Relative strength of thermal and electrical spin currents in a ferromagnetic tunnel contact on a semiconductor

K.-R. Jeon, H. Saito, S. Yuasa, and R. Jansen

Phys. Rev. B 92, 054403 – Published 4 August 2015

Abstract

The relative magnitude of electrical and thermal spin currents is investigated for a ferromagnetic tunnel contact on a semiconductor. A direct quantitative comparison is made by simultaneously generating electrical and thermal spin currents of opposite sign and determining the compensation point at which the sum of both spin currents vanishes. This avoids the need to determine the magnitude of each spin current. Surprisingly, it is found that in a $C o_{70} F e_{30} / MgO / Si$ tunnel contact, the thermal spin current driven by a temperature difference of less than 1 K across the contact is comparable to the electrical spin current induced by a bias voltage of about 22 mV. This suggests that the thermal generation of spin current is more efficient than hitherto assumed and should be considered in the design and analysis of spintronic devices that use spin current or spin transfer torque.

Received 28 May 2015

DOI:https://doi.org/10.1103/PhysRevB.92.054403

Authors & Affiliations

K.-R. Jeon, H. Saito, S. Yuasa, and R. Jansen

National Institute of Advanced Industrial Science and Technology (AIST), Spintronics Research Center, Tsukuba, Ibaraki 305–8568, Japan

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Issue

Vol. 92, Iss. 5 — 1 August 2015

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Images

Figure 1
Sketch of the device layout and the electrical connections used to induce and probe spin currents in the central $C o_{70} F e_{30} / MgO / Si$ tunnel contact.
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Figure 2
(a) Obtained spin signals $(Δ V)$ are shown for heating currents $(I_{heating})$ of $- 3.00, - 1.75$ , 0.00, $+ 1.75$ , and $+ 3.00 mA$ (from left to right), with the bias voltage across the active $C o_{70} F e_{30} / MgO / Si$ tunnel contact fixed by setting the tunnel current $(I_{tunnel})$ at 0.0, $- 0.1$ , and $- 0.4 mA$ for the top, middle, and bottom panels, respectively. A negative sign of $I_{tunnel}$ corresponds to electrons tunneling from the Si into the $C o_{70} F e_{30}$ . The solid (open) symbols represent the spin signal from the Hanle (inverted Hanle) effect obtained for an applied magnetic field $B_{z}$ perpendicular ( $B_{x}$ parallel) to the accumulated spins. All data was measured at a base temperature $(T_{base})$ of 300 K. (b) The top panel shows spin signals obtained at the compensation point using simultaneous thermal and electrical driving forces $(I_{heating} = + 3 mA, I_{tunnel} = - 0.4 mA)$ for applied magnetic fields up to 35 kOe in perpendicular direction (closed circles, Hanle geometry) and in-plane direction (open circles, inverted Hanle geometry). Similar data are shown for a pure thermal driving force (middle panel, $I_{heating} = + 3 mA, I_{tunnel} = 0.0$ ) and a pure electrical driving force (bottom panel, $I_{heating} = 0 mA$ and $I_{tunnel} = - 0.4 mA$ ).
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Figure 3
(a) (Left panel) Total spin signals ( $Δ V_{total Hanle}$ given by the sum of the Hanle and inverted Hanle signals) as a function of the heating current $(I_{heating})$ , with the bias across the tunnel contact set by fixing the tunnel current $(I_{tunnel})$ at $- 0.0$ (navy symbols), $- 0.1$ (blue), $- 0.2$ (olive), and $- 0.4 mA$ (violet), together with fits based on a quadratic scaling (solid lines). (a) (Right panel) The $Δ V_{total Hanle}$ as a function of bias voltage, without heating of the Si (black symbols) and with heating (pink, magenta, and orange symbols), together with fits based on a linear scaling (solid lines). (b) Extracted value of the bias voltage $(V_{com})$ to reach the compensation point (top panel) and calculated temperature difference $(Δ T_{cal})$ across the tunnel contact (bottom panel), both as a function of the heating current $(I_{heating})$ .
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Figure 4
(a) Optical microscope image of the fabricated device with a central active tunnel contact of $40 \times 50 μ m^{2}$ , and (b) schematic of its vertical structure (not to scale) used for the calculation of the temperature difference $(Δ T \equiv T_{Si} - T_{CoFe})$ that develops across the MgO tunnel barrier when the Si device layer is heated by a Joule heating current.
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Figure 5
Calculated values of the temperature difference $(Δ T \equiv T_{Si} - T_{CoFe})$ as a function of the thermal conductance $(G_{th}^{tun})$ of the tunnel barrier at the maximum Joule heating power $(P_{heating})$ of $13.5 μ W μ m^{- 3}$ applied to the Si device layer. The red curve corresponds to the case of 1D heat flow $(F = 1)$ , and the blue curve is for the case of 3D heat flow with the maximum lateral heat flow $(F = 10)$ .
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