The process of the formation of magnetic clusters in perpendicular recording media was investigated through micromagnetic simulation and compared with data for longitudinal recording media. The size of magnetic clusters, which is obtained from the autocorrelation length of demagnetized state by dc reverse field, was almost the same as that for written states. It increased with increasing intergrain exchange coupling and was independent of the process of cluster formation. Also, the thermal switching unit size (activation volume) was found to be smaller than the magnetic cluster size. It remained almost the same as the grain size until the exchange coupling reached a critical value, and increased further. The transition noise, the main part of the medium noise, increased with increasing the magnetic cluster size.

Upper and lower bounds on generalized scattering lengths for static potentials are presented. Their derivation is based on complementary variational principles for a certain class of linear operator equations. Generalized forms of the well-known bounds of Schwinger and of Spruch and Rosenberg are obtained from this approach together with related complementary bounds which are new. The results are illustrated with calculations for screened Coulomb potentials.

The possibility of ultrahigh-density recording higher than 10 Gb/in.2 with perpendicular magnetic recording is investigated by computer simulation for a ring-type head and single-layer medium combination. A nucleation model is used as a media model because it incorporates a nucleation site, which causes irreversible magnetization switching. Fundamental read–write characteristics are found to be entirely different from those of longitudinal recording. Recorded magnetization strongly depends on head field strength; the maximum magnetization appears around the head field strength of media coercivity, and beyond this the recorded magnetization decreases abruptly. Spacing loss in the recording process also depends on head field strength and recording density. However, saturation recording can be attained even at an ultrahigh recording density of 600 kFCI with a spacing of 30 nm. Also, a higher signal-to-noise ratio than in longitudinal recording can be obtained by introducing weak intergrain exchange interaction with a relatively large grain size in the media film. This suggests that perpendicular magnetic recording is stable in thermal fluctuation. © 1996 American Institute of Physics.

The magnetic orientation has been studied for paramagnetic organic radical crystals 1,3,5-triphenyl-6-oxoverdazyl and 1,5-di-p-tolyl-3-phenyl-6-oxoverdazyl in magnetic fields of 2-80 kOe at temperatures of 77-343 K. The X-ray diffraction measurement has revealed that the crystals are oriented with the crystallographic c axis perpendicular to the field. The anisotropic diamagnetic susceptibility arising from the benzene rings has been estimated for the crystals along the principal magnetic chi 1, chi 2, and chi 3 axes. (The chi 1 axis is at a small angle to the a axis in the monoclinic ac plane, and the chi 3 axis is along the b axis.) Since the paramagnetic susceptibility originating from the verdazyl ring is isotropic (though a large absolute value), it is shown that the magnetic orientation occurs by the anisotropy of the diamagnetic susceptibility in the crystals. The diamagnetic susceptibility is found to have a relation of chi 2 < chi 1 < chi 3 < 0.

The micromagnetic simulations for perpendicular recording media with incoherent magnetization rotation was performed by dividing a grain into subgrains in the direction of film thickness. When intra-grain exchange coupling was smaller, incoherent magnetization rotation occurred, leading to stronger thermal fluctuation effect and lower coercive force . It was found that the critical thickness exists, beyond which the incoherent magnetization rotation occurs.is approximately proportional to 2 5 ( ) 1 2 , where is the intra-grain exchange stiffness constant and is the uniaxial perpendicular anisotropy energy. In the incoherent magnetization rotation, the decreasing rate of by logarithmic time (in thermal viscosity slopes) was larger than that for coherent rotation. The calculations were in good agreement with the experimental coercive forces and thermal viscosity slopes.