Magnon, Spinon and Phonon in spin caloritronics

Transcription

1 Magnon, Spinon and Phonon in spin caloritronics Institute of materials research, Tohoku University, Japan WPI-AIMR Tohoku Univ., ASRC JAEA, ERATO - SQR, JST, Japan Eiji SATIOH

2 Contents 1. Introduction spin pumping and spin Seebeck effects 2. Spin current in antiferromagnets 3. Spinon spin-seebek effect in a spin liquid 4. Phonons in spin Seebeck effects Spin current physics/ E. Saitoh

3 Spin Seebeck effect = thermal SP sample: Pt(15 nm )/YIG slab Voltage vs Temperature difference Voltage vs Magnetic field Nature 445(2004)778, APL 97, (2010) Spin current physics/ E. Saitoh

4 Spin Seebeck effect (in Bulk) Pt(5 nm)/bulk YIG Low field Calculation High field (in Bulk) (L x, L y, L z ) = (2 mm, 4 mm, 1 mm) M Spin current physics/ E. Saitoh 4

5 SSE has been reported in a lot of systems 5 SSE is a universal phenomenon in magnetic materials Model system: Pt/Y 3 Fe 5 O 12 (YIG) junction K. Uchida, H. Adahci, T. Kikkawa, A. Kirihara, M. Ishida, S. Yorozu, S. Maekawa, and E. Saitoh, Thermoelectric generation based on spin Seebeck effects Proceedings of the IEEE (2016), DOI: /JPROC Spin current physics/ E. Saitoh

6 development of SSE efficiency since its discovery today s world R.T. (Zaragoza group) ~ 400 times greater our first observation of SSE cf. ZT (n-type Si)~0.005 Spin current physics/ 6 E. Saitoh

7 7 SP can be used to magnetometry for very thin films SP allows us to measure magnetic properties electrically even in very small systems thinner YIG/CoO/Pt: YIG/CoO/Pt d = GHz Cf. χ for CoO film V ISHE Normalized /P ab (10-6 V/W) V ISHE /P ab T. Ambrose and C. L. Chien PRL76, T T (K) (K)

8 For SSE, fluctuation \ is created Creating Fluctuaton H Spinon spin current has Opposite sign FM PM Spin Liquid with singlet correlation M (H 0) M (H 0) M (H 0) Ordered M H T T T Singlet ground state s=0 Elementary excita.on Magnons (spin wave) (Paramagnons or magnon like) Spinons Spinon: excitation from quantum spin liquid

9 For SSE, fluctuation \ is created Creating Fluctuaton H Spinon spin current has Opposite sign FM PM Spin Liquid with singlet correlation M (H 0) M (H 0) M (H 0) First, T magnetic field is applied to T align magnetic moment T Ordered M Singlet FM AFM ground state B B B s=0 PM H fozz Elementary excita.on Magnons (Paramagnons or magnon like) Spinons Spinon: excitation from spin quantum liquid

10 Long range spinon \ correlation in 1D SC Spinon Spin current in Lu,nger liquid in 1D spin chain Long-range (theoretically, ) spincurrent owing to long-range spin correlation due to critical quantum fluctuation (Tomonaga Luttinger) realized in one-dimensional S=1/2 chains Spin Liquid with singlet correlation M (H 0) Singlet ground state s=0 T Spinons Spinon: excitation from spin quantum liquid

11 Spinon excitation in 1D Quantum Spin Liquid (QSL) 1D Spinons collective excitations in one-dimensional QSL In spite of NO magnetic ordering (Paramagnetic) Magnetic susceptibility Individual spinon Pair excitations M. Mourigal, et al., Nature Phys. 9, (2013) domain-wall-like excitation S=1/2 fractional excitation Gapless robust against H Continuous spectrum S. Eggert, et al., PRL. 73, (1994) Gapless excitation 2spinons gapless spinon robust against magnetic fields

12 material in which spinon is well established Paramagnetic Insulator Sr2CuO3 One-dimensional spin chains Neutron scattering Thermal conductivity (5N) Spinon peak A. C. Walters, et al., Nat. Phys. 5, 867 (2009) A. V. Sologubenko, et al., PRB 62, R6108(R) (2000) Cu-O chains along b-axis Continuous spectrum consistent with theory Additional heat conduction along one-dimensional chains Single crystals grown by a TSFZ method were used in this study Ballistic energy transport by spinons T. Kawamata, et al., JPSJ 77, (2008)

13 Spinon spin Seebeck effect Experimental set-up Sr2CuO3 O Cu Pt single crystal (5N purity) Detection of spin currents Sr2CuO3 input output ΔT Js ESHE Pt Detect spin currents electrically via the inverse spin Hall effect in Pt ~ Measure B-, T-dependence of V = V/ΔT

14 Δ T -induced voltage in Pt without Sr2 CuO3 normal Nernst effect in Pt Field dependence Temp. dependence (6/11) YIG/Pt ~ Proportional to field V/B is positive sign: same as YIG SSE (consistent with the normal Nernst effect of bulk Pt)

15 Δ T -induced voltage in Pt with Sr2 CuO3 at low temperatures on Sr 2 CuO 3, V component with Negative sign appeaers in addition to the ordinal Nernst effect (positive) in Pt (7/11) Temp. dependence Nature physics (in press) > V+ V- V = V++V- the normal Nernst effect of Pt O Cu

16 Check if the signal is due to spin current (8/11) Temp. dependence W (Θ<0) V++V+ SHE W/Sr2CuO3 Sr2CuO3 W & Pt convert spin currents into opposite voltages (opposite spin Hall angles) Pt(Θ>0) V++V- SHE W Pt/Sr2CuO3 Pt Sr2CuO3 Positive peak for W/Sr2CuO3 in the vicinity of 20 K Negative voltage signals for Pt are due to spin-current injection from Sr2CuO3 Nature physics (in press)

17 Along vs Across the spin chains (9/11) Temp. dependence J ~ 0.01 K Pt J ~ 2,000 K Suppression of spin-current inj due to anisotropic spin correlation Spin currents through spin chains dominate spin injection Nature physics (in press)

18 Comparison with a theoretical calculation Spin current vs. Field Spinons Due to the curved dispersion Ferromagnetic magnons Greater weight in the positive-ω region Greater weight in the negative-ω region G. Muller, et al., Phys. Rev. B. 24, 1429 (1981) Nature physics (in press) Expanded Bosonization method & Linear response by M. Sato arxiv: iv

19 Spinons vs. Antiferromagnetic magnons Magnetic susceptibility vs. Temp. Spin configurations in each T-region Sr2CuO3 Antiferro. 1D QSL Antiferromagnetic magnons carry spin current Spinons carry spin current Ordered spins aligned along the spin chains below

20 20 Spin Seebeck effect in a sputtered Film of YIG/Pt n High field resolution (ΔH=15mT)

21 21 Temp. dep. Relative peak strength H TA H LA peak at H LA is suppressed (below ~ 10 K).

22 22 Temp. dep. magnon TA LA ω MTA /2π = 0.16 THz (~ 7 K) Frequency of the intersection ω MLA /2π = 0.53 THz k (~ 20 K) B T this suppression suggests that the thermal excitation of spin wave and phonon around this intersection is suppressed below ħω MLA /k~26k, and thus the enhancement peaks vanish.

23 23 Information we can learn from the peak shape N: Number of the Crossing Point between magnon and phonon (TA) N = 2 N = 1 N = 0 Spin Seebeck intensity S(N=0) < S(N=2) < S(N=1)

24 24 Comparison of peak shape with Theoretical Calculation Experiment Calculation Thermal spin-wave flow was calculated using a semi-classical transport theory in which magneto-elastic coupling is taken into consideration. The theory shows that a spin-wave flow is enhanced via the hybridization with phonons, when τ Phonon > τ Magnon. At the point-touching condition, the enhancement effect is maximized because of the maximum phase-space volume around the intersection points. with K.Shen, B.Flebus, R.Duine, and G.Bauer Shen Bauer Duine Flebus

25 25 Summary Magnons Spinons Phonons SSE of magnons Spinon SSE Phonon anomaly 0.04 Pt/YIG-film 3.46 K 0.02 S (µv/k) µ 0 H (T) 0.5