Spin-Filter Tunnel Junction with Matched Fermi Surfaces

T. Harada, I. Ohkubo, M. Lippmaa, Y. Sakurai, Y. Matsumoto, S. Muto, H. Koinuma, and M. Oshima

Phys. Rev. Lett. 109, 076602 – Published 16 August 2012

Abstract

Efficient injection of spin-polarized current into a semiconductor is a basic prerequisite for building semiconductor-based spintronic devices. Here, we use inelastic electron tunneling spectroscopy to show that the efficiency of spin-filter-type spin injectors is limited by spin scattering of the tunneling electrons. By matching the Fermi-surface shapes of the current injection source and target electrode material, spin injection efficiency can be significantly increased in epitaxial ferromagnetic insulator tunnel junctions. Our results demonstrate that not only structural but also Fermi-surface matching is important to suppress scattering processes in spintronic devices.

Received 9 March 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.076602

Authors & Affiliations

T. Harada^1,†, I. Ohkubo^1,*,‡, M. Lippmaa², Y. Sakurai¹, Y. Matsumoto³, S. Muto⁴, H. Koinuma^5,6, and M. Oshima¹

¹Department of Applied Chemistry, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
²Institute for Solid State Physics, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
³Materials and Structures Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Yokohama 226-8503, Japan
⁴Department of Materials, Physics, and Energy Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
⁵Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8568, Japan
⁶Department of Cogno-Mechatronics Engineering, Pusan National University, Korea

^*Author to whom correspondence should be addressed. ohkubo@sr.t.u-tokyo.ac.jp
^†Present address: Institute for Solid State Physics, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan.
^‡Present address: National Institute for Materials Science, 1-1 Namiki Tsukuba-shi, Ibaraki 305-0044, Japan. OHKUBO.Isao@nims.go.jp

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Vol. 109, Iss. 7 — 17 August 2012

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Images

Figure 1
Schematic illustration of the spin filter tunnel junction operation. (a) Simplified band diagram of a SFTJ device. (b) Cross-sectional Fermi surface plots for LNO [26, 27], (c) ${La}_{2 / 3} {Sr}_{1 / 3} {MnO}_{3}$ [25], and (d) Au [28]. The sizes of the illustrations are proportional to the size of the first Brillouin zone, allowing comparison of the Fermi momenta $k_{F}$ . The Fermi surface of ${La}_{2 / 3} {Sr}_{1 / 3} {MnO}_{3}$ depicted in (c) is slightly larger than that of the bottom electrode material ${La}_{0.6} {Sr}_{0.4} {MnO}_{3}$ . (e) Cross-sectional HAADF-STEM image of a device.Reuse & Permissions
Figure 2
Transport and magnetic properties of SFTJs. (a) Resistance of a $Au / PCMCO$ ( $y = 0$ ) $(12 ML) / LSMO$ junction (red line) and magnetization of a PCMCO ( $y = 0$ ) film (blue) as a function of temperature. The junction resistance was measured by applying a bias voltage of $\pm 100 mV$ without an external magnetic field. The magnetization is normalized to the 10 K value. (b) A current-voltage characteristic of a $Au / PCMCO$ ( $y = 0$ ) $(12 ML) / LSMO$ junction at 13 K. (c) Logarithmic plot of the junction resistance as a function of reciprocal temperature. The blue line is a Poole-Frenkel fit for the high temperature region ( $> 100 K$ ). Tunneling conduction is dominant in the highlighted region. (d), (e) Comparison of M-H curves of PCMCO (15 ML) films grown on STO substrates before (gray dashed line) and after depositing either epi-LNO [(d), blue line] or poly-Au [(e), red line] electrodes, measured at 10 K [31]. The vertical axes are normalized to the saturation magnetization of the PCMCO film ( $M_{S 0}$ ). The $d I / d V$ spectra of (f) poly- $Au / PCMCO$ ( $y = 0.2$ ) $(10 ML) / STO$ $(2 ML) / LSMO$ junction and (g) LNO/PCMCO ( $y = 0.2$ ) $(10 ML) / STO$ $(2 ML) / LSMO$ junction measured at 4 K and 0 T. A zero-bias anomaly is indicated by the green arrow. The $d I / d V$ spectra were measured at zero field after cooling in a magnetic field of 0.8 T.Reuse & Permissions
Figure 3
Suppression of electron scattering using epitaxial current injectors. IET spectra of (a) poly- $Au / PCMCO$ ( $y = 0.2$ ) $(10 ML) / STO$ $(2 ML) / LSMO$ junction and (b) LNO/PCMCO ( $y = 0.2$ ) $(10 ML) / STO$ $(2 ML) / LSMO$ junction at 4 K. The solid (dotted) lines are measured in P (AP) state under 0.8 T (0.05 T). Peak positions are indicated by black arrows. Bias voltage dependence of TMR at 4 K in (c) poly- $Au / PCMCO$ ( $y = 0.2$ ) $(10 ML) / STO$ $(2 ML) / LSMO$ and (d) LNO/PCMCO ( $y = 0.2$ ) $(10 ML) / STO$ $(2 ML) / LSMO$ junctions. Lines are visual guides.Reuse & Permissions
Figure 4
Comparison of TMR curves of (a) LNO/PCMCO ( $y = 0.2$ ) $(10 ML) / STO$ $(2 ML) / LSMO$ and (b) poly- $Au / PCMCO$ ( $y = 0.2$ ) $(10 ML) / STO$ $(2 ML) / LSMO$ junctions measured at 4 K under a bias voltage of $- 5 mV$ . The scan direction is indicated by red and blue arrows in (a).Reuse & Permissions

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