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Thermoelectric Signal Enhancement by Reconciling the Spin Seebeck and Anomalous Nernst Effects in Ferromagnet/Non-magnet Multilayers.

Lee KD, Kim DJ, Yeon Lee H, Kim SH, Lee JH, Lee KM, Jeong JR, Lee KS, Song HS, Sohn JW, Shin SC, Park BG - Sci Rep (2015)

Bottom Line: The thermoelectricity in FM/non-magnet (NM) heterostructures using an optical heating source is studied as a function of NM materials and a number of multilayers.It is observed that the overall thermoelectric signal in those structures which is contributed by spin Seebeck effect and anomalous Nernst effect (ANE) is enhanced by a proper selection of NM materials with a spin Hall angle that matches to the sign of the ANE.Moreover, by an increase of the number of multilayer, the thermoelectric voltage is enlarged further and the device resistance is reduced, simultaneously.

View Article: PubMed Central - PubMed

Affiliation: Department of Materials Science and Engineering, KI for the Nanocentury, KAIST, Daejeon, 305-701, Korea.

ABSTRACT
The utilization of ferromagnetic (FM) materials in thermoelectric devices allows one to have a simpler structure and/or independent control of electric and thermal conductivities, which may further remove obstacles for this technology to be realized. The thermoelectricity in FM/non-magnet (NM) heterostructures using an optical heating source is studied as a function of NM materials and a number of multilayers. It is observed that the overall thermoelectric signal in those structures which is contributed by spin Seebeck effect and anomalous Nernst effect (ANE) is enhanced by a proper selection of NM materials with a spin Hall angle that matches to the sign of the ANE. Moreover, by an increase of the number of multilayer, the thermoelectric voltage is enlarged further and the device resistance is reduced, simultaneously. The experimental observation of the improvement of thermoelectric properties may pave the way for the realization of magnetic-(or spin-) based thermoelectric devices.

No MeSH data available.


Locally laser-induced hybrid voltage (V) generation of SSE and ANE of CFB/NM with different hall-bar widths.V of CFB/Pt (a) and CFB/Ta (b) at a laser power of 17 mW. Magneto-thermoelectric voltage (ΔV) of CFB/Pt (c) and CFB/Ta (d) as a function of the Hall-bar resistance R. (e) Circuit model with FM resistance (RF) and NM resistance (RN). The un-excited lateral resistance is represented by Rw.
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f2: Locally laser-induced hybrid voltage (V) generation of SSE and ANE of CFB/NM with different hall-bar widths.V of CFB/Pt (a) and CFB/Ta (b) at a laser power of 17 mW. Magneto-thermoelectric voltage (ΔV) of CFB/Pt (c) and CFB/Ta (d) as a function of the Hall-bar resistance R. (e) Circuit model with FM resistance (RF) and NM resistance (RN). The un-excited lateral resistance is represented by Rw.

Mentions: Local optical induction of thermoelectric voltage reveals the dependence on the Hall-bar width of the CFB/Pt (a) and CFB/Ta (b) samples after eliminating the offset voltage, as shown in Fig. 2. The narrowest sample (w = 0.2 mm), i.e., with the highest resistance is observed to have a larger signal than samples with wider widths. Similarly, highly resistive CFB/Ta samples show a larger signal than the CFB/Pt samples. To clarify the resistance (R) dependence, we plotted the relationship between ΔV and R in Figs. 2(c–e). Both the CFB/Pt and CFB/Ta samples show a linear dependence indicating that the combined signals of the anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE) are linearly scaled with R. Since changing the width of the Hall-bar does not alter the relative resistivity of FM/NM bilayer, the result stems not from the shunting effect between FM and NM but from the local heating. A fixed size of the laser beam kept the excitation width (d) less than the total width (w) of the stripe. Therefore, the thermally-generated voltage (VS) is laterally shunted by the resistance Rw of the un-excited areal width (w - d) of the stripe, thereby resulting in the measured voltage ΔV = (dw−1)VS, as depicted in the circuit model of Fig. 2(e)16. Since SSE (ANE) is generated in NM (FM), the VS is expressed by VS = (ISSE + IANE) × RS, where ISSE ≡ VSSERN−1, IANE ≡ VANERF−1, and RS = RFRN(RF + RN)−1. Note that RF and RN are resistances for the excited width d of FM and NM respectively, and the ISSE and IANE are the equivalent current in the circuit model23. Since d <w, the ISSE and IANE in our sample configuration are thought to be identical with the variation of the stripe width. Considering that the total electrical resistance R = (dw−1)RS, the measured voltages (V = (ISSE + IANE) × R) could be simply proportional to R. Hence, in contrast to a case in which the entire sample area is excited23, the thermoelectric signals generated by local heating could be normalized by total electrical resistance to investigate the ANE and SSE contribution. Namely,


Thermoelectric Signal Enhancement by Reconciling the Spin Seebeck and Anomalous Nernst Effects in Ferromagnet/Non-magnet Multilayers.

Lee KD, Kim DJ, Yeon Lee H, Kim SH, Lee JH, Lee KM, Jeong JR, Lee KS, Song HS, Sohn JW, Shin SC, Park BG - Sci Rep (2015)

Locally laser-induced hybrid voltage (V) generation of SSE and ANE of CFB/NM with different hall-bar widths.V of CFB/Pt (a) and CFB/Ta (b) at a laser power of 17 mW. Magneto-thermoelectric voltage (ΔV) of CFB/Pt (c) and CFB/Ta (d) as a function of the Hall-bar resistance R. (e) Circuit model with FM resistance (RF) and NM resistance (RN). The un-excited lateral resistance is represented by Rw.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4447118&req=5

f2: Locally laser-induced hybrid voltage (V) generation of SSE and ANE of CFB/NM with different hall-bar widths.V of CFB/Pt (a) and CFB/Ta (b) at a laser power of 17 mW. Magneto-thermoelectric voltage (ΔV) of CFB/Pt (c) and CFB/Ta (d) as a function of the Hall-bar resistance R. (e) Circuit model with FM resistance (RF) and NM resistance (RN). The un-excited lateral resistance is represented by Rw.
Mentions: Local optical induction of thermoelectric voltage reveals the dependence on the Hall-bar width of the CFB/Pt (a) and CFB/Ta (b) samples after eliminating the offset voltage, as shown in Fig. 2. The narrowest sample (w = 0.2 mm), i.e., with the highest resistance is observed to have a larger signal than samples with wider widths. Similarly, highly resistive CFB/Ta samples show a larger signal than the CFB/Pt samples. To clarify the resistance (R) dependence, we plotted the relationship between ΔV and R in Figs. 2(c–e). Both the CFB/Pt and CFB/Ta samples show a linear dependence indicating that the combined signals of the anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE) are linearly scaled with R. Since changing the width of the Hall-bar does not alter the relative resistivity of FM/NM bilayer, the result stems not from the shunting effect between FM and NM but from the local heating. A fixed size of the laser beam kept the excitation width (d) less than the total width (w) of the stripe. Therefore, the thermally-generated voltage (VS) is laterally shunted by the resistance Rw of the un-excited areal width (w - d) of the stripe, thereby resulting in the measured voltage ΔV = (dw−1)VS, as depicted in the circuit model of Fig. 2(e)16. Since SSE (ANE) is generated in NM (FM), the VS is expressed by VS = (ISSE + IANE) × RS, where ISSE ≡ VSSERN−1, IANE ≡ VANERF−1, and RS = RFRN(RF + RN)−1. Note that RF and RN are resistances for the excited width d of FM and NM respectively, and the ISSE and IANE are the equivalent current in the circuit model23. Since d <w, the ISSE and IANE in our sample configuration are thought to be identical with the variation of the stripe width. Considering that the total electrical resistance R = (dw−1)RS, the measured voltages (V = (ISSE + IANE) × R) could be simply proportional to R. Hence, in contrast to a case in which the entire sample area is excited23, the thermoelectric signals generated by local heating could be normalized by total electrical resistance to investigate the ANE and SSE contribution. Namely,

Bottom Line: The thermoelectricity in FM/non-magnet (NM) heterostructures using an optical heating source is studied as a function of NM materials and a number of multilayers.It is observed that the overall thermoelectric signal in those structures which is contributed by spin Seebeck effect and anomalous Nernst effect (ANE) is enhanced by a proper selection of NM materials with a spin Hall angle that matches to the sign of the ANE.Moreover, by an increase of the number of multilayer, the thermoelectric voltage is enlarged further and the device resistance is reduced, simultaneously.

View Article: PubMed Central - PubMed

Affiliation: Department of Materials Science and Engineering, KI for the Nanocentury, KAIST, Daejeon, 305-701, Korea.

ABSTRACT
The utilization of ferromagnetic (FM) materials in thermoelectric devices allows one to have a simpler structure and/or independent control of electric and thermal conductivities, which may further remove obstacles for this technology to be realized. The thermoelectricity in FM/non-magnet (NM) heterostructures using an optical heating source is studied as a function of NM materials and a number of multilayers. It is observed that the overall thermoelectric signal in those structures which is contributed by spin Seebeck effect and anomalous Nernst effect (ANE) is enhanced by a proper selection of NM materials with a spin Hall angle that matches to the sign of the ANE. Moreover, by an increase of the number of multilayer, the thermoelectric voltage is enlarged further and the device resistance is reduced, simultaneously. The experimental observation of the improvement of thermoelectric properties may pave the way for the realization of magnetic-(or spin-) based thermoelectric devices.

No MeSH data available.