Nanomagnetism a perspective from the dynamic side

Transcription

1 Nanomagnetism a perspective from the dynamic side Burkard Hillebrands Fachbereich Physik and Research Center OPTIMAS Technische Universität Kaiserslautern Kaiserslautern, Germany TNT 2009 Nanotechnology Barcelona, Spain, 7-11 September 2009

2 Coherent dynamics: spin waves λ Landau-Lifshitz torque equation r 1 dm ( t) γ dt r r = M () t B () t r r r r rr i( qr- ωt) q mrt (, ) = m0( r) e B r eff eff Is intrinsically nonlinear equation! dynamic magnetization r r M () t B eff M r S r M () t

3 Content Background spin waves in a small magnetic stripe with domain wall nanocontacts on spin-valve samples propagating spin waves in a small magnetic stripe Summary

4 Coworkers F. Ciubotaru, S. Hermsdörfer, A. Laraoui, B. Leven, B. Obry, C. Sandweg, S. Schäfer, H. Schultheiss, A. Serga, K. Vogt M. van Kampen, X. Janssens, L. Lagae V. Tiberkevich, A.N. Slavin J. Chapman J. Miltat TU Kaiserslautern Interuniversitaire Micro-Electronica Centrum vzw (IMEC), Leuven, Belgium Dept. of Physics, Oakland University, Rochester, Michigan University of Glasgow LPS, Université Paris-Sud

5 Content Background spin waves in a small magnetic stripe with domain wall nanocontacts on spin-valve samples propagating spin waves in a small magnetic stripe Summary

6 Spin waves Two types of energy contributions exchange energy: generated by twist of neighbored spins dipolar energy: generated by magnetic poles in long-wavelength spin waves S S S N N N N N S S q

7 Spin waves dipolar limit Frequency [GHz] dipolar limit q d B 0 q q q B 0 B 0 magnetostatic surface wave (MSSW) magnetostatic forward volume wave (MSFVW) magnetostatic backward volume wave (MSBVW) TNT 2009, Barcelona September 8, 2009

8 Landau-Lifshitz equation: uur M t uur uur = γ M H uur r uur r r uur r ( ) ( ) 3 2A uur 2 Heff ( r) = Happl + G % r, r' M r' dr' + M M V Spin waves - dipolar-exchange dipolar interaction S eff exchange interaction

9 Brillouin light scattering (BLS) process = inelastic scattering of photons from spin waves scattered photon L q L q L q L phonon or spin wave q r r r q = q ± q SC L ω sc = ω L ±ω spectrum of scattered light incident photon proportional to the spin wave intensity ϕ 2 BLS-Intensity [Counts] Stokes anti-stokes Spin wave frequency Frequensy shift [GHz]

10 Brillouin light scattering spectrometer high-resolution interferometry with high contrast for measurements of acoustic phonons and spin waves

11 On macroscopic scale so far a lot of experience in development of spin-wave based concepts, such as: spin-wave logic nonlinear excitations (solitons, bullets) parametric amplification magnonic crystals... Realize these concepts on submicrometer scale

12 Content Background spin waves in a small magnetic stripe with domain wall nanocontacts on spin-valve samples propagating spin waves in a small magnetic stripe Summary

13 Lateral spin-wave quantum size effect x z y Standing lateral modes propagating dipolar spin-wave modes quantization condition due to the lateral edges: w = n λ spin wave /2; q n = 2π/λ spin wave = πn/w; n = 1,2,3,... boundary conditions (open pinned)

14 Ni 81 Fe 19 nanostripes Nucleation of a domain at protuberance applying a field sequence Observation of thermal spin waves Experiment: BLS spectra measured along a line indicated by the red dots, focus diameter 250 nm OOMMF simulations:

15 Lorenz microscopy Comparison to OOMMF simulation: in cooperation with J. Chapman group, Glasgow

16 Technique: BLS Microscopy Frequency Analysis optical resolution: 250nm 2D piezo stage controlling sample while measuring frequency range: 1GHz 1THz spectral resolution: 200MHz Sample Stage Viewing System Active Stabilization + Positioning position stability: infinite accuracy: better than 20nm high reproducibility

17 BLS Microscopy - experimental setup sample

18 Measurement procedure Stokes part anti-stokes part Colour maps are created by assembling the spectra of each scan point Only the Stokes part of the spectrum is used

19 Ni 81 Fe 19 nanostripes: thermal spectrum Thermal spin wave spectrum without domain wall with domain wall at protuberance C. W. Sandweg et al., J. Phys. D 41, (2008) TNT 2009, Barcelona September 8, 2009

20 Ni 81 Fe 19 nanostripes: thermal spectrum H parallel C. W. Sandweg et al., J. Phys. D 41, (2008) TNT 2009, Barcelona September 8, 2009

21 Content Background spin waves in a small magnetic stripe with domain wall nanocontacts on spin-valve samples propagating spin waves in a small magnetic stripe Summary

22 Nano-contact sample layout Samples provided by M. van Kampen, X. Janssens and L. Lagae, IMEC Point contact diameter: 80 nm SEM images of the sample (top view) TNT 2009, Barcelona September 8, 2009

23 AC induced magnetization dynamics Measuring at fixed position near the nanocontact RF frequency = 2 GHz, H = 150 Oe BLS Intensity (counts) 3x10 3 2x10 3 1x10 3 reference BLS Frequency (GHz)

24 AC induced magnetization dynamics Measuring at fixed position near the nanocontact Sweeping the RF current frequency for a fixed applied power (= 20 mw) BLS frequency [G H z] f 3f 2f 3f/2 f ½ f 150 Oe RF frequency [GH z] Higher frequency generation: 2ƒ, 3ƒ, 4ƒ Non-integer-half frequency generation: 1/2 ƒ, 3/2 ƒ

25 Nonlinear magnetic phenomena ƒ Three-magnon scattering processes 2ƒ ƒ ½ƒ ƒ Confluence Splitting ½ƒ BLS frequency [G H z] f f ½ f 150 Oe RF frequency [GH z] TNT 2009, Barcelona September 8, 2009

26 Field-dependent excitation spectra BLS frequency [G H z] BLS frequency [G H z] Oe RF frequency [GH z] 250 Oe RF frequency [GH z] Frequency [GHz] BLS frequency [G H z] Oe RF frequency [GH z] k 2 0 1x10 5 2x10 5 k B 0 B 0 Wave vector [cm -1 ] 250 Oe 225 Oe 150 Oe

27 Splitting process threshold frequency Frequency [GHz] e - -4 ma 0 ma 4 ma x10 5 2x10 5 Frequency limit [GHz] Applied field: 281 Oe Applied field: 281 Oe Direct current [ma] H = H + total external H Oe Wave vector [cm -1 ] Oersted field contribution of 16 Oe/mA to the internal field

28 TNT 2009, Barcelona September 8, Origin of the Oersted field contribution Y [nm] H x component X [nm] [Oe] Y [nm] H y component [Oe] X [nm] Asymmetry for the component parallel to the applied field x Contribution of top electrode to the free layer is ~ 13 Oe/mA y H In agreement with experimental findings from f /2 mode frequency limit (~16 Oe/mA)

29 Splitting process threshold power RF Frequency = 8.9 GHz; H = 245 Oe BLS intensity [counts, log] f f/2 threshold power RF power [mw] BLS frequency [G H z] f/2 245 Oe RF frequency [GHz] The resonance mode increases linearly with the applied RF-power The f/2 mode shows clearly threshold behaviour f

30 Content Background spin waves in a small magnetic stripe with domain wall nanocontacts on spin-valve samples propagating spin waves in a small magnetic stripe Summary

31 Phase-resolved BLS Inelastically scattered light contains phase information Interference between sample beam and reference beam A. A. Serga et al., APL 89, (2006)

32 Phase-resolved BLS microscopy Phase-resolved detection of propagating spin waves in small Py microstripe Permalloy stripe: width: 2.5 µm length: ~ 100 µm thickness: 40 nm Interference picture: proof of propagating spin-wave nature information on spin-wave wavelength

33 Phase profile of spin waves Spin-wave phase profile: four measurements required [1] slope yields spin-wave wavelength [1] A. A. Serga et al., APL 89, (2006)

34 Propagating spin waves Comparison with theory yields perfect agreement: Material parameters: M S = 860 G γ = GHz/Oe A = erg/cm Phase-resolved BLS microscopy is a powerful tool for the detection of propagating spin waves

35 Summary Background spin waves in a small magnetic stripe with domain wall nanocontacts on spin-valve samples propagating spin waves in a small magnetic stripe Summary nano-magnonics: spin dynamics on the nano-scale