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

Ultrafast Phenomena in Cooperative Systems 5-10 February Buellton, CA

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

Università Cattolica del Sacro CuoreDipartimento di Matematica e Fisica, Via Musei 41, Brescia, Italy.

Claudio [email protected],

http://www.dmf.unicatt.it/elphos

mailto:[email protected]

http://www.dmf.unicatt.it/elphos


ARRAYS OF MAGNETIC DISKS

Introduction

•Fundamental physics → Vortex configurationT. Shinjo et al., Science 289, 930 (2000).

Magnetic eigenmodes on permalloy squares and disksK. Perzlmaier et al., Phys. Rev. Lett. 94, 057202 (2005).

•Technological interest → Candidates to MRAMR. Cowburn, J. Phys. D: Appl. Phys. 33, R1 (2000).

1m

Fe20Ni80


THERMODYNAMICS AT NANOSCALE

Introduction

Cylindrical disks, in thermal contact with the substrate, are suitable to study the mechanical properties and the dynamical heat exchange at the solid interface. Si substrate

Py disk

•Fundamental physics → limits of classical thermodynamicsC. Bustamante et al., Physics Today 58, 43 (2005)

•Technological problems → measuring without perturbing the nano-systemT.S. Tighe et al., Appl. Phys. Lett. 70, 20 (1997)


Diffraction by ordered arrays

DIFFRACTIONThe contribution from the periodic structure is decoupled from the substrate contribution

= 800 nm =120 fs76 MHz

Ti:Sapphireoscillator

modulation 50 kHz1/f noise reduction

time-resolved reflectivity → S/N<10-6

and time-resolved MOKE

pump

probe

→ S/N<10-5


Standing waves induced by lattice heating

The laser-induced non-adiabatic heating triggers radial acoustic standing waves

4.5x10-5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

R/R

7006005004003002001000delay (ps)

The background at negative delays is related to the mean heating of the sample

~245 J/cm2

Oscillations in the transient reflectivity on the diffraction pattern

170 ps 2a=400 nm

TIME-RESOLVED REFLECTIVITY

3.2x10-5

2.8

2.4

2.0

1.6

1.2

R/R

50403020100-10-20delay (ps)

~10 ps

osci

llatio

n am

plitu

de

24020016012080400laser fluence (µJ/cm2)


h

a

Si substrate

Py disk

Standing waves induced by lattice heating

)(

)()(

latellat

lat

latelel

elel

TTGtTC

tPTTGtTT

420

400

380

360

340

320

300

tem

pera

ture

(K)

109876543210

time (ps)

Tel

Tlat

electronic specific heatelectron-phonon coupling

excitation intensity

Impulsive heating striggers acoustic longitudinal standing waves

ELASTIC OSCILLATION OF CYLINDRICAL FUSESG.D. Mahan et al., J. Appl. Phys. Lett. 70, 20 (1997)


SIMPLE COMPRESSION MODEL:

Yu rrrr

rrzz upuu

Oscillation period

0

00

arr

ru

Ya 4

Tran

sien

t ref

lect

ivity

(arb

itrar

y un

its)

50045040035030025020015010050delay (ps)

Mechanical properties

1.2

1.0

0.8

0.6

0.4

Dia

met

er (µ

m)

450400350300250200150Oscillation period (ps)

Y ~ 230 GPa

Frequency dependance on the dot size

1080 nm

600 nm

500 nm

400 nm

300 nm

Young modulus

z

r

rtkrur ˆsinsin

L.D. Landau and E.M. Lifshitz, Theory of Elasticity

ur

Radial displacement


Thermodynamics at nanoscale

We use an harmonic oscillator model, where the radial displacement ur(t) depends on the temperature of the disk.

)(2)]()([)( 020 tutututu rrrr

)sincos()( / teteetu tttr

/0 )( tr etu

The solution is given by:

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

Heat exchange with the substrate6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

Tran

sien

t ref

lect

ivity

(10-5

)

8007006005004003002001000delay(ps)

=10 ns

fit: =0.90+0.05 ns

=10 ps

2a=300 nm

where 2=02-2 and =1/-


Thermodynamics at nanoscale

THERMAL DECOUPLING: ACCESSING CRTherm

Isothermal nanodisk in contact with Si substrate through intrinsic thermal resistance RTherm:

ThermS

t

RlCzeTtT

/0

provided Biot number 1ThermkRlBiNanodisk isothermal on

ps to ns time scale

a0

l

Si substrate

Py disk

true in our caseRTherm10-8 Km2/Wkel=91 W/Km

From the measured we are able to obtain the specific heat of a mesoscopic physical system:

Cs ~ 3106 J/(m3K)

Bi~0.03

Measured specific heat

Cs ~ 2.2106 J/(m3K)Specific heat of a Ni thin film


Magneto-optical Kerr microscopy

The excitation modes of the vortex state phase can be studied by TR-Kerr microscopy

Ultrafast SC switch

Magnetic field pulse

H

dynamics of the excited magnetization vortex

K. Perzlmaier et al., Phys. Rev. Lett. 94, 057202 (2005)

Is it possible to excite the magnetic spectrum without magnetic pulses?

Magnetoelastic interaction lkijklijkijkij MMMdMg ),(

piezomagnetism magnetostrictionthermodynamic potential


Kerr hysteresis cycles

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0K

err e

llipt

icity

(arb

. uni

ts)

-600 -400 -200 0 200 400 600H (Oe)

Ellipticity_600nm

vortex configuration

single-domain

The hysteresis cycle can be reproduced via micromagnetic simulation software OOMMF

Vortex expulsion

KERR ELLIPTICITY


-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

Transient Kerr ellipticity

-400 -200 0 200 400H (Oe)

=490 ps

vortex configuration

single-domain

Kerr ellipticity at fixed delay

LASER INDUCED VARIATION of KERR ELLIPTICITY

Dynamical hysteresis cycles

MM

RR

''

MMMM

''

''

21

Ellipticity variation

non-magnetic contribution

•Subtracting measurements taken at opposite values of the external magnetic field, eliminates non-magnetic contributions

•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 magnetoelastic coupling

•We measure transient hysteresis cycles as a function of the delay between the pump and probe pulses

-11

-10

-9

-8

-7

-6

-5

-4

M/M

x 10-4

6004002000delay (ps)

Averaged magnetization as a function of the pump-probe delay

R/R

(ar

bitra

ry u

nits

)6005004003002001000

delay (ps)

M/M

(arbitrary units)

M/M

R/R

510-5

After subtraction of the background, a small oscillation of the magnetization averaged over the cycle is evidenced

OSCILLATION in the AVERAGED MAGNETIZATION

• Improving of the experimental resolution to discriminate magnetoelastic coupling in the different magnetic configurations


Isothermal nanodisk@ 50 oC

photon-e-

e--phononcoupling

Nanodisk-substratecoupling 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)

psps nsns 10 ns10 nstime time delaydelay

PHYSICS TIME-SCALE

nanodisk heating

Pump excitation

Conclusions


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 leaderFulvio Parmigiani

•TR-MOKEAlberto Comin (LBL)

•SamplesP. 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

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

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