Strategic enhancement of anomalous Nernst effect in Co 2 MnAl 1-x Si x Heusler compounds

Strategic enhancement of anomalous Nernst effect in Co 2 MnAl 1-x Si x Heusler compounds Y. Sakuraba 1,2, K. Hyodo 3, A. Sakuma 3 and S. Mitani 1 1 Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki, 305-0047 Japan 2 PRESTO, Japan Science and technology Agency, Saitama 332-0012, Japan 3 Department of Applied Physics, Tohoku University, Aoba-ku, Sendai, 980-8579, Japan Abstract The anomalous Nernst effect (ANE), a thermoelectric phenomenon in magnetic materials, has potential for novel energy harvesting applications because its orthogonal relationship between the temperature gradient and the electric field enables us to utilize the large area of non-flat heat sources. In this study, the required thermopower of ANE for practical energy harvesting applications is evaluated in simulations of electric power obtainable from ANE using a low-temperature heat source. A strategy for finding magnetic materials having large ANEs is proposed, which suggests that a large ANE originates from the constructive relationship between large Seebeck, anomalous Hall, and transverse Peltier effects. This strategy leads us to investigate the electric and thermoelectric properties in Co 2 MnAl 1-x Si x. As a result, it is found that Co 2 MnAl 0.63 Si 0.37 has the largest ANE thermopower ever reported, 6.2 µv/k. A first principles calculation of the anomalous Hall conductivity and transverse Peltier coefficient in B2 and L2 1 -ordered Co 2 MnAl 0.63 Si 0.37 shows close agreement with the experimental results, indicating that the observed large ANE arises from the intrinsic Berry phase curvature of Co 2 MnAl 1-x Si x. This study can be a guide for developing materials for new thermoelectric applications using ANE.

1. Introduction 1.1 Energy harvesting using anomalous Nernst effect There is an urgent need for ambient energy harvest technology to supply electricity to autonomous wireless sensor nodes working in the Internet of Things (IoT). Thermoelectric power generation (TEG) using various heat sources such as environmental, waste, and body heat is one of the most promising ways of energy harvesting. In particular, TEG using the Seebeck effect (SE) has been extensively studied and is viewed as being able to overcome various remaining problems including low energy conversion efficiency, poor mechanical endurance and flexibility, etc. [1,2] Since the required electric energy for harvesting applications is usually of a hundred µw to mw order, if the utilizable area of the heat source can be enlarged economically, it would be possible to obtain enough power even with poor energy conversion efficiency. However, the Seebeck effect is a one-dimensional phenomenon in which the electric field appears in the coaxial direction of the temperature gradient, T ; therefore, an SE-based TEG system must consist of a complex matrix thermopile structure, which is in principle not suitable for utilizing a large area of a non-flat heat source. Although TEG using flexible organic materials is being developed, the difficulty of obtaining a large temperature difference DT in a thin flexible module is a critical issue that makes it difficult to obtain sufficient electric output power. On the other hand, it has been recently proposed that the anomalous Nernst effect (ANE), which is a thermoelectric phenomenon unique to magnetic materials, has several important advantages as an energy harvesting

! technique. [3,4] Here, the electric field of ANE ( ) can be expressed by the following equation, E ANE!! E ANE = Q S µ 0 M T ( ), (1) where Q S and µ 0! M represent the anomalous Nernst coefficient and magnetization, respectively. As equation (1) indicates, ANE generates an electric field in the direction of the outer product of the magnetization µ 0! M and temperature gradient T. This three dimensionality of ANE enables us to increase the serial voltage by using thermopiles consisting of simple laterally connected magnetic wires, because! E ANE appears along the surface of a heat source, which is a significant advantage when it comes to enlarging the size of the TEG module and utilizing large-area heat sources covering hundreds to thousands square centimeters. Furthermore, in conventional SE-based TEG, the voltage of SE (V SE ) is proportional to S SE T t(= S SE ΔT ) ; thus, the TEG module t must be sufficiently thick for obtaining a large temperature difference DT. In contrast, the voltage of ANE (V ANE ) is proportional to S ANE T L, with L being the length of magnetic wire; i.e., V ANE depends on T, not DT. Thus, even a thin papery ANE-based thermopile can generate a large electric voltage if T is large. Therefore, ANE-based TEG has the potential to be a different type of thermoelectric energy harvesting technique using large non-flat heat source thanks to its simple laterally connected structure, high flexibility, mechanical endurance, and cost-performance. These unique advantages have stimulated studies on ANE not only for gaining a fundamental understanding of the phenomenon, but also for practical thermoelectric applications. [5-16] However, studies on ANE are still

much fewer than those on SE, and the reported thermopowers of ANE at around room temperature are usually less than 1µV/K (the highest value to date is 3µV/K, in Fe 0.85 Ga 0.15 [13]), which is obviously insufficient for practical energy harvesting applications. In addition, although there is a clear need for materials showing high thermopower of ANE, a strategy for finding such large ANE materials has not been established. In this paper, first, we show the results of a simulation on the electric power obtainable using ANE-based TEG in a realistic energy-harvesting situation in order to estimate the required thermopower of ANE. Second, we suggest a strategy for finding magnetic materials having large ANEs on the basis of the linear response equation of the electric field under a temperature gradient and the Mott formula [17]. According to the proposed strategy, we find that Co 2 MnAl 1-x Si x Heusler compound is a promising material because of its potentially large intrinsic anomalous Hall effect and large Seebeck effect. Third, we show the results of an experimental systematic analysis of the crystal structure and electric and thermoelectric properties of Co 2 MnAl 1-x Si x epitaxial thin films. Finally, we report the highest ever ANE thermopower of +6.2 µv/k, from Co 2 MnAl 0.63 Si 0.37. Our first principles calculation involving the transverse Peltier tensor agrees with the experimental results, which validates the suggested strategy for obtaining a large ANE. 1.2. Simulation of required thermopower of ANE for practical applications In this section, we show the results of a simulation of electric power producible from ANE in a realistic IoT situation. We simulate the case of giving a temperature gradient

T in the z direction in a magnetic material (MM) having width w, depth d, thickness t, electric resistivity r, and thermal conductivity k as shown in Figure 1. When the magnetization is aligned in the y-direction, the V ANE generated in the x direction is expressed as S ANE T w, and the internal electric resistance R int in the x direction is written as R int = rw/td. Here, the work resistance R ext is connected to the left and right 2 sides; thus, the power P obtainable by ANE is expressed as P =V ANE / (R int + R ext ). Here, we consider the case for optimal impedance matching R ex = R int, wherein P can be expressed using the following equation. P = (S T )2 ANE wtd (2) 2ρ Thus, P is proportional to the square of S ANE and the volume of MM. To simulate the S ANE dependence of P, we need to estimate T in a specific situation. Thus, as a typical example of energy harvesting for IoT, we estimated T using a low-temperature heat source (HS) of 60 C and a heat bath (HB) of 20 C. We also took r and k to be 300 µw cm and 10 W/mK, respectively, as these are typical values for magnetic metals having a relatively high r such as the Heusler compounds studied in this work. T in a magnetic material generated by heat flowing from HS to HB depends on the interfacial thermal resistance between the magnetic material and HB (R MM/HB, unit is K/W). The product of R MM/HB and area A (= wd) is expressed as 1/h, where h is the heat transfer coefficient at the MM/HB interface. When the HB is air, h is usually smaller than about 30 W/Km 2, [18] which limits the generated heat current density j Q passing through the MM and results in a small T. On the other hand, if HB is a liquid such as water, h can be 50 10000

W/m 2 K depending on the flow velocity of the liquid. Thus, here, we simulated three cases with different h to consider a variety of thermal flows, i.e., (i) h = 10 W/Km 2 assuming a situation of free convection into air, (ii) 30 W/Km 2 for forced convection into air, and (iii) 500 W/Km 2 for forced convection into water. Since R MM/HB is usually much greater than R MM/HS, T is estimated to be 0.04, 0.12, and 1.9 K/mm for cases (i), (ii), and (iii), respectively. Figure 1 shows the S ANE dependence of obtainable P for the above three cases. In this simulation, we consider 1-mm-thick MM with a relatively large area, A = wd = 50 50 cm 2, for cases (i) and (ii) and with a small area, 5 5 cm 2 for (iii). It should be noted that, although 50 50 cm 2 is too large for conventional SE-based TEG, it is not an unrealistic size for ANE-based TEG because of its expandability as mentioned in the first section. From Figure 1, we can see that when HB is air, we require an S ANE over 20-40 µv/k to obtain in excess of 100µW in this situation. On the other hand, if heat can convect to a flowing liquid HB such as water, 10 µv/k is able to provide over 100 µw output even when using 5 5 cm 2 HS. Therefore, we concluded from this simulation that achieving an S ANE in excess of 10µV/K is a realistic target for ANE-based energy harvesting applications with high heat convection. On the other hand, much higher thermopower, at least 20-40µV/K, would be required for applications with poor heat convection. 2. Strategy for finding magnetic materials having large ANE Although it is necessary to explore magnetic materials having very large S ANE values for developing practical TEG applications using ANE, the previous studies did not state

any guidelines for materials development. Therefore, here, we propose a strategy to obtain a large ANE based on the linear response equation of the electric field E under a temperature gradient T. When we apply E and T together to an electrically conductive material, the generated electric current J can be expressed using the electric conductivity tensor!σ and Peltier tensor!α, J =!σ E!α T (2) In the open circuit condition (J = 0), the electric field E induced by T is written as E =!α T (3)!σ Therefore, the transverse field component E y, which mainly originates from ANE in the case of a magnetic material, can be expressed as E y = σ α σ α xx xy xy xx T (4) 2 σ 2 xx +σ xy Since usually σ 2 xx >> σ 2 xy, the thermopower of ANE S ANE can be expressed using the diagonal and off-diagonal parts of the resistivity tensor, ρ xx (=1/σ xx ) and ρ xy (= σ xy /σ 2 xx ), as follows: S ANE = ρ xx α xy + ρ xy α xx (5) This formula tells us that there are two different phenomenal sources in ANE. For simplifying the following explanation, we denote the first and second terms in eq. (5) as S I = ρ xx α xy and S II = ρ xy α xx. Although previous studies on CuCr 2 Se 4-x Br x and Ga 1- xmn x As quantitatively analyzed these two contributions of ANE [19,20], there is no mention of a strategy/policy regarding materials having a large S ANE from the view point

of thermoelectric applications of ANE. Therefore, here, we elaborate the phenomenological picture of S I and S II and propose guidelines for materials development. As for the second term, the product of S II and T, i.e., ρ xy α xx T, can be converted into ρ xy j xx. Since j xx is regarded as flowback of the Seebeck current originating from a heat driven charge accumulation on the cold side, the contribution of S II to V ANE can be considered to be an anomalous Hall voltage originating from the Seebeck effect-induced charge current. Thus, S II can be also expressed as S II = S SE tanq AHE by using the Seebeck coefficient S SE (= r xx a xx ) and anomalous Hall angle q AHE (= r xy /r xx ). On the other hand, the product of the first term S I and T, i.e., ρ xx α xy T, is transformed into ρ xx j xy, indicating that the contribution of S I is considered to be a direct generation of the transverse current j xy from the transverse Peltier term a xy, which leads to the transverse electric field E ANE. Though the contribution of S I is often regarded as an intrinsic part of ANE apart from the contribution of AHE, here, we have to consider the magnitude and sign of both S I and S II for obtaining a large S ANE. It is known that the solution of the Boltzmann equation leads to a relationship between!α and!σ, and a xx and a xy are expressed as follows. [17] α xx = π 2 3 k 2 B T e σ xx ε (6) EF α xy = π 2 3 k 2 B T σ xy e ε (7) E F

From the above equations, we propose three required properties for possessing a large anomalous Nernst effect; a large S ANE would originate from (i) a large S II, i.e., large S SE (~ large energy derivative of s xx at around E F ) and anomalous Hall angle q AHE, (ii) a large S I, i.e., large r xx and a xy (i.e., a large energy derivative of s xy around E F ), and (iii) a constructive relationship between S I and S II. The third requirement is the most important, because S ANE becomes tiny if S I and S II have different signs and they destructively affect each other. These three requirements collectively form a strategy for finding magnetic materials showing large ANEs. 3. Anomalous Nernst effect in Co 2 MnAl 1-x Si x 3.1. Potential of large anomalous Nernst effect in Co 2 MnAl 1-x Si x According to the above-mentioned strategy, we noticed that there are several promising Co-based Heusler compounds for obtaining a large ANE, particularly Co 2 MnAl 1-x Si x (CMAS). Kübler and Tung et al. calculated the intrinsic anomalous Hall effect originating from the Berry phase curvature in Co-based Heusler compounds and predicted a large s xy in Co 2 MnAl [21,22], which was clearly confirmed in a subsequent experiment performed by Vidal et al. on epitaxial Co 2 MnAl thin film. [23] The calculated Fermi level (E F ) dependence of s xy in Co 2 MnAl shown in ref. [21] indicates that Co 2 MnAl should have not only a large s xy but also a large σ xy / ε around E F, which is important for obtaining a large S I. In addition, since the E F of Co 2 MnAl is also predicted to be around the edges of a half-metallic gap in the minority spin-band, S SE could be enlarged by tuning

of E F due to its large σ xx / ε. First principles calculations and several experiments have already confirmed that the replacement of Al by Si in Co 2 MnAl shifts the E F to higher energy, while preserving the electronic structure around E F. [24,25] Therefore, Co 2 MnAl 1-x Si x (CMAS) with an optimal x should have both a large S I and a large S II stemming from its intrinsic electronic structure, which could lead to a large ANE if the S I and S II effects work constructively. Thus, in this study, we fabricated a series of epitaxial CMAS thin films having different values of x and systematically investigated their crystal structure and electric and thermoelectric properties to validate the proposed strategy for finding a material with a large ANE. 3.2. Experimental procedure (001)-oriented epitaxial Co 2 MnAl 1-x Si x thin films having different Al to Si ratios were grown on a MgO (001) substrate using a co-sputtering technique with Co 2 MnSi and Co 2 MnAl sputtering targets. All films were deposited at ambient substrate temperature and then in-situ annealed at 600 C. The composition of the films was measured by a combination of inductively coupled plasma mass spectrometry (ICP-MS) and x-ray fluorescence analysis (XRF). The thickness of the films was fixed at 30 nm. The crystal structure and atomic ordering were investigated by x-ray diffraction with a Cu K a x-ray source. Longitudinal and transverse electric and thermoelectric transport properties including ANE were investigated with a physical property measurement system (PPMS) for films patterned by photolithography and Ar ion milling. The electric resistivity r xx was measured using a dc four-probe method by flowing a constant dc current of 3 ma. A

systematic study of the anisotropic magnetoresistance effect (AMR) in Co-based Heusler thin films recently showed that the sign of the AMR ratio clearly reflects the position of E F ; i.e., AMR is positive (negative) when E F is inside (outside) the half-metallic energy gap. [25,26] Thus, to see the expected shift of E F with increasing x, we systematically investigated the AMR ratio for the CMAS thin films while rotating the external magnetic field 360 within the film plane direction. ANE (AHE) was measured by flowing a heat (electric) current in the film plane direction and applying a magnetic field in the perpendicular direction. As for ANE, the temperature gradient T was carefully measured through a calibration using an infrared camera and black body coating to correct the emissivity of the samples. The Seebeck effect was simultaneously measured with ANE. Note that, for actual ANE-based TEG applications, T is perpendicular to a thin ferromagnetic wire to obtain V ANE from the remanent in-plane magnetization without applying an external magnetic field. However, here, we applied T in the in-plane direction because it is strictly measurable in this configuration. We investigated the annealing temperature dependence from 500 to 700 C only for the Co 2 MnAl 0.63 Si 0.37 thin film showing the highest ANE. 3.3. Crystal structure and atomic ordering in CMAS films Out-of-plane XRD patterns for the CMAS films annealed at 600 C are shown in Figure 2(a). We clearly detected only (002) and (004) peaks from all CMAS films, indicating (001)-oriented growth in the whole range of x. A clear (002) super lattice peak indicates

the existence of atomic ordering between Co and (Mn,Al/Si) sites, which is called the B2 structure. Although we also tried to measure the (111) super lattice peak of the ideal L2 1 - ordered structure by tilting the film plane at a 54.7 from the normal direction, none of the films, except for the Co 2 MnSi film, showed a detectable (111) peak, indicating almost no ordering between Mn and Al/Si atoms. The out-of-plane lattice constant, as evaluated from the (004) peak position, is plotted against x in Figure 2(b). The lattice constant almost linearly decreases with increasing Si composition ratio, following Vegard s law. 3.4. Anisotropic magnetoresistance in CMAS films In magnetic materials, the electric resistivity r xx changes with the relative angle between the magnetization and applied electric current, which is called the AMR effect. The AMR ratio is defined by (ρ // ρ ) / ρ //, where r // (r ) is the resistivity when the magnetization is parallel (perpendicular) to the direction of the electric current. Thus, when r // < r, the AMR ratio becomes negative. It was recently proved theoretically and experimentally that the sign of the AMR ratio in Co-based Heusler compounds is a useful signature for monitoring Fermi level tuning against the total number of valence electrons, because of its half-metallic nature. [25,27,28] Figure 3(a) shows the AMR curves for the CMAS films. Co 2 MnAl film exhibits a large positive AMR ratio, which suggests that the E F of Co 2 MnAl is at the valence band edge of the half-metallic gap, as indicated by first principles calculations and experiments. [24] With increasing x, AMR gradually decreases and its sign turns from positive to negative around x = 0.4. At x = 1, i.e.,

Co 2 MnSi shows the largest negative AMR ratio, a signature property for which E F is in the half-metallic energy gap as reported in previous studies. [29 31] Therefore, from the analysis of the AMR ratio, we indirectly confirmed that E F shifts with increasing x. 3.5. Anomalous Hall effect in CMAS films Figure 4(a) shows the perpendicular magnetic field dependence of the anomalous Hall resistivity r xy for the CMAS thin films. The Co 2 MnAl film had the largest r xy of about 18 µw cm. r xy almost monotonically decreases upon replacing x with Si, as shown in Figure 4(b), and the Co 2 MnSi film had the smallest r xy, 0.7 µw cm. Co 2 MnAl also had the largest magnitude of anomalous Hall angle, q AHE of -7.3%, whereas q AHE decreases with x to -0.8% in Co 2 MnSi. It should be noted that the s xy values obtained for Co 2 MnAl and Co 2 MnSi are 295 and 96 S/cm, respectively, which are lower than the calculated intrinsic s xy values of 1265 and 193 S/cm [22]. Observed lower s xy must have been caused by the enlargement of r xx due to electron scattering at the film surface, film/substrate interface, and grain boundaries. The almost complete lack of L2 1 -ordering in our CMAS films also causes a deviation from the s xy calculated for the ideal L2 1 structure. However, since the x dependence of the magnitude of AHE qualitatively agrees with the calculated intrinsic AHE in Co 2 MnAl and Co 2 MnSi, there should be a large AHE in Co 2 MnAl from the intrinsic contribution due to the electronic structure.

3.6. Seebeck and anomalous Nernst effect in CMAS films ANE was measured in the same geometry as AHE was measured, by applying an inplane heat current instead of an electric current. The obtained transverse voltage V y was normalized by the width of the patterned film (w = 3 mm) and T evaluated by an infrared camera, i.e., (V y /w)/ T. Since V y is mainly generated by ANE, (V y /w)/ T is regarded as the thermopower of ANE (S ANE ). Figure 5 shows the external magnetic field dependence of (V y /w)/ T for the CMAS films. The x dependence of S ANE is summarized in Figure 6(b). Interestingly, the Co 2 MnAl film having the largest AHE exhibits a very small S ANE of +0.9 µv/k, and S ANE gradually grows as more Al is substituted with Si. The largest S ANE of +3.9 µv/k was observed for Co 2 MnAl 0.63 Si 0.37. The value follows x, reaching + 0.7 µv/k for Co 2 MnSi. The Seebeck coefficient S SE was measured at the same time as ANE, and the results are summarized in Figure 6(a). The sign of S SE is negative in the whole range of x, and the magnitude gradually increases with x from -16.8 µv/k in Co 2 MnAl to -43.8 µv/k in Co 2 MnAl 0.49 Si 0.51, the highest value of all of the prepared films. To analyze the x dependence of S ANE, Figure 6(c) plots S II estimated from S SE and a q AHE. Although we cannot see a direct relationship between the magnitudes of AHE and ANE in the CMAS films, the x dependence of S II is similar to that of S ANE, which suggests a contribution of S II to ANE. An important point here is that the magnitude of S II is smaller than the observed S ANE except for Co 2 MnAl. Thus, we evaluated S I from equation (5) and plotted it in Figure 6(d). The result indicates that the largest S ANE of +3.9 µv/k at x = 0.37 arises from the constructive and comparative contributions of S I

(+1.8 µv/k) and S II (+2.1 µv/k). It is interesting to see that a destructive relationship between S I (-0.3 µv/k) and S II (+1.2 µv/k) was only observed in the Co 2 MnAl film. According to the calculated energy dependence of s xy in Co 2 MnAl shown in ref. [21], E F is predicted to be very close to the peak of the s xy vs. electron energy curve. Therefore, it can be inferred from equation (7) that our Co 2 MnAl film intrinsically has a negative S I because of the position of E F, but the sign changes to positive when E F moves to a higher energy through replacement of Al with Si and reaches a maximum at x = 0.37. 3.7. Annealing temperature dependence of anomalous Nernst effect in CMAS films: Comparison of experiment and calculation Finally, we investigated the annealing temperature dependence of the Co 2 MnAl 0.63 Si 0.37 film showing the highest ANE for annealing at 600 C. Since the atomic disordering must result in convolution of the electronic structure against electron energy [32], the thermoelectric responses are expected to improve as a result of enhancing L2 1 -atomic ordering. Figure 7 shows the XRD patterns for the Co 2 MnAl 0.63 Si 0.37 films annealed at temperatures from 500 to 700 C. Although the (111) super lattice peak coming from the L2 1 -ordered structure was not observed below 600 C, a clear (111) signal was detected above 650 C. The order parameter S $%& of the L2 1 structure was % evaluated as 0.48 at 650 C from S $%& = (I *+, &&& /I *+,../ )/(I,12 &&& /I,12 *+,/,12../ ), where I 345 indicates the observed/simulated integrated diffraction intensity of the (hkl) peak, indicating that there is a mixture phase of B2 and partial L2 1 over 650 C. As a result of

systematic measurements of the AHE, ANE, and Seebeck effect in these films, we found that S ANE dramatically increased from +3.9 µv/k at 600 C to + 6.2 µv/k at 650 C, the highest S ANE ever reported arising from a large S I (+3.3 µv/k) and S II (+2.8 µv/k). To see if the experimental results and the theoretical values agree, we performed a first principles calculation to evaluate s xy and a xy. The first-principles technique was the tight binding-linearized muffin-tin orbital method under the local spin-density approximation [33]. To consider the AHE effect, the spin-orbital-coupling term under the Pauli approximation was added to the non-relativistic Hamiltonian. s xy was calculated from the Kubo-Bastin formula consisting of Fermi-surface and sea terms [34]. The electron scattering effect originating from disordered occupation of the constituent atoms in both systems was taken into account in the coherent-potential-approximation [33]. About 5 10 9 k-points were used for the Fermi-surface term and from 1 10 : to 5 10 9 depending on the energy variable in the integration for the Fermi-sea term in the full Brillouin zone [34]. Figures 8(a) shows the annealing temperature dependence of the observed s xy and a xy as estimated from a xy = S I /r xx. s xy monotonically increases from 132 to 285 S/cm with increasing annealing temperature from 500-700 C, which is as expected from the calculated s xy at E F or B2 to L2 1 -ordered Co 2 MnAl 0.63 Si 0.37 shown in Figure 8(b). The smaller experimental s xy might be due to electron scattering at the surface, film/substrate interface, and impurities in the thin film. The experimental a xy shows a dramatic increase from about +0.5 to +1.2 A/mK in going from below 600 to above 650 C, together with an enhancement in L2 1 -ordering. a xy is theoretically evaluated from the energy

dependence of s xy using the following equation by setting T=300 K. [17] α xy = 1 et σ xy (ε)(ε E ) df dε (8) F dε By setting ±2 10 =% Ry from E F as the integration range of calculation, the theoretical a xy for B2-Co 2 MnAl 0.63 Si 0.37 becomes +0.57 A/mK, which is close to the experimental value below 600 C. L2 1 -ordering significantly enhances a xy to 3.09 A/mK because of the enlargement in σ xy / ε around E F. This tendency is qualitatively consistent with the annealing temperature dependence of atomic ordering and a xy, suggesting that the observed large ANE in Co 2 MnAl 0.63 Si 0.37 can be attributed to not an extrinsic factor such as impurity/defect scattering but rather the intrinsic nature of Co 2 MnAl 0.63 Si 0.37 originating from its electronic structure. This result also validates our strategy for finding materials having large ANE. Note that the deviation of a xy for L2 1 -ordered Co 2 MnAl 0.63 Si 0.37 between the experiment and calculation might be due to imperfect L2 1 - ordering in our Co 2 MnAl 0.63 Si 0.37 thin film even after annealing above 650 C. In other words, an even larger S ANE might be realized by improving atomic ordering in Co 2 MnAl 0.63 Si 0.37. Because of the intrinsic origin of the large ANE, the same magnitude or larger S ANE can be expected in bulk Co 2 MnAl 0.63 Si 0.37 having similar/ideal atomic ordering; it will be important to confirm this for practical energy-harvesting applications.

4 Conclusion We performed a simulation on generating electric power from ANE by placing an inplane magnetized magnetic material between a low-temperature heat source and an air/water heat bath. The results suggest that a thermopower of 10 µv/k at least is required for practical energy-harvesting applications. We also proposed a strategy to find magnetic materials having large ANEs for the first time. The linear response equation indicates that a large ANE arises from the constructive relation between the large Seebeck effect, anomalous Hall effect, and transverse Peltier coefficient. This strategy led us to investigate the electric and thermoelectric properties of Co 2 MnAl 1-x Si x. Single phase epitaxial Co 2 MnAl 1-x Si x was grown on MgO substrate and a Fermi level shift with x was indirectly confirmed from an AMR measurement. Finally, Co 2 MnAl 0.63 Si 0.37 was found to have an ANE thermopower of 6.2 µv/k, the largest ever reported. This thermopower is still lower than our target (a few tenths of µv/k) but is exceptionally large compared with previous general values (<1 µv/k). First principles calculations involving the anomalous Hall conductivity and transverse Peltier coefficient in B2 and L2 1 -ordered Co 2 MnAl 0.63 Si 0.37 matched the experimental results, suggesting that the observed large ANE arises from a comparable contribution of the intrinsic large anomalous Hall effect and transverse Peltier effect in Co 2 MnAl 0.63 Si 0.37 due its large Berry phase curvature Therefore, the validity of the proposed strategy was demonstrated by the present experiment on Co 2 MnAl 1-x Si x. Further materials exploration based on the proposed strategy could enhance the thermopower of ANE and pave the way to practical thermoelectric applications using ANE.

[1] Q. Zhang, Y. Sun, W. Xu, and D. Zhu, Adv. Mater. 26, 6829 (2014). [2] K. Koumoto, R. Funahashi, E. Guilmeau, Y. Miyazaki, A. Weidenkaff, Y. Wang, and C. Wan, J. Am. Ceram. Soc. 96, 1 (2013). [3] Y. Sakuraba, K. Hasegawa, M. Mizuguchi, T. Kubota, S. Mizukami, T. Miyazaki, and K. Takanashi, Appl. Phys. Express 6, 033003 (2013). [4] Y. Sakuraba, Scr. Mater. 111, 29 (2016). [5] K. Hasegawa, M. Mizuguchi, Y. Sakuraba, T. Kamada, T. Kojima, T. Kubota, S. Mizukami, T. Miyazaki, and K. Takanashi, Appl. Phys. Lett. 106, 252405 (2015). [6] K. I. Uchida, T. Kikkawa, T. Seki, T. Oyake, J. Shiomi, Z. Qiu, K. Takanashi, and E. Saitoh, Phys. Rev. B 92, 094414 (2015). [7] Y. P. Mizuta and F. Ishii, Sci. Rep. 6, 28076 (2016). [8] D. J. Kim, K. D. Lee, S. Surabhi, S. G. Yoon, J. R. Jeong, and B. G. Park, Adv. Funct. Mater. 26, 5507 (2016). [9] M. Ikhlas, T. Tomita, T. Koretsune, M.-T. Suzuki, D. Nishio-Hamane, R. Arita, Y. Otani, and S. Nakatsuji, Nat. Phys. 13, 1085 (2017). [10] H. Narita, M. Ikhlas, M. Kimata, A. A. Nugroho, S. Nakatsuji, and Y. Otani, Appl. Phys. Lett. 111, 202404 (2017). [11] S. Tu, J. Hu, G. Yu, H. Yu, C. Liu, F. Heimbach, X. Wang, J. Zhang, Y. Zhang, A. Hamzić, K. L. Wang, W. Zhao, and J.-P. Ansermet, Cit. Appl. Phys. Lett 111, 222401 (2017). [12] T. C. Chuang, P. L. Su, P. H. Wu, and S. Y. Huang, Phys. Rev. B 96, 174406 (2017). [13] Z. Yang, E. A. Codecido, J. Marquez, Y. Zheng, J. P. Heremans, and R. C. Myers, AIP Adv. 7, 095017 (2017). [14] R. Ando and T. Komine, AIP Adv. 8, 056326 (2018). [15] P. Gautam, P. R. Sharma, Y. K. Kim, T. W. Kim, and H. Noh, J. Magn. Magn. Mater. 446, 264 (2018). [16] S. Isogami, K. Takanashi, and M. Mizuguchi, Appl. Phys. Express 10, 073005 (2017). [17] N. F. Mott and H. Jones, in Clarendon Press. Oxford (1936), pp. 308 314. [18] Eng. ToolBox https://www.engineeringtoolbox.com/convective (2003). [19] W. L. Lee, S. Watauchi, V. L. Miller, R. J. Cava, and N. P. Ong, Phys. Rev. Lett.

93, 226601 (2004). [20] Y. Pu, D. Chiba, F. Matsukura, H. Ohno, and J. Shi, Phys. Rev. Lett. 101, 117208 (2008). [21] J. Kübler and C. Felser, Phys. Rev. B - Condens. Matter Mater. Phys. 85, 012405 (2012). [22] J. C. Tung and G. Y. Guo, New J. Phys. 15, 033014 (2013). [23] E. Vilanova Vidal, G. Stryganyuk, H. Schneider, C. Felser, and G. Jakob, Appl. Phys. Lett. 99, 132509 (2011). [24] Y. Sakuraba, K. Takanashi, Y. Kota, T. Kubota, M. Oogane, A. Sakuma, and Y. Ando, Phys. Rev. B 81, 144422 (2010). [25] Y. Sakuraba, S. Kokado, Y. Hirayama, T. Furubayashi, H. Sukegawa, S. Li, Y. K. Takahashi, and K. Hono, Appl. Phys. Lett. 104, 172407 (2014). [26] F. J. Yang, Y. Sakuraba, S. Kokado, Y. Kota, A. Sakuma, and K. Takanashi, Phys. Rev. B 86, 020409(R) (2012). [27] S. Kokado, M. Tsunoda, K. Harigaya, and A. Sakuma, J. Phys. Soc. Japan 81, 24705 (2012). [28] S. Kokado, Y. Sakuraba, and M. Tsunoda, Jpn. J. Appl. Phys. 55, 108004 (2016). [29] T. Iwase, Y. Sakuraba, S. Bosu, K. Saito, S. Mitani, and K. Takanashi, Appl. Phys. Express 2, 063003 (2009). [30] Y. Sakuraba, M. Hattori, M. Oogane, Y. Ando, H. Kato, A. Sakuma, T. Miyazaki, and H. Kubota, Appl. Phys. Lett. 88, 192508 (2006). [31] T. Ishikawa, H. Liu, T. Taira, K. Matsuda, T. Uemura, and M. Yamamoto, Appl. Phys. Lett. 95, 232512 (2009). [32] Y. Kota, H. Tsuchiura, and A. Sakuma, J. Phys. Conf. Ser. 200, 052012 (2010). [33] I. Turek, Electronic Structure of Disordered Alloys, Surfaces and Interfaces, 1997th Ed. (Springer, Berlin, 1996). (1996). [34] I. Turek, J. Kudrnovsk, and V. Drchal, Phys. Rev. B 89, 064405 (2014). Acknowledgements Authors thank K. Takanashi, M. Mizuguchi, S. Maekawa, K. Hono, and K. Uchida for valuable discussions and N. Kojima and H. Ikeda for a technical support. This work was

supported by a JSPS KAKENHI Grant-in-Aid for Young Scientists (A) (No. JP2670945) and PRESTO from the Japan Science and Technology Agency (No. JPMJPR17R5). Figure 1. Simulated S ANE dependence of thermoelectric power from magnetic material having t of 1 mm, k = 10 W/mK, and r = 300 µwcm. The heat source (HS) and heat bath (HB) were 60 and 20 C, respectively. Three different situations of heat convection from the magnetic material to HB and the area of MM are simulated: (i) h = 10 W/Km 2, wd = 2500 cm 2 for free convection into air, (ii) h = 30 W/Km 2, wd = 2500 cm 2 for forced convection into air, and (iii) h = 500 W/mK 2, wd = 25cm 2 for forced convection into air.

Figure 2 (a) Out-of-plane XRD patterns for Co 2 MnAl 1-x Si x thin films. (b) Si composition x dependence of lattice constant evaluated from position of (004) peak. AMR ratio (%) 0.2 0.1 0.0-0.1 300 K Co 2 MnAl (x = 0) x = 0.11 0.14 0.15 0.22 0.23 0.29 0.32 0.37 0.48 0.51 AMR ratio (%) E F 0.2 0.1 0.0 Co 2 MnAl D D E 10K 300K Co 2 MnSi E F D D E -0.1-0.2 (a) 0 90 Co 2 MnSi (x = 1) 180 φ (degree) 270 360 (b) -0.2 0.0 Co 2 MnAl 0.2 0.4 0.6 0.8 Si composition ratio 1.0 Co 2 MnSi Figure 3 (a) AMR curves for Co 2 MnAl 1-x Si x thin films. (b) Si composition x dependence of AMR ratio.

Figure 4 (a) Perpendicular magnetic field H dependence of anomalous Hall resistivity r xy for Co 2 MnAl 1-x Si x thin films. Si composition dependence of r xy (b), r xx (c), and anomalous Hall angle q AHE (d).

Figure 5 Perpendicular magnetic field H dependence of anomalous Nernst voltage V ANE normalized by the width w and the given temperature gradient T for Co 2 MnAl 1- xsi x thin films. S SE (µv/k) S ANE (µv/k) 0-10 -20-30 -40-50 5 4 3 2 1 0-1 -1 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Co 2 MnAl Co 2 MnSi Co 2 MnAl Co 2 MnSi Si composition, x (a) (b) θ AHE, θ ANE (%) S I, S II (µv/k) 12 10 8 6 4 2 0 5 4 3 2 1 0 (c) (d) θ ANE θ AHE S I = α xy ρ xx S II = S SE tanθ AHE Si composition, x Figure 6 Summary of Seebeck and ANE measurements on Co 2 MnAl 1-x Si x thin films. Si composition dependence of S SE (a), S ANE (b), q AHE and q ANE (c) and S I and S II (d).

25x10 3 Intensity (counts) 20 15 10 5 0 (111) (002) 30 40 (a) 50 700 C 650 C 600 C 500 C 2 (degree) 60 (004) 70 S SE (µv/k) 0-10 (c) -20-30 -40-50 -60 500 550 600 T ann ( C) 650 700 (V ANE /w) / T (µv/k) 6 4 2 0-2 -4-6 (b) 500 C 600 C 650 C 700 C -20x10 3-10 0 10 20 H (Oe) S ANE,S I,S II (µv/k) 7 6 5 4 3 2 1 0 500 (d) 550 S ANE S I S II 600 T ann ( C) 650 700 Figure 7 (a) XRD patterns for Co 2 MnAl 0.63 Si 0.37 thin films annealed at 500 700 C. (b) perpendicular magnetic field H dependence of V ANE normalized by the width w and the given temperature gradient T for the Co 2 MnAl 0.63 Si 0.37 thin films. Annealing temperature dependence of S SE (c), S ANE, S I and S II (d).

3.5 (a) 350 1500 (b) α xy (A/mK) 3.0 2.5 2.0 1.5 1.0 L2 1 structure α xy = 3.05A/mK 300 250 200 150 100 σ xy (S/cm) 1200 900 600 300 L2 1 B2 0.5 B2 structure α xy = 0.57A/mK 0.0 500 600 T ann ( C) 50 0 700 0-300 -2x10-2 -1 0 1 2 E-E F (Ry) Figure 8 (a) Annealing temperature dependence of a xy for Co 2 MnAl 0.63 Si 0.37 thin films. (b) Calculated energy dependence of s xy for B2 and L2 1 -ordered Co 2 MnAl 0.63 Si 0.37.