Tomohiko Niizeki

Tunnel magnetoresistance with improved bias voltage dependence in lattice-matched Fe/spinel MgAl2O4/Fe(001) junctions

We fabricated fully epitaxial Fe/MgAl2O4/Fe(001) magnetic tunnel junctions using plasma oxidation of an Mg/Al bilayer. The MgAl2O4 showed a (001)-oriented spinel-type structure, and there were few misfit dislocations at the interfaces between the MgAl2O4 and the two Fe layers due to a small lattice mismatch (∼1%). Tunnel magnetoresistance (TMR) ratios up to 117% (165%) were obtained at room temperature (15 K). The bias voltage for one-half of the zero-bias TMR ratio (Vhalf) was relatively large, ranging from 1.0 to 1.3 V at room temperature, which is attributed to the small misfit dislocation density.

Extraordinarily large perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite CoxFe3−xO4(001) (x = 0.75, 1.0) thin films

Perpendicular magnetic anisotropy (PMA) of cobalt-ferrite CoxFe3-xO4 (x = 0.75 and 1.0) epitaxial thin films grown on MgO (001) by a reactive magnetron sputtering technique was investigated. The saturation magnetization was found to be 430 emu/cm3 for x = 0.75, which is comparable to that of bulk CoFe2O4 (425 emu/cm3). Torque measurements afforded PMA constants of Kueff=9.0 Merg/cm3 (Ku=10.0 Merg/cm3) and Kueff=9.7 Merg/cm3 for x = 0.75 and 1.0, respectively. The value of Kueff extrapolated using Miyajimas plot was as high as 14.7 Merg/cm3 for x = 1.0. The in-plane four-fold magnetic anisotropy was evaluated to be 1.6 Merg/cm3 for x = 0.75. X-ray diffraction measurement revealed our films to be pseudomorphically strained on MgO (001) with a Poisson ratio of 0.4, leading to a considerable in-plane tensile strain by which the extraordinarily large PMA could be accounted for.

Large perpendicular magnetic anisotropy at Fe/MgO interface

A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m3 was observed from ultrathin Fe/MgO(001) bilayers grown on Cr-buffered MgO(001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.

Selective growth of Fe3O4 and γ-Fe2O3 films with reactive magnetron sputtering

A selective method for growing high-quality spinel-type epitaxial Fe3O4(0 0 1) or γ-Fe2O3(0 0 1) films was developed using conventional planar-type sputtering by controlling the flow rate of oxygen gas . Although magnetization of the oxide films is close to that of Fe3O4 or γ-Fe2O3 for all , the room-temperature resistivity and the Fe3+ and Fe2.5+ composition ratios of the films are dependent on . The films for and are identified as Fe3O4 and γ-Fe2O3 films, respectively. All the results suggest that Fe3O4 films are obtained only when sputtered in the metal mode, and γ-Fe2O3 films are obtained in the oxide mode.

Observation of longitudinal spin-Seebeck effect in cobalt-ferrite epitaxial thin films

The longitudinal spin-Seebeck effect (LSSE) has been investigated in cobalt ferrite (CFO), an exceptionally hard magnetic spinel ferrite. A bilayer of a polycrystalline Pt and an epitaxially-strained CFO(110) exhibiting an in-plane uniaxial anisotropy was prepared by reactive rf sputtering technique. Thermally generated spin voltage in the CFO layer was measured via the inverse spin-Hall effect in the Pt layer. External-magnetic-field (H) dependence of the LSSE voltage (VLSSE) in the Pt/CFO(110) sample with H ∥ [001] was found to exhibit a hysteresis loop with a high squareness ratio and high coercivity, while that with H∥[110] shows a nearly closed loop, reflecting the different anisotropies induced by the epitaxial strain. The magnitude of VLSSE has a linear relationship with the temperature difference (ΔT), giving the relatively large VLSSE /ΔT of about 3 μV/K for CFO(110) which was kept even at zero external field.

Perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite (001) thin films

We investigated the dependencies of both the magnetization characteristics and the perpendicular magnetic anisotropy of CoxFe3–xO4(001) epitaxial films (x = 0.5 and 0.75) on the growth conditions of the reactive magnetron sputtering process. Both saturation magnetization and the magnetic uniaxial anisotropy constant Ku are strongly dependent on the reactive gas (O2) flow rate, although there is little difference in the surface structures for all samples observed by reflection high-energy electron diffraction. In addition, certain dead-layer-like regions were observed in the initial stage of the film growth for all films. Our results suggest that the magnetic properties of CoxFe3–xO4 epitaxial films are governed by the oxidation state and the film structure at the vicinity of the interface.

Control of magnetization in spinel-type Fe3O4 thin films by N2 ion implantation

The magnetism of a typical spinel ferromagnetic oxide, Fe3O4, was controlled via ion implantation. Nitrogen ions were accelerated at 6–10 kV and irradiated to the 13-nm-thick Fe3O4 thin films with dosages of 2 × 1016 to 6 × 1016 ions/cm2. The magnetization decreased with the increase in ion dosage, and there was almost no magnetization when 6 × 1016 ions/cm2 of nitrogen was irradiated, irrespective of the acceleration voltages. The results of the temperature dependence of the magnetization and the Mossbauer study suggest that the transition from ferromagnetic to nonmagnetic phases in the Fe3O4 thin film upon N2 ion irradiation proceeds abruptly without the formation of intermediate states.

Magnetotransport properties in epitaxial Fe3O4(001) thin films with current perpendicular to the plane geometry

We investigated the fundamental transport properties of epitaxial magnetite (Fe3O4) films with applied current perpendicular to the plane geometry. The devices with a junction area of 36 μm2 were fabricated with TiN/Fe3O4/ TM (TM: Ti and Fe) stacking. Both the temperature dependence and the magnetic field dependence of the junction resistance were measured. Most of the resistance properties were independent of whether the electrode was magnetic (Fe) or nonmagnetic (Ti). Below the Verwey point, the junction resistance abruptly increased with decreasing temperature, and the resistance reached maximum at approximately 30 K. The magnetoresistance showed a peak of ∼27% at T≈55 K. The observed resistance behavior was consistent with a hopping conduction mechanism with a Coulomb interaction.

Nanofabrication of magnetic tunnel junctions by using side-edge thin film deposition

Abstract Nanostructured double ferromagnetic tunnel junctions (MTJs) are indispensable for investigation of spin-dependent single-electron transport at low temperature. A new fabrication process that enables us to reduce the size of MTJs down to nanometer scale by using the side edge of a patterned film were developed. The multilayers of MTJ partially replaced by thick Al2O3/Cu double layer were prepared by using electron beam lithography and lift-off, then Pt film was vacuum-evaporated onto the side edge of Al2O3/Cu film, which masked MTJ during following Ar ion milling. As a result, the double MTJs with the dimension of 10 nm £ 10 mm were formed beneath the Pt film. The large tunnel magnetoresistive ratio of 35% and symmetrical I–V characteristics were obtained at room temperature.

Ferromagnetic Resonance in Magnetite Thin Films

We have been investigating the spin motive force induced by magnetization dynamics in ferromagnetic materials. Ferrimagnetic materials are expected to exhibit even larger spin motive force due to their spatially staggered magnetic structure and high frequency spin wave modes. We believe that magnetite (Fe₃O₄) is one of the candidates for this purpose. Here, we report magnetic resonance in Fe₃O₄ thin films epitaxially grown on a MgO (001) substrate. The gyromagnetic ratio and the effective saturation magnetization are estimated to be 1.71 ×10¹¹ s^-1T^-1 and 364 emu/cm³, respectively. The Gilbert damping constant of 0.02 is obtained from the magnetic field dependence of the linewidth. We expect to investigate the spin motive force induced from this magnetic resonance.