Magnetoelastic effects in permalloy nano-dots induced by laser-driven acoustic standing waves

1. Magnetoelastic effects in permalloy nano-dots induced by laser-driven acoustic standing waves Claudio Giannetti c.giannetti@dmf.unicatt.it, http://www.dmf.unicatt.it/elphos Università Cattolica del Sacro Cuore Dipartimento di Matematica e Fisica, Via Musei 41, Brescia, Italy.

2. Fe20Ni80 1m Introduction ARRAYS OF MAGNETIC DISKS • Fundamental physics → Vortex configuration • T. Shinjo et al., Science289, 930 (2000). • Magnetic eigenmodes on permalloy squares and disks • K. Perzlmaier et al., Phys. Rev. Lett.94, 057202 (2005). • Technological interest → Candidates to MRAM • R. Cowburn, J. Phys. D: Appl. Phys. 33, R1 (2000).

3. Introduction THERMODYNAMICS AT NANOSCALE • Cylindrical disks, in thermal contact with the substrate, are suitable to study the mechanical properties and the dynamical heat exchange at the solid interface. Py disk Si substrate • Fundamental physics → limits of classical thermodynamics • C. Bustamante et al., Physics Today58, 43 (2005) • Technological problems → measuring without perturbing the • nano-system • T.S. Tighe et al., Appl. Phys. Lett.70,20 (1997)

4. probe pump Diffraction by ordered arrays DIFFRACTION The contribution from the periodic structure is decoupled from the substrate contribution modulation 50 kHz 1/f noise reduction Ti:Sapphire oscillator • = 800 nm t=120 fs 76 MHz → S/N<10-6 time-resolved reflectivity → S/N<10-5 and time-resolved MOKE

5. ~10 ps Standing waves induced by lattice heating TIME-RESOLVED REFLECTIVITY The laser-induced non-adiabatic heating triggers radial acoustic standing waves Oscillations in the transient reflectivity on the diffraction pattern 2a=400 nm 170 ps ~245 J/cm2 The background at negative delays is related to the mean heating of the sample

6. a Py disk h Si substrate Standing waves induced by lattice heating Impulsive heating striggers acoustic longitudinal standing waves electron-phonon coupling electronic specific heat excitation intensity ELASTIC OSCILLATION OF CYLINDRICAL FUSES G.D. Mahan et al., J.Appl. Phys. Lett.70,20 (1997)

7. SIMPLE COMPRESSION MODEL: Oscillation period Mechanical properties Frequency dependance on the dot size 1080 nm 600 nm 500 nm 400 nm Young modulus 300 nm Radial displacement z q ur r L.D. Landau and E.M. Lifshitz, Theory of Elasticity

8. Heat exchange with the substrate 2a=300 nm Thermodynamics at nanoscale We use an harmonic oscillator model, where the radial displacement ur(t) depends on the temperature of the disk. The solution is given by: where 2=02-2 and =1/- We are able to estimate the relaxation time between the nano-sized system and the substrate.  damping → dephasing between disks oscillations  relaxation → heat exchange between the disk and the substrate

9. a0 Py disk l Si substrate Thermodynamics at nanoscale • THERMAL DECOUPLING: ACCESSING CRTherm • Isothermal nanodisk in contact with Si substrate through intrinsic thermal resistance RTherm: • provided Biot number • Nanodisk isothermal on ps to ns time scale • true in our case • RTherm10-8 Km2/W • kel=91 W/Km Bi~0.03 • From the measured  we are able to obtain the specific heat of a mesoscopic physical system: Measured specific heat Specific heat of a Ni thin film Cs~ 3106 J/(m3K) Cs~ 2.2106 J/(m3K)

10. Magneto-optical Kerr microscopy The excitation modes of the vortex state phase can be studied by TR-Kerr microscopy Magnetic field pulse dynamics of the excited magnetization vortex H Ultrafast SC switch K. Perzlmaier et al., Phys. Rev. Lett.94, 057202 (2005) Is it possible to excite the magnetic spectrum without magnetic pulses? Magnetoelastic interaction thermodynamic potential piezomagnetism magnetostriction

11. single-domain vortex configuration Kerr hysteresis cycles • KERR ELLIPTICITY • The hysteresis cycle can be reproduced via micromagnetic simulation software OOMMF Vortex expulsion

12. Subtracting measurements taken at opposite values of the external magnetic field, eliminates • non-magnetic contributions Ellipticity variation non-magnetic contribution • The S/N ratio is increased by adding the difference of all the points in the cycle Magnetization is averaged over different magnetic configurations: only qualitative information Dynamical hysteresis cycles • LASER INDUCED VARIATION of KERR ELLIPTICITY Kerr ellipticity at fixed delay single-domain vortex configuration

13. After subtraction of the background, a small oscillation of the magnetization averaged over the cycle is evidenced 510-5 Averaged magnetization as a function of the pump-probe delay Dynamical magnetoelastic coupling • OSCILLATION in the AVERAGED MAGNETIZATION • We measure transient hysteresis cycles as a function of the delay between the pump and probe pulses • Improving of the experimental resolution to discriminate magnetoelastic coupling in the different magnetic configurations

14. Conclusions • PHYSICS TIME-SCALE time delay ps ns 10 ns Pump excitation photon-e- e--phonon Isothermal nanodisk @ 50 oC Nanodisk-substrate coupling through interface resistance RTherm gives R/R decay: access to CRTherm R/R oscillations: access to elastic properties and coupling to the magnetization Steady-state : access to RTherm (in process) coupling nanodisk heating

15. Future • Improving of the experimental resolution to discriminate magnetoelastic coupling in the different magnetic configurations • Different Fe-Ni composition to investigate the coupling between elastic and spin modes • Study of the shape of the transient hysteresis cycles to investigate the photon-electron interaction • Mechanical and thermodynamical properties of nanometric systems across a phase transition

16. Acknowledgements • Group leader • Fulvio Parmigiani • TR-MOKE • Alberto Comin (LBL) • Samples • P. Vavassori (Università di Ferrara) • V. Metlushko (University of Illinois) • Thermodynamics • F. Banfi and B. Revaz (University of Genève) • Ultrafast optics group (Università Cattolica, campus di Brescia) • Gabriele Ferrini, Stefania Pagliara, Emanuele Pedersoli, Gianluca Galimberti