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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.
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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.
Fe20Ni80 1m 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).
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)
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
~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
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)
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
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
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 • RTherm10-8 Km2/W • kel=91 W/Km 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~ 3106 J/(m3K) Cs~ 2.2106 J/(m3K)
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
single-domain vortex configuration Kerr hysteresis cycles • KERR ELLIPTICITY • The hysteresis cycle can be reproduced via micromagnetic simulation software OOMMF Vortex expulsion
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
After subtraction of the background, a small oscillation of the magnetization averaged over the cycle is evidenced 510-5 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
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
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
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