Chaehyun Kim

FePt nanodot arrays with perpendicular easy axis, large coercivity, and extremely high density

Ordered FePt nanodot arrays with extremely high density have been developed by physical vapor deposition using porous alumina templates as evaporation masks. Nanodot diameter of 18nm and periodicity of 25nm have been achieved, resulting in an areal density exceeding 1×1012dots∕in.2. Rapid thermal annealing converts the disordered fcc to L10 phase, resulting in (001)-oriented FePt nanodot arrays with perpendicular anisotropy and large coercivity, without the need of epitaxy. High anisotropy and coercivity, perpendicular easy axis orientation and extremely high density are desirable features for future magnetic data storage media applications.

Giant positive magnetoresistance in Co@CoO nanoparticle arrays

We report the magnetotransport properties of self-assembled Co@CoO nanoparticle arrays at temperatures below 100 K. Resistance shows thermally activated behavior that can be fitted by the general expression of R∝ exp{(T0/T)ν}. Efros–Shklovskii variable range hopping (ν=1/2) and simple activation (hard gap, ν=1) dominate the high and low temperature region, respectively, with a strongly temperature-dependent transition regime in between. A giant positive magnetoresistance (MR) of >1400% is observed at 10 K, which decreases with increasing temperature. The positive MR and most of its features can be explained by the Zeeman splitting of the localized states that suppresses the spin dependent hopping paths in the presence of on-site Coulomb repulsion.

Magnetization reversal in epitaxial MnAs thin films

The magnetization reversal process of MnAs epitaxial films grown on semiconductor substrates was investigated by magneto-optical Kerr effect (MOKE) microscopy, magnetic hysteresis, and magnetoresistance measurements. While the stripe patterns with opposite magnetization in the α-phase of MnAs have been observed previously, we have observed domain dynamics on a vastly different length scale. MOKE images revealed domain nucleation and avalanche domain reversal, which are a collective behavior resulting from interactions among a large number of α-phase regions. The observed reversal processes are reflected in the magnetic hysteresis and magnetotransport measurements. These effects are strongly film thickness dependent.