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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NNANO.2016.182

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Supplementary information

Magnetoelectric interaction and transport behaviors in magnetic nanocomposite thermoelectric materials under the intrinsic excitationWenyu Zhao, Zhiyuan Liu, Ping Wei, Qingjie Zhang, Wanting Zhu, Xianli Su, Xinfeng Tang, Jihui Yang, Yong Liu, Jing Shi, Yimin Chao, Siqi Lin, and Yanzhong Pei

Supplementary Information Contents:

Supplementary Figure 1: XRD patterns of Ba0.3In0.3Co4Sb12 matrix and xBaM/Ba0.3In0.3Co4Sb12

Supplementary Figure 2: Effect of BaM-NPs on electron structure

Supplementary Figure 3: Effect of BaM-NPs on crystal structure

Supplementary Figure 4: Seebeck coefficient of MNC00 and MNC35 under applied magnetic field

Supplementary Figure 5: Magnetization versus magnetic field at room temperature

Supplementary Figure 6: Schematic diagram of magnetoelectric interaction

Supplementary Figure 7: Effect of permanent magnet nanoparticles on electrical transport

Supplementary Figure 8: The schematic diagram of the magnetization for MNC00 and MNC35

Supplementary Figure 9: Magnetoelectric interaction mechanism

Supplementary Table SI: Electric and thermal transport properties of MNC00 and MNC35 at ZTm

Supplementary References

1

Magnetoelectric interaction and transport behaviours in magnetic nanocomposite thermoelectric materials

http://dx.doi.org/10.1038/nnano.2016.182


2 NATURE NANOTECHNOLOGY | www.nature.com/naturenanotechnology

SUPPLEMENTARY INFORMATION DOI: 10.1038/NNANO.2016.182

1. Effects of BaM-NPs on composition and structure

As shown in Figure S1, all the resolved XRD diffraction peaks of xBaM/Ba0.3In0.3Co4Sb12 can be indexed to those of CoSb3,

indicating that Ba0.3In0.3Co4Sb12 matrix remains good chemical stability in the presence of BaM-NPs during the spark plasma

sintering process. The zoomed XRD patterns in the 2θ range of 30°~40° clearly indicate that the BaM-NPs have no any effect on the

crystal structure of Ba0.3In0.3Co4Sb12 matrix. However, all the diffraction peaks of BaM were not present because the highest content

of BaM-NPs is only 0.45% and far less than the detection limit of XRD technique (about 1%).

To investigate the effect of BaM-NPs on the electron structure of Ba0.3In0.3Co4Sb12 matrix, XPS of Sb 3d5/2 and 3d3/2 core levels of

Ba0.3In0.3Co4Sb12 matrix and xBaM/Ba0.3In0.3Co4Sb12 were measured and shown in Figure S2. XPS of Sb 3d5/2 and 3d3/2 core levels

for Ba0.3In0.3Co4Sb12 matrix are composed of double peaks, a similar phenomenon was observed in our previous work1. The peaks of

Sb 3d5/2 and 3d3/2 core levels of xBaM/Ba0.3In0.3Co4Sb12 have no any chemical shift as compared with those of Ba0.3In0.3Co4Sb12

matrix, suggesting that BaM-NPs did not affect the electron structure of Ba0.3In0.3Co4Sb12 matrix.

To investigate the effect of BaM-NPs on the crystal structure of Ba0.3In0.3Co4Sb12, the Raman spectra of Ba0.3In0.3Co4Sb12 matrix and

xBaM/Ba0.3In0.3Co4Sb12 were measured and shown in Figure S3. Four obvious Raman peaks of Ba0.3In0.3Co4Sb12 matrix and

xBaM/Ba0.3In0.3Co4Sb12 are almost the same, indicating that BaM-NPs do not affect the symmetry of the Sb4 rectangle ring of CoSb3

in Ba0.3In0.3Co4Sb12 matrix. Based on the Raman scattering study of CoSb3 by Nolas et al.2, the four peaks originate from four

vibration modes of Sb4 rectangle ring. The peaks at about 106.8, 132.9, 143.1 and 173.5 cm-1 correspond to the F2g mode of the

in-plane shear motion of the short Sb-Sb bonds, the Eg mode from the elongation and/or shrinkage of all Sb-Sb bonds, the Ag mode

of the elongation of two Sb-Sb bonds and the shrinkage of other two Sb-Sb bonds, and the F4g mode of the out-of-plane shear

motion of Sb4 rectangle ring.

Figure S1 XRD patterns of Ba0.3In0.3Co4Sb12 matrix and xBaM/Ba0.3In0.3Co4Sb12.

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Figure S2 Effect of BaM-NPs on electron structure.

Figure S3 Effect of BaM-NPs on crystal structure.

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2. Magnon-drag effect induced by BaM-NPs

Compared with the matrix, besides electron-phonon interaction, there must exist electron-magnon interaction in

xBaM/Ba0.3In0.3Co4Sb12. Therefore, the magnon-drag effect must occur and produce an extra thermopower (Sm) in the

nanocomposties, similar to that in FeNi nanowires3 and NiCu alloy4. Namely, the α of xBaM/Ba0.3In0.3Co4Sb12 is the sum of S from

Seebeck effect and Sm from magnon-drag effect. To investigate the contribution of BaM-NPs to improving the α, the temperature

dependences of Seebeck coefficient of MNC00 and MNC35 were measured with a thermal transport option of PPMS-9 (Quantum

Design INC., USA) in the range of 90-300 K under zero field and a magnetic field of 0.4 T, respectively. As shown in Figure S4,

BaM-NPs can significantly increase the α of MNC35 in the range of 90-300 K, as compared with that of MNC00. At the same time,

the applied magnetic field of 0.4 T has no effect on α of MNC35 in the range of 90-300 K because the α of MNC35 under zero field

and a magnetic field of 0.4 T are almost the same. The phenomenon had been also observed in the magnetized MNC35 in the range

of 300-675 K, as shown in Figure 5. The origin that BaM-NPs have no effect on the electric transport of xBaM/Ba0.3In0.3Co4Sb12 in

the range of 90-675 K may be because the working temperature is still less than the TC of BaM-NPs. The magnon-drag effect may

reasonably explain why BaM-NPs can cause the significant increase in α of the nanocomposties.

2/3 2/32 2 2 2*

2 2 3 2

8 83 3 3 3

B Bk k e TS m T T Aeh n eh n nπ ππ τ π

m m = ⋅ ⋅ ⋅ = ⋅ ⋅ ⋅ = ⋅

( )5/2 3/2 3

2 3/26m B

mk T TS L y B

T ne D nαΠ

π= = ⋅ ⋅ = ⋅

Figure S4 Seebeck coefficient of MNC00 and MNC35 under applied magnetic field.

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3. Magnetoelectric interaction in xBaM/Ba0.3In0.3Co4Sb12

To ensure whether xBaM/Ba0.3In0.3Co4Sb12 is magnetic TE materials, the magnetization versus magnetic field of (a) BaM-NPs, (b)

Ba0.3In0.3Co4Sb12 matrix (MNC00), and (c) 0.45%BaM/Ba0.3In0.3Co4Sb12 (MNC45) were measured with a PPMS-9 (Quantum

Design INC., USA) at room temperature. As shown in Figure S5, BaM-NPs reveals the strongest ferromagnetism in three samples

and have a very well-defined hysteresis loop. The saturation magnetization (Ms), remnant magnetization (Mr), and coercivity (HC) of

BaM-NPs are about 16.7 emu/g, 6.9 emu/g, and 2218 Oe, respectively. The Ms is much lower than the theoretical value of BaM

single crystal about 72 emu/g5, however, the HC is much higher than that of micron-sized BaM about 400 Oe6. The remarkable

decrease in Ms and great increase in HC of BaM-NPs can be reasonably explained with surface effect and/or spin-canting.

Ba0.3In0.3Co4Sb12 matrix shows very weak ferromagnetism, which the Ms, Mr and HC are only about 7.5×10-3 emu/g, 2.5×10-4 emu/g,

and 40 Oe, respectively. Chen et al.7 suggested that Ba0.44Co4Sb12 should be paramagnetism owing to the spin of Co2+ with d7

electron configuration. The very weak ferromagnetism of the matrix may be attributed to Fe impurity originating from iron can and

hammer used during the preparation process of the matrix. The Mr and HC of MNC45 are remarkably greater than those of the

matrix owing to the presence of strong ferromagnetic BaM-NPs but much lower than those of the BaM-NPs, which are about

9.7×10-4 emu/g and 136 Oe, respectively. It is worth noting that a negative differential magnetization was observed in both the

matrix and MNC45 under high magnetic field. This is a typical feature of field-induced antiferromagnetic coupling effect, which

was reported in discontinuous Ni nanoparticles in Al matrix8. As compared with the matrix, the antiferromagnetic coupling in

MNC45 was enhanced according to its bigger negative differential magnetization.

Figure S5. Magnetization versus magnetic field at room temperature.

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4. Electron spiral motion induced by Magnetoelectric interaction

As shown in Figure S6a, the c axis is the easy magnetization direction of BaM. Therefore, an effective magnetocrystalline

anisotropy field (HA) parallel to the c axis is generated in BaM-NPs owing to the electron spin of ferric ions (Fe3+). The HA is given

by HA=2K1/Ms9, where K1 is the magnetocrystalline anisotropy constant. For a BaM molecule, the total magnetic moment (µT)

induced by electron spin of Fe3+ is 20µB (µB is the Bohr magneton). The direction of the total magnetic moment is parallel to the c

axis in the absence of applied magnetic field. According to the relation between MS and µT (MS=2µT/V, where V is the volume of

BaM unit cell), the HA can be expressed as HA=K1V/μT. As shown in Figure S6b, the majority carriers (electrons) of

xBaM/Ba0.3In0.3Co4Sb12 will drift from the hot side with higher temperature (TH), to the cold side with lower temperature (TL) as

driven by thermal energy. At the same time, the magnons in the TH side are moved to the TL side when the BaM-NPs are subjected

to a temperature gradient field. Therefore, a large proportion of electrons will be confined to circular paths or helixes and even

trapped if the drift velocities are unparallel to the HA owing to the FL. There are at leat three kinds of electrons with different

velocities v1, v2 and v3 in xBaM/Ba0.3In0.3Co4Sb12 owing to the random oriention of BaM-NPs. The electrons with v1 move linearly

and are not affected by the HA owing to the velocities parallel to magnetic field. However, the electrons with v2 and v3 must be

affected by the FL because the angle between the HA and electron velocity (θ) is nonzero. As a result, the electrons with v2

perpendicular to the HA are confined to circular paths, the electrons with v3 having components parallel and perpendicular to the HA

move in a spiral pattern and are trapped by BaM-NPs. The spiral motion may reasonably explain why BaM-NPs can lower the n of

xBaM/Ba0.3In0.3Co4Sb12 in the low temperature range. Therefore, the reduction in the n at room temperature from 4.9×1020 cm-3 for

the matrix to 3.8×1020 cm-3 for the MNC15 sample is attributed to the electron trapping effect of the HA induced by BaM-NPs.

Figure S6. Schematic diagram of magnetoelectric interaction. (a) The spin state of ferric ion (Fe3+) in BaM-NPs, (b) The movements of electrons and magnons in xBaM/Ba0.3In0.3Co4Sb12 magnetic nanocomposite TE materials.

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5. Electron repository role of BAM-NPs

The electrons with the velocities parallel to magnetic field move linearly and are not affected by the HA as shown in Figure S7e and

S7f. The electrons with the velocities perpendicular to the HA are confined to circular paths in the ferromagnetism state as shown in

Figure S7h and move linearly in the paramagnetism state as shown in Figure S7i. The electrons with the velocities having

components parallel and perpendicular to the HA move in a spiral pattern and are trapped by BaM-NPs in the ferromagnetism state

as shown in Figure S7k, and move linearly and released the trapped electrons in the paramagnetism state as shown in Figure S7l.

Namely, BaM-NPs in fact play an “electron repository” role. The understanding has been experimentally confirmed by the

significant increase in the n of xBaM/Ba0.3In0.3Co4Sb12 above 675 K (Figure 2b) and may reasonably explain why BaM-NPs can

lower the n of xBaM/Ba0.3In0.3Co4Sb12 in the low temperature range.

As shown in Figure S8, to reveal the effects of magnetoelectric interaction on the transport properties, after having measured the

data of Figure 3, the samples Ba0.3In0.3Co4Sb12 matrix (MNC00) and 0.35%BaM/Ba0.3In0.3Co4Sb12 (MNC35) were first magnetized

for 24 h under an applied magnetic field of about 0.2 T along the A direction and measured to obtain the σ and α, and then repeated

the magnetization and measurement process along the B and C directions, respectively. The samples magnetized along the A, B and

C directions are labeled as A_MNC00, B_MNC00, C_MNC00 and A_MNC35, B_MNC35, C_MNC35.

As shown in Figure S9a, in Ba0.3In0.3Co4Sb12 matrix (MNC00) the electrons with vl move parallel to the temperature gradient field,

most of electrons with v2 and v3 move along the different directions due to the interface scattering and the lattice scattering.

However, after magnetization along the A direction an effective magnetic field (Hp) parallel to the applied magnetic field will occur

in A_MNC00 because of the existence of remanence as shown in Figure S5b. Therefore, the electrons with v1 are not affected by

the Hp owing to the v1 parallel to the Hp, other electrons with v2 and v3 must be affected by the FL. As a result, the electrons with v2

are confined to circular paths owing to the v2 perpendicular to the Hp, but the electrons with v3 move in a spiral pattern owing to the

v3 having components parallel and perpendicular to the Hp, as shown in Figure S9b. After magnetization along the B and C

directions, an effective magnetic field (Hp) parallel to the applied magnetic field in B_MNC00 and C_MNC00 will turn to the B and

C directions, respectively. Therefore, the electrons with v1 must do circular motion because the v1 is vertical to the Hp, other

electrons with v2 and v3 must be affected by the FL induced by the Hp and moved in a spiral pattern (Figure S9c and S9d). Since the

HA induced by BaM-NPs are randomly oriented before magnetization, in MNC35 the electrons with v1 are not affected by the HA

since the v1 is parallel to the HA, the electrons with v2 are confined to circular paths because the v2 is perpendicular to the HA, and

the electrons with v3 move in a spiral pattern and be trapped by BaM-NPs owing to the v3 having components parallel and

perpendicular to the HA, as shown in Figure S9e. Different from the un-magnetized MNC35, the randomly oriented HA had been

rearranged and were parallel to the applied magnetic field in the magnetized MNC35. Obviously, the effect of the HA on the

electrons in the magnetized MNC35 should be the same as that of the Hp in the magnetized MNC00. Therefore, in A_MNC35 the

electrons with v1 are not affected by HA and Hp, the electrons with v2 are confined to circular paths and the electrons with v3 move in

a spiral pattern and are trapped by BaM-NPs, as shown in Figure S9f. In the B_MNC35 and C_MNC35, the electrons with v1 do

circular motion because the v1 is vertical to the Hp and HA, and other electrons with v2 and v3 are affected by the FL induced by Hp

and HA and moved in a spiral pattern and even captured by the HA, as shown in Figure S9g and S9h. It is worth noting that as

compared with the situations in A_MNC00, B_MNC00, and C_MNC00, the thread pitch and radius of electron spiral motion must

be decreased in A_MNC35, B_MNC35, and C_MNC35 because of the bigger FL induced by HA and Hp. However, both HA and Hp

must be disappeared when the TW is higher than the TC of BaM-NPs and Ba0.3In0.3Co4Sb12 matrix. Obviously, the magnetic transition

can not only lead to the disappearance of the magnetoelectric interaction, but also make the electrons trapped by BaM-NPs become

conduction electrons again. Therefore, BaM-NPs in fact play an “electron repository” role in xBaM/Ba0.3In0.3Co4Sb12, which trap

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electrons below the TC and release the trapped electrons above the TC. The “electron repository” role can reasonably explain the

significant increase in the n of xBaM/Ba0.3In0.3Co4Sb12 above 675 K, as shown in Figure 2b.

Figure S7. Effect of permanent magnet nanoparticles on electrical transport. (a) the magnetic transition of permanent magnetnanoparticles from ferromagnetism to paramagnetism, (b) non-uniform spherical magnetic field around a permanent magnetnanoparticle in the ferromagnetism state, (c) permanent magnet nanoparticles in the paramagnetism state, (e) and (f) the motions of electrons parallel to the magnetic field as shown in (d), (h) and (i) the motions of electrons perpendicular to the magnetic field as shown in (g), (h) and (i) the motions of electrons inclined to the magnetic field as shown in (j).

Figure S8. The schematic diagram of the magnetization for MNC00 and MNC35.

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Figure S9 Magnetoelectric interaction mechanism. Schematic diagram of magnetic field distribution and electron motion patternin MNC00 (a-d) and MNC35 (e-h) before and after magnetization along the A, B and C directions, respectively.

6. Contribution of BAM-NPs to enhanced ZTmax

Unlike the usual situation10,11, where the majority of the enhancement in ZT is the reduction of the κL, in our present work, it is the

significant reduction in the κE, associated with the reduced σ, because of almost the same κL of MNC35 and MNC00 as shown in

Table SI. In addition, the α is increased by about 12 % which is consistent with the decrease of σ. Therefore, the majority of the

enhancement in ZT of xBaM/Ba0.3In0.3Co4Sb12 is derived from the decreased κE because of the presence of BaM-NPs.

Table SI Electric and thermal transport properties of MNC00 and MNC35 at ZTmax

κ(W/m·K)

κE(W/m·K)

κL(W/m·K)

α(μV/K)

σ(×104 S/m)

α2σ(mW/m·K) ZTmax

MNC00 2.76 1.94 0.813 -194 11.44 4.33 1.33MNC35 2.33 1.51 0.814 -218 9.41 4.47 1.64

InSb/InxCeyCo4Sb12[10] 2.91 2.40 0.51 -207 12.3 5.22 1.43

BaxLayYbzCo4Sb12[11] 2.63 2.24 0.39 -194 13.46 5.08 1.65

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Supplementary references

1. Zhao, W. Y., Wei, P., Zhang, Q. J., Dong, C. L., Liu, L. S. & Tang, X. F. Enhanced thermoelectric performance in barium and indium double-filled skutterudite bulk materials via orbital hybridization induced by indium filler. J. Am. Chem. Soc. 131, 3713–3720 (2009).

2. Nolas, G. S., Kendziora, C. A. & Takizawa, H. Polarized Raman-scattering study of Ge and Sn-filled CoSb3. J. Appl. Phys. 94,7440–7443 (2003).

3. Costache, M. V., Bridoux, G., Neumann, I. & Valenzuela, S. O. Magnon-drag thermopile. Nature Mater. 11, 199–202 (2012).4. Grannemann, G. N. & Berger, L. Magnon-drag Peltier effect in a Ni-Cu alloy. Phys. Rev. B 13, 2072–2079 (1976).5. Shirk, B. T., Buessem, W. R. Temperature dependence of Ms and Kl of BaFe12O19 and SrFe12O19 single crystals. J. Appl.

Phys. 40 1294–1296 (1969).6. Dho, J., Lee, E. K., Park, J. Y. & Hur, N. H. Effects of the grain boundary on the coercivity of barium ferrite BaFe12O19. J. Magn.

Magn. Mater. 285, 164–168 (2005).7. Chen, L. D., Kawahara, T., Tang, X. F., Goto, T. & Hirai, T. Anomalous barium filling fraction and n-type thermoelectric

performance of BayCo4Sb12. J. Appl. Phys. 90, 1864–1868 (2001).8. Fonda, E., Texeira S. R., Geshev J., Babonneau D., Pailloux F. & Traverse A. Negative differential magnetization for Ni

nanoparticels in Al. Phys. Rev. B 71, 184411 (2005).9. Harris, I. R. Hard magnets. Mater. Sci. Technol. 6, 962–966 (1990).10. Li, H., Tang, X. F., Zhang, Q. J., & Uher, C. High performance InxCeyCo4Sb12 thermoelectric materials with in situ

forming nanostructured InSb phase, Appl. Phys. Lett. 94, 102114(2009).11. Shi, X., Yang, J., Salvador, J. R., Chi, M. F., Cho, J. Y., Wang, H., Bai, S. Q., Yang, J. H., Zhang, W. Q. & Chen, L. D.

Multiple-filled skutterudites: high thermoelectric figure of merit through separately optimizing electrical and thermal transports, J. Am. Chem. Soc. 133, 7837(2011).

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