1. MAGNETIC DEFLAGRATION
AND DETONATION?
J. Tejada, F.Macià, J.M. Hernández, P.V.Santos,
V. Moschalkov, J. Vanacken and W. Decelle

2. Contents
 Magnetic avalanches: Nanomagnets and manganites
(before 2005)
 Quantum magnetic deflagration in nanomagnets
 Quantum astroid
 Spin dynamics combining SAW and HFEPR
 Deflagration to detonation transition in nanomagnets
 Magnetic deflagration in manganites
 Colossal and fast magnetoresistance variation

3. Magnetic avalanches: Nanomagnets
and manganites (before 2005)
The magnetization process can occur in two ways, depending on the value of
the sweep rate and the size of the crystal:
1. slow: regular steps in the magnetization curve (red circles)
2. fast: at a certain field the sample experiences an avalanche (black squares)
1.0
0.5
4HR
M/Ms
0.0
-0.5 2.0 K
3HR
2.2 K
HR 2HR 2.4 K
-1.0 2.6 K
0.0 0.5 1.0 1.5 2.0
µ BH (T)

4. Magnetic avalanches: Nanomagnets and
manganites (before 2005)
At low temperatures and under fast varying fields the
magnetization change occurs in a very short time.
1.0 T=3K
 Accompanied by a huge
heat release from the 0.5
sample.
0.0
 Resistivity also abruptly M/MS
-0.5
changes with the
avalanche. -1.0
-4 -3 -2 -1 0 1 2 3 4
H (kOe)

5. Quantum magnetic deflagration in
nanomagnets
Magnetic deflagration:
Propagation of a front of reversing
spins at constant velocity along the
crystal
The conventional theory of
deflagration yields the following
A. Hernández-Mínguez et. al. PRL 95 17205 (2005)
expression for the velocity of the
flame front:
Problem: Sweeping H we
cannot control the magnetic
κ field at which it occurs.
v =
τ
κ  U (H ) 
Y. Suzuki et. al. PRL − , 147201 
v= exp 95 (2005)
τ0  2k T 
 B f 

6. Quantum magnetic deflagration in
nanomagnets
• The speed of the avalanche
increases with the applied
magnetic field.
• At resonant fields the
velocity of the flame front
presents peaks.

7. Quantum magnetic deflagration in
nanomagnets
The speed shows peaks at the
magnetic fields at which spin levels
become resonant.
κ  U(H) 
v = exp −
 2k T 

τ0  B f 
This velocity is well fitted:
κ = 0.8·10-5 m2/s
Tf (H = 4600 Oe) = 6.8 K
Tf (H = 9200 Oe) = 10.9 K
PRL 95 17205 (2005)

8. Quantum astroid
1
H = − ( S cos(θ ) ) 2 − hz S cos(θ ) − hx S sin(θ )
2

9. Quantum astroid
 MPMS system Key parameters for deflagration threshold:
 Magnetic fields up to 5 T •Relaxation,
 Temperatures down to 1.8 K
•Magnetic energy
Therm.
H
Saturate the sample
⇓
Sweep the magnetic field
⇓
Detect the temperature
variations.

10. Quantum astroid
Measured avalanches
• Always occurring through superposition
of states
• There is a critical angle

11. Spin dynamics combining SAW and HFEPR
Surface acoustic waves (SAWs) are low frequency acoustic phonons (below 1 GHz)
The coaxial cable is connected to an Agilent microwave signal generator.
The change of the magnetic moment is registered by a rf-SQUID magnetometer.
Hz
coaxial cable
IDT Mn12 crystal
c-axis
conducting LiNbO3
stripes substrate

12. Spin dynamics combining SAW and HFEPR
Hz
Hz

13. Spin dynamics combining H -3T to 3 T
SAW and HFEPR T 2K
Pulse time 1 ms to 100ms
Optical detection
Frequency 150–350 GHz
f = 269 GHz
SAW dissipation Sample perturb. Aval.
ignition.
12/02/13

14. Spin dynamics combining SAW and HFEPR
Metastable well

15. Spin dynamics combining SAW and HFEPR
Stable well

16. Spin dynamics combining SAW and HFEPR
Population in thermal equilibrium
Temperature Different
dependence Energy levels
(9 - 8)
PRB (R) 77, 020403 2008

17. Deflagration to detonation transition in
nanomagnets

18. Magnetic deflagration in manganites
• The basic concept underlying the colossal magnetoresistance
effect in manganites is phase separation
• In a broad region of parameter space, the ground state is actually
a nanoscale mixture of phases
• There is still a local tendency
toward either FM or AFI short-
distance correlations. However,
globally neither of the two states
dominates
• The fragility of the state shown
here implies that several
perturbations besides magnetic
fields should induce dramatic
changes, including pressure,
strain, and electric fields
[E. Dagotto, et al., Science 309, 257 (2005)]

19. Magnetic deflagration in manganites
La0.225Pr0.4Ca0.375MnO3
1.5
H = 10 kOe
ZFC
FCC
FCW
1.0
M (emu)
0.5
0.0
0 50 100 150 200 250 300
T (K)

20. Magnetic deflagration in manganites
Below 2 Tesla the FM-AF phase ratio is frozen
NO RELAXATION
At higher fields
phase
1.00
concentration
55
slowly relax.
H = 30 kOe
0.75 50
45
M/Ms
0.50
3.0 K 40
x (%)
3.5 K
35
0.25 4.0 K
4.5 K 30
5.0 K
0.00 25
0 20000 40000
3.0 3.5 4.0 4.5 5.0
H (Oe) T (K)

21. Magnetic deflagration in manganites
At low temperatures and under fast varying fields the
magnetization change occurs in a very short time.
 Accompanied by a
huge heat release 1.0 T=3K
from the sample. 0.5
0.0
M/MS
 Resistivity also
-0.5
abruptly changes
with the avalanche. -1.0
-4 -3 -2 -1 0 1 2 3 4
H (kOe)

22. Magnetic deflagration in manganites
Experimental setup
 Commercial MPMS
SQUID magnetometer
 Three pick-up coils detect
the magnetic flux
Sample
variation.

23. Magnetic deflagration in manganites
1.0
0.8 Sample
coil A
Vcoil / Vcoil,max
0.6
coil B
coil C
0.4
T = 3.5 K
0.2
0.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
t (ms)
 Evidence of propagation
 Deflagration begins at the center of the sample

24. Magnetic deflagration in manganites
30
 Velocity decreases
for high 20
v (m/s)
temperatures
10
3.0 3.5 4.0 4.5
55
H = 30 kOe
T (K)
50
45
At high temperatures the initial
40
concentration of ferromagnetic
x (%)
35
phase is bigger.
30
25
It is like burning again a partially
burned forest.
3.0 3.5 4.0 4.5 5.0
T (K)

25. Magnetic deflagration in manganites
 Field cooling process.
 Initial concentration of the FM phase
2.0 36
12
34
1.5
Ha (kOe)
32
M (emu)
1.0
36
30
H a (kOe)
32
0.5 28
28
26
0.0 0 201 40 60 0 8050 100
100
xa (%)
xa (%)
0 10 20 30 40 50
H (kOe)

26. Colossal and fast magnetoresistance
variation
AF-CO
8
(insulator) OC
750
6
500
T (K)
R (kΩ )
FM-CD
(metallic) 250
4
0
2
-1 0 1
t (s)
Initial FM-CD phase concentration smaller than 10%

27. Colossal and fast magnetoresistance
variation
Dependence on the initial state
Initial FM-CD phase concentration bigger than 10%
O.C.
6
1000
R (k Ω )
100
T (K)
4
10
1
2
-6 -4 -2 0 2
t (s)

28. Colossal and fast magnetoresistance
variation
High Temperature results
At high temperature, no magnetic avalanche occurs
But we still have some resistivity jumps
140 6
120
100 5
80
R (kΩ )
4
T (K)
60
40
3
20
0
2
0 10 20 30 40 50
t (s)

29. Resistivity avalanches
Percolation •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

30. Conclusions
Completely new experiments:
 SAW +HFEPR +spin dynamics
 Magnetic deflagration is observed in manganites.
 Resistivity avalanches are associated to
percolation of conducting paths (new ingredient).

31. References
 J. Tejada, E. M. Chudnovsky, J. M. Hernandez, R. Amigó, Appl. Phys. Lett. 84, 2373 (2004).
 A. Hernández-Mínguez, J. M. Hernandez, F. Macià, A. García-Santiago, J. Tejada, and P. V. Santos, Phys.
Rev. Lett. 95, 217205 (2005)
 J. M. Hernandez, P. V. Santos, F. Macià, A. García-Santiago, and J. Tejada, Appl. Phys. Lett. 88, 012503
(2006)
 A. Hernández-Mínguez, F. Macià, J. M. Hernandez, J. Tejada, L. H. He, and F. F. Wang, Europhys. Lett. 75,
811 (2006)
 W. Decelle, J. Vanacken, V. V. Moshchalkov, J. Tejada, J. M. Hernández, and F. Macià, Phys. Rev. Lett. 102,
027203 (2009)
 F. Macià, G. Abril, A. Hernández-Mínguez, J. M. Hernandez, J. Tejada, and F. Parisi , Phys. Rev. B 77,
77,
012403 (2008)
 F. Macià, J. Lawrence, S. Hill, J. M. Hernandez, J. Tejada, P. V. Santos, C. Lampropoulos, and G. Christou,
Phys. Rev. B 77, 020403 (2008)
 F. Macià, A. Hernández-Mínguez, G. Abril, J. M. Hernandez, A. García-Santiago, J. Tejada, F. Parisi, and P.
V. Santos, Phys. Rev. B 76, 174424 (2007)
76,
 F. Macià, G. Abril , N. Domingo, J. M. Hernandez, J. Tejada, and S. Hill, Europhys. Lett. 82 37005 (2008)