Susumu Soeya

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).

Distribution of blocking temperature in bilayered Ni81Fe19/NiO films

Exchange paths were investigated for unidirectional exchange coupled 40 nm Ni81Fe19/50 nm NiO films by performing several field cooling experiments. Our experimental data were very consistent with the assumed existence of a variety of exchange paths. Each exchange path seemed to produce its own local unidirectional anisotropy and different local blocking temperature. The measureable exchange coupling could be described as consisting of the sum of the respective exchange paths, each with its own local blocking temperature. On the other hand, an observed blocking temperature of about 230 °C was determined from the exchange paths having the highest local blocking temperature. The local blocking temperatures were thought to be widely distributed, ranging from room temperature to about 230 °C, and the maximum existence probability was most likely at about 215 °C. This indicated that the exchange paths having the local blocking temperature of 215 °C made the largest contribution to the exchange coupling field a...

Exchange coupling between ferromagnetic fcc Ni81Fe19 and antiferromagnetic bcc CrMnPt films

We studied an antiferromagnetic CrMnPtx [(Cr:Mn≂1:1) in atomic percent] film for an exchange‐biased layer, focusing especially on the relationships between the exchange coupling properties of the CrMnPtx (top)/Ni81Fe19(bottom) films and the character of its CrMnPtx film. The best Pt content to obtain a large exchange coupling of the CrMnPtx film was 5.0–8.0 at. %. Typically, the exchange coupled 50 nm CrMnPt5−8/40 nm Ni81Fe19 films exhibited a relatively large exchange coupling field of ∼22 Oe and a high blocking temperature of ∼380 °C. Besides, the CrMnPt5−8 film deposited on the Ni81Fe19 film had a considerably high resistivity of ∼300–350 μΩ cm. These large exchange coupling and high resistivity values were obtained only when the α‐phase with a disordered bcc structure was stabilized in the CrMnPtx film by the underlying fcc Ni81Fe19 film. The Pt within the CrMnPtx film might localize the Mn magnetic moment. As to why the CrMnPtx film having the Pt content of 5.0–8.0 at. % could give the Ni81Fe19 film ...

Magnetic exchange coupling for bilayered Ni81Fe19/NiO and trilayered Ni81Fe19/NiFeNb/NiO films

Exchange coupling between a ferromagnetic film and an antiferromagnetic NiO film was investigated. Bilayered ferromagnetic Ni81Fe19/antiferromagnetic NiO films had a large exchange coupling field and blocking temperature of about 200 °C. In trilayered Ni81Fe19/ferromagnetic (Ni81Fe19)100−xNbx/NiO films, a way could be developed to control the exchange coupling field at a small value by increasing the Nb of the intermediate film. The most important factor in the control seemed to be that the numbers of magnetic Fe and Ni atoms of (Ni81Fe19)100−xNbx, which contributed to the exchange coupling between (Ni81Fe19)100−xNbx and NiO, varied with the existence of nonmagnetic Nb at their interface. From experimental results with other trilayered Ni81Fe19/ferromagnetic (Ni100−xFex)93Nb7/NiO films, it was ascertained that the exchange coupling field seemed to be independent of the magnetic moment of the ferromagnetic film although unidirectional anisotropy constant was proportional to it. As for blocking temperature,...

Rotational hysteresis loss study on exchange coupled Ni81Fe19/NiO films

Investigations on unidirectional anisotropy and rotational hysteresis loss of exchange coupled Ni81Fe19/NiO films have been conducted to clarify the nature of the exchange coupling mechanism. The interfacial exchange coupling regions, which had been considered to be scattered among the nonexchange coupling regions matrix, were found to be composed of many local regions of two kinds: (i) blockable regions which can give the Ni81Fe19 film a unidirectional anisotropy and (ii) unblockable regions which can have exchange coupling, but cannot give the Ni81Fe19 film unidirectional anisotropy. These unblockable regions begin to change gradually to blockable regions on decreasing the temperature below around 100–110 K. This change is probably caused by the antiferromagnetic NiO anisotropy of unblockable regions being strengthened below that temperature. Moreover, the decrease in size of the exchange coupling field and lowered blocking temperature for tNiO<50 nm (tNiO:NiO film thickness) seems to originate from a d...

NIO STRUCTURE-EXCHANGE ANISOTROPY RELATION IN THE NI81FE19/NIO FILMS AND THERMAL STABILITY OF ITS NIO FILM

We studied an antiferromagnetic (AF) NiO film for an exchange‐biased layer, focusing especially on the relationships between the exchange coupling properties of the Ni81Fe19(top)/NiO(bottom) films and the character of its NiO film. Among the variable sputtering conditions, our experimental data showed that the dominant factor determining the exchange coupling properties was the Ar pressure during the NiO film preparation. Better exchange coupling properties resulted when the NiO film was deposited at low Ar pressure which was attributed to: (i) the smooth surface of the NiO film and (ii) the presence of relatively large particle sizes within it. The former was thought to bring about not only an increase in the number of unidirectional exchange coupled Ni81Fe19/NiO spins, but also the appearance of exchange paths having large local exchange anisotropies. The latter was thought to produce an increase in the AF clusters with a particle volume larger than KeiA/KAFi, where Kei, A, and KAFi are local unidirecti...

Spin-glass behavior in amorphous BiFeO3

Short‐range antiferromagnetic (speromagnetic) behavior was observed for amorphous BiFeO3 by static magnetic measurements, ac susceptibility (χac) measurement, and 57Fe Mossbauer spectra measurements. The magnetic behavior is classified into three temperature ranges: (i) paramagnet for T≳220 K; (ii) local clustering of spins for 20 K<T<220 K; and (iii) speromagnet in which all spins are frozen into random directions for T<20 K. In the spin freezing state the magnetization shows irreversibility and the χac−T curve has a cusp. The variation of the spin freezing temperature against measuring magnetic field and measuring frequency indicate that the speromagnetic order is very similar to that of spin glass.

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.

Spin-Valve Films Using Exchange-Coupled CrMnPt/Co Structure

Spin-valve films using CrMnPt/Co structure were investigated. The exchange coupling field (Hex) applied on 3 nm thick Co pinned layers was increased by annealing from 190 Oe to 320-380 Oe. The annealing treatment necessary for the enhancement of the Hex was typically 230/spl deg/C, 1 hr. The annealing distorted the crystal lattice of the CrMnPt films, which is likely to enhance the Hex. The blocking temperature was 320/spl deg/C. The resistivity of the CrMnPt films was as high as 300 /spl mu//spl Omega/cm resulting in low current shunting to antiferromagnetic layers in spin-valve films.

Effect of metallic additives (M) on the exchange coupling of antiferromagnetic CrMnMx films to a ferromagnetic Ni81Fe19 film

Among metal additives M(M:Pt, Pd, Rh, Ir, and Cu) of antiferromagnetic CrMnMx films, Pd was the most suitable M for obtaining large exchange coupling. For the 50 nm CrMnPd5/40 nm Ni81Fe19 films, the exchange coupling field of ∼28 Oe and the blocking temperature of ∼380 °C could be obtained. The CrMn(Pt, Pd, or Rh)x films having the optimum content of ∼8, ∼5, or ∼11 at.% exhibited the same high blocking temperature. The high blocking temperature of the CrMnPt8, CrMnPd5, and CrMnRh11 films was attributed to the nearest neighbor Mn–Mn within the respective films being at the same distance as that at which the Mn–Mn exchange integral showed the maximum negative value.