Paamagnetic spin pumping with micowave magnetic fields Steven M. Watts Physics of Nanodevices Mateials Science Cente Univesity of Goningen the Nethelands http://nanodevices.fmns.ug.nl/ s.watts@ug.nl
Mesoscopic metal spintonics team. J. van Wees C. H. van de Wal Postdoc: J. Gollie * S. Watts Ph.D. Student: M. V. Costache Students: M. Sladkov * Now at Unite Mixte de Physique CNRS/Thales
Techniques to obtain spin accumulation: Spin injection into metals and semiconductos: Optical (ciculaly polaied light Not good fo Si, metals o devices Electical (injection fom feomagnets Poblems with mismatch & intefaces between FM and SC New method: Spin pumping, the spin battey No chage cuents equied! Spin cuents geneated by pecessing magnetiation in a feomagnet Ou new appoach: Spin pumping with f magnetic fields No feomagnet equied!
Outline Intoduction: ackgound 1: Spin injection and spin accumulation ackgound : Spin battey device Theoetical model: Time-dependent field gives spin accumulation loch equations in a unifom system The univesal esult h Some ealistic numbes Inteface-enhanced spin accumulation The spin-injection MASER
All-electical spin injection by chage cuent Cobalt-aluminum mesoscopic spin valve with tunnel junctions E F E E Spin accumulation - E E ex σ σ σ σ N (E N (E N (E N (E N (E N (E
Spin accumulation in Al Spin diffusion length: λ sf D sf Spin elaxation time: sf Non-local spin valve measuement: F. J. Jedema et al., Natue 416 (00
Spin battey : inteface scatteing model Spin tansfe effect : spin polaied cuent magnetiation motion Spin battey effect : magnetiation motion spin polaied cuent H dc m (t H ac F N Fo long sf, pumped spin accumulation has the univesal value h A. ataas, Y. Tsekovnyak, G. E. W. aue, &. I. Halpein, PR 66 (00 Y. Tsekovnyak, A. ataas and G. E. W. aue, PRL 11 (00
Coplana wave guide to apply localied f field: CPW I f m Electical detection: efeence feomagnet 4 m CPW m Detecto Co Injecto Co1 Al 300nm
Outline Intoduction: ackgound 1: Spin injection and spin accumulation ackgound : Spin battey device Theoetical model: Time-dependent field gives spin accumulation loch equations in a unifom system The univesal esult h Some ealistic numbes Inteface-enhanced spin accumulation The spin-injection MASER
Spin accumulation in time-dependent magnetic fields The spin-esolved density of states of nonmagnetic, conducting mateial in eo magnetic field: E E F N (E N (E
Tun on a magnetic field, the states ae Zeeman - split: An instantaneous spin accumulation develops time >> E g afte elaxation, we have magnetiation with no spin accumulation
Rate equation fo spin accumulation geneated by an oscillating magnetic field: t sint ( 0 d dt + I souce Souce tem fo spin accumulation: d I souce EZeeman g dt d dt Solution: oscillates with field but out-of-phase by 1 φ tan 1
Now conside a otating magnetic field: ( cost, sint,0 At t0, field geneates collinea spin accumulation
Now conside a otating magnetic field: ( cost, sint,0 The spin elaxation time causes the spin accumulation vecto to lag behind the field The accumulated spins will pecess aound poducing accumulation pependicula to the plane of otation (See also A. Abagam, The Pinciples of Nuclea Magnetism
Spin pumping in a bulk conducto loch-type equations fo spin accumulation (t Souce tem Spin pecession d g + I ( t dt h Spin elaxation Key ingedient, the souce tem: I ( t d dt E Zeeman g d dt
+ h g t I dt d (, sin, cos ( t t θ x y g g h h The field configuation:
Steady-state solution in the otating efeence fame: ( 0 ( + + dt d h h h h (( ( 1 ( (( ( 1 ( (( ( 1 ( ( + + + + + + In-phase with field Out-of-phase dc component
Analytic solution fo steady-state dc spin accumulation: ( h 1+ ( + (( In geneal, we get only some small faction of the univesal esult h Special conditions to obtain the univesal esult: Resonance, : >> 1 No dc field, 0:, 1and >> >>
Some ealistic numbes Standad NMR/ESR techniques can be used Nea esonance, linea f field left + ight otating fields Simulated signal fo Al metal: 0.1ns 0.1 ( 6mT 10 ( f 16GH x10-3 1x10-3 ~x10-3 of h f h 0 h 66 V -1x10-3 V n exp spins 150nV ~ 10 15 cm 3 -x10-3 -0-10 0 10 0 g h
Outline Intoduction: ackgound 1: Spin injection and spin accumulation ackgound : Spin battey device Theoetical model: Time-dependent field gives spin accumulation loch equations in a unifom system The univesal esult h Some ealistic numbes Inteface-enhanced spin accumulation The spin-injection MASER
Now add inteface, and diffusion acoss inteface g + I ( x, t D + t h Spin diffusion Pumped egion No pumping -L 1 cost sint 0 x0 L x 1-D system to be solved numeically
Unbounded system 10 GH Weak feomagnet model: 1 T, 100 T h 10 0 10-1 10-10 -3 10 s 10-4 10-10 s -5-4 -3 - -1 0 1 x/λ
Pumped spin cuent acoss the inteface
Dwell time, no longe pecesses in egion II
ack-flow cuent andomies components nea inteface The andomiation leads to patial cancellation of the component nea the inteface This can be thought of as giving an effective anisotopic elaxation time: Reduced by back-flow cuent Not effected by back-flow cuent >>
Analytical model with anisotopic elaxation ( ( y x y x y x y x dt d dt d dt d h,,,, h ( 1 + + Relevant egime: Stongest effect off-esonance and fo egions bounded at << π λ D L
Outline Intoduction: ackgound 1: Spin injection and spin accumulation ackgound : Spin battey device Theoetical model: Time-dependent field gives spin accumulation loch equations in a unifom system The univesal esult h Some ealistic numbes Inteface-enhanced spin accumulation The spin-injection MASER
Enegy flows Enegy is absobed fom the f field to geneate spin accumulation powe to spin sink f powe in medium f powe out dissipation
Can we evese the pocess? An injected spin cuent dives the medium to poduce gain: powe fom spin injection souce f powe in f powe out medium dissipation
I s M feomagnet tunnel baie paamagnet x y v v d d v h + + dt dt I 1 3 4 s (t 1. Pumping. Relaxation 3. Pecession 4. Injection
Spin pumping with a spin injected cuent I s Solutions: ( ( ( h + s 1+ ( + (( 1+ ( s 1+ ( ( + (( ( + (( s ˆ ( h + s ( h + s Dispesive component Absoptive component With With s h s < h Absoption we can tun off effect of spin pumping! we can change the sign of the components Emission
An LRC cicuit model: C 0 The sample magnetiation couples to the cicuit via the inductance: ~ L R 0 L L0 (1 + ηχ Define the complex susceptibility : χ χ' + iχ'' χ' χ'' m 0 0m 1 4 gn F χ0 ( h + s h 0 χ 0
The total impedance: C 0 Z R R 0 0 + il L ηχ'' + il 0 + ( ic 0 1 0 (1 + ηχ' + ( ic 0 1 ~ R 0 R L L R 0 L The condition fo MASER opeation: R0 + R' < 0 Expessed in tems of the quality facto: L Q 0 > ηχ0 ( h + s R0 h 1 Fo Al metal: Q ~ > 1/ ( ev s ~ 00
Conclusions We descibe a new way to poduce dc spin accumulation in nonmagnetic metals and semiconductos Spin pumping with a otating magnetic field can poduce dc spin accumulations as lage as h Spin accumulation can be inteface-enhanced by engineeing anisotopic elaxation S. M. Watts, J. Gollie, C. H. van de Wal, and. J. van Wees, PRL 96, 07701 (006 Spin injection + spin pumping spin-injection injection MASER S. M. Watts and. J. van Wees, submitted to Natue Physics.