Femtomagnétisme Marie Barthelemy

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1 Femtomagnétisme Marie Barthelemy Département d Optique ultrarapide et Nanophotonique Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS) UMR754 Ecole femto 217

2 Modify and study the dynamics of magnetization with light pulses how small? how fast? which physical mechanisms? 1 Térabits/inch 2 and 1 GHz 1 THz Nanoscale Magneto-Optics Nano-dots (MBE and e-lithography) Nanoparticles Molecular magnets Doped Alloys thin films Ultrafast Magnetism Dynamics of phase transitions Coherent Spin-photon interaction Precession and Magneto-optical switching

3 Outline 1. Ultrafast magnetization dynamics and related physical mechanisms 2. How to control magnetization? 3. What happens on very short time scales? 4. Probing magnetization dynamics with a chemical selectivity

4 Magnetization dynamics timescales Phys. Rev. Lett. 76, 425 (1996) Ni 1 fs Thermalization Transient re-magnetization SO, Exch, Coulomb Ultrafast Demagnetization ps 1ps - ns µs time Precession of magnetization around Heff Domains (wall propag, nucleation)

5 Interaction with a short laser pulse: T approach r(e) Non thermal Distribution E F hn Electronic excitation : non thermal distribution Plasma oscillations t < 2 fs r(e) Fermi-Dirac hot : Te ~ 1 K E F Electrons thermalisation : e-e scattering (Coulomb interaction) t= t ee ~ 2 fs r(e) T e =T l E F Energy transfert from electrons to lattice: electron-phonon interaction t= t ep ~ ps Heat diffusion 1ps- ns

6 Metallic system: Electron/lattice dynamics using 2 bath model Temperature (K) fs laser = heat pulse Hypothesis: electrons thermalized instantaneously dt e(t, r) Ce(T e) G dt dt l(t, r) Cl Gel e dt T e el T (t, r) T (t, r) l coupling T (t, r) T (t, r) T (t, r) l G el e P(t, r) l l T l e T (t, r) e Laser pulse Telectrons Optical excitation 6 TLattice Time (ps)

7 Magnetic system : spin temperature - 3 bathes model Spin bath (at temperature T S T e ) te C T T c Pierre Weiss T C dt e(t, r) Ce(T e) G dt dt l(t, r) Cl Gel e dt dts (t, r) CS(TS ) dt G Se G SO Coupling? T (t, r) T (t, r) Magnons propag. e S el T (t, r) T (t, r) T (t, r) T (t, r) T (t, r) Sl l T (t, r) T (t, r) l l e S P(t, r) l l e T (t, r) e Optical excitation

8 Ultrafast ferro paramagnetic phase transition Kerr Signal (a.u.) r E S (t) t r E P ( t t ) Time resolved Ellipsometry sample with magnetization M H r Kerr Q, h reflection Faraday Q, h transmission 1. Rotation q and ellipticity h are measured for each pump-probe delay t DM(t)/Ms 2. Reflection/Transmission CoPt 15 nm nickel film 3 thin film (M ) Excitation : 1 fs pulses CoPt 3 5 fs No pump T e : Electron temperature T S : Spin temperature H (koe) Phys. Rev. Lett. 76, 425 (1996) Paramagnetic in 5 fs For high density of energy (typically ~mjcm -2 ) demagnetization is complete and faster than the lattice heating

9 Field x 1-5 ( A/m ) Optically induced precession A modification of H eff can be induced by an ultrashort pulse triggering a dynamics of precession from ps to ns time scale. H eff M dm dt dm dt DPol/Pol x H 2 1 M Delay (ps) Precession in Co (16nm) / Al 2 O 3 vs H magnitude 2.5 Landau Lifshitz Gilbert dm dt = γμ M H eff α M dm M s dt Field x 1-5 ( A/m ) H eff H anis H

10 Shape anisotropy ie demagnetizing field contribution Vomir et al. Nano Letters 16, 5291 (216) B μh M However: divb H d H H d : demagnetizing field with Nd demagnetizing field tensor related to shape Film x z M s y N d Ms cos Mssin H d N d M NANOSTRUCTURATION

11 Magneto crystalline anisotropy Cubic (Ni cfc K 1 = Jm -3, Fe cc or cfc) E anis i : K K cos directors relative to edges Low anisotropy: soft materials Hexagonal compact (Ex:Co: K 1 = Jm -3 ) E anis K 1 sin ² a K2 q sin ² q Minimum of energy Magnetization along C axis (easy axis) High anisotropy: hard materials a c q a Related to crystalline structure via SO coupling K i can depend on temperature

12 Precession dynamics in 3 dimensions 16 nm Co/Al 2 3 D Pol (1-2 ) -4-4 Polar D Long (1-2 ) Longitudinal D Trans (1-2 ) Transverse Delay (ps) Delay (ns) Vomir et al. PRL 94, (25)

13 How to control magnetization?

14 Spin Photonics : manipulating the spins with the laser pulses CoPt 3 /Al 2 O 3 What about the time or the smallest dimension? Ideally, as small as few nanometers -> Terabits/inch 2 as fast as few femtoseconds -> TeraHertz

15 Time Resolved Magneto-Optical Microscope DMM PM F 5kHz amplified Ti:Sa Pulse duration ~12 fs F PM Analyzer l/2 Polarization bridge R = 5x1-4 R (4 nm) Scanning piezo Pinhole (2 mm). Magnet (±.4T) y x Pump (8 nm) H Sample Focal plane Objective lens: N.A =.65 (x 4) Dichroic Beam splitter t MO image of a CoPt Polarizer 3 dot (D = 5 nm) Probe (4 nm). -.5 t e(spin)-l = 5.2 ps Resolutions spatial:3nm temporal:12fs t Diff =63 ps A. Laraoui, M. Albrecht, J.-Y. Bigot. Opt. lett. 32, (27) 1 2 Delay (ps)..5 Delay (ns) 1.

16 DM/M Magneto-Optical Pump-Probe Imaging (MOPPI) Reading the written dots with the pump-probe signal for low ( 1 mj.cm -2, Oe) intensities of the pump and a fixed delay between pump and probe fs 2 ps 1 ps 5 Delay (ps) 1 CoPt 3 /Al 2 O 3 Advantages of the MOPPI technique compared to a static approach: - Differential imaging : better signal to noise ratio and stability - Time resolved imaging : read the spatial information at different temporal delays

17 M / Ms M reversal on CoPt 3 / Al 2 O 3 film M / Ms M / Ms M / Ms Reversal of M for I P = 8 mj.cm -2 at H = + 5 Oe. Local demagnetization for I P = 8 mj.cm -2 at H = Oe Re-switch towards initial state for I P = 8 mj.cm -2 and H = - 5 Oe. 1 Initial saturation 1 Laser demag H: switch Reversal remag 1 1 DM/M H DM/M DM/M H DM/M H (koe) H H (koe) H (koe) H (koe)

18 Reading magnetic optically induced domains on a CoPt 3 film Magneto-Optic Imaging for different delays on a CoPt 3 /Al 2 O 3 I read = 1 mj.cm -2 for H = 3 fs 2 ps 1 ps Example of a diffraction grating patterned on a CoPt3 thin film

19 Polarization dependent switching vs thermal demagnetization N=3 Switching of magnetization related to polarization of the pump field! GOAL: small absorption, single pulse AOS athermal process involved => coherence between laser field and magnetic sample ~1-1fs!! BUT minimum duration of AOS is about 1ps in GdFeCo AO helicity-dependent switching in TM multilayers: Single pulse AOS in transparent garnets with linearly polarized IR pulses N=8 N=5 N=3 [Co(.4 nm)/pt(.7 nm)] N multilayer C-H. Lambert et al. Science 345, 1337 (214) Stupakievicz et al. Nature 71,542 (217)

20 Athermal modification of effective magnetic field in garnets Photoinduced anisotropy with linearly polarized laser pulse in garnet Hansteen et al. Phys Rev B (26) Inverse Faraday effect with circularly polarized pulses in DyFeO3 Kimel et al.nature Vol 435 (25) a Hi ( ) ijkl E j ( ) Ek ( ) Ml () M.Deb et al Appl. Phys. Lett. 17, (215) H IFE ijk E 16 i ( ) E j * ( )

21 Magnetization dynamics at very short time scales: What happens before thermalization?

22 Interaction with a short laser pulse: T approach r(e) Non thermal Distribution E F hn Electronic excitation : non thermal distribution Plasma oscillations t < 2 fs r(e) Fermi-Dirac hot : Te ~ 1 K E F Electrons thermalisation : e-e scattering (Coulomb interaction) t= t ee ~ 2 fs r(e) T e =T l E F Energy transfert from electrons to lattice: electron-phonon interaction t= t ep ~ ps Heat diffusion 1ps- ns

23 What happens before and during thermalization? Nature Physics 5, 461 (29) Short pulses: Low thermal effects 8nm 48 fs, 5 khz Detectors S 1 S 2 low repetition rate+ static magnetic field H : back to the fundamental state l/2 H T Single pulse experiment Kerr single pulse configuration (48 fs) -Rotation and ellipticity measured as a function of absorbed laser energy E abs ~I

24 Coherent Magnetization Dynamics : single pulse excitation (48 fs) Nature Physics 5, 461 (29) Loss of magnetic momentum during the pulse duration (48 fs) : spin momentum must be modified by a coherent process!!

25 Faraday / Kerr pump probe configuration (48 fs) 8nm 48 fs, 5 khz Detectors S 1 S 2 Ellipsometry Beam splitter I l/2 H T Rotation q K (t), q F (t) Ellipticity h K, h F M (t) Transmission T (t) Reflectivity R (t) (t) e diag. (t) (t) Delay line (t) t

26 Pump probe Kerr experiment : coherent optical and magneto-optical response Bigot et al. Nature Physics 5, 461 (29) Ni 7.5 nm CoPt 3 15 nm. a) c). z y H Dqq q pp = 9 q pp = x pump probe Kerr rotations (mrad) b) optic magnetic d) Pump - probe delay t (fs) Distinction between the pump polarization and the external field orientation : magnetic [(S qpp=,+h - S qpp=,-h ) - (S qpp=9,+h - S qpp=9,-h )]/2 Interaction with ultrashort pulse gives rise to a electronic [ (S qpp=,+h + S qpp=,-h ) - (S qpp=9,+h + S qpp=9,-h )]/2 coherent interaction between spins and photons

27 DR/R r t tot 1 ih H H dipolar, r tot Pump probe signal: r (3) ab 3rd order in E t 1 < t 2 <t 3 ( t, t ) 1 4h 2 3 (3) (3) Macroscopic P ( t ) NTr polarization r ( t ) 2 S( t ) Detected PTOT ( t, signal t) dt dt 1 dt 2 dt 3 [ H int e( t t 1 ] E( t t E( t t 1 1 t ) E( t ) e( t ) E 1 * ( t 1 1 t 2 t t ) E t ) E 2 2 ) e( t 3 * * ( t ( t 2 2 t 2 t t 3 3 ) ) t ) 1,,8,6,4,2, (3) (2) (1) -, Delay (fs) (1) Pump probe signal: populations dynamics (T 1 ) (2) Coherent peak : electron dephasing time in the metal (T 2 ) (3) Pump perturbed free induction decay (T 2 )

28 Spin photon interaction in a 8 levels system Hydrogen: 2s 1/2,3p 3/2 Relativistic Dirac 2 nd order DL in 1/m L z, S z Spin orbit analogous linked to local potential induced by the laser Zeeman Photo induced anisotropy High field basis Spin orbit interaction with ionic potential H.Vonesh and J.-Y Bigot Phys. Rev. B 85, 1847 (212)

29 8-level system: MO pump probe configuration Faraday rotation 3 rd order in E field r t tot 1 ih H TOT, r tot P (3) ( t ) Tr( r (3) ( t ). ) 2 nd order in E field: Population generation At resonance PPFD Coh Pop Total (2) L ( t ) Tr( r (2) ( t ). L) Out of resonance H.Vonesh and J.-Y Bigot Phys. Rev. B 85, 1847 (212)

30 Four wave mixing experiment r 2k k 1 3 * ( t, t ) dt1dt2dt3[ Hint] E1( t t1) E1( t1 t2) E2 ( t 2 2 t3 4h (3) 12 t 1 2 ) S( t ) e t T 2 coherences 2 nd order = diffraction grating 3rd order: diffraction of the E field (self diffraction)

31 2 beams degenerate MO-FWM in Bi doped garnet 9 fs@8nm 8MHz r E S (t) t r E P ( t t ) H r 2kp-ks Faraday Sample: 7µm (GdTmPrBi) 3 (FeGa) 5 O 12 film low absorption at 8nm high SO coupling 2ks-kp Faraday: coherences+populations MO-FWM: coherences only Simulation of optical time shift: 2-level system with dipolar interaction and inhomogeneous broadenning time shift dependence on pulse duration and spectral phase in the error bar T 2 /2

32 Magneto-Optical dephasing time Closer to MO resonance (53nm) M.Barthelemy et al. Optica 4, 2334 (217) Coherent magnetism T2 MO ~3 fs Study of decoherence dynamics of magnetic states vs sample T (decoherence due to phonons?) M.Sanches Piaia et al. in preparation

33 Probing magnetization dynamics with a chemical selectivity: Goal: study of transferts mechanisms between chemical elements

34 Distinguishing elemental momenta dynamics in Transition Metals alloys 3d(1) IR pump 3d(2) XMCD from femto slicing at synchrotron (BESSY, SOLEIL) IR/VIS s L E F 4-7 ev XMCD HHG 3p(1) 3p(2) Table top Magneto Optical measurement probed with HHG 5-7 ev E 2p(1) 2p(2)

35 XMCD : selection rules at L2 and L3 edges of Co 3d L 3 : 2p 3/2 L2 : 2p1/2.75

36 Time resolved XMCD measurements at L2 and L3 edges of Co (femto-slicing) Boeglin et al. Nature 465, (21) 7eV, 1fs,1.5kHz 79nm, 6fs, 1.5kHz t Sz =28 2 fs t Lz =22 2 fs 15nm Co.5 Pd.5 film on Si 3 N 4 1. Spin orbit coupling is involved in demagnetization process R. W. Schoenlein, et al. Science 287, 2237 (2) 2. Magneto crystalline anisotropy varies with time

37 Examples of XMCD results: L IR pump s L-edge probe Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins Systematic study of demagnetization in TM/TM and TM/RE alloys I.Radu et al. Nature 472, (211) I. Radu, et al., SPIN 5, 1554 (215)

38 Magnetization dynamics probed with High order Harmonics 3.5 mm cell target Al foil 2 nm 4mJ 25 fs 8nm 1kHz Driving pulses M edges of transition metals (Ni, Fe, Co): 5-7 ev H17 H37 ( ev) H17 H47 ( ev)

39 Spectro temporal T-MOKE with IR 25 fs pump/ XUV probe 3.5 mm cell target 8 25 fs W/cm² 4mJ 25 fs 8nm 1kHz Driving pulses Al foil 2 nm T-MOKE configuration t 2q TG q=35 q=43 ~ 3 counts/s DM ( t, H M q ) I H I ( t, H H ( H q q ) ) I I H H ( t, H ( H q ) q ) Ni 8 Fe 2

40 Demagnetization of Ni vs Ni 8 Fe 2 at M-edge counts x Pure Ni Ni 8 Fe 2 H+ H- counts x H+ H Energy (ev) T c, Ni = 63 K Energy (ev) T c, NiFe = 85 K 8 T c, Ni < T c, NiFe J NiNi (1-21 J) J FeFe J NiFe Py Ni Ni and Fe are strongly ferromagnetically coupled GOAL: evaluate the importance of exchange interaction in demagnetiztation dynamics

41 25fs 1.55 pump / 66eV probe T-MOKE in 1nm Ni and Py DMM DM/M DM/M DM/M 1nm Ni/Al 2 O 3 1nm Ni 8 Fe 2 /Al 2 O 3 counts x H+ H- counts x H+ H Energy (ev) Energy (ev) 8 t M (fs) mJ/cm² 3.2mJ/cm² 4.1mJ/cm² 25 Time delay (fs) Ni pump fluence (mj/cm²) t s-l (ps) pump fluence (mj/cm²) delay (fs) 2.5mJ/cm² delay (fs) 2 3.8mJ/cm² 4 delay (fs) 4.7mJ/cm² o Ni Fe

42 ) Elemental demagnetization of Ni vs Ni 8 Fe 2 T c, Ni < T c, NiFe in permalloy: J NiNi < J NiFe.4 Tc Py.8 Tc Ni τ M Ni (pure) < τ M Ni (Permalloy) μ Ni (pur) < μni (permalloy) μ Ni(py) 2μ Ni(pure) τ Mi α μ i BUT τ M Ni (permalloy)~τ M Fe (Permalloy) μ Ni (permalloy) < μfe (permalloy) μ Fe 4μ Ni τ Mi ~ μ i 2α i γ i E exch i Maghraoui et al. in preparation

43 Elemental momenta dynamics in Ni 8 Fe 2 : mean field exchange + LLB with Langevin dynamics + heat step Landau Lifshitz Bloch with Mean Filed Approx of exchange fields Neglecting dipolar interactions (I) (II) (I):Transverse dynamics: precession motion (II) : Longitudinal dynamics: Mz I and II Connected to the same microscopic parameter : Gilbert damping Hinzke et al., Phys. Rev. B. 92, (215) Exchange

44 DM/M DM/M DM/ Experimentally: Dqq Ni x1-3 Dqq Ni x1-3 Dqq Ni x t Ni M delay = (fs) t Fe M mJ/cm² 4 delay (fs) 2 3 delay (ps) 4 delay (fs) 1.5mJ/cm² Ni delay = 4.7mJ/cm² (ps) Fe delay (ps) Ni Fe 2.5mJ/cm² 6 Ni Fe 3.8mJ/cm² 6 4 Ni Fe Dqq Fe x1-3 Dqq Fe x1-3 Dqq Fe x1-3 Hinzke et al., Phys. Rev. B. 92, (215) τ M Ni τ M Fe μ Fe μ Ni 4 ~ μ Niα Fe γ Fe E exch Fe μ Fe α Ni γ Ni E exch Ni E exch Ni = 4E exch Fe Effective exchange interaction sensed by each sublattice Maghraoui et al. in preparation

45 M edge Elemental demagnetization measurements in Py DM/M DM/M DM/M 1. Both Ni and Fe demagnetize simultaneously within our time resolution 2. t M is increased with fluence exchange reduced? 3. We do not observe a clear delayed Ni demagnetization at lowest fluences Favors the high hybridization of Ni and Fe d Bands mJ/cm² 2 4 delay (fs) mJ/cm² delay (fs) 4.7mJ/cm² delay (fs) Dependent on fluence? Mathias et al PNAS 212

46 DM/M Conclusion Thermal vs all optical switching Thermal vs optical manipulation of precession XUV for L-edge or M-edge probing of TMs Coherent coupling of E laser with spins states. Ni 7.5 nm a) CoPt 3 15 nm c) mJ/cm² 6 delay (fs) HHG vs XMCD Dqq Kerr rotations (mrad) q pp = 9 q pp = b) optic magnetic Pump - probe delay t (fs) d)

47 Merci! M.Vomir M.Barthelemy V.Halte Y.Brelet J.-Y.Bigot M.Albrecht G.Versini J.Faerber G.Dekyndt A.Maghraoui M. Sanches Piaia J.Kim E.Terrier M.Deb J.Panigrani O.Kovalenko V.Shokeen Équipe Femtomag en 215