08.07.2011

Heat generation by electric current, which is ubiquitous in electronic devices and circuits, raises energy consumption and will become increasingly problematic in future generations of high-density electronics. The control and re-use of heat are therefore important topics for existing and emerging technologies, including spintronics. Recently it was reported that heat flow within a ferromagnet can produce a flow of spin angular momentum—a spin current—and an associated voltage¹. This spin Seebeck effect has been observed in metallic^{1, 2}, insulating³ and semiconductor ferromagnets⁴ with temperature gradients across them. Here we describe and report the demonstration of Seebeck spin tunnelling—a distinctly different thermal spin flow, of purely interfacial nature—generated in a tunnel contact between electrodes of different temperatures when at least one of the electrodes is a ferromagnet. The Seebeck spin current is governed by the energy derivative of the tunnel spin polarization. By exploiting this in ferromagnet–oxide–silicon tunnel junctions, we observe thermal transfer of spins from the ferromagnet to the silicon without a net tunnel charge current. The induced spin accumulation scales linearly with heating power and changes sign when the temperature differential is reversed. This thermal spin current can be used by itself, or in combination with electrical spin injection, to increase device efficiency. The results highlight the engineering of heat transport in spintronic devices and facilitate the functional use of heat.

Figures at a glance

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Figure 1: Basic concept of Seebeck spin tunnelling.

In a tunnel contact between a ferromagnetic electrode and a non-magnetic electrode, for instance silicon, a temperature difference between the electrodes causes a transfer of spin angular momentum from the ferromagnet to the non-magnetic electrode. The thermal spin current requires no tunnel charge current. The induced spin polarization is reversed when the temperature difference is reversed and is parallel or antiparallel to the magnetization of the ferromagnet.

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Figure 2: Observation of thermal spin current from ferromagnet to silicon by Seebeck spin tunnelling.

a, Device layout, showing the 3-μm-thick, 800-μm-wide, 4-mm-long silicon strip (purple), the 1.5-nm-thick SiO₂/Al₂O₃ tunnel barrier (light grey) and the ferromagnetic Ni₈₀Fe₂₀ electrode (green). The silicon is heated by Joule dissipation from a constant current applied using two Cr/Au contacts at opposite ends, resulting in T_Si > T_FM. The voltage across the tunnel contact is measured in the absence of a tunnel charge current. The voltage lead simultaneously acts as a thermal sink that keeps the ferromagnet cold. b, Thermally induced spin accumulation in silicon, detected as a voltage change, ΔV_TH, in an external magnetic field, B_z, applied perpendicular to the tunnel interface (Hanle geometry). The plot shows results for heating current in two opposite directions. The magnetization (M) of the ferromagnet remains in the plane of the thin film owing to shape anisotropy. c, Measured ΔV_TH (symbols) versus heating current density, J_heating, together with a quadratic fit (solid line). d, Measured ΔV_TH (symbols) as a function of Joule heating power, and a linear fit (solid line). All data were measured for p-type silicon with a resistivity of 11 mΩ cm at 300 K, for a tunnel contact area of 180 μm × 400 μm and for a base temperature of 300 K.

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Figure 3: Origin of Seebeck spin tunnelling and model calculation of salient characteristics.

a, Spin-dependent density of states and its occupation for a tunnel contact with a hot silicon electrode and a cold ferromagnetic electrode, and the electron flows (four red arrows) that are induced by the temperature difference. For the cold ferromagnet, taken to be at T_FM = 0, all states below the Fermi energy, E_F, are filled (dark blue) and those above E_F are empty (light blue). For the hot silicon, a finite number of electrons with excess energy above E_F exist, together with a fraction of empty states below E_F. The sum of the four tunnel currents, two for each spin, results in a net flow of spin because the tunnel spin polarizations above and below E_F are unequal. b, Calculated value of the spin accumulation, Δμ, induced in the silicon electrode of a tunnel contact between the silicon and the ferromagnet, shown as a function of the temperature difference, ΔT = T_Si − T_FM, under the condition of zero tunnel charge current. Results are shown for three different profiles of TSP versus energy (E), as depicted on the right. For ΔT > 0, T_FM = 300 K and T_Si is increased, whereas for ΔT < 0, T_Si = 300 K and T_FM is increased. See Supplementary Information for details and parameters. a.u., arbitrary units.

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Figure 4: Sign reversal of thermal spin current by heating the silicon or the ferromagnet.

a, Layout of the device for silicon heating. b, Thermally induced spin accumulation for silicon heating in the same junction as in Fig. 2, but now detected with an external magnetic field, B_x, applied parallel to the tunnel interface (inverted Hanle geometry). c, d, Measured ΔV_TH (symbols) versus heater current density (c) and heating power (d), with corresponding fits. e, Layout of the device for heating of the ferromagnetic electrode (FM). In the tunnel area, the metal electrode consists of 10-nm-thick Ni₈₀Fe₂₀ and 10-nm-thick gold. f–h, Similar sets of inverted Hanle data as in b–d, but now with the ferromagnetic electrode heated. For all data, the inverted Hanle signal is defined as the voltage at a given B_x minus the saturation voltage, V_SAT, at large B_x, with voltage polarity consistently defined as V_Si − V_FM. All data were measured for p-type silicon with a resistivity of 11 mΩ cm at 300 K, for a tunnel contact area of 200 μm × 200 μm and for a base temperature of 300 K.

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Thermal spin current from a ferromagnet to silicon by Seebeck spin tunnelling