Fabrication of Spin Valve Junctions Based on Fe 3 Si / FeSi 2 / Fe 3 Si Trilayered films

Yuki Asai, Ken-ichiro Sakai*, Kazuya Ishibashi, Kaoru Takeda, and Tsuyoshi Yoshitake** 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Control and Information Systems Engineering, Kurume National College of Technology, Kurume, Fukuoka 830-8555, Japan 3Department of Electrical Engineering, Fukuoka Institute of Technology, Fukuoka 811-0295, Japan


Introduction
Since the discovery of giant magnetoresistance (GMR) [1,2] and tunnel magnetoresistance (TMR) [3][4][5][6][7][8][9] effects, spin-dependent carrier transport has received attention from physical and engineering viewpoints.Whereas GMR and TMR films generally employ nonmagnetic metal and insulator spacers, respectively, we have studied Fe3Si/FeSi2 artificial lattices, in which FeSi2 is semiconducting and its employment as spacers is specific to our research.The combination of Fe3Si and FeSi2 has the following merits [10][11][12][13][14][15][16]: (i) a magnetoresistance effect in the current-perpendicular-to-plane (CPP) structures is easily detectable since the electrical resistivity of FeSi2 spacer layers is distinctively larger than that of Fe3Si layers; (ii) a spin injection efficiency might be higher than that in TMR films; (iii) the epitaxial growth of Fe3Si layers on Si(111) substrates is successively kept up to the top Fe3Si layer across FeSi2 spacer layers, which is beneficial to the coherent transportation of spin-polarized electrons; (iv) Fe3Si is feasible for a practical use since it has a high Curie temperature of 840 K and a large saturation magnetization, which is half of that of Fe.
The generation and control including filtering of spin currents are key for the application to devices [17][18][19][20][21][22][23].Spin currents are classified into three types: spin-polarized current, fully spin-polarized current, and pure spin current.Although pure spin current is physically curious and received much attention these days, even its detection is not easy.On the other hand, spin-polarized current has been studied through GMR and TMR effects thus far and its research is familiar with existing electronics.The switching between the parallel and antiparallel magnetization alignments of ferromagnetic layers in multilayered films, so-called spin valve, is an important treatment for modulating spin-polarized current [24].
Another way for realizing the formation of parallel/antiparallel alignments of layer magnetizations is the employment of ferromagnetic layers with different coercive forces combined with the application of magnetic fields.This way is beneficial from the viewpoints of flexible design of junction structures and no restriction of spacer layer thickness.There have been few studies on the fabrication of Fe-Si system spin valves comprising ferromagnetic layers with different coercive forces and their evaluation as spin valves.In this work, CPP structural Fe3Si/FeSi2/Fe3Si trilayered films were prepared by sputtering combined with a mask method and the magnetic properties as spin valves were studied.

Experimental procedure
Fe3Si (700 nm)/FeSi2 (0.75 nm)/Fe3Si (100 nm) trilayer films were deposited by facing target direct-current sputtering (FTDCS), combined with a mask method (Fig. 1(a)).First, a p-type Si(111) substrate with a specific resistance range of 1000-3000 Ω･cm, which was produced by a floating zone (FZ) method, was cleaned with 1% hydrofluoric acid and rinsed in deionized water before it was set into a chamber together with a mask.Fe3Si bottom layer (100 nm) was deposited on the Si(111) substrate, using the 1st mask with a line width of 0.4 mm.After that, the sample was temporarily took out from the sputtering apparatus to change the 2nd mask.After changing the mask, FeSi2 (0.75 nm) and Fe3Si (700 nm) layers were successively deposited.All the depositions were carried out at a substrate temperature of 300C.The base pressure was lower than 3×10 5 Pa and the film deposition was carried out at 1.33×10 1 Pa.The crystalline structure of the films was characterized by X-ray diffraction (XRD) using Cu K radiation.The magnetization curves were measured at room temperature using a vibration sample magnetometer (VSM).The magnetic field was applied parallel to the line of bottom Fe3Si layer with the thickness of 100 nm.The electrical resistance as a spin valve was measured in an electrical circuit as shown in Fig. 2. was confirmed that 111-oriented grains are also in-plane ordered.Totally considering these results and our previous research, wherein Fe3Si thin films are epitaxially grown on Si(111) substrate even at room temperature, the bottom Fe3Si layer should epitaxially be grown on Si(111) substrate.On the other hand, although the top Fe3Si layer deposited on FeSi2 layer might partially be oriented with the same orientation relationship as the bottom layer [30], it contains polycrystalline grains due to the temporal exposure to air for the masks being exchanged.The Fe3Si-220 peak is attributable to the polycrystalline grains.

Results and discussion
A typical magnetization curve is shown in Fig. 5.The shape of the magnetization curve has clear steps that evidently indicate the formation of antiparallel alignment of magnetizations owing to the difference in the coercive force between the top and bottom Fe3Si layers.The soft ferromagnetic layers should be the bottom Fe3Si layer epitaxially grown on Si(111) from the previous study [25,26].The top Fe3Si layer comprising polycrystalline grains and oriented grains with the same orientation relationship with the epitaxial Fe3Si grains in the bottom layer probably has the larger coercive force.In addition, note that a large difference in the thickness between the top and bottom layers might facilitate the generation of the coercive force difference.
Figure 6 shows a magnetoresistance (MR) curve under a bias current of I = 10 mA.Clearly, the MR curve exhibits a hysteresis loop.High and low electrical resistance values are owing to the antiparallel and parallel alignments of the Fe3Si layers magnetization, respectively.It was experimentally demonstrate that the CPP-structural Fe3Si/FeSi2/Fe3Si trilayered films act as a spin valve that can modulate spin-polarized currents.

Conclusion
CPP-structural Fe3Si/FeSi2/Fe3Si trilayered films were prepared by FTDCS combined with a mask method.Owing to a difference in the coercive force between the bottom and top Fe3Si layers, antiparallel and parallel alignments of magnetizations were realized in the magnetization curve and a signal due to spin-polarized currents was evidently detected in the MR curve.

Acknowledgment
The measurement of magnetization curves was performed at Fukuoka Institute of Technology.

Figure 3 (Fig. 3 .Fig. 4 .
Figure 3(a) and 3(b) show the 2θ-θ XRD patterns of a Si(111) substrate as a background and CPP film deposited on the Si(111) substrate.The diffraction peaks of Fe3Si-220 and 222 are observed.A pole-figure concerning the Fe3Si-422 plane with a rotation axis of Fe3Si [222] is shown in Fig. 4. It