Fabrication of Spin Valve Junctions Based on Fe3Si/FeSi2/Fe3Si Trilayered films

Yuki Asai1, Ken-ichiro Sakai1,2*, Kazuya Ishibashi1, Kaoru Takeda3, and Tsuyoshi Yoshitake1**

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

E-mail: *kenichiro_sakai@kyudai.jp; **tsuyoshi_yoshitake@kyudai.jp

(Received July 28, 2014)

Fe3Si/FeSi2/Fe3Si trilayered junctions were prepared on Si(111) by facing targets direct-current

sputtering combined with a mask method, and spin valve signals in current-perpendicular-to-plane

(CPP) geometry was investigated for the change of the magnetization alignment. The shape of

magnetization curves evidently exhibited that an antiparallel alignment is realized owing to a

difference in the coercive force between the top and bottom Fe3Si layers. The electrical resistance

was alternately changed for the formation of parallel and antiparallel alignments with the magnetic

field. The spin valve signals in the Fe3Si/FeSi2/Fe3Si trilayered junctions were experimentally

demonstrated.

1. Introduction

Since the discovery of giant magnetoresistance (GMR) [1,2] and tunnel magnetoresistance (TMR)

[3-9] 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 [10-16]: (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 [17-23]. 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].

Previously, we have prepared Fe3Si/FeSi2 artificial films, wherein interlayer coupling was

JJAP Conf. Proc. (2015) 011501©2015 The Japan Society of Applied Physics

3Proc. Int. Conf. and Summer School on Advanced Silicide Technology 2014

011501-1

induced between Fe3Si layers across FeSi2 spacers [25-27]. The parallel and antiparallel alignments

of layer magnetizations have been realized owing to the interlayer coupling and applied magnetic

fields. In addition, current-induced magnetization switching (CIMS) was observed for

antiferromagnetically interlayer coupled CPP structural artificial lattice films [27-29].

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.

2. 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

Junction area = 0.16 mm2

Line width = 0.4 mm

(a) (b)

Fig. 1. (a) Preparation procedure of CPP trilayered junctions with masks,

(b) top view of CPP trilayered junction.

VFe3Si

Fe3Si

FeSi2↓

Fig. 2. Electrical circuit for measuring electrical conductivity as a spin valve.

011501-2JJAP Conf. Proc. (2015) 0115013

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 300C. The base pressure was lower than 3×105 Pa and the

film deposition was carried out at 1.33×101 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 external

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.

3. Results and discussion

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

20 40 60 80

2[degree]

Inte

nsity [

arb

. unit]

Si 111

Si 222

20 40 60 80

2[degree]

Inte

nsity [

arb

. unit]

Fe

3S

i 222

Fe

3S

i 2

20

Si 222

Si 111

(a) (b)

Fig. 3. 2θ-θ X-ray diffraction patterns of (a) Si(111) substrate (background)

and (b) CPP trilayered film deposited on the Si(111) substrate.

0180

90

270

2040 60 80

α

β

Fig. 4. Pole figure concerning Fe3Si-422 planes.

011501-3JJAP Conf. Proc. (2015) 0115013

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.

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.

4. 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.

–20 0 20

–1

0

1

M/M

s

Magnetic field [Oe]

–10 0 10105.38

105.4

105.42

105.44

Resis

tance [Ω

]

Magnetic field [Oe]

Fig. 5. Typical magnetization curve of CPP Fig. 6. MR curve of CPP trilayered film,

trilayered film, measured at room temperature. measured at room temperature.

011501-4JJAP Conf. Proc. (2015) 0115013

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