Diapositiva 1

1
Magnetoelastic effects in permalloy nano-dots
induced by laser-driven acoustic standing waves
Claudio Giannetti c.giannetti_at_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
Introduction
ARRAYS OF MAGNETIC DISKS

• Fundamental physics ? Vortex configuration
• T. Shinjo et al., Science 289, 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 Today 58, 43
  (2005)
• Technological problems ? measuring without
  perturbing the
• nano-system
• T.S. Tighe et al., Appl. Phys. Lett. 70, 20
  (1997)

4
Diffraction by ordered arrays
DIFFRACTION
The contribution from the periodic structure is
decoupled from the substrate contribution
modulation 50 kHz 1/f noise reduction
TiSapphire oscillator

• 800 nm
• t 120 fs
• 76 MHz

? S/Nlt10-6
time-resolved reflectivity
? S/Nlt10-5
and time-resolved MOKE
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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
2a400 nm
170 ps
245 ?J/cm2
The background at negative delays is related to
the mean heating of the sample
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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)
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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
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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
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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 kel91 W/K?m
Bi0.03
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
Kerr hysteresis cycles
KERR ELLIPTICITY
The hysteresis cycle can be reproduced via
micromagnetic simulation software OOMMF
Vortex expulsion
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Dynamical hysteresis cycles
LASER INDUCED VARIATION of KERR ELLIPTICITY
Kerr ellipticity at fixed delay
single-domain
vortex configuration
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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

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Conclusions
PHYSICS TIME-SCALE
time delay
ps
ns
10 ns
Pump excitation
Isothermal nanodisk _at_ 50 oC
photon-e- e--phonon
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
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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

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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