Ayumu Kida

Development of half-metallic ultrathin Fe3O4 films for spin-transport devices

We attempted to fabricate a high-quality Fe3O4 film while satisfying both low-thermal preparation (≦573 K) and film thinness (≦500 A). X-ray diffractometry showed that our prepared Fe3O4 film was epitaxially grown onto a MgO (100) substrate. The saturation magnetization, resistivity, and Verwey point were, respectively, ∼438 emu/cm3, ∼10 000 μ Ω cm, and ∼110 K. These values were comparable to those of the Fe3O4 bulk. Our experimental results suggested that a high-quality Fe3O4 film could be obtained even under the crucial conditions of the deposition temperature being low (∼523 K) and the film being ultrathinned (∼100 A).

Magnetic properties of fcc-Fe artificial superlattices

Abstract Fcc-Fe/Cu(M)(001) (M = Au,Ni) artificial superlattices were prepared. The interatomic distance of fcc-Fe was changed by the epitaxial growth of Cu(Au) or Cu(Ni) alloy layers. The magnetic moment of Fe is increased up to 2.7μB for only 1% increment of interatomic distance. Furthermore, non-ferromagnetic state of fcc-Fe was observed at 1.8% decrement.

Fabrication and magnetoresistive effect of current perpendicular to plane devices using half-metallic Fe3O4 thin films on metallic films

The current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) devices having half-metallic Fe3O4 for their magnetic layers were investigated along with the fabrication of Fe3O4 films on Au layers at low temperature (523 K). The 10–50-nm-thick Fe3O4 films that were grown on a 100 nm Au (111) layer were Fe3O4 (111) oriented. These films showed Verwey transition at ∼120 K. Using these films, the relation between the magnetoresistive (MR) effect of CPP-GMR and the Fe3O4 layer thickness was examined with 2×2 μm2 samples of Ni80Fe20/Au/Fe3O4 trilayers on Au bottom electrode films. At the Fe3O4 layer thickness of 20 nm, the MR ratio was 0.04% and the area magnetoresistance-change product (ΔRA) was 1.5 mΩ μm2. The MR ratio was increased with decrease in the Fe3O4 thickness. The CPP-GMR of Fe3O4/Au/Fe3O4 on the Au layer showed that the MR ratio was 0.04% and the ΔRA was 3.9 mΩ μm2. This MR ratio was four times larger than that of the NiFe-type CPP-GMR for the same Fe3O4 bottom layer thickness.

Atomic Rearrangement and Magnetism of [Fe/Cu] Multilayers on Cu(001)

Multilayers of [7ML Fe/14ML Cu] on Cu(001) were prepared. A rearrangement of Fe atoms is observed, when Fe is deposited on a flat Cu-buffer surface. The degree of rearrangement depends on the flatness of the buffer surface. The structure of the rearrangement is 3D-( n ×1), n = 2, 3, 4, 5 etc. and the mean magnetic moment of the Fe atoms in a sample which contains many rearranged atoms is about 0.7 µ B , while that of a sample mainly with fcc structure is 1.6 µ B . The various values of Fe moments for multilayers with the same design are corresponding to the mixture of the 3D-( n ×1) and fcc structures in the film. The fcc structure of Fe is obtained when Fe is deposited on a rough Cu-buffer surface.

Magnetic properties of fcc‐Fe multilayer (invited) (abstract)

Recently, magnetic properties of fcc‐Fe ultrathin films have been extensively investigated. The results, however, are complicated and inconsistent with one another. These are antiferromagnetism, ferromagnetism, different Fe magnetic moment for different atomic sites in film or a rearrangement of surface Fe atoms etc. In this article the recent results for fcc‐Fe multilayers will be reported and discussed to obtain a unified picture for magnetism of fcc‐Fe ultrathin films. Epitaxially grown multilayers of fcc‐Fe with Cu or Cu‐M (M=Au,Ni) spacer on Cu(001) and Cu(111) substrates were prepared by molecular beam epitaxy method. The Cu‐M spacer was adopted to change the mean interatomic distance between Fe atoms at interfaces by changing the composition of M (=Au,Ni), where Au was used to increase the distance and Ni to decrease it. Fe layers on a pure Cu spacer were fcc and the magnetic moment was at most 2.0μB. Fe layers on a Cu‐Au spacer, however, were fct with c/a<1 and those on Cu‐Ni with c/a≳1, where a i...

Study on Magnetoresistance for Trilayer with Fe3O4 Film

High quality Fe3O4 thin film was synthesized by the magnetron sputtering method using Ar-H2 gas. Giant magneto resistance of the trilayer using Fe3O4 was investigated, where the observed MR ratio was less than 1%. A phenomenological calculation was made to make the origin of the small MR ratio clear, using a parallel current model assuming five passes. It is concluded that the non-magnetic layer surrounded by two magnetic layers is necessary to have the same order resistivity and similar thickness with Fe3O4 layer in order to obtain the large MR ratio for the trilayer using Fe3O4.