James Rantschler

Sulfur and Saccharin Incorporation into Electrodeposited CoFe Alloys: Consequences for Magnetic and Corrosion Properties

Incorporation of sulfur into 2.4 T CoFe alloys during galvanostatic deposition has been studied. The results indicate that the main source of sulfur in magnetic deposit is saccharin used as an additive in the plating solution. The atomic percent of sulfur in deposit and the sulfur incorporation rate show strong dependence on saccharin concentration in the plating solution. This dependence has pronounced maximum at Csac 0.12 g L �1 , while for Csac 1.5 g L �1 the incorporation rate approaches constant value. A simple physical model is developed to describe the sulfur incorporation as a function of saccharin concentration having an excellent qualitative agreement with experimental data. The corrosion and magnetic properties of the electrodeposited 2.4 T CoFe alloys were found to be the strong function of the sulfur incorporation rate, and these results are discussed within the framework of the proposed sulfur incorporation model.

Carbon Combustion Synthesis and Magnetic Properties of Cobalt Ferrite Nanoparticles

Cobalt ferrite CoFe2O4 crystalline nanoparticles (50-100 nm) were produced by carbon combustion synthesis of oxides (CCSO). In this combustion synthesis process, the exothermic oxidation of carbon generates a thermal reaction wave that propagates through the solid reactants mixture of CoO and Fe2O3 converting it to cobalt ferrite. The extensive emission of CO2 increased the porosity and friability of the product. The quenching front method combined with XRD and VSM characterization revealed that crystalline CoFe2O4 particles formed in the early stage of the combustion, before the temperature reached its maximum. The maximum value of the coercivity of the quenched product within the front region was 940 Oe with a magnetization of 15 emu/g. The as-synthesized ferrites had hard magnetic properties with coercivity of 700 Oe and saturation magnetization of up to 47 emu/g

Recording Physics, Design Considerations, and Fabrication of Nanoscale Bit-Patterned Media

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.

Micromagnetics of signal propagation in magnetic cellular logic data channels

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.

He+ ion irradiation study of continuous and patterned Co/Pd multilayers

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.

Micromagnetic study of domain wall dynamics in bit-patterned nanodots

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.

Low temperature vacuum annealing study of (Co∕Pd)n magnetic multilayers

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.

Size Distribution and Anisotropy Effects on the Switching Field Distribution of Co/Pd Multilayered Nanostructure Arrays

We use ion beam proximity lithography (IBPL) to produce 4 mm times 4 mm arrays of 220-nm dots in perpendicularly oriented Co/Pd multilayered media. A novel technique of overlapping neighbors is used with IBPL to generate samples with controllable size distribution sigmaD of the nanoarrays. The switching field distribution sigmaHcr/HCr is measured for each sample before and after irradiation, and a linear relationship is experimentally found between sigmaHcr/HCr and sigmaD. Empirical calculations support our determination that self-demagnetization fields are responsible for a portion of the switching field distribution. Using the zero intercept of the experimental data, we find the inherent anisotropy distribution in the Co/Pd samples is on the order of 10%. We further irradiate the samples with He+ ions to alter the anisotropy and better understand the role of shape anisotropy. As interface anisotropy is reduced in the Co/Pd bit-patterned medium (BPM) samples, shape anisotropy has a much greater contribution on the switching field distribution of the BPM samples.

Design and Fabrication of High Anisotropy Nanoscale Bit-Patterned Magnetic Recording Medium for Data Storage Applications

Design considerations for 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. Optimized patterned multilayers used in this study had coercivities in excess of 12- 14kOe. 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.

Intergranular interactions of low temperature atmosphere annealed Co∕Pd magnetic multilayers

We present evidence that low temperature atmosphere annealing of Co∕Pd magnetic multilayers reduces both the dipolar and exchange interactions, but the dipolar interaction decreases very rapidly at the lowest temperatures. We also infer cobalt oxide formation at grain boundaries. This results in a reduction of the exchange between grains, reduces the length scale of ordering in the ac demagnetized state of the films. We demonstrate that this is the result of the decoupling the grains.