Magnetization dynamics in lanthanides new frontiers in spin-dependent band mapping at BESSY VSR

Transcription

1 1 FEMTOMAGNETISM BESSY VSR 1 Magnetization dynamics in lanthanides new frontiers in spin-dependent band mapping at BESSY VSR Martin Weinelt

2 2 FEMTOMAGNETISM BESSY VSR 2 Photon flux Ti:Sa UV 10 mw 86 MHz, 0.1 nj 10 9 photons / pulse 80 MHz pump, 1 nj BESSY photons / s (@ 0.1A, 0.1% BW) 500 MHz 10 5 photons / pulse 1.25 MHz HHG 10 khz 10 5 photons / pulse pump, 500 µj ~ J / cm photons / s, 50 fs photons / s, 3 ps 10 9 photons / s, 700 fs 10 9 photons / s, 100 fs Space-charge problem: S. Passlack et al., J. Appl. Phys. 100 (2006) A. Pietzsch et al., New J. Phys. 10 (2008) limit at E kin ~ 80 ev: 10 7 photons / pulse

3 Two-photon photoemission occupied state unoccupied state E vac hν E vac E vac χ 3hν XUV VBM VBM VBM core level delay 3 FEMTOMAGNETISM BESSY VSR 3

4 4 FEMTOMAGNETISM BESSY VSR 4 Our Goal: Follow the signatures of phase transitions in the transient electronic band structure Our BESSY VSR Goal: Spin- and time-resolved ARPES

5 5 FEMTOMAGNETISM BESSY VSR 5 Motivation: Magnetic Switching Time ns Conventional switching: Domain-wall nucleation and propagation > 1 ns ps Coherent rotation of magnetization: precessional switching > 10 ps fs Laser-induced magnetic switching! I. Radu et al., Nature 472 (2011) 205. T.A. Ostler et al., Nature Communications 3 (2012) 666.

6 Spin lattice relaxation time τ SL in iron Spin polarization 30 ps 20 ns τ SL > 30 ps A. Vaterlaus et al., Phys. Rev. B 46 (1992) FEMTOMAGNETISM BESSY VSR 6

7 7 FEMTOMAGNETISM BESSY VSR 7 Three Temperature Model optical excitation Electrons T e ns-pulse ps-pulse ~ 2 ps Lattice T p Spins T S > 30 ps, G ps spin and orbital momentum Heat S.I. Anisimov et al., Sov. Phys. JETP 39 (1974) 375 A. Vaterlaus et al., Phys. Rev. Lett. 67 (1991) 3314

8 Femtosecond magnetization dynamics 8 FEMTOMAGNETISM BESSY VSR 8 MOKE 20 nm Ni film 60 fs, 7 mj/cm 2 τ < 1 ps E. Beaurepaire, J.-C. Merle, A. Daunois, and J.-Y. Bigot., Phys. Rev. Lett. 76 (1996) 4250

9 Three Temperature Model optical excitation Electrons T e < 1 ps, G es ~ 2 ps Lattice T p Spins T S > 30 ps, G ps spin and orbital momentum S.I. Anisimov et al., Sov. Phys. JETP 39, (1974) 375 A. Vaterlaus et al., Phys. Rev. Lett. 67 (1991) 3314 E. Beaurepaire et al.; Phys. Rev. Lett. 76 (1996) FEMTOMAGNETISM BESSY VSR 9

10 Experimental Setup 10 FEMTOMAGNETISM BESSY VSR 10

11 Higher harmonic spectrum of Argon 11 FEMTOMAGNETISM BESSY VSR 11 7x10 4 E M = 210 mev ev 0.2 ev XUV photons / pulse XUV sensitive silicon photodiode (AXUV100)

12 12 FEMTOMAGNETISM BESSY VSR 12 VUV photoemission beamline 35 ev XUV-pulse, 100 fs pulse duration, 150 mev energy resolution, 10 8 photons/s Rev. Sci. Instrum. 84 (2013)

13 Pulse broadening 13 FEMTOMAGNETISM BESSY VSR 13 c t Pulse tilt large IR focal length: 600 mm small divergence of VUV beam: 4 mrad slit - grating distance: 330 mm 200 lines / mm decrease in bandwith: O.E. Martinez, Opt. Comm. 59 (1986) 229 Pulse broadening (35 nm): ~ 90 fs Group-velocity dispersion: Energy - chirp: ~ 8 fs ~ 30 mev

14 Ray tracing Temporal broadening (fs) Energy resolution (mev) Harmonic order Harmonic order Olaf Schwarzkopf, Helmholtz Zentrum Berlin REFLEC CODE, F. Schäfers, Technical Report 201, BESSY, (1996) 14 FEMTOMAGNETISM BESSY VSR 14

15 15 FEMTOMAGNETISM BESSY VSR 15 Electronic Structure of Gd Gd: (5d6s) 3 exchange splitting

16 Space 35 ev 16 FEMTOMAGNETISM BESSY VSR 16 XUV probe-pulse IR pump-pulse 35 ev XUV-pulse, 100 fs pulse duration, 150 mev energy resolution, 10 8 photons/s khz repetition rate

17 (5d6s)³ Exchange Splitting Gd (0001) on W(110) (TC=293K) ferromagnetic paramagnetic T=100K T=300K k is measured in Γ-M direction FEMTOMAGNETISM BESSY VSR Binding energy (ev) 17

18 18 FEMTOMAGNETISM BESSY VSR 18 Thermal phase transition Gd / W(110) Binding energy (ev) majority spin bulk-band 2 minority spin bulk-band 1.0 ex (ev) 0.5 exchange splitting ex Temperature (K) C. Schüßler-Langeheine, PhD thesis, FU Berlin, 1999

19 Ultrafast Demagnetization of Gd PRL 109, (2012) 19 FEMTOMAGNETISM BESSY VSR 19

20 Gd: band position vs exchange splitting 20 FEMTOMAGNETISM BESSY VSR 20 delay time: ps

21 Ultrafast Demagn.: Hysteresis Loop Hysteresis: valence and 4f spins not in equilibrium during demagnetization Instantaneous response of minority spin band 1 ps delayed response of the majority spin band PRL 109, (2012) 21 FEMTOMAGNETISM BESSY VSR 21

22 Ultrafast Microscopic Processes Three Temperature Model Electrons T e a) a) Superdiffusive spin transport Battiato et al., PRL 105 (2010) Rudolf et al., Nature Comm. 3 (2012) 1037 Spins 1-2 ps b) 4f Gd 10 nm W(110) substrate Lattice T p (5d6s)³ b) Electron-phonon spin-flip scattering Koopmans et al., Nature Mat. 9 (2010) 259 Beaurepaire et al. PRL 76 (1996) 4250 Hübner, Bennemann PRB 53 (1996) FEMTOMAGNETISM BESSY VSR 22

23 Magnetism in the 4f system MLD PRL 100, (2008) 23 FEMTOMAGNETISM BESSY VSR 23

24 24 FEMTOMAGNETISM BESSY VSR 24 Magnetic Linear Dichroism (MLD) O. Krupin, PhD-Thesis (2004), FU-Berlin

25 - towards a complete picture 26 FEMTOMAGNETISM BESSY VSR 26 valence spins exchange 4f spins spin currents (5d6s)³ 4f τ SL Electrons T e τ e,ph Lattice T p

26 Spin mixing and E ex in Gd 27 FEMTOMAGNETISM BESSY VSR 27 Spin up Spin down T = 120 K T = 280 K K. Maiti et al., Phys. Rev. Lett. 88 (2002)

27 30 FEMTOMAGNETISM BESSY VSR 30 Thank you! AG Weinelt, FU-Berlin Robert Carley Björn Frietsch Martin Teichmann John Bowlan Kristian Döbrich Jan Wolter Collaboration Partners Phillipe Wernet, Olaf Schwarzkopf Helmholtz Zentrum Berlin Christian Tusche Max-Planck-Institut für Mikrostrukturphysik, Halle Gerd Schönhense Johannes Gutenberg Universität Mainz WE2037/4-1