G. Hu

Microstructure and magnetic properties of cobalt ferrite thin films

In contrast to films grown on spinel structure substrates, epitaxial cobalt ferrite films grown on MgO substrates provide a model system to study the effects of microstructural defects on the magnetic properties. The anomalous magnetic behaviors observed in as-grown films are explained by the structural and magnetic disorder across the antiphase boundaries. During post deposition annealing, magnesium interdiffusion between the cobalt ferrite film and underlying substrate was observed and resulted in phase segregation.

Observation of inverse magnetoresistance in epitaxial magnetite/manganite junctions

We have fabricated and characterized epitaxial Fe3O4/CoCr2O4/La0.7Sr0.3MnO3 magnetic trilayer junctions (MTJs) grown on (110) and (100) SrTiO3 substrates. Large inverse junction magnetoresistance (JMR) as high as −25% has been observed at a field of 4 kOe in (110) oriented MTJs. The improvement of the JMR over that observed in previous epitaxial magnetite junctions is attributed to the choice of the spinel structure CoCr2O4 barrier, which minimizes structural disorder at the barrier/Fe3O4 interface. As a comparison, studies on (100) MTJs elucidate the effect of the magnetic domain state of the ferromagnetic electrodes on the JMR.

In-plane uniaxial magnetic anisotropy of cobalt-doped Y3Fe5O12 epitaxial films

We have grown magnetically harder single-crystalline thin films of Y3Fe5O12 doped with Co2+ on (110)-oriented Ga3Gd5O12 substrates. Ge4+ and Ce4+ have been substituted to compensate for the charge differential between Co2+ and Fe3+. These garnet films, prepared using pulsed-laser deposition, exhibit excellent crystallinity as determined from x-ray diffraction and Rutherford backscattering spectroscopy. The addition of Co2+ in Y3Fe5O12 films enhances the in-plane uniaxial anisotropy over an order of magnitude, depending on composition.

Magnetotransport in exchange-coupled magnetite junctions

Epitaxial magnetic junctions of theoretically half-metallic electrode material magnetite were fabricated with a cobalt chromite paramagnetic insulating tunnel barrier. This spinel structure barrier was chosen to be isostructural to the magnetite electrodes. Highly crystalline trilayers with low surface roughness were grown. Strong exchange coupling between the electrodes is observed across the barrier. Magnetoresistance measurements are reminiscent of a single ferromagnetic layer and not a junction.