Y. Uesaka

Fluctuation fields and medium noise in CoCrTa and CoCrPt films

Abstract The correlation between magnetic viscosity and medium noise in CoCrTa and CoCrPt longitudinal thin-film media was investigated by measuring the time dependence of the remanence coercivity H r and the read/write characteristics. The media were prepared by do magnetron sputtering under various conditions. Fluctuation fields H f of the magnetic viscosity at H / H r = 1 were obtained from the slopes of the H r versus In t plots. The medium noise decreases with kT / H f (the product of activation volume and saturation magnetization per unit volume), and is independent of other magnetic properties, such as the coercivity and the remanent magnetization per unit area. The medium noise thus primarily depends on the size of the minimum unit of the magnetic moment in reversal.

Interparticle interactions and the switching field distribution of barium ferrite media

A series of barium ferrite disks with a variation in volumetric packing density from 1.5% to 35% were investigated to determine the effect of interparticle interactions on the switching field distribution (SFD) of the media. Kelly et al. (1989) introduced a parameter Delta M(H) to highlight deviations from the theoretical non-interacting assembly. The degree of interparticle interactions were determined from the principal remanence curves and the subsequent calculation of the Delta M(H) parameter. It was found that the SFD of these disks became narrower as the volumetric packing density increased, and that the SFD width strongly correlated with the strength of the interparticle interactions as measured from the Delta M(H) plot. Examination of scanning electron micrographs of these disks revealed that for the low-packing-density disks, separation between the stacks of particles was large and that the separation decreased as the volumetric packing density was increased. It is believed that the properties of the media result not only from the large interparticle interaction arising from particle stacking, but also from the interaction between the stacks. >

Effect of Grain Size Dispersion of Read/Write Properties in Thin Film Recording Media Affected by Thermal Fluctuation

The effect of thermal fluctuation and particle-size dispersion on longitudinal and perpendicular magnetic recording media is investigated by computer simulation. In the case of longitudinal recording media the coercive force depends largely on the size of the particle rather than its distribution while it depends on both particle size and its distribution in the case of perpendicular recording media. Calculations performed to obtain read/write properties show that removal of particle-size dispersion results in 92% and 229% increase in the recording density in the case of longitudinal and perpendicular recording media, respectively, when particle size is 120 /spl Aring/.

Computer simulations of switching fields and magnetization reversal mechanisms of interacting cubic particles: Cases with fields applied parallel to the easy axes

Abstract Computer simulations were performed to investigate switching fields and magnetization reversal mechanisms of two, three, and four interacting cubic particles arranged in three configurations. The easy axes of all particles were oriented in the same direction, and fields were applied parallel to the easy axis. When three or four particles are aligned in the hard axis direction, the magnetizations of the outer particles switch at a field lower than the field at which the center particle(s) swith(es). When the particles are aligned in the easy axis direction, the magnetizations of all the particles switch at the same field, but the magnetizations of the end particles switch faster than the magnetization of the center particle(s).

Magnetic viscosity of oriented barium ferrite media

Abstract The magnetic viscosity of unoriented and oriented barium ferrite media has been investigated in order to understand the mechanism of magnetic reversal. The activation volume of magnetic reversal for the unoriented medium gradually increases with increasing test field. In contrast, the activation volumes for the oriented media depend on their degree of orientation. In the media with relatively low orientation, the activation volumes increase sharply in narrow field regions. In the higly oriented media, however, the amount of demagnetization varies nonlinearly against ln( t ) and the activation volume cannot be obtained as a function of field. In these cases, Richter-type demagnetization curves are obtained. The influence of magnetic viscosity on magnetostatic properties is also described.

Two interacting cubic particles: effect of placement on switching field and magnetisation reversal mechanism

The Landau-Lifshitz-Gilbert equation has been solved for two interacting particles in which each particle was divided into large number of elements, so that a detailed investigation of the effect of interparticle interactions on the magnetisation reversal mechanisms of these particles could be investigated. Interparticle interactions were found to have a drastic effect on the switching field. Positive interactions were observed when the particles were separated in the z direction. The reversal mechanisms for the isolated particle and the positively interacting pair were very similar, despite a large difference in switching field. When the particles were displaced in the x direction, the switching field of the interacting pair was reduced from that of the isolated particle. The reversal mechanism for this particle configuration was very complex and asymmetric, due to the nonuniformity of the interaction field. An interesting feature was observed when the particles were separated in the x and z directions. The switching field for this arrangement was lower than that of the isolated particle, indicating a net negative interaction effect. >

Dependence of switching time on temperature, applied field, and material parameters

The switching time of a single-domain particle with uniaxial anisotropy subjected to an applied field along the easy axis is studied by solving Browns Fokker-Planck equation numerically. The equation is modified in order to be integrated by a finite-difference method. The validity of this method is verified by comparing the frequency prefactors in the exponential decay calculated by this method with the values from Browns formula for the prefactor. Switching times for several values of the applied field and the temperature are calculated. Curves of the inverse of the switching time are fitted to a simple expression by using a least-squares method. The expression consists of two terms: a linear function of the applied field and a function proportional to the square root of the temperature. The dependence of the switching time on the magnetization, the volume of the particle, the anisotropy field, the Gilberts damping constant, and the gyromagnetic constant is also presented.

Thermal stability of longitudinal and perpendicular recording media obtained by Monte Carlo simulation (abstract)

The thermal stability of longitudinal and perpendicular recording media was investigated using Monte Carlo simulation method.12 In longitudinal recording, the easy directions of the grains are distributed randomly in the film plane. In perpendicular recording, the easy directions are aligned almost perpendicular to the film plane. The material parameters, Ms=400 emu/cm3, Ku=2 ×106u2002erg/cm3, and α=1, were used. The exchange stiffness constant between adjacent grains was assumed to be zero. The effects of thickness, grain size in-plane, and distribution of grain size in-plane of the media on decay of the magnetic properties and on decay of the output signal with time were calculated. The rate of signal decay of a longitudinal medium is similar to the decay rate of the squareness and it is much smaller than the decay rate of the coercivity. The same relationship holds for a perpendicular medium. In longitudinal recording, a high density signal is more affected by thermal fluctuation than a low density signal,...

Effect of Co- gamma Fe/sub 2/O/sub 3/ particle-size on high density rigid disks

The effects of particle size on the read/write characteristics of Co- gamma Fe/sub 2/O/sub 3/ particulate rigid disks are investigated. Smaller particle size reduces PW/sub 50/, the half-width of an isolated pulse wave, as well as disk noise. The transition region widens as the particle size increases. The change in the transition region seems to originate from the particle size itself, rather than from any inhomogeneity of the magnetic properties. The thickness dependence of the read/write characteristics of ultrathin coated disks using fine Co- gamma Fe/sub 2/O/sub 3/ particles was also investigated. In a 0.12- mu m-thick disk, D/sub 50/ is 36.5 kFCI at 0.20- mu m head flying height. >

Computation of switching fields of stacked magnetic hexagonal platelet particles

Abstract Switching fields of magnetic hexagonal platelet particles, which were stacked to the easy axis directions, were investigated by computer simulation. The particles had the easy axes perpendicular to the hexagonal planes. The angular dependence of the switching fields became smaller when more particles were stacked and when the particles were stacked closer together. This result is considered as one of the reasons for the small angular dependence of the remanence coercivity of a longitudinally oriented barium ferrite medium. Absence of exchange interaction between adjacent layers in a particle also reduces the angular dependence of the switching field.