H. Fukushima

Volume average demagnetizing tensor of rectangular prisms

An exact analytic formula is presented for the magnetostatic field produced by a three-dimensional array of uniformly magnetized rectangular prisms. It is derived from the volume integration of a point-function demagnetizing tensor. The values of the point-function demagnetizing tensor at the centers of rectangular prisms of different aspect-ratios are compared with the volume averaged ones. This difference for the zz-component of demagnetizing tensors reaches 35% at its maximum when the aspect-ratio is 2.

Computer simulation of thermal fluctuation of fine particle magnetization based on Langevin equation

Abstract Thermal fluctuation after-effect is simulated using the Langevin equation. The relations between the number of reversals of a fine magnetic particle and the material parameters are investigated, and an experimental formula, p = 60α/(1 + α2)exp((−kuv0.979)/(kT)) is obtained for the transition probability due to the thermal fluctuation after-effect. The formula is found to depend on the damping constant and the transition probability is 30 times as large as that predicted theoretically.

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/.

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.

Protective circuit for electroacoustic transducer with hall element and voice coil temperature rise time constant circuit

The output signal of an l-f (low-frequency) power amplifier is supplied to an electroacoustic transducer, through an inductive element for preventing parasitic oscillation, to cause the inductive element to generate a magnetic flux whose density depends on the output current signal of the power amplifier while a current according to the magnitude of the output voltage signal of the power amplifier is supplied to the current input terminals of a Hall-effect element disposed in the magnetic field induced by the inductive element, so that a voltage according to the output power of the l-f power amplifier is taken out of the output terminals of the Hall-effect element. When the voltage exceeds a predetermined level, the l-f signal supplied to the electroacoustic transducer is attenuated by a signal attenuating circuit so that the transducer is prevented from being damaged.

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 ×106 erg/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,...

Evaluation of the base metal activity of oxide cathodes

Activity of the base metal is evaluated by identifying reaction products with the oxide layer, measuring Ba vapor pressure from the oxide, measuring of emission current properties, and by analyzing the surface composition of the oxide. Reducing capabilities of Zr, W, and Mo activators in Ni base metal are determined. Their activators are alloyed to the Ni core metal up to the solubility limits. The experimental results are evaluated systematically from the standpoint of base metal activity, together with the thermochemical treatment for the various reactions and the emission mechanism of the oxide coated cathode.

Effect of grain size on cluster size

A cluster size of a perpendicular medium was calculated from the correlation between grains in M‐H loops. The cluster size was affected by physical properties as follows. When grain size is small and exchange stiffness constant A is not zero, the cluster size increases with decreasing grain-size. The increment of the cluster size decreases with increasing grain size distribution. The cluster size divided by the average grain size (normalized cluster size) increases with an increasing intergranular exchange coupling constant he. The normalized cluster size can be estimated from the grain-size distribution and the intergranular exchange coupling constant.

Computer simulations of magnetization switching of elongated magnetic particles with shape defects

The switching fields of elongated magnetic particles with various shape defects were investigated by means of computer simulations. The magnetic particles were approximated by rectangular parallelepipeds that have groove or chip defects. We modeled 13 types of particle shape defects for two particle sizes. The particle sizes used were “small particle” 10 nm×10 nm×50 nm and “large particle” 40 nm×40 nm×200 nm. The magnetocrystalline anisotropy field in the magnetic particles was set to zero. The switching fields of the small and large particles were found to decrease with the number of groove defects, but they were independent of the total number of chip defects. The magnetic moments of all of the small particles rotated almost coherently. The switching fields of the small particles almost coincided with the average of the difference between the demagnetizing fields in all cells along two directions: An easy-axis direction and one of the hard-axis directions. That is, we obtained the switching fields with ...

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

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.