M.R. Ibarra

Institute of Nanosciencie of AragónLaboratory of Advanced MicroscopiesCondensed Matter Physics Department

Outline

• The concept of spin currrents• The thermoelectric conversion• Introduction to Thermospin effects• SSE effects in [Fe3O4/Pt]n multilayers• Spin Peltier effect in [Fe3O4/Pt]n multilayers• Thermoelectric power: thermopiles• Conclusions

Charge and spin currents

JS = 0

Jc

Jc = 0

JS JS

Jc = 0

Jc: charge current JS: spin current

Conduction-electronspin current

Spin wave (magnons)spin current

Pure Spin Currents

Net electron spin flow

Non-magneticMetal

MagneticInsulator

Magnon flow

Spin Hall effect (SHE)Interconversion of charge – spin currents in materials with high spin orbit coupling (high Z)(Dyakanov & Parel 1971,Hirch 1999)

(Js) Spin (Jc) Charge

Jc

JS

US-L US-L

Inverse Spin Hall effect (ISHE)Interconversion of spin currents – charge currents in non-magnetic metals with high spin orbit coupling (high Z)

(JS) Spin (Jc) Charge

Jc

JS US-L

Saitoh, E., Ueda, M., Miyajima, H., & Tatara, G. (2006). Conversion of spin current into charge current at room temperature: Inverse spin-Hall effect. Applied Physics Letters, 88(2006), 1–4.

Thermoelectric effects

Seebeck effect: 𝑆𝑆 = 𝐸𝐸𝛻𝛻𝛻𝛻

𝜵𝜵𝜵𝜵

𝑬𝑬𝐽𝐽 = 𝜎𝜎(𝐸𝐸 − 𝑆𝑆 𝛻𝛻𝛻𝛻) = 0

Peltier effect: Π = 𝑆𝑆 𝛻𝛻

𝜵𝜵𝜵𝜵

+

Thermoelectric power generation Thermoelectric cooling

𝑍𝑍𝛻𝛻 =𝑆𝑆2𝜎𝜎𝜅𝜅

𝛻𝛻

𝜅𝜅 = 𝜅𝜅𝑒𝑒 + 𝜅𝜅𝑙𝑙 𝜅𝜅𝑒𝑒 = 𝐿𝐿𝜎𝜎𝛻𝛻

𝐿𝐿 =𝜋𝜋2

3𝑘𝑘𝐵𝐵𝑒𝑒

2

= 2.44 × 10−8 �𝑊𝑊Ω𝐾𝐾2

Figure of merit

𝜵𝜵𝑽𝑽

Would it be posible thermoelectriceffect due to spin?

𝜵𝜵𝜵𝜵

𝜵𝜵𝜵𝜵

JS

Spin Peltier Effect (SPE)Spin Seebeck Effect (SSE)

JS

Heat vs. Electricity

To get Electricity To get Heat

Charge

Spin

Seebeck effect Peltier effect

Spin Seebeck effect

Uchida 2008

Spin Peltier effectFlipse 2014

Daimon 2016

Spin Seebeck effect effect: Spin current generation by heat

)( NFB

SS TTkGI −−=

H. Adachi et al. PRB 83, 094410 (2011), Rep. Prog. Phys. 76, 036501 (2013)

J. Xiao et al. PRB 81, 214418 (2010)

Spintronics

Spin Seebeck

Effect

SeebeckEffect

16

Longitudinal spin Seebeck effect (LSSE)

Longitudinal SSE setup

paramagnet

ferromagnet

Inverse spin Hall effect:

K. Uchida et al.,Appl. Phys. Lett. 97, 172505 (2010).

EISHE = θSHρ(JS × σ)

Spin Seebeck basic principles

• Spin current proportional to applied thermal gradient

• Injected spin current converted in electric voltage by the inverse spin Hall effect

H. Adachi et al. Phys. Rev. B 83, 094410,& Rep. Prog. Phys. 76, (2013) 036501

𝐸𝐸𝐼𝐼𝐼𝐼𝐼𝐼𝐸𝐸 =𝜃𝜃𝐼𝐼𝐼𝐼𝜌𝜌𝐴𝐴

2𝑒𝑒ℏ

𝐽𝐽𝐼𝐼 × �⃗�𝜎

𝐼𝐼𝐼𝐼 = −𝐺𝐺𝐼𝐼𝑘𝑘𝐵𝐵ℏ

(𝛻𝛻𝐹𝐹 − 𝛻𝛻𝑁𝑁)

J. Xiao et al. Phys. Rev. B 81, 214418

Magnon emission associatedwith spin accumulation at themetal-ferromagnet interface (Takahasi et al ICM 2009)

Spin angular momentumtransfer at the interface:Magnon and elecronspin currentinterconversion(Steven et al. PRB 86 (2012) 214424)

SPIN CURRENT AT THE INTERFACES

Fe3O4

Pt

Fe3O4

SSE in [F/N]n multilayers

Pt

Combined PLD & Sputtering

KrF Laser (λ = 248 nm)

PLD-sputtering (Neocera Llc) P180-sys

(Neocera Llc)

Sputtering module

21

Atomic resolution morphological characterization

MgO/(Fe3O4/Pt)

STEM-HAADF image

0,21 nm MgO

Fe3O4

Interface Fe3O4/MgO

0,39 nm

Pt

a~ 0,83 nm

Fe3O4

Pt

Fe3O4

50 nm

MgO

Fe3O4

Pt

FIB-Pt-C

23Atomic resolution chemical mapping of the interfaces

SSE vs number of Fe3O4/Pt bilayers

-8 -6 -4 -2 0 2 4 6 8-10

-5

0

5

10

V ML /

∆T

(µV/

K)

H (kOe)

n = 1 n = 2 n = 3 n = 6

T = 300 K

n = 1

n = 6

SSE voltage enhancement with incresing number of Fe3O4/Ptbilayers

50 nmMgO

Fe3O4

Pt

50 nm

Ramos et al. Phys. Rev. B 92, 220407(Rap. Comm.) (2015)

-6 -4 -2 0 2 4 6

-0.5

0.0

0.5

1 x Pt/Fe3O4(tF), 300K

tF = 40 nm

(Vy/∆

T) (L

z/Ly)

(µV

/K)

H (kOe)

tF = 160 nm

tFPt

Fe3O4MgO

-6 -4 -2 0 2 4 6

-0.5

0.0

0.5 n = 4

H (kOe)

(Vy/∆

T)(L

z/Ly)

(µV/

K)

n = 1

n x Pt/Fe3O4(40), T=300 K

PtFe3O4

MgO

4 x

Fe3O4

Pt (15 nm)t (nm)

2mm

7mm

SSE dependence on Fe3O4 thickness

A. Anadón et al. 2016) APL (2016)

Dependence of SSE versus metal/insulator interlayer

Spin current across the multilayer must be considered

-8 -6 -4 -2 0 2 4 6 8-30

-20

-10

0

10

20

30

V ML /

∆T

(µV/

K)

H (kOe)

n x Fe3O4/Pt(tN) n = 1 n = 6

GSSE

T = 300 K

Pt 15 nm

Pt 7 nm

Optimized configuration

-8 -6 -4 -2 0 2 4 6 8

-30

-20

-10

0

10

20

30 n = 12n = 6

V ML/∆

T (µ

V/K)

H (kOe)

T = 300 K

n = 1

[Fe3O4(23)/Pt(7)]n

Largest SSE voltage measured in a thin film based structure!!

VML ≈ 28 µV/ K !!

29

Mechanism of LSSE enhancement in multilayer systems

Essence of LSSE enhancement: Boundary conditions for spin currents flowing normal to P/F interfaces

(i) spin currents must disappear at the top and bottom surfaces(ii) spin currents are continuous at the interfaces

Spin

cur

rent

PtFe3O4 Fe3O4

T = 300 K

z

PtFe3O4

Magnon spin current

Electron spin current

Qualitative agreement with experimental results

0 1 2 3 4 5 6 70

2

4

6

8

10

experiment

T = 300 K

S SSE (µV

/K)

Number of Fe3O4/Pt bilayers (n)

0.00

0.02

0.04

0.06

0.08

0.10

model

<JS>

Average SSE voltage measured:

𝐽𝐽𝐼𝐼 =1𝑡𝑡𝑁𝑁𝑛𝑛

�𝑖𝑖=1

𝑛𝑛�

𝑧𝑧𝑖𝑖=0

𝑡𝑡𝑁𝑁

𝑑𝑑𝑑𝑑 𝐽𝐽𝑠𝑠(𝑖𝑖)(𝑑𝑑)

Maximum spin current at central interlayers

Envelope curve(JS(z)):

Ramos et al. Phys. Rev. B 92, 220407(Rap. Comm.) (2015)

(Ilustration from Graham Jones)

K. Uchida et al. Phys. Rev.B 95, 184437 (2017)

Spin peltier effect

K. Uchida et al. Phys. Rev.B 95, 184437 (2017)

Strong enhancement of the spin peltier effect in multiple bilayers

Spin Seebeck device

Wide area, low cost thermoelectric devices

Conventional charge thermoelectric device:

Many thermocouples necessaryHigh cost, difficulty in integration

T-gradient over centimeter scale neededThin film device difficult

Spin-Seebeck thermoelectric device

Many thermocouples unnecessaryLow cost, ultimate integration

T-gradient over nanometer scale is sufficientThin film device possible

(IMR, Tohoku Univ. / NEC / ASRC, JAEA/Zaragoza)

SSE thermopilesBilayer Multilayer

TP2TP1

Spin current conversión at the interfaces F/N gives rise to anstrong enhancement of the thermospin effects in multiplebilayers and constitutes an excellent play ground for thestudy of new physical phenomena and promising for devicesapplication

K. Uchida et al. review Proceedings of the IEEE 104, 1499 (2016)

LSSE Termopower LSSE power factor

Monographic issue in Journal PhysD: Applied Physics on

CALORITRONICS to appear soon

Enhanced thermo-spin effects in iron-oxide/metal multilayers

R Ramos1, I. Lucas2,3,4, P. A. Algarabel4,5, L. Morellón2,3,4,K. Uchida6,7,8, E. Saitoh1,8,9,10 and M. R. Ibarra2,3,4,11

Development of thin-film thermoelectric SSE baseddevices

Strategic Japanese-Spanish Cooperative Research Program Nanotechnologies and new materials for environmental

challenges

http://ina.unizar.es ibarra@unizar.es

THANK YOU FOR YOUR ATENTION