Thermoelectric performance of spin Seebeck effect in Fe3O4/Pt-based thin film heterostructures

We report a systematic study on the thermoelectric performance of spin Seebeck devices based on Fe3O4/Pt junction systems. We explore two types of device geometries: a spin Hall thermopile and spin Seebeck multilayer structures. The spin Hall thermopile increases the sensitivity of the spin Seebeck effect, while the increase in the sample internal resistance has a detrimental effect on the output power. We found that the spin Seebeck multilayers can overcome this limitation since the multilayers exhibit the enhancement of the thermoelectric voltage and the reduction of the internal resistance simultaneously, therefore resulting in significant power enhancement. This result demonstrates that the multilayer structures are useful for improving the thermoelectric performance of the spin Seebeck effect.

Thermoelectrics is one of the promising energy harvesting technologies for waste heat management and recovery, having potential applications in thermoelectric generation or temperature sensing. Thermoelectric generation usually relies on the Seebeck effect, a phenomenon that refers to the generation of an electromotive force parallel to the direction of an applied temperature gradient in electrically conducting materials [see Fig. 1(a)].
In spintronics, 1,2 a different thermoelectric principle was recently discovered. The principle is based on a spin counterpart of the Seebeck effect, in analogy named the spin Seebeck effect (SSE). 3 The SSE refers to the generation of spin currents 4 in a ferromagnetic material (F) upon application of a temperature gradient. The spin current induced by the SSE is injected into a non-magnetic metal (N) in contact with F and converted into an electromotive force by means of the inverse spin Hall effect (ISHE) in suitable N materials 5-8 [ Fig. 1(b)]. The SSE has been observed in a range of ferromagnetic materials: metals, 3 semiconductors 9,10 and insulators. 11-13 Thus, the SSE has been experimentally established as a general transport phenomenon in ferromagnets. Since the discovery of the SSE, the study of the interaction among spin, charge, and heat currents has been strongly invigorated, which is the main focus of spin caloritronics. 14,15 This has resulted in the discovery of various thermospin effects, such as the spin Peltier effect 16 and the spin dependent versions of the ordinary Seebeck 17 and Peltier 18 effects among others. [19][20][21] Theoretically, the SSE 22-24 is understood as an effect resulting from the thermal nonequilibrium between magnons in F and conduction electrons in N, generating a spin current at the F/N interface proportional to the effective temperature difference between magnons in F and electrons in N. Theories from other view points have also been developed, [25][26][27] where roles of magnon spin currents inside the ferromagnet has been highlighted. 26,27 This is supported by several recent experiments: the dependence of SSE voltage with the thickness of F, 28 the enhancement of the SSE voltage in magnetic multilayers, 29 and the suppression of the SSE at high magnetic fields. 30,31 The SSE has potential advantages over conventional thermoelectric technologies for thermal energy harvesting. The experimental geometry, having paths for heat and electric currents independent and perpendicular to each other, allows to have two different materials that can be optimised independently and it is also advantageous for the implementation of thin film devices over large surfaces, for example, simply by thin film coating. 32 Moreover, the observation of the SSE in magnetic insulators implies the potential to generate pure spin currents with lesser dissipation losses due to mobile charge carriers and further expanding the range of possible materials where spin mediated thermoelectric conversion can be studied. However, a main disadvantage of the SSE is the low magnitude of the thermoelectric output. Strategies to overcome this limitation are currently being explored. One possibility is to increase the spin current detection efficiency by taking advantage of the spin Hall angle characteristics of different N materials, 33-36 as it has been shown in the spin Hall thermopiles. 37 A different approach can be directed towards increasing the thermal spin current generation, as recently shown in the SSE of magnetic multilayers. 29 In this paper, we have investigated the thermoelectric performance of two types of SSE devices: spin Hall thermopiles 37 and magnetic multilayers formed by repeated growth of n number of [F/N] bilayers, hereafter referred to as [F/N] n . We have performed room temperature measurements of the thermoelectric voltage and power output characteristics for both the structures. The materials chosen to form the basic F/N bilayer structure are magnetite, Fe 3 O 4 , as the F layer and platinum, Pt, as the N layer. Magnetite is a ferrimagnetic oxide, commonly employed in spintronics 1,2 due to its half-metallic character and high Curie temperature. 38,39 These properties have inspired several studies of its magnetotransport properties. [40][41][42][43][44] Platinum is a material commonly used for spin current detection, due to its relatively large spin Hall angle. 45,46 The Fe 3 O 4 films were grown on magnesium oxide, MgO(001), substrates by pulsed laser deposition using a KrF excimer laser with 248 nm wavelength, 10 Hz repetition rate, and 3 × 10 9 W/cm 2 irradiance in an ultrahigh-vacuum chamber (UHV). The Pt films were deposited in the same UHV chamber by DC magnetron sputtering. Further details on the growth can be found in Ref. 47. The film thickness was measured by x-ray reflectivity and its structural quality was confirmed by x-ray diffraction and transmission electron microscopy (TEM). The film cross sections were prepared by Focused Ion Beam and measured by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). The measurements were carried out in a probe aberration corrected FEI Titan 60-300 operated at 300 kV. The fabrication of the spin Hall thermopile structure was carried out using optical lithography, following a two-step process consisting of Ar milling of the Fe 3 O 4 /Pt bilayer and fabrication of Al interconnection wires by metal evaporation and lift-off.
The SSE experiments were performed using the longitudinal configuration. 48 The sample dimensions are L x = 2 mm, L y = 7 mm and L z = 0.5 mm, except for the spin Hall thermopile structure in which the in plane sample geometry is defined by the lithography pattern. Figure 2(a) shows a schematic illustration of the longitudinal SSE measurement geometry. The sample is placed between two AlN plates with high thermal conductivity, where the top plate has a resistive heater and the lower plate works as a heat sink. By passing an electric current to the heater, a temperature gradient (∇T ) is applied in the z direction. This generates a temperature difference (∆T ) between the bottom and top of the sample, which are stabilized at temperatures T and T + ∆T , respectively. All the measurements were carried out at T = 300 K. The temperature difference is monitored by two thermocouples connected differentially. The SSE voltage in the Pt film is measured along the y direction with a nanovoltmeter, while a magnetic field is applied in the x direction.
The measurement mechanism is as follows. In the longitudinal configuration, a spin current is injected into the Pt layer with the spatial direction J S perpendicular to the Fe 3 O 4 /Pt interface (parallel to ∇T ) and the spin-polarization vector σ (parallel to the magnetization of Fe 3 O 4 ). Then, an electric field (E ISHE ) is generated in Pt due to the ISHE, which can be expressed as:    The electric power output characteristics of the SSE in the Fe 3 O 4 /Pt single bilayer and spin Hall thermopile were also investigated. This is performed by attaching a variable load resistance (R L ) to the samples and measuring the voltage drop across R L , as schematically shown in the inset of Fig. 3. Then, we estimate the output power as V 2 L /R L . Figure 3 shows the comparative results obtained for the single bilayer and spin Hall thermopile as a function of R L ; the output power is maximized when R L equals the internal resistance of the sample, as expected from impedance-matching criterion for maximum power transfer and also in agreement with previous reports. 51 We can see that, despite the increased voltage response in the spin Hall thermopile structure, there is no advantage in terms of extractable electrical power output from the device when compared to the response of the single Fe 3 O 4 /Pt bilayer. This is a consequence of the fact that the voltage enhancement is due to the increase in effective length of the sample, which is associated to an increase in the internal resistance. 51 In the case of ANE-based thermopile structures 52 it was proposed that, if the ANE voltage is independent of sample thickness, the performance can be improved by reducing the internal resistance of the devices, by just increasing the ferromagnet thickness. However, this approach cannot be used in the case of spin Hall thermopiles, since increasing the thickness of the N layers will result in a reduction of the ISHE voltage 53 . Therefore, in order to increase the electric output power of the SSE, we must follow a different approach. To realize the enhancement of the output power of the SSE, we have investigated the thermoelectric performance of the SSE multilayers ([F/N] n ) formed by repeated growth of n number of F/N bilayers [ Fig. 4(a)]. This type of structure has recently been reported to present an enhancement of the transversal thermoelectric voltage in various magnetic multilayer systems based on oxide/metal, 29 all-metallic, 54,55 and all-oxide 56 junctions. The advantage of the multilayer structure is that along with the voltage enhancement, the internal resistance of the structure decreases with increasing the layer number, due to parallel connection be-tween the layers. Therefore, a significant enhancement of the extractable electric power is expected.
Here, we show the measured results of voltage and output power characteristics of the SSE in [Fe 3 O 4 (23)/Pt (7) Fig. 4 (d). We found that the power largely increases in the multilayer systems, reaching more than two orders of magnitude enhancement compared to the single bilayer case: the power is estimated to be 0.01, 1 and 2 pW/K 2 for n = 1, 6 and 12, respectively.
In summary, we have performed thermoelectric voltage and power measurements of the spin Hall thermopile and SSE multilayer structures based on Fe 3 O 4 /Pt junction systems. Our results show that the spin Hall thermopile can enhance the voltage, but cannot improve the power output due to the increase of the internal resistance of the devices. However, the voltage increase is beneficial to increase the sensitivity of the SSE and might have possible sensor applications. In contrast, the multilayer SSE devices significantly enhance both thermoelectric voltage and power compared to the conventional bilayer systems, leading to an efficiency improvement in SSE-based thermoelectric generation. Although the obtained power is still small, we believe that the SSE in multilayers can be further enhanced by careful choice of materials and device geometry. Since the observed SSE enhancement in multilayers is expected to depend on spin transport properties across the multilayers, using materials with longer spin transport characteristic lengths might lead to further enhancement in the SSE.