We demonstrate that the crystallization of ferromagnetic CoFeB layers originates at the interface with a MgO layer in CoFeB/ MgO/CoFeB magnetic tunnel junctions by annealing using cross-sectional transmission electron microscopy and electron diffraction. The CoFeB layers, which are amorphous in the as-deposited state, crystallize with a (001) out-of-plane texture by annealing at 360 C. Crystal grains of 15 -20 nm in the CoFeB layers are observed at the interface with the MgO layer, but not at the interface with a Ta or Ru layer. Much smaller crystal grains with random crystal orientations are formed in a region away from the MgO interface in the CoFeB layers. The depth profiles obtained by X-ray photoelectron spectroscopy show that boron diffuses from the crystallized region at the interface into the MgO layer and the rest of the region in the CoFeB layers during crystallization, indicating that crystal grains have much lower B contents than the original composition.

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The authors have prepared Ba–ferrite film disks and evaluated their magnetic read/write characteristics. The c-axis-oriented Ba–ferrite thin films were formed epitaxially on c-axis-oriented ZnO films by means of a targets-facing-type sputtering system. The direction of easy magnetization in the film with perpendicular crystalline anisotropy is almost perpendicular to the film plane. Thus, the perpendicular remanence is more than 60 times as large as the in-plane one. The simple Ba–ferrite films, however, have a coercive force, Hc⊥ above 2 kOe which is too large to write on by means of a conventional head. The substitution of In3+ in the Ba–ferrite films makes the Hc⊥ values decrease to about 1 kOe without increasing the critical temperature, Tcritical, of c-axis orientation. The read/write characteristics of the films were measured with a MnZn–ferrite ring head. The reproduced signal exhibits clear dipulsive waveforms as the dipulse ratio exceeds 0.7. Furthermore, the overwrite ratio decreases below −28 dB for the In3+-substituted Ba-ferrite film disk. The Ba–ferrite film disks have promising potential as a high-density perpendicular magnetic recording medium.

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