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Nickel cobalt spinel oxide, NiCo2O4, is an important material for spintronics because it exhibits both ferrimagnetic characteristics and electrical conductivity. Because high oxygen pressure during deposition is necessary to obtain useful properties, pulsed laser deposition has been used to fabricate NiCo2O4 films. In this study, we investigated the physical properties of NiCo2O4 films synthesized by reactive molecular beam epitaxy and annealing in a high-pressure oxygen atmosphere. The characteristics of the films strongly depended on the oxygen pressure, and a larger pressure tended to give larger magnetization and higher conductivity. Crystal structure analysis by x-ray diffraction and transmission electron microscopy revealed that the films consisted of a rock salt (Ni,Co)O phase and a spinel NiCo2O4 phase. This phase separation caused the small magnetization and large electric resistance because the rock salt phase was an antiferromagnetic insulator.

Fe3O4 possesses unique properties such as high Curie temperature and half-metallic nature and therefore can function as a high-spin-polarized electrode in spintronic devices. In order to use this compound in such devices, the magnetic characteristics of multilayers used in these devices, which include Fe3O4, should be highly controllable. In this study, we fabricated Fe3O4/Pt or Cr (0-10 nm)/Fe systems through epitaxial growth on MgO(110) substrates in order to investigate the magnetic coupling in Fe3O4 and metallic hybrid systems. We found that these two systems show a marked difference in the dependence of the coercive field (H-c) of the Fe layer on the nonmagneticmetal- layer thickness. Hc for the system with Pt showed variation characteristics of interlayer exchange coupling in metal systems, while H-c increased monotonically with the thickness of the Cr layer.

The frequency dependence of tunneling magnetocapacitance (TMC) in magnetic tunnel junctions (MTJs) is investigated theoretically and experimentally. According to the calculation based on Debye-Frohlich model combined with Julliere formula, the TMC ratio strongly depends on the frequency and it has the maximum peak at a specific frequency. The calculated frequency dependence of TMC is in good agreement with the experimental results obtained in MgO-based MTJs with a tunneling magnetoresistance (TMR) ratio of 108%, which exhibit a large TMC ratio of 155% at room temperature. This calculation also predicts that the TMC ratio can be as large as about 1000% for a spin polarization of 87%, while the TMR ratio is 623% for the same spin polarization. These theoretical and experimental findings provide a deeper understanding on AC spin-dependent transport in the MTJs and will open up wider opportunities for device applications, such as highly sensitive magnetic sensors and impedance-tunable devices.

Fe$_3$O$_4$ is a ferrimagnetic spinel ferrite that exhibits electric conductivity at room temperature (RT). Although the material has been predicted to be a half metal according to ab-initio calculations, magnetic tunnel junctions (MTJs) with Fe$_3$O$_4$ electrodes have demonstrated a small tunnel magnetoresistance effect. Not even the sign of the TMR ratio has been experimentally established. Here, we report on the magnetic properties of epitaxial Fe$_3$O$_4$ films with various crystal orientations. The films exhibited apparent crystal orientation dependence on hysteresis curves. In particular, Fe$_3$O$_4$(110) films exhibited in-plane uniaxial magnetic anisotropy. With respect to the squareness of hysteresis, Fe$_3$O$_4$ (111) demonstrated the largest squareness. Furthermore, we fabricated MTJs with Fe$_3$O$_4$(110) electrodes, and obtained an TMR effect of -12\% at RT. The negative TMR ratio corresponded to the negative spin polarization of Fe$_3$O$_4$ predicted from band calculations.

In order to combine the photovoltaic and piezoelectric activities in a single film, we fabricated tricolor superlattices consisting of an organic p-n junction and a piezoelectric polymer with sub-nm thickness control. The photovoltaic response of each repeating unit was saturated at 10 nm each as a function of the layer thicknesses. The capacitance across the 50 periods of superlattice changed by 0.009 pF upon photo-irradiation, which corresponds to a 2 nm thickness change of a 1-μm thick film. The characteristic time dependent behaviors of the capacitance of the superlattice consist of photocarrier generation, electrostriction, and poling of the piezoelectric polymer.

Ultrathin [Co/Pt]n and [Co/Pd]n superlattice films consisting of 0.14–0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3–9)×106 ergs/cm3] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2–2.4 nm. The films were stable against annealing up to 370 °C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Ω μm2) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory.

We fabricated fully epitaxial magnetic tunnel transistors (MTTs) consisting of a Fe(001)/MgO(001) emitter and a Fe(001) based on a n-GaAs(001) collector. We observed much higher transfer ratio (α) and magnetocurrent (MC) ratio than those of the previously reported MTTs that had an amorphous AlO barrier. These improvements are due to the coherent tunneling and single-crystalline base layer. In addition, a pronounced peak structure appeared in the emitter bias dependence of α, which might be attributed to the interface resonance states at the Fe(001)/GaAs(001) interface.

Topological surface analysis using in situ scanning tunneling microscopy was performed for highly oriented polycrystalline (textured) MgO(001) tunneling barrier layers grown on amorphous CoFeB electrode layers. The microscopy revealed a MgO surface structure in which nanosized grains were dispersed on clusters that originated from the CoFeB underlayer. In situ annealing reduced this surface roughness. Local tunneling spectroscopy measurements revealed the formation of a nearly perfect and uniform tunneling barrier in spite of grain boundaries in the textured MgO(001) layer, which is consistent with the fact that textured CoFeB∕MgO∕CoFeB and fully epitaxial MgO-based magnetic tunnel junctions exhibit comparable spin-dependent tunneling properties.

The authors fabricated fully epitaxial Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (MTJs) with various MgO thicknesses (tMgO) and investigated spin-dependent transport properties. Both the tunneling resistance in the parallel magnetic state (RP) and that in the antiparallel magnetic state (RAP) exhibited short-period oscillations as functions of tMgO with the same period of 3.2A and different phases. RAP also showed a long-period oscillation with a period of 9.9A. As a result, tMgO dependence of magnetoresistance is expressed as a superposition of the short- and long-period oscillations. These results provide important clues for understanding the oscillatory tMgO dependence of the tunneling magnetoresistance effect.

The microscopic structures of MgO(001) barrier layers in magnetic tunnel junctions showing giant tunneling magnetoresistance were characterized by in situ scanning tunneling microscopy. The MgO thin films formed exceedingly flat surfaces, and their terraces were made even flatter by annealing after deposition. This flattening of MgO surfaces apparently promotes coherent transport of electrons, which should enhance the tunneling magnetoresistance ratio. Local tunneling spectroscopy revealed that an annealed MgO layer has a critical thickness between 3 and 5 ML (monolayer), and a continuous film without pinholes can be formed over the thickness.

Fully epitaxial bcc Fe1−xCox(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (x=0, 0.5, 1) were fabricated with molecular-beam epitaxy and microfabrication techniques. While the bcc Fe(001) and Fe0.5Co0.5(001) electrodes had similar magnetoresistance (MR) ratios of about 180% at room temperature, the bcc Co(001) electrode exhibited a higher MR ratio up to 271% at room temperature (353% at 20 K). The fact that the MR ratio for a bcc Co electrode is much higher than that for a bcc Fe electrode is consistent with first-principle calculations, indicating the importance of electrode band structure in the k‖=0 direction.

Observation using in situ scanning tunneling microscopy of the layers constituting a magnetic tunnel junction with a naturally oxidized aluminum barrier layer revealed an extremely flat aluminum-oxide surface. It was clarified from line-scan images that the aluminum-oxide barrier layer has atomic steps. This flatness, which is surprising given that the aluminum-oxide film is amorphous, reduced electron scattering within the barrier, leading to momentum-dependent tunneling, which should enable the fabrication of advanced devices, such as spin-polarized resonant tunneling transistors.