Seebeck Rectification Enabled by Intrinsic Thermoelectrical Coupling in Magnetic Tunneling Junctions

An intrinsic thermoelectric coupling effect in the linear response regime of magnetic tunneling junctions (MTJ) is reported. In the dc response, it leads to a nonlinear correction to Ohm’s law. Dynamically, it enables a novel Seebeck rectification and second harmonic generation, which apply for a broad frequency range and can be magnetically controlled. A phenomenological model on the footing of the Onsager reciprocal relation and the principle of energy conservation explains very well the experimental results obtained from both dc and frequency-dependent transport measurements performed up to GHz frequencies. Our work refines previous understanding of magnetotransport and microwave rectification in MTJs. It forms a new foundation for utilizing spin caloritronics in high-frequency applications.

Figure 1

The Thomson thermoelectric conductor (TTC) connected in (a) an open circuit, (b) a closed circuit, and (c) a closed circuit with a supporting material. (e),(f),(g) The corresponding temperature profiles of the TTC. (d) Schematic MTJ circuit and (h) MTJ structure used in this work.

Figure 2

(a) Asymmetric and (b) symmetric combination of the dc voltage $V_{+}$ and $V_{-}$ measured on sample $A$ with a positive and negative tunneling current $I$, respectively. (c) and (d) are the same as (a) and (b) but measured on sample $B$. Circles and squares are measured at the AP and $P$ alignments of the MTJ, respectively. Lines are fits to the Eq. (2).

Figure 3

(a) The first- and (b) the second-harmonic voltage measured on sample $B$ with a low-frequency ac tunneling current. The Seebeck rectification voltage $V_r$ measured with the microwave tunneling current being (c) modulated and (d) in continuous wave. Circles and squares are measured at AP and $P$ alignments, respectively. Lines are fits to Eq. (3).

Figure 4

The TMR of sample $A$ measured as a function of (a) the external magnetic field strength and (b) the field direction. The Seebeck rectification voltage measured at $\omega /2\pi =10.0\mathrm{GHz}$ as a function of (c) the external magnetic field strength and (d) the field direction.