超短パルス レーザーを用いた磁化ダイナミクス計測と円偏光誘起磁化反転

Transcription

1 SPring-8 利用推進協議会先端磁性材料研究会第 4 回研究会 スピンダイナミクスと光誘起磁化過程 総評会館 201 会議室, Japan 超短パルス レーザーを用いた磁化ダイナミクス計測と円偏光誘起磁化反転 塚本新 1,2 1 College of science and technology,, Japan; 2 PSTO, Japan Science and Technology Agency, Japan atsuka@ecs.cst.nihon-u.ac.jp 1/14

2 Dynamic property under the precessional motion Magnetization reversal Damping term Proportional to the dm/dt? Landau-Lifshitz-Gilbert equation d d eff M dt M 0.2 My dt μ μ μ μ μ Critical damping =1 Conventional recording media Low damping (CoCrPt ~0.04 Inaba) Mz Mx

3 4. Precessional switching by ultra-short pulse laser Magnetization reversal How to gain the speed? Idea Interplay of ultra fast heating and large temperature dependence of magnetic resonance - Ferrimagnet Angular momentum compensation High-speed and strongly damped precessional switching triggered with ultrafast heating of a GdFeCo

4 With angular momentum Angular momentum compensation Spinning Top ( KO MA in Japanese) Without angular momentum Gravity Starting precessional motion Rapid fall Ferro magnetism L M / M L Long time survive Ferri magnetism sub-lattice magnetization anti-parallel tight coupling,, different γ L M net net M M / M / M L A M B M A L B

5 Magnetization dynamics in ferrimagnetics (sub-lattice) 1 2 phenomenological Landau-Lifshitz-Gilbert equation d d eff M dt M effective gyromagnetic ratio M x M eff x M x M dt 1 x 1 x anti-parallel tight coupling Net M M Net ANet x ferromagnetic resonance (FMR) branch FMR eff eff effective Gilbert damping parameter* eff 2 2 M x M 1 x Net angular momentum x 0 A ANet

6 As a fast recordable media idea As a fast recordable media, acceleration of dynamic response with adequate net magnetization is realized by controlling net angular momentum. Gd: huge 4f-moment S=7/2 Anti-para with FeCo Amorphous: uniform

7 All-optical Pump probe set up Measurements:monitering Faraday F / F, Kerr K / K, TransmittanceT/T, Reflectance R/R Δt: 3.3 fs FWHM : 90fs : 420nm and 800nm Note that only the magnetization of the transition metal subsystem is probed by the linear Faraday-effect at laser wave length of 800 nm. It makes possible to measure the magnetic dynamics near magnetic compensation point.

8 Employ femtosecond pulsed laser Ultrafast heating Ultrafast detection of M

9 Optical pump-probe probe measurements Excitation of precession / spin waves t 0 t 700 fs t 700 fs GdFeCo Heat up: t = 0 Pomp light Probe light t Delay time

10 Composition dependence of precessional motion Experimental results [a. u.] : 420nm FWHM : 90fs Gd 20 Fe 70 Co 10 Room temperature Delay time [ps]

11 Composition dependence of precessional motion Divergent tendency of precession frequency and damping was appeared closing to compensation composition ratio T A < RT T A > RT : 800nm FWHM : temp.

12 Results Does truly accelerate the magnetization reversal? Pump-probe measurement? How to generate Ultra-fast magnetic field switching? Ultrafast demagnetization by ultra short pule laser?? Ultrafast magnetic field switch

13 3. Thermo-magnetic recording by ultra-short single laser pulse Demonstration of thermo-magnetic recording by ultra-short single laser pulses (FWHM: 90 fs) Rapid heating by laser Gd 26.0 Fe 64.7 Co 9.3 M - M + Polarized microscope Stray field from surrounded magnetization

14 Pomp-Probe measurements Measurements: Faraday F / F, Kerr K / K, TransmittanceT/T, Reflectance R/R A B B C F, K, T/T, R/R [a. u.] light Gd 20 Fe 70 Co 10 : 420nm FWHM : 90fs Over lap F K Faraday Refrectance Kerr Transmittance T/T R/R Delay time [ps] Three time region: A ~100fs Direct interaction: Coherent, non-thermal B ~few ps Charge/spin dynamics: equilibrations Main part of this talk C ~ns LLG-like motion on meta-stable state

15 Faraday rotation: F 2 l M 0 Opto-magnetism (-) (+) M M Inverse Faraday effect W 0 E( ) E ( ) H eff W M 1 (0) W (0) 0 M * 0E( ) E ( ) M M ij jj H eff (0) jj M M M Light-wave energy M magnetization H eff - magnetic field * (0) E( ) E ( ) 0 L. Onsager (1931) Light acts as a magnetic field L. P. Pitaevskii, Sov. Phys. JETP 12, 1008 (1961). J. P. van der Ziel et al, Phys. Rev. Lett. 15,190 (1965).

16 1. Ultrafast laser excitation / direct angler momentum transfer A Direct interaction: non-thermal effect Deference with B and C process A B F, K, T/T, R/R [a. u.] light Over lap Gd 20 Fe 70 Co 10 : 420nm FWHM : 90fs F K Faraday Refrectance Kerr Transmittance T/T Delay time [ps] Ultra-fast pass light to spins R/R Information writing on GdFeCo media Magnetization reversal depend on Angular momentum of light Non-thermal effect

17 Radboud University Nijmegen Conclusions - ferrimagnetic alloy (GdFeCo) with femtosecond pulsed laser (FWHM~90fs) Ultrafast demagnetization of and components around 1ps Divergent tendency of precession frequency and damping (~0.32 ) was appeared closing to angular momentum compensation Precessional switching can be triggered by ultra-short pulse laser Angular momentum compensation is a vital point for the magnetization switching speed of magnetic and magneto optical data storage devices! Demonstrate that magnetic information can be recorded by non-thermal way, with combination of inverse Farady like effect, ultrafast heating across compensation temperature Acknowledgements This work is partially supported by a grant-in-aid from the Nihon university Multidisciplinary Research Grant for (2009). Collaborations! Light Magnetism Thermo