Volume 17 Issue 2
Jul.  2019
Article Contents

Kohei Kataoka, Nobuki Tezuka, Masashi Matsuura, Satoshi Sugimoto. Fabrication of Co2Fe(Al,Si) and Co2Fe(Al,Si)/MgO on Ge(111) Substrate and Its Magnetic Properties[J]. Journal of Electronic Science and Technology, 2019, 17(2): 109-115. doi: 10.11989/JEST.1674-862X.71128011
Citation: Kohei Kataoka, Nobuki Tezuka, Masashi Matsuura, Satoshi Sugimoto. Fabrication of Co2Fe(Al,Si) and Co2Fe(Al,Si)/MgO on Ge(111) Substrate and Its Magnetic Properties[J]. Journal of Electronic Science and Technology, 2019, 17(2): 109-115. doi: 10.11989/JEST.1674-862X.71128011

Fabrication of Co2Fe(Al,Si) and Co2Fe(Al,Si)/MgO on Ge(111) Substrate and Its Magnetic Properties

doi: 10.11989/JEST.1674-862X.71128011
Funds:  This work was supported by JSPS KAKENHI under Grant No. 15H05699
More Information
  • Author Bio:

    Kohei Kataoka received his B.S. degree from Sendai National College of Technology, Sendai, Japan in 2013 and M.S. degree from Tohoku University, Sendai, Japan in 2015, both in materials science. He is currently pursuing the Ph.D. degree in materials science with the Graduate School of Engineering, Tohoku University. His research interests include magnetic thin films, spintronics, and quantum information

    Nobuki Tezuka received his B.S., M.S., and Ph.D. degrees in applied physics from Tohoku University in 1993, 1995, and 1998, respectively. He is currently an associate professor with the Graduate School of Engineering, Tohoku University. His research interests include magnetic thin films, nano-sized magnets, and their devices

    Masashi Matsuura received his B.S., M.S., and Ph.D. degrees in materials science from Tohoku University in 2007, 2009, and 2012, respectively. He is currently an assistant professor with the Graduate School of Engineering, Tohoku University. His research interests include permanent magnets, microstructure, and magnetism

    Satoshi Sugimoto received his B.S. and M.S. degrees in materials science from Tohoku University in 1982 and 1984, respectively. Now he is a doctor of engineering, a professor with the Graduate School of Engineering, and the Director of Research Center for Rare Metal and Green Innovation, Tohoku University. His research interests include permanent magnets and high-frequency magnetic materials

  • Corresponding author: K. Kataoka, N. Tezuka, M. Matsuura, and S. Sugimoto are with the Graduate School of Engineering, Tohoku University, Sendai 980-8576, Japan (e-mail: kohei.kataoka.t1@dc.tohoku.ac.jp; tezuka@material.tohoku.ac.jp; m-matsu@material.tohoku.ac.jp; sugimots@material.tohoku.ac.jp).
  • Received Date: 2017-11-28
  • Rev Recd Date: 2018-05-08
  • Publish Date: 2019-06-01
  • We investigated the interfacial effects on magnetic properties in Co2Fe(Al,Si)/Ge (CFAS/Ge) and CFAS/MgO/Ge systems to demonstrate the effects of the interface structure on magnetic properties. CFAS and CFAS/MgO were deposited on the i-Ge(111) substrate. In-situ reflection high energy electron diffraction (RHEED) patterns showed epitaxially grown CFAS and MgO on Ge(111). According to the X-ray diffraction (XRD) ϕ-scan of CFAS(220), we determined that the crystallographic orientation relationships were CFAS(111)<–110>//Ge(111)<–110> and CFAS(111)<–110>//MgO(111)<–110>Ge(111)<–110>. The magnetic properties were measured by the vibrating sample magnetometer (VSM) and the saturation magnetization Ms value of CFAS with 2-nm thick MgO reached the value of L21 ordered one. A uniaxial magnetic anisotropy behavior was observed both in CFAS/Ge and CFAS/MgO/Ge structures after annealing. We confirmed the behavior did not only originate from the CFAS/Ge interface but also CFAS/MgO and the ordering structure.
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Fabrication of Co2Fe(Al,Si) and Co2Fe(Al,Si)/MgO on Ge(111) Substrate and Its Magnetic Properties

doi: 10.11989/JEST.1674-862X.71128011
Funds:  This work was supported by JSPS KAKENHI under Grant No. 15H05699
  • Author Bio:

  • Corresponding author: K. Kataoka, N. Tezuka, M. Matsuura, and S. Sugimoto are with the Graduate School of Engineering, Tohoku University, Sendai 980-8576, Japan (e-mail: kohei.kataoka.t1@dc.tohoku.ac.jp; tezuka@material.tohoku.ac.jp; m-matsu@material.tohoku.ac.jp; sugimots@material.tohoku.ac.jp).

Abstract: We investigated the interfacial effects on magnetic properties in Co2Fe(Al,Si)/Ge (CFAS/Ge) and CFAS/MgO/Ge systems to demonstrate the effects of the interface structure on magnetic properties. CFAS and CFAS/MgO were deposited on the i-Ge(111) substrate. In-situ reflection high energy electron diffraction (RHEED) patterns showed epitaxially grown CFAS and MgO on Ge(111). According to the X-ray diffraction (XRD) ϕ-scan of CFAS(220), we determined that the crystallographic orientation relationships were CFAS(111)<–110>//Ge(111)<–110> and CFAS(111)<–110>//MgO(111)<–110>Ge(111)<–110>. The magnetic properties were measured by the vibrating sample magnetometer (VSM) and the saturation magnetization Ms value of CFAS with 2-nm thick MgO reached the value of L21 ordered one. A uniaxial magnetic anisotropy behavior was observed both in CFAS/Ge and CFAS/MgO/Ge structures after annealing. We confirmed the behavior did not only originate from the CFAS/Ge interface but also CFAS/MgO and the ordering structure.

Kohei Kataoka, Nobuki Tezuka, Masashi Matsuura, Satoshi Sugimoto. Fabrication of Co2Fe(Al,Si) and Co2Fe(Al,Si)/MgO on Ge(111) Substrate and Its Magnetic Properties[J]. Journal of Electronic Science and Technology, 2019, 17(2): 109-115. doi: 10.11989/JEST.1674-862X.71128011
Citation: Kohei Kataoka, Nobuki Tezuka, Masashi Matsuura, Satoshi Sugimoto. Fabrication of Co2Fe(Al,Si) and Co2Fe(Al,Si)/MgO on Ge(111) Substrate and Its Magnetic Properties[J]. Journal of Electronic Science and Technology, 2019, 17(2): 109-115. doi: 10.11989/JEST.1674-862X.71128011
  • For avoiding the limit of Moore’s law, alternative development ways need to be concerned and semiconductor (SC) spintronics devices are attracting the attention as one of the candidates. Spin-based metal-oxide-semiconductor field-effect transistors (spin MOSFETs) consist of ferromagnetic metal (FM) source and drain electrodes and work as a logic and memory device[1]. To realize spin MOSFETs, it is necessary to inject spin-polarized current from FM into SC with high efficiency. However, because of the conductance mismatch between the metallic FM and SC, it is difficult to obtain high efficiency for spin injection from FM to SC[2],[3]. The use of FM with a high spin polarization or the insertion of a tunnel barrier between FM and SC is beneficial for improving the spin injection efficiency. Some Co-based Heusler alloys are theoretically predicted to show half-metallicity and have a relatively high Curie temperature[4]. Intensive research for Co-based Heusler alloys was carried out especially in giant-magnetoresistance and tunnel-magnetoresistance fields[5]-[9]. Our group previously reported that L21 ordered Co2FeAl0.5Si0.5 (CFAS) showed a high spin polarization measured by the tunnel-magnetoresistance effect. Therefore, CFAS is considered as one of the candidates for FM electrodes for spin injection. In the spin injection experiment, a non-local spin signal in the CFAS/GaAs junction was detected at room temperature[10]. Recently, a non-local signal in CFAS/Ge at room temperature was detected as well[11],[12]. Although it was successfully detected at room temperature, it was also mentioned that CFAS deposited by low-temperature molecular beam epitaxy (LT-MBE) mainly had the B2 structure[13]. For improving the spin signal in this device, highly ordered CFAS is preferable. In addition, the combination use of highly spin-polarized FM and inserting tunnel barrier between FM and SC can be considered to improve spin-injection efficiency. In this paper, we focus on evaluating the effects of the interface on magnetic properties of highly spin-polarized FM/SC interfaces for the device design. In the previous reports, several origins of uniaxial magnetic anisotropy were reported. One is uniaxial magnetic anisotropy originated from bond-orientation anisotropy[14]-[21]. Furthermore, an ordered alloy deposited on Si(111) or Ge(111) shows uniaxial magnetic anisotropy as well[22]-[26] and uniaxial magnetic anisotropy of Fe3Si/Si(111) deposited by the oblique deposition method[23] is depending on the deposition direction. The other one is uniaxial magnetic anisotropy originated from the competition of surface magnetic anisotropy, first-order magnetocrystalline anisotropy, and step induced magnetic anisotropy from symmetry breaking and dipolar interactions[27],[28]. In this study, we inserted the 1-nm and 2-nm thick MgO between CFAS/Ge samples for the evaluation of the interface effect on magnetic anisotropy. It can be considered that MgO works as not only the tunnel barrier which improves spin injection efficiency by preventing inter-diffusion during annealing, but also a different interface which affects on magnetic anisotropy. We clarified the crystallographic relationship and magnetic properties in the structures with comparing with two different interfaces.

  • All CFAS and CFAS/MgO layers were prepared by molecular beam epitaxy (MBE) and RF-sputtering. The base pressure of each chamber was 5×10−7 Pa and 5×10−6 Pa, respectively. First, the i-Ge(111) substrate was chemically cleaned by dipping in 10% H2SO4, 10% NH4OH, and 10% H2O2 aqueous solutions, sequentially. Then, the substrate was loaded into the MBE chamber and annealed at 600 °C for surface cleaning. CFAS (20 nm) by RF-sputtering and MgO with the thicknesses of tMgO=0 to tMgO=2 nm by electron beam evaporation were fabricated on the substrate at room temperature without air exposure. After the deposition, the samples were capped with 3-nm thick Ta and then annealed at Ta=100 °C to Ta=500 °C in vacuum.

    The crystal structure was analyzed by in-situ reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD). The magnetic properties were measured by a vibrating sample magnetometer at room temperature. To eliminate the shape magnetic anisotropy, we fabricated disk-shaped samples with a diameter of 4.5 mm by using conventional lithography and Ar ion milling etching, as shown in Fig. 1 (a). For the vibrating sample magnetometer (VSM) measurement, the magnetic field was applied along the in-plane directions in the Ge(111) substrate as described in Fig. 1 (b).

    Figure 1.  Fabricated sample and its crystallographic orientation: (a) schematic image of the disk-shaped sample and (b) crystallographic orientation in the Ge(111) substrate.

  • We first observed the in-situ RHEED patterns of CFAS/Ge and MgO/Ge samples after each deposition. Fig. 2 shows the RHEED patterns along each azimuth of CFAS grown on a Ge substrate and MgO with tMgO=2 nm also on a Ge substrate. The patterns of the cleaned Ge substrate are shown as well. The streak patterns were observed for the Ge substrate, indicating that the surface of the substrate was atomically flat. We observed spot-like patterns of the CFAS(111) plane in Figs. 2 (c) and (d). It is considered that the CFAS is epitaxially grown on Ge substrates with CFAS(111)<–110>//Ge(111)<–110> orientation relationship. The MgO layer also grew epitaxially on Ge substrates as shown in Figs. 2 (e) and (f). The crystallographic orientation relation is MgO(111)<–110>//Ge(111)<–110>. Then, we observed RHEED patterns of sequentially deposited CFAS on MgO(111)/Ge which showed that it did not crystallize. After annealing, the XRD ϕ-scan of CFAS(220) was carried out to determine the crystallographic orientation relations between the annealed CFAS, Ge, and MgO/Ge. Fig. 3 shows ϕ-scan patterns of CFAS(220) and clear 6-fold symmetry diffraction patterns of CFAS(220) in both tMgO=0 and tMgO=2 nm samples are obtained, which correspond to Ge(220) 6-fold peaks on the Ge substrate. Consequently, we can finally determine that the orientation relationships in this structure are CFAS(111)<–110>//MgO(111)<–110>//Ge(111)<–110>.

    Figure 2.  In-situ RHEED patterns of flushed Ge(111) substrate taken along (a) [–110] and (b) [–1–12], of CFAS deposited on Ge substrates taken along (c) [–110] and (d) [–1–12], of MgO deposited on Ge substrates taken along (e) [–110] and (f) [–1–12], and of CFAS deposited on MgO/Ge substrates taken along (g) [–110] and (h) [–1–12].

    Figure 3.  XRD ϕ-scan of CFAS(220) on annealed 20-nm thick CFAS with tMgO=0 and tMgO=2 nm. The tMgO=0 sample annealed at Ta=300 °C and the tMgO=2 nm sample annealed at Ta=400 °C are shown in blue and red, respectively. A Ge(220) pattern on Ge substrate is also shown.

    Next, the phases in these stacking structures were identified by out-of-plane XRD. The XRD patterns of the annealed samples are shown in Fig. 4. The diffraction peaks of CFAS are not clearly observed because the lattice parameter of CFAS is close to that of Ge. In the tMgO=0 samples annealed at Ta=100 °C to Ta=300 °C, there is no difference with the Ge substrate. However, CoGe peaks are observed in the tMgO=0 sample annealed at Ta=400 °C, indicating that CFAS and Ge are inter-diffused by annealing. It can be considered that MgO with tMgO=2 nm prevents inter-diffusion between CFAS and Ge. Despite the diffusion prevention by the 2-nm thick MgO, we find CoGe peaks in the tMgO=1 nm sample annealed at Ta=400 °C. In this case, there might not have been enough MgO to fully cover the Ge surface.

    Figure 4.  XRD patterns of annealed samples with tMgO=0 and tMgO=2 nm. The samples were annealed at Ta=300 °C and 400 °C, respectively. For the reference, the Ge substrate is shown as well.

    For measuring the magnetic properties of deposited CFAS, VSM was used for micro-fabricated samples at room temperature. Fig. 5 shows Ta dependent saturation magnetization Ms for the tMgO=0, tMgO=1 nm, and tMgO=2 nm samples. Note that before annealing, all Ms values for different tMgO thicknesses were the same. In the tMgO=0 and tMgO=1 nm samples, Ms values were similar up to Ta=200 °C and then started to decrease. For the Ta=400 °C annealing, Ms drastically decreased to around 10% of the Ms value of the as-deposited sample. On the contrary, the Ms value of the tMgO=2 nm samples increased with the increasing Ta and reached 5.4 μB/f.u. which corresponds to the Ms value of L21 ordered CFAS, suggesting it had the L21 ordered structure. The inset figure shows hysteresis loops of the tMgO=2 nm sample annealed at Ta=400 °C measured along the [–110] and [–1–12] directions. We clearly observed that the hysteresis loops were getting more anisotropic with the increasing Ta. For the determination of the magnetic anisotropy of CFAS, the ratios of residual magnetization to saturation magnetization (Mr/Ms) values are measured for each direction shown in Fig. 1 (b). Fig. 6 shows polar plotted Mr/Ms curves of the tMgO=0 and tMgO=2 nm samples annealed at Ta=300 °C and Ta=400 °C, respectively. For comparison, the Mr/Ms curves before annealing are also shown in black. Before annealing, weak uniaxial anisotropy was observed both in the tMgO=0 and tMgO=2 nm samples. After annealing, it was enhanced and Mr/Ms reached 1 in the tMgO=2 nm sample annealed at Ta=400 °C along the [–110] direction. This type of magnetic anisotropy has been previously reported in Fe3Si on Ge(111)[14] and Co2FeSi on Si(111) and Ge(111)[22]-[26], and two contributions were discussed, first, surface and/or interfacial effects and, second, the ordering of FM. Because it is observed both in the tMgO=0 and tMgO=2 nm samples, the uniaxial magnetic anisotropy is not only caused by the FM/SC(111) interface in this case. After annealing, the direction of easy axis changed both in the tMgO=0 and tMgO=2 nm samples and enhanced. Thus, we speculate that it originates from both the competition of surface magnetic anisotropy, first-order magnetocrystalline anisotropy, and step induced magnetic anisotropy from symmetry breaking and the ordering effect of FM. And the latter one affects stronger than the former. Despite the results, further research is necessary to clarify the origin of this behavior.

    Figure 5.  Saturation magnetization Ms dependence on Ta. The Ms value of L21-CFAS is 5.4 μB/f.u. In-plane hysteresis loops in [–110] and [–1–12] directions of the tMgO=0 sample annealed at Ta=400 °C are shown in the inset.

    Figure 6.  Polar plotted Mr/Ms curves of (a) tMgO=0 and (b) tMgO=2 nm samples annealed at Ta=300 °C and Ta =400 °C, respectively. Mr/Ms curves of the as-deposited samples are shown as well in black.

  • We have fabricated CFAS/Ge(111) and CFAS/MgO/Ge(111) structures and investigated the crystallographic orientation relationship and magnetic properties. CFAS on Ge(111) grew epitaxially and the orientation relationship was CFAS(111)<–110>//Ge(111)<–110>. Inserted MgO also grew epitaxially on Ge(111) along with the (111) plane of MgO, but CFAS on MgO did not crystallize. After annealing CFAS was crystallized and the crystallographic orientation relationship was CFAS(111)<–110>//MgO(111)<–110>//Ge(111)<–110>. The Ms value reached the value of L21 ordered CFAS in the tMgO=2 nm samples after annealing at Ta=400 °C and above. In-plane uniaxial magnetic anisotropy behavior was observed as previously reported and it was also observed in the CFAS/MgO/Ge structure. This result suggests that the uniaxial magnetic anisotropy behavior is not caused by the FM/SC(111) interface.

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