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

skillfulbuyerUrban and Civil

Nov 16, 2013 (3 years and 11 months ago)

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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 Cuore

Dipartimento di Matematica e Fisica, Via Musei 41, Brescia, Italy.

Claudio Giannetti

c.giannetti@dmf.unicatt.it
,

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


Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

ARRAYS OF MAGNETIC DISKS

Introduction


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).


1

m

Fe
20
Ni
80

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

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 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)


Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

Diffraction by ordered arrays

DIFFRACTION

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


= 800 nm

t

=120 fs

76 MHz

Ti:Sapphire

oscillator

modulation 50 kHz

1/f noise reduction

time
-
resolved reflectivity


→ S/N<10
-
6

and
time
-
resolved MOKE


pump

probe

→ S/N<10
-
5

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

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
700
600
500
400
300
200
100
0
delay (ps)
The background at negative delays is related to the mean heating of the sample

~245

J/cm
2

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
50
40
30
20
10
0
-10
-20
delay (ps)
~
10 ps

oscillation amplitude
240
200
160
120
80
40
0
laser fluence (µJ/cm
2
)
Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

h

a

Si substrate

Py disk

Standing waves induced by lattice heating

)
(
)
(
)
(
lat
el
lat
lat
lat
el
el
el
el
T
T
G
t
T
C
t
P
T
T
G
t
T
T













420
400
380
360
340
320
300
temperature (K)
10
9
8
7
6
5
4
3
2
1
0
time (ps)
T
el
T
lat
electronic specific heat

electron
-
phonon coupling

excitation intensity

Impulsive heating striggers acoustic
longitudinal standing waves

ELASTIC OSCILLATION OF CYLINDRICAL FUSES

G.D. Mahan
et al
.,
J.

Appl. Phys. Lett.

70
,

20 (1997)

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

SIMPLE COMPRESSION MODEL:

Y
u
rr
rr


rr
zz
u
p
u
u




Oscillation period

0
0
0


a
rr
r
u

Y
a

t
4

Transient reflectivity (arbitrary units)
500
450
400
350
300
250
200
150
100
50
delay (ps)
Mechanical properties

1.2
1.0
0.8
0.6
0.4
Diameter (µm)
450
400
350
300
250
200
150
Oscillation 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



r
t
kr
u
r
ˆ
sin
sin




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

u
r

Radial displacement

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

Thermodynamics at nanoscale

We use an harmonic oscillator model,
where the radial displacement
u
r
(t)

depends on the temperature of the disk.

)
(
2
)]
(
)
(
[
)
(
0
2
0
t
u
t
u
t
u
t
u
r
r
r
r









)
sin
cos
(
)
(
/
t
e
t
e
e
t
u
t
t
t
r






t






t
/
0
)
(
t
r
e
t
u


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



t

relaxation →
heat exchange between the disk


and the substrate


Heat exchange with the substrate

6.0
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
Transient reflectivity (10
-5
)
800
700
600
500
400
300
200
100
0
delay(ps)
t
=10 ns
fit:
t
=0.90
+
0.05 ns
t
=10 ps
2a=300 nm

where

2
=

0
2
-

2

and

=1/
t
-


Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

Thermodynamics at nanoscale

THERMAL DECOUPLING: ACCESSING

CR
Therm

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













Therm
S
t
R
lC
z
e
T
t
T
t
t


/
0
provided Biot number

1


Therm
kR
l
Bi
Nanodisk

isothermal

on

ps

to

ns

time

scale

a
0

l

Si substrate

Py disk

true in our case

R
Therm

10
-
8

K

m
2
/W

k
el
=91 W/K

m

From

the

measured

t

we

are

able

to

obtain

the

specific

heat

of

a

mesoscopic

physical

system
:

C
s

~ 3

10
6

J/(m
3

K)

Bi
~0.03

Measured specific heat

C
s

~ 2.2

10
6

J/(m
3

K)

Specific heat of a Ni thin film

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

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

l
k
ijkl
ij
k
ijk
ij
M
M
M
d
M
g






)
,
(
piezomagnetism

magnetostriction

thermodynamic potential

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

Kerr hysteresis cycles

-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Kerr ellipticity (arb. units)
-600
-400
-200
0
200
400
600
H (Oe)
Ellipticity_600nm
vortex configuration

single
-
domain

The hysteresis cycle can be reproduced via micromagnetic
simulation software OOMMF

Vortex expulsion

KERR ELLIPTICITY

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
Transient Kerr ellipticity
-400
-200
0
200
400
H (Oe)


t
=490 ps

vortex configuration

single
-
domain

Kerr ellipticity at fixed delay

LASER INDUCED VARIATION of KERR ELLIPTICITY

Dynamical hysteresis cycles

M
M
R
R





'
'

















M
M
M
M
'
'
'
'
2
1




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

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

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
600
400
200
0
delay (ps)
Averaged magnetization as a
function of the pump
-
probe delay


R/R (arbitrary units)
600
500
400
300
200
100
0
delay (ps)

M/M (arbitrary units)

M/M



R/R
5


-
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

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

Isothermal


nanodisk

@ 50
o
C

photon
-
e
-

e
-
-
phonon

coupling

Nanodisk
-
substrate

coupling through

interface resistance
R
Therm

gives

R/R decay
:

access to CR
Therm



R/R oscillations:


access to elastic
properties and coupling
to the magnetization

Steady
-
state :

access to R
Therm

(in process)

ps

ns

10 ns

time
delay

PHYSICS TIME
-
SCALE

nanodisk
heating

Pump

excitation

Conclusions

Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

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


Ultrafast Phenomena in Cooperative Systems

5
-
10 February Buellton, CA

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