the role of angular momentum

Transcription

1 Ultafast ast magnetization at dynamics: the ole of angula momentum Andei Kiilyuk, The Nethelands 1

2 Magnetization dynamics and switching N S enegy gain: E = M dl H toque: = T dt with damping: M γ = dm dt = γl T = [ M H ] H [ M H ] Landau & Lifshitz, =γ 1935 e g 2 mc = 28 GHz/T dm = γ α eff dt M dm dt ( ) M H + M 2

3 Consequence 1: Inetia-fee motion dm dt =γ [ ] γ M H The motion happens only as long as the field is thee 3

4 Consequence 2: consevation of angula momentum Einstein de Haas effect A. Einstein & W.J. de Haas, Expeimentelle Nachweis de Ampeèschen Molekülstöme, Vehandl. Deut. Phys. Ges. 17, (1915). SJBanettMagnetization S. J. Banett, by Rotation, Physical Review 6, (1915). 4

5 Consequence 3: Pecessional magnetization evesal Kaka et al, APL 80,, 2958 (2002); Geits et al, Natue 418, 509 (2002); Schumahe et al, PRL 90, (2003). The fastest way to evese the magnetization is via pecession 5

6 Angula momentum tansfe and two ways of evesal usual (pactical) pecessional (fast) M M H H dm ( M H ) + M dm = γ α eff dt M dt fom spins to lattice dm ( M H ) + M dm = γ α eff dt M fom spins to field dt 6

7 Outline of the lectue Angula momentum gone, inetia ecoveed: antifeomagnets Tuning angula momentum in feimagnets: faste pecession / switching Angula momentum consevation vs exchange inteaction 7

8 Outline of the lectue Angula momentum gone, inetia ecoveed: antifeomagnets Tuning angula momentum in feimagnets: faste pecession / switching Angula momentum consevation vs exchange inteaction 8

9 Ballistic (inetial) magnetic dynamics? 9

10 Consequence of the LL equation: Inetia-fee motion d M [ =γ ] M H dt The motion happens only as long as the field is thee Inetia may appea when the angula momentum is gone! 10

11 Feomagnet and antifeomagnet ω ~ γ ( H H ) FM H A GHz no inetia Lagangian ω ~ γ AFM H A H ex GHz M G M M 1 2 m = M M l = M 1 M M 2 inetia! 0 Equation of motion Andeev and dmachenko, Sov. Phys. Usp. 23, 21 (1980) 11

12 Magnetic phases in HoFeO 3 Γ 12 Γ 24 M x H M x l y θ l z y θ δϕ z Γ 12 Γ 24 Γ 12 Γ 24 δt H(t) Magnetic pulse Inetial spin motion t 12

13 Inetia-diven spin eoientation in HoFeO 3 2 a) Theoy 0.04 b) Expeiment ϕ(t) Magnetic field pulse H=+1.05H C H=+0.95H C H=-1.05H C ωt/2π Faad day ota ation (de eg) σ (+) σ (-) T=50.5 K Time delay (ps) Kimel et al., Natue Physics 5, 727 (2009) 13

14 Ultafast eoientation in HoFeO 3 σ (+) σ ( ) σ (+) 40 K 0.03 deg 44 K H Γ 24 x S 1 m z Faa aday o otation σ ( ) σ (+) ( ) σ σ (+) σ (+) σ ( ) ( ) 48 K 51 K Γ 12 S 2 m x y z S 1 δh S 2 δt y σ (-) σ (+) σ ( ) 55 K Time (ps) 52 K What oute is faste? T

15 Heat diven vs field-diven dynamics 4 (a) z-hofeo 3 48 K (b) Heat-diven 3 Field-diven M Z /M Field- diven 52 K M Z /M (% %) 2 1 Heat-diven (c) Tempeatue (K) Heat-diven ~ 400 ps Field-diven diven ~ 3 ps Lase pulse ~ 0.1 ps 0 M Z /M Field-diven Heat-diven Time delay (ps) Time delay (ps) 15

16 Outline of the lectue Angula momentum gone, inetia ecoveed: antifeomagnets Tuning angula momentum in feimagnets: faste pecession / switching Angula momentum consevation vs exchange inteaction 16

17 To evese the magnetization fast(e): d M apply stonge toque =γ [ M H ] o dt educe associated angula momentum 17

18 Solution: make the field stonge? 2 ps, seveal Teslas end of ultafast t magnetism? 18

19 Any way aound? 19

20 Ultafast lase-induced demagnetization thin Ni film 20

21 Ultafast lase-induced demagnetization (Ni film) M S L Beauepaie et al. (1996) Stamm et al. (2007) see any diffeence?? 21

22 Simple model to descibe the pocess localized atomistic spin model with a Heisenbeg exchange exchange inteaction electon tempeatue is intoduced via stochastic field tem 22

23 Example: themalization of Gd spins in GdFe alloy tempeatue is applied as a step at t=0 T.A. Ostle et al., PRB (2011) 23

24 Landau-Lifshitz-Bloch equation longitudinal elaxation tansvese elaxation Assuming that the heat bath (phonons o electons) acts much faste than the spins, the bath degees of feedom can be aveaged out. The ensemble-aveaged spin polaization gives the magnetization m Gaanin, Phys. Rev. B 55, 3050 (1997). 24

25 to summaize hee: lase does change M (and L) vey fast but only to disode the system can we still do something useful with it? 25

26 Feimagnet with a compensation point(s) samples in ou expeiments: Gd 20-30% Fe 65-75% Co 5% M RE (Gd) M RE T M T A SiN (60nm) GdFeCo (20 nm) SiN (5nm) AlTi (10 nm) Glass substate A TM T C ~500 K M A 0.3 'up' A RE netization n 0.0 TM (FeCo) M TM Mag -0.3 'down' Tempeatue Field (koe) g Gd < g FeCo 26

27 GdFeCo static measuements T M samples by A. Tsukamoto and A. Itoh 27

28 Feimagnetic esonance Landau Lifshitz Gilbet equation fo two sublattices: dm eff TM = γ [ M ( )] TM HTM λm RE dt dm eff RE = γ [ M ( )] RE H RE λm TM dt H eff dm RE dt = M R y M TM E x γ ( T ) M RE M RE γ ( T ) M TM ( T ) M ( T ) ( T ) M TM ( T ) A ( T ) eff = = RE γ TM dm TM dt z ω FMR = γ H eff eff if = also αeff T T A 28

29 Dynamics of magnetization in GdFeCo. H ext = 0.29 T Phys. Rev. B 73, (2006) 29

30 High fequency + high damping nea T A Fast switching Stop Angula momentum compensation is the key to incease the evesal speed Phys. Rev. B 73, (2006) 30

31 Can this be ealized in pactice? 31

32 Tempeatue incease ove T comp Gd Fe Gd M tot Fe dynamics? 32

33 Hysteesis loops with pump lase on Phys. Rev. Lett. 99, (2007) 33

34 Lase-induced magnetization evesal Contast eveses in 0.7 ps Also M Gd is affected by the lase pulse on a sub-ps time scale (?) Decease of the angula momentum does wok! Stanciu et al, Phys. Rev. Lett. 99, (2007) 34

35 Outline of the lectue Angula momentum gone, inetia ecoveed: antifeomagnets Tuning angula momentum in feimagnets: faste pecession / switching Angula momentum consevation vs exchange inteaction 35

36 To distinguish the two sublatices 36

37 XMCD contast 37

38 Static hysteesis loops of Gd &Fe Fe Gd Gd M tot Fe 38

39 Femto slicing (BESSY) optical pump X-ay pobe fs time-esolved measuements 39

40 Ultafast dynamics of sublattices Fe: 100±23 fs Gd: 427±102 fs feomagnetic GdFeCo! 40

41 Atomistic simulations localized atomistic spin model with a Heisenbeg exchange exchange paametes (Fe-Fe, Gd-Gd, and Fe-Gd) obtained by fitting static M Fe,Gd (T) dependencies. stochastic tem added to the effective field evesing field is pesent duing the pocess simulations by the goup of R.W. Chantell 41

42 Simulation esults feomagnetic state epoduced! 42

43 43

44 Simulation esults feomagnetic state epoduced! 44

45 dl dt RE dl dt TM diffeent demagnetization times of Fe and Gd consevation of angula momentum FM state initiates the evesal Fe Gd dl dt RE dl dt TM t 45

46 Theefoe: angula momentum contols the switching diffeent demagnetization times of the sublattices exact compensation is not equied 46