Magneto-optical design of anomalous Nernst thermopile

Abstract

The introduction of spin caloritronics into thermoelectric conversion has paved a new path for versatile energy harvesting and heat sensing technologies. In particular, thermoelectric generation based on the anomalous Nernst effect (ANE) is an appealing approach as it shows considerable potential to realize efficient, large-area, and flexible use of heat energy. To make ANE applications viable, not only the improvement of thermoelectric performance but also the simplification of device structures is essential. Here, we demonstrate the construction of an anomalous Nernst thermopile with a substantially enhanced thermoelectric output and simple structure comprising a single ferromagnetic material. These improvements are achieved by combining the ANE with the magneto-optical recording technique called all-optical helicity-dependent switching of magnetization. Our thermopile consists only of Co/Pt multilayer wires arranged in a zigzag configuration, which simplifies microfabrication processes. When the out-of-plane magnetization of the neighboring wires is reversed alternately by local illumination with circularly polarized light, the ANE-induced voltage in the thermopile shows an order of magnitude enhancement, confirming the concept of a magneto-optically designed anomalous Nernst thermopile. The sign of the enhanced ANE-induced voltage can be controlled reversibly by changing the light polarization. The engineering concept demonstrated here promotes effective utilization of the characteristics of the ANE and will contribute to realizing its thermoelectric applications.

Figure 1

(a) Schematic of the anomalous Nernst thermopile consisting of two ferromagnetic materials (FM1 and FM2) with different anomalous Nernst coefficients. Here, E_ANE stands for the electric field driven by the ANE, M the magnetization, and ∇T the temperature gradient. (b) Schematic of the zigzag structure consisting of a single ferromagnetic material with uniform out-of-plane magnetization. (c) Schematic of the anomalous Nernst thermopile consisting of a single ferromagnetic material with alternately reversed out-of-plane magnetization realized by all-optical helicity-dependent switching of magnetization. σ⁺ (σ⁻) indicates right (left) circularly polarized light.

Figure 2

(a) Layer configuration of the [Co/Pt]₄ film. (b) Magnetization M curves of the [Co/Pt]₄ film, measured when a magnetic field H was applied along the easy axis (out-of-plane direction of the film) and the hard axis (in-plane direction of the film) at room temperature. The [Co/Pt]₄ film exhibits strong perpendicular magnetic anisotropy with an out-of-plane coercive field of 2.5 kOe and an anisotropy field of 25.1 kOe. (c) MOKE image of the [Co/Pt]₄ film illuminated with right (σ⁺) and left (σ⁻) circularly polarized laser light as well as linearly (L) polarized laser light at room temperature. The bright (dark) contrast represents the area with M along the + z (− z) direction perpendicular to the film plane. (d) Hall resistance R_H of the [Co/Pt]₄ film with a Hall bar shape at room temperature. The red line shows the out-of-plane H dependence of R_H. The blue stars indicate the R_H values before and after illuminating the entire Hall bar with σ⁻ light at zero field, where the Hall bar was uniformly magnetized by applying an external field H =  + 20 kOe before light illumination.

Figure 3

(a) Schematic of the zigzag-shaped [Co/Pt]₄ film with the alternately reversed magnetization configuration, where the M direction is reversed between the neighboring wires. (b–e) Representative local MOKE images of the zigzag-shaped [Co/Pt]₄ film. The magnetization configuration was designed by alternately irradiating the wires with left (σ⁻) circularly polarized laser light, where M was along the + z direction before light illumination. The bright (dark) contrast in the MOKE images represents M along to the + z (− z) direction.

Figure 4

(a,b) Schematics of the zigzag-shaped [Co/Pt]₄ film uniformly magnetized along the + z (a) and − z (b) directions. (c) Temperature difference ΔT dependence of thermoelectric voltage in the zigzag-shaped [Co/Pt]₄ film uniformly magnetized along the − z or + z direction. (d) ΔT dependence of V_ANE (= (V₊ – V_–)/2) and V_SE (= (V₊  + V_–)/2), calculated from the data in (c). V₊ (V_–) denotes the thermoelectric voltage measured when M is along the + z (− z) direction. (e,f) Schematics of the zigzag-shaped [Co/Pt]₄ film in the alternately reversed magnetization configurations designed via the AO-HDS. The configuration in (e) ((f)) was designed by irradiating the wires uniformly magnetized along the − z (+ z) direction with right (σ⁺) (left (σ⁻)) circularly polarized light. (g) ΔT dependence of the thermoelectric voltage in the zigzag-shaped [Co/Pt]₄ film in the alternately reversed magnetization configurations. (h) ΔT dependence of V_ANE in the zigzag-shaped [Co/Pt]₄ film in the alternately reversed magnetization configurations, calculated by subtracting the SE background in (d) from the raw data in (g).