Masayoshi Hiramoto

Fabrication and Magnetoresistance Properties of Spin-Dependent Tunnel Junctions Using an Epitaxial Fe3O4 Film

Magnetoresistance of spin-dependent tunnel junctions has been studied using a high-quality epitaxial Fe3O4 film. The bottom magnetic electrodes of epitaxial Fe3O4 were grown onto the TiN-buffered (110) surface of MgO single-crystal substrates, and trilayer junctions of Fe3O4/AlOx/CoFe mesa were fabricated by sequential sputtering and Ar ion etching. The junctions showed the magnetoresistance (MR) ratio of more than 10% at room temperature with butterfly-like hysteresis which arose from the different coercive fields between Fe3O4 and CoFe when the field was applied along the easy axis of the epitaxial Fe3O4 layer. The MR ratio remained almost constant against the temperature down to nearly 100 K. Below 100 K, the decrease of MR and the increase of junction resistance were observed, which may be related to the Verwey transition that inevitably occurs in the characteristic of high-quality Fe3O4 samples.

Thermally stable exchange-biased magnetic tunnel junctions over 400 °C

Exchange-biased magnetic tunnel junctions (MTJs) with interposed Fe1−xPtx metal alloy layers between the Al oxide barrier and the ferromagnetic electrodes maintain large tunneling magnetoresistance (TMR) after thermal treatment in excess of 400 °C, owing to an improved barrier interface. After 400 °C annealing, TMRs of MTJs with Fe1−xPtx (x=0.1–0.2) exhibit over 40% and retain 30% TMR after 420 °C annealing. The tunnel barrier height derived from the current–voltage curve fitted to the Simmons equation increases with richer Pt content. Secondary ion mass spectroscopy depth profiles and cross-section transmission electron micrographs of MTJs with Fe0.85Pt0.15 show a clear interface around the Al oxide barrier even after annealing at 400 °C.

Microstructure and soft magnetic properties of FeSiAl(Ti/Ta)(O)N

The microstructure and magnetic properties of FeSiAl(Ti/Ta)(O)N films prepared by rf magnetron reactive sputtering were studied. The microstructure was controlled by changing the N2 gas flow rate ratio [η=N2/(Ar+N2)] in the sputtering gas. The soft magnetic properties were observed after thermal treatment at around η of 2% and η of 20%, where the saturation magnetostrictions and the intrinsic stresses took small values. The film with η of 20% had a granular structure as mostly observed on nanocrystalline materials. On the other hand, the film prepared with η of 2% had the structure which consisted of needle shaped grains or dendritic grains. The composition of the granular grains and needle shaped grains were mainly Fe and FeSi, respectively. The film with η of 2% exhibited Bs of 1.3 T, permeability of 3000–8000 at 1 MHz, respectively, and realized a high corrosion resistance. The soft magnetic properties of the films with needle shaped grains are explained by two-dimensional random anisotropy effect.

Improvement of Thermal Stability of Metal/Oxide Interface for Electronic Devices

The nano-meter controlled iron/iron-oxide multilayer materials have been successfully obtained by the pulse reactive sputtering method with high deposition rate. These multilayer demonstrated a good thermal stability of its structure and magnetic properties up to 500 C when a small amount of Si was doped in the structure, whereas the non-doped multilayer degraded at above 300 C. The difference of the oxidation energy between Fe and Si increases the thermal stability of the interface between Fe and Fe-O layer.

Magnetic thin films with anisotropic grain-growth in the 1 or 3-dimensional direction

Publisher Summary This chapter investigates nanocrystalline magnetic materials, made of small grains under 20nm, to realize soft magnetic materials with high saturation magnetic flux density, Bs. It investigates the magnetic, structural, and corrosive properties of sputtered fesialtion. Though the general nanocrystalline materials are obtained from the amorphous state with isotropic grain growth by thermal treatment, the developed materials have fine crystalline textures as prepared state. According to the TEM observations, the microstructures of the film annealed at 500°C consist of needle shape fine grains in 1-dimensional growth and/or dendritic in 3-dimensional growth. The typical grain is over 200nm in height and 20nm in width. The developed materials exhibit high corrosion resistance with high Bs as well as good soft magnetic properties caused by their compositions and structures. The developed materials have Bs of 1.3T, permeability of 3000–8000, respectively, and have higher corrosion resistance than FeSiAl with Bs of 1T.