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A complex transverse ac susceptibility ([[chi].sub.T]) technique based on magneto-optical detection has been developed with sufficient sensitivity for thin-film recording media. The technique has been used to measure the dispersions in grain size and the anisotropy field of perpendicular magnetic recording media. Experiments have been designed to study the key factors contributing to the anisotropy field distribution ([sigma][H.sub.k]) in perpendicular media. The correlation between anisotropy dispersion, stacking faults, and recording performance have also been studied. A good correlation between stacking faults and [sigma][H.sub.K] has been established. Lower stacking faults result in smaller anisotropy distribution and better media signal-to-noise ratio. The main factors controlling stacking faults are studied. Theoretical studies of the effect of interactions on the [[chi].sub.T] are also presented. Index Terms--AC suseptibility, anisotropy distribution, magnetic recording, perpendicular media, stacking faults.

A complex transverse alternating-current (ac) suspectibility [X.sub.T] technique based on magneto-optical detection is developed with sufficient sensitivity for thin film recording media. The technique is used to measure the dispersions in grain size and anisotropy field in perpendicular magnetic recording media.

Numerical and analytical calculations of rotational process in perpendicular recording media are presented. The work supports recent experimental studies that suggest that the measurement of rotational magnetization processes can be used to determine the value of the anisotropy constant. An expression for the rotational magnetization for a noninteracting system is derived taking into account the dispersion of K and the easy-axis orientation. The calculations show that the experiments determine the mean value of [H.sub.K], essentially independent of the angular dispersion. A numerical (Monte-Carlo based) micromagnetic model is used to study the effects of magnetostatic and exchange interactions at nonzero temperatures. It is shown that for small values of KV/kT, irreversible magnetization processes take place, which precludes the use of the rotational magnetization method to determine K values. This effect is enhanced by the presence of the magnetostatic interaction. However, the presence of exchange interactions is found to enforce coherent rotation in small fields, reducing the irreversible processes and allowing the determination of [H.sub.K]. Under these circumstances, it is shown that the exchange does not significantly affect the value of [H.sub.K], and that a well-defined demagnetization correction of 4[pi] M is appropriate. Finally, a comparison with experimental data gives good agreement for multilayer and granular media and shows the role of domain formation on the rotational magnetization process. Index Terms--Anisotropy, micromagnetics, perpendicular media, rotational magnetization.

The design of media for ultrahigh-density perpendicular recording is discussed in depth. Analytical and semianalytical models are developed to determine the constraints upon the media to fulfill requirements of writability and thermal stability, and the effect of intergranular exchange coupling is examined. The role of vector fields during the write process is examined, and it is shown that one--dimensional models of perpendicular recording have significant deficiencies. A micromagnetic model is described and the results of simulations of recording undertaken with the model are presented. The paper demonstrates that there is no physical reason why perpendicular recording should not be possible at or above 1 Tb/[in.sup.2]. Index Terms--Magnetic recording, micromagnetic modeling, perpendicular recording, recording media.

A generalised deltaM method is used to investigate and characterize interparticle interactions.

We have investigated the impact of Ag surface coating on the microstructure of CoAg core-shell nanoparticles, and the consequences for the magnetic properties. Atomic structures were simulated using a molecular dynamics approach utilizing the embedded atom method. The magnetic properties were then simulated using an atomistic approach using a classical spin Hamiltonian, taking into account the long-range nature, atomic separation and directional and phase dependence of the exchange interactions in Co. For pure cobalt nanoparticles with a diameter less than approximately 3 nm the internal crystal structure showed multiple twinned regions and the morphology of an icosahedron. The addition of a monolayer silver coating alters the internal cobalt crystal structure to a regular planar form along the cubic [111] direction with a mixture of face centred cubic (fcc) and hexagonal close packed (hcp) atomic arrangements. The local atomic environment was used to assign anisotropies to the atoms on a site by site basis. Moreover, the capping layer influences the shape of the particle and yielded a morphology similar to that of a truncated octahedron. Taking into account the effects of surface anisotropy in addition gives an overall picture of anisotropy for CoAg nanoparticles. We present the results of calculations estimating the energy barrier to magnetization reversal and the intrinsic coercivity for these particles. Index Terms--Cobalt, core-shell, crystal structure, energy barrier, magnetic anisotropy, nanoparticles, nanostructured materials, nanotechnology.

The time variation of magnetostatic fields generated by space and time varying magnetization configurations in small perpendicular pole tips is studied. The magnetization configurations are a response to external fields driving the pole tip and soft under layer (SUL). When the system damping is sufficiently small the magnetization excitations persist for a long time after reversal. The effects of damping parameter, position in the media, and discretization cell size on the magnitude of the time varying magnetostatic fields will be given. Decreasing the damping parameter increases the magnitude of the magnetostatic field variation. Index Terms--Damping parameter, head field, micromagnetic, perpendicular recording.

For acceleration of the calculations of thermal magnetic switching, we report the use of the Voter method, recently proposed in chemical physics (also called 'hyperdynamics of the infrequent events'). The method consists of modification of the magnetic potential so that the transition state remains unchanged. We have found that the method correctly describes the mean first passage time even in the case of small damping (precessional case) and for an oblique angle between the anisotropy and the field directions. Due to the costly evaluation of the lowest energy eigenvalue, the actual acceleration depends on its fast computation. In the current implementation, it is limited to intermediate time scale and to small system size. Index Terms--Computational magnetics, numerical methods, spin dynamics and relaxation.

The anisotropy field of perpendicular media with a well-defined demagnetizing state has been determined with the help of magneto-optical measurements. The theoretical studies indicate that the effects of easy axis angular dispersion are small, about 1% error for 5 degrees dispersion, and the rotation method measures the mean value of anisotropy field H(sub k) for media with anisotropy field distribution.

A new method for evaluation of the statistical and mean interaction fields in a particulate system is described. The Integral Generalized [Delta]M plot is introduced as the area of the Generalized [Delta]M plot against the starting remanent magnetic moment for the corresponding remagnetization process. This new plot is sensitive to the shape of the interaction distribution and to the value of the mean interaction field. The Integral Generalized [Delta]M plot provides an efficient way of distinguishing between the effects of the statistical and mean field interactions. Index Terms--Magnetization processes, inter-particle interactions, Preisach modeling, magnetic recording media.