Keikichi Ando

Fine Metallic Particles for Magnetic Domain Observations.

Fine domain structures of a ferromagnetic specimen can be visualized by deposition of ultrafine ferromagnetic particles onto its surface. We have applied various ferromagnetic particles, such as Co, Fe, Co–Fe alloy and Permalloy, to observation of high-density recording states of thin-film recording media. These studies have revealed that single-domain particles mainly contribute to the domain pattern contrast, while very small superparamagnetic particles are so insensitive to the leakage field from the recording media that they deteriorate the image quality of the domain patterns. Therefore, suppressing the formation of the superparamagnetic particles is effective in improving the resolution and the image quality of the observed domain patterns. In order to suppress the formation of superparamagnetic particles, we have attempted to synthesize hcp-Co particles with large uniaxial anisotropy by alloying Co with some elements which increase the fcc hcp transformation temperature, but all the Co and Co-alloy particles exhibit a pure high-temperature fcc phase.

Evolution of magnetic domain patterns at low temperature

Abstract The domain observation technique using fine magnetic particles as small as 10 nm is very effective in investigating the high density recording states of various kinds of magnetic and magneto-optic recording media. Theoretical calculations predict that suppression of thermal agitation of spins within each particle greatly improves the image quality of observed domain patterns. This prediction has been experimentally verified by varying the sample temperature during evolution of the domain patterns. As the sample temperature decreases, high density recording states can be visualized more clearly and definitely owing to the suppression of thermal agitation of the spins.