Spin Valve behavior in Current-Perpendicular-to-Plane Crossover Structural Fe3Si/FeSi2/Fe3Si Trilayered junctions

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 8110295, Japan

We have studied Fe3Si/FeSi2 artificial lattices, in which the ferromagnetic Fe3Si and semiconducting FeSi2 layers were alternatively accumulated in nano scale by facing targets direct current sputtering (FTDCS) and their spin-dependent electrical properties accompanied by a change in the interlayer coupling induced between the Fe3Si layers across the FeSi2 layer.The combination of Fe3Si and FeSi2 has the following merits [10][11][12][13][14][15][16]: (i) magnetoresistance effects in the currentperpendicular-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 of 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.
Spin-dependent scattering of polarized carriers have been studied through GMR and TMR effects thus far.A switch between the parallel and antiparallel magnetization alignments of ferromagnetic layers in multilayered films, so-called spin valve, is a key operation for modulating spin-polarized current [17][18][19][20].Spin valves are classified into two types.One is a multilayered film wherein antiferromagnetic interlayer coupling is induced between ferromagnetic layers at a zero magnetic field.The antiparallel alignment is switched to a parallel alignment by applying magnetic fields.The other is multilayer films comprising ferromagnetic layers with different coercive forces.The antiparallel/parallel alignments, which depends on the applied magnetic field, are induced owing to the different coercive forces.Practically, a difference in the coercive force is produced as follows: (i) the combination of different materials for ferromagnetic layers; (ii) the employment of pining layers; and (iii) purposely changing the crystalline quality (polycrystalline or epitaxial) and film thickness between ferromagnetic layers, for instance between top and bottom layers for trilayered films.
In this work, the (iii) method was employed.In addition, in order to produce antiparallel alignments in the wide range of applied magnetic field, Fe3Si/FeSi2/Fe3Si trilayered films were prepared in crossover structure by employing a mask method.It was experimentally demonstrated that the magnetic field range for producing antiparallel alignments can be extended owing to a magnetic shape anisotropy.

Experimental procedure
Fe3Si (700 nm)/FeSi2 (0.75 nm)/Fe3Si (100 nm) trilayer films were deposited by FTDCS, combined with a mask method in a procedure schematically shown in 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 FTDCS apparatus together with a mask.An Fe3Si bottom layer (100 nm) was deposited on the Si(111) substrate, using the 1st mask with line widths of X (X = 0.4, 0.6, and 0.8) mm.After the deposition, the sample was temporarily took out from the FTDCS apparatus to replace the mask.After replacing the mask, FeSi2 (0.75nm) 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 external magnetic field was applied parallel to the line of bottom Fe3Si layer with the thickness of 100 nm.It corresponds to the horizontal direction in Fig. 1(b).

Results and discussion
The 2θ-θ XRD patterns of a Si(111) substrate as a background and CPP film deposited on the Si(111)   are observed.The Fe3Si-220 peak is attributable to non-oriented crystallites.Figure 3 shows a polefigure concerning the Fe3Si-422 plane with a rotation axis of Fe3Si [222].It was confirmed that oriented crystallites are also in-plane ordered.Totally considering the results including those of 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 the Si(111) substrate.On the other hand, although the top Fe3Si thick layer deposited on FeSi2 layer might partially be oriented with the same orientation relationship as the bottom layer, it might be predominantly polycrystalline due to the temporal exposure to air for the replacement of the masks.
Figure 4 shows the top view of trilayered films prepared with no masks and three types of masks.It is confirmed that the trilayered films prepared with the masks have crossover structures, as expected.
Figure 5 shows the hysteresis loops of the trilayered films.As shown in Fig. 5(a), the trilayered film with no crossover structure exhibits the hysteresis loop with a weak step, which indicated that a difference in the coercive force between the top and bottom Fe3Si layers is not so large.
The hysteresis loops of the crossover structural films exhibit steps clearly as shown in Fig. 5(b), 5(c), and 5(d), as compared with that of non-crossover structural film.The clearness of the step is evidently enhanced with decreasing line width, which implies that the effective magnetic field is strongly affected by the geometry, concretely line width.The effective magnetic field is defined as follows: =   +     : effective magnetic field   : external magnetic field   : demagnetizing field

011504-4
The demagnetization field is strongly dependent on the shape of ferromagnetic materials.With decreasing line width, the difference in the demagnetizing field between the top and bottom Fe3Si layers is increased, which results in the effective magnetic field being further differently applied to the top and bottom Fe3Si layers.Since the magnetic field is applied parallel to the line of the bottom Fe3Si layer, the demagnetization field in the bottom layer is weak.On the other hand, for the line of the top Fe3Si layer, the magnetic field is applied perpendicular to the line.Thus, the demagnetizing field become so large.With decreasing line width, the difference in the demagnetizing field between the top and bottom Fe3Si layers become extreme, in other words, the effective magnetic field is remarkably reduced for the top Fe3Si layer and is hardly changed for the bottom Fe3Si layer.
Considering the above-mentioned magnetic shape anisotropy, the change in the magnetization alignment can qualitatively considered as follows.The magnetization of the bottom Fe3Si layer follows the reverse applied magnetic field at first, from the parallel alignment.As a result, the magnetization of the bottom Fe3Si layer become parallel with the reverse magnetic field while the magnetization of the top Fe3Si layer keeps antiparallel with the reverse magnetic field.In the reverse magnetic field range, antiparallel alignments are produced between the top and bottom Fe3Si layers magnetizations.With increasing reverse magnetic field, the magnetization of the top Fe3Si layer is reversed, and both magnetizations become parallel.The reverse of the top layer magnetization from the antiparallel to parallel alignment is further delayed by the enhanced demagnetizing field.Since the demagnetizing field is enhanced with decreasing line width, the crossover structural films with the small line widths exhibits the clear steps in the hysteresis loops.

Conclusion
CPP-structural Fe3Si/FeSi2/Fe3Si trilayered junctions with crossover structures were prepared on Si(111) by FTDCS combined with a mask method.The hysteresis loops evidently exhibited steps that originates from the antiparallel alignment between the top and bottom Fe3Si layers magnetizations owing to a difference in the coercive force of the Fe3Si layers.The difference in the coercive force is caused by differences in the layer thickness and crystalline quality (epitaxial or polycrystalline).In addition, it was experimentally demonstrated that the magnetic field range for producing the antiparallel alignment can be extended by forming crossover structures.It should be attributable to a magnetic shape anisotropy, and the line width dependent change in the hysteresis loop profile with the applied magnetic field was qualitatively explained well.