N. Hayashi

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/.

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.

Bit-Percolation Studies on CoPtCr–SiO

Bit percolation degrades the performance of high-density perpendicular magnetic recording. The magnetic cluster size determines the limit of the recordable bit length. In order to examine the parameter, which influences the high-density recording characteristics, we have investigated the relationship between magnetic cluster size and the recording bit length at which bit percolation begins to occur in a CoPtCr-SiO2 perpendicular magnetic-recording medium to the range of 1500 kfci. Our experimental data indicate that the onset of bit percolation takes place at a recording bit length close to the magnetic cluster size of the medium

_{2}

Computer simulation was used to examine the accuracy of anisotropy energy constant, K/sub 2/ of thin film media obtained from the 45/spl deg/ torque method. The easy axes of the media were distributed randomly in plane. The obtained K/sub u/ value depends on range of the applied field. For 2D random media with H/sub K/=1-40 kOe, M/sub s/=100-600 emu/cm/sup 3/, A=0-1/spl times/10/sup -6/ erg/cm, separation between adjacent grains=0-a nm, the difference between obtained K/sub u/ value and exact K/sub u/ was at most 20%, if the range of applied field was larger than 10-20 kOe. Here, the standard values of the physical parameters were H/sub K/=10 kOe, M/sub s/=460 emu/cma, A=0, separation between adjacent grains=0, and each parameter was varied successively.

Perpendicular Magnetic-Recording Media

The Gilbert equation expressed in conventional polar angle notation is solved for one-dimensional Neel and Bloch walls in a thin Permalloy film, using different numerical methods to compare the extent of stability. The methods examined include the Euler, so-called modified Dufort-Frankel, and backward Euler methods. The backward Euler method is found to be stable under arbitrary magnitude of time difference for the case of Neel walls if the contribution of the magnetization of the nearest neighboring cells to the demagnetizing field is treated implicitly. The method is also stable in the case of Bloch walls, though the stability limit for the time step is only 10 times as large as that of the Euler method. The Dufort-Frankel method is quite unstable. Two-dimensional computation is feasible using the backward Euler method. The principle and the results of the calculations are given. >

Accuracy of 45/spl deg/ torque method for obtaining anisotropy constant of 2D random films

The thermal stability of written bits on single- and double-layered perpendicular magnetic recording media were compared from micromagnetic simulation. It was found that, although double-layered media show a higher thermal stability than single-layered media, the difference is not large. Under the calculation conditions used here the gain in thermal stability by double-layering was found to correspond to 7.5% increase in the volume of a magnetic grain. It was also found that this difference tends to decrease at the low and high ends of recording density. Investigation of the magnetostatic field exerted in the recording layer shows that the change in the thermal stability is due to the change in the layer-thickness component of the static field exerted on the recording layer. Although the increase in the dispersion in the easy axes of magnetization was found to decrease thermal stability, the decrease was greater in single-layered films than double-layered films.

Direct solution of Landau-Lifshitz-Gilbert equation for domain walls in thin Permalloy films

The magnetization reversal of a nanometer size ferromagnetic particle with surface anisotropy is investigated by micromagnetic computer simulation. Two kinds of calculations were performed: calculations with and without thermal fluctuation. In the case without thermal fluctuation a surface anisotropy of several ergs/cm/sup 2/ was found to increase the magnetization reversal field of the particle by a factor of ten as compared with the case without surface anisotropy. The introduction of surface anisotropy was also found to increase the reversal field, and hence, improve thermal stability.

Thermal stability of written bits in double-layered perpendicular recording media

The formation and growing process of magnetic clusters in perpendicular thin films were investigated using micromagnetic simulation. Two estimation methods of cluster size were proposed. CS(area) was defined as the diameter of the circle which has the same area as the reversed region. CS(corr) was defined from the correlation between magnetizations of adjacent grains. These cluster sizes were calculated in the demagnetizing process. The shape of the cluster can be estimated from comparison between CS(area) and CS(corr). If CS(area) is larger than CS(corr), string shape clusters are formed. If CS(area) coincides with CS(corr), there are nonstring shape clusters. In order to make cluster size small in the demagnetizing process it is useful for the perpendicular thin film to have grain size dispersion

Effect of thermal fluctuation on reversal field of magnetic fine particle with surface anisotropy

In this study, we investigated the magnetic cluster formation in perpendicular thin films in demagnetizing process using micromagnetic simulation.