Superimposed contributions to two-terminal and nonlocal spin signals in lateral spin-transport devices

R. Jansen, A. Spiesser, Y. Fujita, H. Saito, S. Yamada, K. Hamaya, and S. Yuasa

Phys. Rev. B 104, 144419 – Published 22 October 2021

Abstract

The spin voltages produced by spin accumulation and Hanle spin precession in a lateral spin-transport device with a silicon channel and ferromagnetic tunnel contacts (Fe/MgO) are probed for a wide range of magnetic fields. Signal analysis reveals that for the interpretation of the two-terminal magnetoresistance and the nonlocal spin signals, one needs to consider various superimposed contributions, namely, (i) spin signals arising from spin transport of mobile carriers through the Si channel from one ferromagnetic contact to the other, thus depending on the relative magnetization of the two contacts, (ii) spin signals also arising from the spin accumulation of mobile carriers in the Si channel but generated at each of the ferromagnetic contacts separately, and (iii) spin signals originating from spin accumulation of carriers that are confined at or near the Si/MgO interface of the magnetic tunnel contacts, with rather different spin precession characteristics. Perhaps surprisingly, in the nonlocal spin signal a clear broad Hanle signal from confined electrons is also observed, and argued to be mediated by heat flow from the injector to the nonlocal detector.

Received 12 August 2021
Revised 5 October 2021
Accepted 12 October 2021

DOI:https://doi.org/10.1103/PhysRevB.104.144419

Physics Subject Headings (PhySH)

Electrical spin injection Magnetism Spin accumulation Spin current Spintronics

Spin valves

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

R. Jansen¹, A. Spiesser¹, Y. Fujita^1,2, H. Saito¹, S. Yamada³, K. Hamaya³, and S. Yuasa¹

¹Research Center for Emerging Computing Technologies, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305-8568, Japan
²Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan
³Center for Spintronics Research Network, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan

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Issue

Vol. 104, Iss. 14 — 1 October 2021

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Images

Figure 1
Two-terminal magnetoresistance (2T MR) of a lateral spin-transport device with a Si channel and two Fe/MgO tunnel contacts at low magnetic field (a), medium field (b), and high field (c). Data is shown for the spin-valve configuration, with the external magnetic field oriented in plane, parallel to the sample surface (green data) and for the Hanle configuration, with the magnetic field perpendicular to the sample surface (pink and blue data, corresponding, respectively, to the parallel and antiparallel alignment of the magnetization of the two Fe contacts). All data was obtained at 10 K for a current of $+ 0.66$ mA. We subtracted the spin-independent part ( $+ 1734$ mV) of the voltage, which arises from the Ohmic voltage drop across the Si channel plus the two tunnel contacts for a nonzero current. Note that the data in (b) and (c) were obtained using a magnetic field sweep with a larger step size ( $\sim$ 20 Oe), so the narrow Hanle signals at low field are not resolved.
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Figure 2
Four-terminal nonlocal spin signals of a silicon device with Fe/MgO tunnel contacts at low magnetic field (a), medium field (b), and high field (c). Data is shown for the spin-valve configuration, with the external magnetic field oriented in plane, parallel to the sample surface (green data) and for the Hanle configuration, with the magnetic field perpendicular to the sample surface (pink and blue data, corresponding, respectively, to the parallel and antiparallel alignment of the magnetization of the two Fe contacts). All data was obtained at 10 K for a current of $+ 0.66$ mA. A nonlocal offset voltage of $+ 3.25$ mV was subtracted. In panel (b), we added the high-resolution Hanle data for the P and the AP configuration taken at low fields [pink and blue open symbols, respectively. This is the same data as in panel (a)].
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Figure 3
Two-terminal magnetoresistance data (symbols) together with fits (solid lines) of the broad Hanle and inverted Hanle signals produced by electrons confined at the MgO/Si interfaces of the magnetic tunnel contacts. Experimental data obtained at 10 K for a current of $+ 0.66$ mA is shown for medium (a) and high magnetic field (b) for the spin-valve configuration (green symbols) and for the Hanle configuration (pink symbols). For the spin-valve data, only one trace (field sweep from $+$ to $-$ ) is shown.
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Figure 4
Nonlocal spin signals (symbols) together with fits (solid and dashed lines) at medium (a) and high magnetic field (b). For the dashed lines, only the contribution of mobile electrons in the Si channel was considered. For the solid lines, also a contribution from electrons confined at the MgO/Si interfaces of the magnetic tunnel contacts was included, using the same parameters as for the fit of the 2T MR data in Fig. 3, except for the prefactor, which is adjusted to match the overall magnitude. Experimental data obtained at 10 K for a current of $+ 0.66$ mA is shown for the spin-valve configuration (green symbols) and for the Hanle configuration (pink symbols). For the spin-valve data, only one trace (field sweep from $+$ to $-$ ) is shown.
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Figure 5
Schematic illustration of the mechanisms that can produce a broad Hanle signal from interface-confined electrons in a nonlocal measurement. (a) Joule or Peltier heating in the ferromagnetic injector contact, which is modulated by the spin precession of the confined electron spins near the injector interface. The resulting magnetic field-dependent heat flow through the device is converted into a magnetic field-dependent charge voltage in the nonlocal detector circuit via the Seebeck effect. (b) Heat generated by the injection current causes a heat flow across the detector Si/MgO/Fe junction, which, via thermal spin injection by Seebeck spin tunneling, creates a spin accumulation of confined electrons near the detector interface. These confined electron spins, in turn, produce a broad nonlocal Hanle signal
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Figure 6
(a) Layout of the spin-transport device and the nonlocal measurement configuration, with the lateral dimensions of the channel and the ferromagnetic contacts indicated. Panels (b) and (c) depict a schematic view of the distribution of the charge flow in an ideal device (b) and in a device in which the length of the ferromagnetic contacts is smaller than the width of the channel (c). The current in the injector circuit is assumed to be positive, corresponding to the injection of electrons from the ferromagnet into the Si channel. The arrows indicate the direction of the electron flow.
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