/sub 12/ film, we have fabricated sub-80-nm size islands that are single domain and with a narrowed switching field distribution and an enhanced thermal stability. Magnetic isolation of the islands is shown to be a result of vanishing of magnetic remanence and coercivity in the irradiated region and not a result of sputtering. Recording measurements using a quasi-static giant magnetoresistive head demonstrate the sensitivity to detect single 80-nm islands. The readback jitter from the patterned region is dramatically reduced compared to that measured for continuous media at the same linear density.

Writing of high-density patterned perpendicular media with a conventional longitudinal recording head

We have fabricated arrays of magnetic islands in perpendicular CoCrPt media with ∼100 nm lateral dimension using a focused ion beam. A quasistatic write/read tester was used to study aspects of the recording physics of these patterned media. We present results on the variation of the readback signal as the phase of the written square wave changes with respect to the patterned array as a function of island size and write current. Using an analytic near-field expression for head field we are able to model how the observed dependence between phase shift and readback signal as a function of write current arises. This analysis allows us to gain an insight into the role of the island switching field distribution and the write head magnetic field gradient in the patterned media writing process.

Angular dependence of the switching field of thin-film longitudinal and perpendicular magnetic recording media

An experimental survey of the angular dependence of the remanent switching fields for longitudinal and perpendicular magnetic recording media is presented and compared to the Stoner–Wohlfarth and Kondorsky models of magnetic switching. An experimental procedure is used where the remanence measured in the easy-axis direction is used to define the switching field for all angles of the applied field. Clear differences between different types of media are found. CoPtCrB alloy longitudinal recording media are found primarily to have a Stoner–Wohlfarth switching character. In contrast, the switching of perpendicular Co/Pd multilayer media is a mixture of Stoner–Wohlfarth rotation and domain-wall motion.

Lithography and self-assembly for nanometer scale magnetism

Abstract The limits to scaling the relevant physical dimensions required to increase the areal density of magnetic storage devices will be reached soon, if the storage density continues to double annually. Two approaches to overcoming the limit of the minimum particle size required for thermal stability are presented. In the first approach, a narrow particle size distribution is produced using self-assembled layers of magnetic Fe–Pt nanoparticles. The very narrow particle size distribution offers the potential for increased storage density by utilizing a smaller mean particle size and ultimately storage of one bit per individual nanoparticle. The second approach involves patterned magnetic Co–Cr–Pt nanostructures produced using a focused ion beam, which offers the possibility of single bit per island storage on thermally stable sub-100-nm islands.

X-ray absorption and diffraction studies of thin polymer/FePt nanoparticle assemblies

We have produced assemblies of monodisperse 4 nm FePt nanoparticles using polymer-mediated layer–by–layer deposition at room temperature. The process leads to good control of particle assembly thickness and offers great potential for future fabrication of ultra-high density magnetic storage media. Vibrating sample magnetometry with fields up to 9 T was applied to study the magnetic properties of the particle assemblies as a function of annealing condition while near edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray diffraction (XRD) were used to investigate the chemical nature and structural properties within particles. It was found that the coercivity can be as high as 22.7 kOe for samples annealed at 800u200a°C, the moment density (normalized to the particle volume) for the sample annealed at 650u200a°C is estimated close to the value for bulk FePt, at 1140 emu/cm3. NEXAFS spectroscopy shows that the Fe in the as-deposited assemblies is partly oxidized, and the oxidation is greatly reduced by...

X-ray studies of magnetic nanoparticle assemblies

Monodisperse FePt nanoparticles were prepared using high-temperature solution phase synthesis. Polymer-mediated layer-by-layer growth leads to precise control of the particle self assembly. The narrow particle size distribution (σ⩽5%) offers the potential for increased data storage density by utilizing a smaller mean particle size and ultimately storage of one bit per individual nanoparticle. We have studied self-assembled multilayers of magnetic FePt nanoparticles. The L10 phase of FePt has a very high magnetic anisotropy that allows the magnetization of particles of about 4 nm diameter to be thermally stable at room temperature. Magnetic measurements using a vibrating sample magnetometer were combined with x-ray diffraction (XRD) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy to study the annealed FePt nanoparticle assemblies and to optimize annealing conditions. NEXAFS spectra showed that a fraction of the iron in the as-deposited particles was oxidized, and this fraction was reduc...

Angle dependent magnetization reversal of thin film magnetic recording media

The results of angular measurements of the remanent switching fields for granular longitudinal and perpendicular magnetic recording media are presented and compared to idealized models of magnetic switching. Co alloy longitudinal and perpendicular recording media are found primarily to have a Stoner–Wohlfarth switching character at vibrating sample magnetometer time scales. Since the Stoner–Wohlfarth model does not consider the effects of thermal activation, the angular dependence of the time independent switching field Hswro(θ) was determined from a Sharrock analysis. This approach shows a closer agreement between experimental data and model. For the case of a representative high density longitudinal recording medium, we additionally investigate the switching field distribution.

Thermal stability and recording properties of sub-100 nm patterned CoCrPt perpendicular media

Patterned magnetic media, where the bit cells are predefined, offer a potential path to ultrahigh density data storage. Here we report results on the magnetic and recording properties of islands with lateral dimensions on the order of 80 nm. Prototype nanometer-scale magnetic structures were made by patterning single-layer Co70Cr18Pt12 perpendicular media using a focused ion beam of Ga+. The effects of patterning were investigated by comparing patterned and unpatterned regions of the same medium. The magnetic reversal properties of the patterned media were obtained from magnetic force microscopy images, while those for the unpatterned media were obtained from vibrating sample magnetometer measurements. At remanence the unpatterned region decays at 1.4% per decade, whereas the patterned region shows no measurable change over the course of the experiment (5×105u2009s). We have also studied aspects of the recording physics of patterned media using a quasistatic write/read tester with a giant magnetoresistance he...

Magnetic recording on patterned media

The storage density of conventional thin film recording media, where each bit is comprised of several hundred grains, is limited by superparmagnetism, since thermal excitation of the magnetization may degrade recorded information. Patterned magnetic media where magnetic bits are recorded on pre-defined, single-domain islands is one of the proposed approaches for extending magnetic storage densities beyond the limit set by thermal decay for conventional media. In this paper we discuss writing and reading from single domain magnetic islands with a conventional recording head, compare the performance to unpatterned media, and present a possible manufacturable approach to achieving run-scale patterning over disk sized areas.