Download - Deflagration in Magnetism
J. Tejada, A. Hernández-Mínguez, F. Macià, S. Vélez and
J.M. Hernández
Grup de Magnetisme, Dept. de Física Fonamental, Universitat de Barcelona
V. Moschalkov, J. Vanacken, Wim Decelle
INPAC, Katholieke Universiteit Leuven
P. V. Santos
Paul-Drude-Institut für Festkörperelektronik, Berlin
Introduction
What is a deflagration?
From Magnetisation jumps to magnetic deflagration.
Molecule Magnets
Manganese Oxides
Intermetallic Compounds
There are two characteristic timescales which are
important here. The first is the thermal diffusion
timescale is approximately equal to
The second is the burning timescale that strongly
decreases with temperature, typically as
Energy released E
Ignition (barrier overcoming) U
Thermal diffusion
Characteristic length of propagation Metastable
State
U
Deflagration is a technical term describing subsonic combustion that usually propagates
through thermal conductivity
E
Estable
State
When the burning timescale greater exceed the difussion timescale, the huge
amount of energy realized by the metastable spins could lead to the ocurrence of
a Magnetic Deflagration.
-30 -20 -10 0 10 20 30
-1.0
-0.5
0.0
0.5
1.0
M/M
s
H (kOe)
T = 1.8 K
Molecule magnets
Field jumps 1999
Deflagration-like description
2005
0 10 20 30
0.0
0.5
1.0
M/M
S
H (kOe)
T = 3 K
Manganites
Field jumps 1999
Deflagration-like description 2007
Intermetallic compounds
Field jumps 2002
Deflagration-like description 2010
0 5 10 15 20 25 30 35 40 45 50
0.0
0.2
0.4
0.6
0.8
1.0
M/M
S
H (kOe)
Magnetic deflagration:
Propagation of a front of reversing
spins at constant velocity along the
crystal
Problem: Sweeping H we
cannot control the magnetic
field at which it occurs.Y. Suzuki et. al. PRL 95, 147201 (2005)
A. Hernández-Mínguez et. al. PRL 95 17205 (2005)
H
ΔE
IDT
LiNbO3
substrate
conducting
stripes
coaxial cable
Mn12 crystalc-axis
Hz
The coaxial cable is connected to an Agilent microwave signal generator.
The change of the magnetic moment is registered by a rf-SQUID magnetometer.
Surface acoustic waves (SAWs) are low frequency acoustic phonons (below 1 GHz)
• The speed of the avalanche
increases with the applied
magnetic field.
• At resonant fields the
velocity of the flame front
presents peaks.
• The ignition time shows peaks at
the magnetic fields at which spin
levels become resonant.
Optical detection
Frequency 150-350 GHz
• Space is needed to place piezoelectric
devices and ignite avalanches
•NO cavities can be used
f 9,8= 269 GHz
H=12 kOe
0 10 20 30 40 50 60
8.3
8.4
8.5
8.6
8.7
8.8
8.9
9.0
9.1
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
E-10,-9
E8,7
E6,5
E7,6
Sig
n. A
mpl
. (
arb.
u.)
H (kOe)
E9,8
Sig
n. A
mpl
. (
arb.
u.)
F. Macià et. al. PRB 77 020403R (2008)
(a)
(d)(c)
0 50 100
-0.4
-0.2
0.0
Sig
n. A
mpl
. (
arb.
u.)
t (ms)
Avalanche
SAW pulse
0 50 100 150
-0.4
0.0
Avalanche
SAW pulse
Sig
n. A
mpl
. (
arb.
uni
ts)
t (ms)
0 25 50
-0.10
-0.05
0.00
Sig
n. A
mpl
. (
arb.
uni
ts)
t (ms)
-0.1
0.0
0 50 100 150
t (ms)
Sig
n. A
mpl
. (
arb.
uni
ts)
(b)
• Surface Acoustic Waves allow us
to control magnetic avalanches
Very fast sweeping magnetic fields
Decelle et al. Phys. Rev. Lett. 102, 027203 (2009)
Superradiance
H-M. et al. Europhys. Lett. 69, 270 (2005)
100 200 300 400 500 600 700 800
-4
-3
-2
-1
0
V (
mV
)
time ( s )
During the second of two field pulses with the same polarity.
During the second of two field pulses with opposite polarity.
• Is the described deflagration-like process in molecular clusters unique?
• Among the variety of compounds presenting steps in the magnetisation
curves… are there also spatial propagation involved?
Antiferromagnetic and Isolating
Ferromagnetic and Conductor
The fragility of the state shown here implies that
several perturbations besides magnetic fields should
induce dramatic changes, including pressure, strain,
and electric fields.
PS Manganites
(La,Pr,Ca)-MnO3
0 10 20 30
0.0
0.5
1.0
M/M
S
H (kOe)
T = 3 K
0 20 40
0.00
0.25
0.50
0.75
1.00
AF-CO initial state
FM-CD final state
M/M
s
H (kOe)
3.0 K
3.5 K
4.0 K
4.5 K
5.0 K
x = M / Mferro
x, fraction of the ferromagnetic phase
0 10 20 30 40 50
0.0
0.5
1.0
1.5
2.0
0 50 100
28
32
36
12
M (
emu)
H (kOe)
1
Ha (
kOe)
xa (%)
Commercial MPMS SQUID
magnetometer
Three pick-up coils detect the magnetic
flux variation.
Recorded by an oscilloscope
Sample
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0
0.2
0.4
0.6
0.8
1.0
Vco
il /
Vco
il,m
ax
t (ms)
coil A
coil B
coil C
T = 3.5 K
Evidence of propagation
Avalanche begins at the centre of the sample
0.0 0.2 0.4 0.6 0.8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8
Vco
il /
Vco
il,m
ax
t (ms)
z = 4.0 cm z = 5.5 cm
t (ms)
z = 6.5 cm
t (ms)
Energy released
Thermal diffusion
Ignition
(barrier overcoming)
0 10 20 30
0.0
0.5
1.0
M/M
S
H (kOe)
T = 3 K
AF
FM
Energy Barrier
Energy released
80 90 100 110 120
0.00
0.01
0.95
1.00
0 200 400 600
0.00
0.25
0.50
0.75
1.00
2
4
6
8
10
12
M (
arb.
u.)
t (ms)
t (ms)
M (
arb.
u.) T
(K)
?
The larger the initial FM phase concentration, the slower the deflagration velocity
0
200
400
600
20 25 30 35 40 45 50 55
6
8
10
12
t ig (
ms)
(a)
(b)
x (%)
t d (
ms)
H=28 kOeInitial FM fraction dependence
0 10 20 30 40 50
0.0
0.5
1.0
1.5
2.0
0 50 100
28
32
36
12
M (
emu)
H (kOe)
1
H
a (
kOe)
xa (%)
Macià et al. Phys. Rev. B 76, 174424 (2007)
As the field increases the energy
barriers decrease and
deflagration becomes faster.
28 30 32
5
10
15
20
0
100
200
500
600
28 30 32
20
40
60
t d (
ms)
H (kOe)
(b)
(a)
t ig (
ms)
x (
%)
H (kOe)
0 100 200 300 400
0.00
0.01
0.96
0.98
1.00
27.0 kOe
27.5 kOe
28.0 kOe
28.5 kOe
29.0 kOe
29.5 kOe
30.0 kOe
30.5 kOe
31.0 kOe
M (
arb
.u.)
t (ms)
T = 2 K T = 4 KT = 3 K
• Initially sample is in the AF-CO phase.
• As field increases FM-CD phase begins
to grow.
• At some time a conducting path
appears.
• It is not necessarily associated with the
magnetic avalanche
0 10 20 30
0.1
1
10
100
O.C.
0.0
0.2
0.4
0.6
0.8
1.0
resistance
(k
)
H (kOe)
T=3 K
magnetisation
M/M
s
AF-CO
(insulator)
FM-CD
(metallic)
Initial FM-CD phase concentration smaller than 10%
T = 2.5 K
-2 -1 0 1
0
250
500
750
OC
2
4
6
8
-200 0 200 400
0
400
800
O.C.
R (
k)
t (s)
T (
K)
R (
k)
t ( s)
Macià et al. Phys. Rev. B 77, 012403 (2008)
Initial FM-CD phase concentration larger than 10%
T = 3 K
0 10 20 30 40 50
0
20
40
60
80
100
120
140
2
3
4
5
6
R (
k)
t (s)
T (
K)
T = 3 K
Zero field cooled Field cooled H = 15 kOe
-6 -4 -2 0 2
1
10
100
1000
O.C.
2
4
6
R (
k)
t (s)
T (
K)
Coax. Resonator f 3 GHz, Q 100
10 20 30
0.0
0.2
0.4
3.80
3.84
3.88
3.92
0 1 2
0.20
0.25
0.30
S1
1 (
arb.
uni
ts)
H (kOe)
f (
GH
z)
T=3 K
S1
1 (
a. u
.)
t (ms)
f=3.88 GHz
Macià et al. Europhys. Lett. 82, 37005 (2008)
• In the ac plane the spins are aligned
ferromagnetically
• Two different crystal structures: AFM or FM
coupling between the spin layers along the b axis
• At low temperatures, the field driven AFM FM
transition undergoes via an avalanche due to the arrest
of kinetics of the crystal structure.
Dynamical study It is a magnetic deflagration?
Outline: Magnetic and crystallographic Properties
• Hability to control the initial FM phase (x) cooling the sample at different HFC
• Avalanches appear at HFC smaller than HFC ~11.0 kOe (x~0.5)
T = 2 K
Velez et al. Phys. Rev. B 81, 064437 (2010)
•Set-up
•For spontaneous avalanches: The signals detected with the two coils are almost simultaneous:
The Nucleation should take place at the middle of the sample
•For ignited avalanches, a time diference is observed between the coils:
A phase deflagration front is generated an it propagates through the sample changing the magneto-
crystallographic structure of the system: Magnetostructural Deflagration.
•Ignited avalanche by sending a heat pulse:
Magnetic time-evolution of the sample for different ignition fields (Hign) at T=2 K
The quasi-linear time evolution of M(t) indicates that the phase-front traverses
the sample at a constant speed.
T = 2KHign = 22 KOe
Speed of the flame vs Hign at two dif. HFC Speed of the flame vs HFC different xini
T = 2K
• The velocity increases with the Hign
• The velocity decreases with the initial FM phase x: losses of the flammability of the
system
smTTUTkvfffB
/1.0/ 21
Kk
ExT
DB
D
f30
3
)1(54
1KUE 200
smTf
/1025
1410/exp sTkUT
fBf
KD
120
AFM
FM
U
ΔE
IDT
LiNbO3
substrate
Conducting
stripes
coaxial
cable Gd5Ge4 SC
Hz
Magnetic deflagrations have been induced by means of controlled SAW pulses.
They were induced on Gd5Ge4 Single Crystals with diferent geometries and under different sample
configurations.
The magnetic time evolution of the sample was taken directly from the SQUID-voltmeter
a 1.17 mm
b
2.45 mm c
1.04 mm
c
1.07 mm
b 1.29 mm
a
2.40 mm
Velez et al. Submitted
16 18 20 22 24 26 28 300
10
20
30
S1 H b, SAW a
S1 H b, SAW c
S2 H a, SAW c
S2 H a, SAW b
t d~
L/ v
p(m
s)
Hig (kOe)
Same direction of applied magnetic field + different sample configuration:
Anisotropic Magnetic Deflagration attributed to the Anisotropy of the Thermal Difussivity
Clear different speed for different crystallographic direction of the applied magnetic field +
Correlation with the magnetic anisotropy of the sample:
Anisotropic Magnetic Deflagration attributed to the Magnetic Anisotropy
Due to the sample’s
geometry, L is approx the
same for all cases
Magnetic deflagration is observed in several magnetic materials.
Molecular magnets: New methods to study spin dynamics (SAW+HFEPR) and to
correlate experiments with theory.
Radiation emission associated to deflagration and detonation (SUPERRADIANCE TASER)
Manganites: Colossal MagnetoResistance associated to the phase deflagration
Intermetallic compounds Fast Magnetostructural transitions
Anisotropic Magnetic Deflagration: Magnetic and Thermal diffusivity