This paper describes the noise spectra in the GHz range of tunneling magnetoresistive (TMR) read heads as a function of both bias current and external magnetic field in order to understand the effect of spin-torque (ST) on the thermally excited magnetization fluctuation noise (thermal mag-noise) in the TMR heads. A strong noise spectral peak at around 3-4 GHz is either enhanced or reduced depending on both the bias current and the external magnetic field. This result demonstrates that the ST influences the thermal mag-noise originated from the free layer under the positive (or negative) bias current in case the magnetization of the free and reference layers are nearly anti-parallel (or parallel) to each other.

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  The magnetic properties of perpendicular magnetic recording media after ion irradiation modification were studied. CGC (Coupled Granular and Continuous) PMR media with a line-and-space pattern were dosed with Ar ions at a density of 1e+16 ions/cm². A nano-imprint technique was used to fabricate precise minute nano-scale patterns. Examination of the Kerr loop of the line-and-space pattern after ion irradiation revealed that the coercivity and nucleation field were reduced, and the loop shape was similar to the sum of the loops between the magnetic film has ion irradiated and the resist stopped ions was observed by MOKE. As expected, the interactions were quite small between the ion-irradiated area, where magnetic modification was applied, and the non-ion-irradiated area. This indicates that the exchange coupling between the track, which preserves its magnetic properties, and the groove, which is magnetically modified, was significantly smaller than that expected in the continuous layer. To evaluate the read/write characterization, a spin-stand was used. There was no abnormal head flying. It was confirmed that there was no abnormal noise, such as Barkhausen noise, in the line-and-space pattern, except at the edge of the pattern. Analysis of the readback waveform signal in FFT showed that the patterned (ion-irradiated) area has several peaks that can be observed at lower frequency than the harmonics peaks, unlike the non-patterned (non-ion-irradiated) area.

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  The authors did some experiments and calculations on the initial magnetization curves for Fe-3mass%Si powder cores. The initial magnetization curves of the powder cores with gas-atomized and water-atomized powders were measured. The magnetic flux density was high for water-atomized powder cores at low magnetic fields, but it was high for gas-atomized powder cores at high magnetic fields like 10 kA/m. The initial magnetization curves were calculated taking into account the distribution of the demagnetizing field, which was supposed to have a log-normal distribution. The calculated curves were found to be in good agreement with the measured curves by adjusting the mean and standard deviation of the demagnetizing factor. The gas-atomized powder cores were estimated to have smaller standard deviations than the water-atomized powder cores. This suggested that dispersions of the gaps between the particles and the particle shapes largely affected the demagnetizing factors of the powder cores.

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  Li_1+x(Ni_1/2Mn_1/2)_1-xO₂ was prepared by co-precipitation and calcination technique. The compounds had a layered rock-salt structure with R3m symmetry, and the degree of non-stoichiometry x was an important factor for the structural and magnetic properties. The degree of cationic ordering was improved with an increase in x and simultaneously, the ferrimagnetic characters were reduced. That is, the magnetic interaction network is converted from three-dimensional to two-dimensional ones, and is restricted within each c-plane. Hence, the long range magnetic correlation as well as the local magnetic interaction is weakened. The spontaneous magnetization was evaluated using a ferrimagnetic cluster model. As the degree of non-stoichiometry was increased, the cluster density was reduced, and it resulted in the lowering in the spontaneous magnetization at low temperature.

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