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MAGNETIC DEFLAGRATION MAGNETIC DEFLAGRATION AND DETONATION? AND DETONATION?

J. Tejada, F.Macià, J.M. Hernández, P.V.Santos,J. Tejada, F.Macià, J.M. Hernández, P.V.Santos, V. Moschalkov, J. Vanacken and W. Decelle V. Moschalkov, J. Vanacken and W. Decelle

ContentsContents

Magnetic avalanches: Nanomagnets and manganites Magnetic avalanches: Nanomagnets and manganites

(before 2005)(before 2005)

Quantum magnetic deflagration in nanomagnetsQuantum magnetic deflagration in nanomagnets

Quantum astroidQuantum astroid

Spin dynamics combining SAW and HFEPRSpin dynamics combining SAW and HFEPR

Deflagration to detonation transition in nanomagnetsDeflagration to detonation transition in nanomagnets

Magnetic deflagration in manganitesMagnetic deflagration in manganites

Colossal and fast magnetoresistance variationColossal and fast magnetoresistance variation

Magnetic avalanches: Nanomagnets Magnetic avalanches: Nanomagnets and manganites (before 2005)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)

0.0 0.5 1.0 1.5 2.0

-1.0

-0.5

0.0

0.5

1.0

4HR

2HR

3HR

M/M

s

µBH (T)

2.0 K 2.2 K 2.4 K 2.6 K

HR

Magnetic avalanches: Nanomagnets and Magnetic avalanches: Nanomagnets and manganites (before 2005)manganites (before 2005)

Accompanied by a huge Accompanied by a huge heat release from the heat release from the sample.sample.

Resistivity also abruptly Resistivity also abruptly changes with the changes with the avalanche. avalanche.

-4 -3 -2 -1 0 1 2 3 4

-1.0

-0.5

0.0

0.5

1.0

M/M

S

H (kOe)

T = 3 K

At low temperatures and under fast varying fields the magnetization change occurs in a very short time.

Quantum magnetic deflagration in Quantum magnetic deflagration in nanomagnetsnanomagnets

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.

The conventional theory of deflagration yields the following expression for the velocity of the

flame front:

−=

fBTkHU

v2

)(exp

0τκ

τκ

=v

Y. Suzuki Y. Suzuki et. alet. al. PRL . PRL 9595, 147201 (2005), 147201 (2005)

A. Hernández-Mínguez A. Hernández-Mínguez et. al.et. al. PRL 95 17205 (2005) PRL 95 17205 (2005)


• The speed of the avalanche

increases with the applied

magnetic field.

• At resonant fields the

velocity of the flame front

presents peaks.


−=

fB0 T2kU(H)

expτκ

v

The speed shows peaks at the magnetic fields at which spin levels become resonant.

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)PRL 95 17205 (2005)


( ) )sin()cos()cos(2

1 2 θθθ ShShSH xz −−−=


MPMS systemMPMS system Magnetic fields up to 5 TMagnetic fields up to 5 T Temperatures down to 1.8 KTemperatures down to 1.8 K

Saturate the sample

⇓

Sweep the magnetic f ield

⇓

Detect the temperature variations.

HHTherm.Therm.

Key parameters for deflagration threshold:

•Relaxation,

•Magnetic energy


Measured avalanches• Always occurring through superposition of states• There is a critical angle


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)


Hz

Hz

12/02/13

Spin dynamics combiningSpin dynamics combining SAW and HFEPR SAW and HFEPR

H -3T to 3 TT 2 KPulse time 1 ms to 100ms

SAW dissipation Sample perturb. Aval. ignit ion.

Optical detection

Frequency 150–350 GHz

f = 269 GHz


Metastable well


Stable well


Different Energy levels

Temperature dependence

(9 - 8)

Population in thermal equilibriumPopulation in thermal equilibrium

PRB (R) 77, 020403 2008PRB (R) 77, 020403 2008

Deflagration to detonation transition in Deflagration to detonation transition in nanomagnetsnanomagnets


• 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)]


0 50 100 150 200 250 3000.0

0.5

1.0

1.5

M (

emu)

T (K)

H = 10 kOe ZFC FCC FCW

La0.225Pr0.4Ca0.375MnO3

At higher fields phase

concentration slowly relax.


0 20000 400000.00

0.25

0.50

0.75

1.00

M/M

s

H (Oe)

3.0 K 3.5 K 4.0 K 4.5 K 5.0 K

3.0 3.5 4.0 4.5 5.0

25

30

35

40

45

50

55

x (%

)

T (K)

H = 30 kOe

Below 2 Tesla the FM-AF phase ratio is frozenNO RELAXATION


Accompanied by a Accompanied by a huge heat release huge heat release from the sample.from the sample.

Resistivity also Resistivity also abruptly changes abruptly changes with the avalanche. with the avalanche.

-4 -3 -2 -1 0 1 2 3 4

-1.0

-0.5

0.0

0.5

1.0

M/M

S

H (kOe)

T = 3 K

At low temperatures and under fast varying fields the magnetization change occurs in a very short time.


Experimental setupExperimental setup

Commercial MPMS Commercial MPMS SQUID magnetometerSQUID magnetometer

Three pick-up coils detect Three pick-up coils detect the magnetic flux the magnetic flux variation. variation. Sample

Evidence of propagationEvidence of propagation Deflagration begins at the center of the sampleDeflagration begins at the center of the 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 / V

coil,

max

t (ms)

coil A coil B coil C

T = 3.5 K


Sample


Velocity decreases Velocity decreases for high for high temperaturestemperatures

3.0 3.5 4.0 4.5

10

20

30

T (K)

v (m

/s)

3.0 3.5 4.0 4.5 5.0

25

30

35

40

45

50

55

x (%

)

T (K)

H = 30 kOe

At high temperatures the initial concentration of ferromagnetic phase is bigger.

It is like burning again a partially burned forest.


Field cooling process.Field cooling process. Initial concentration of the FM phaseInitial concentration of the FM 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 (%)

0 20 40 60 80 10026

28

30

32

34

36

Ha (

kOe

)

xa (%)

Colossal and fast magnetoresistance Colossal and fast magnetoresistance variationvariation

-1 0 1

0

250

500

750

OC

2

4

6

8

R (

kΩ)

t (s)

T (

K)

AF-CO(insulator)

FM-CD(metallic)

Initial FM-CD phase concentration smaller than 10%

Colossal and fast magnetoresistance Colossal and fast magnetoresistance variationvariation Dependence on the initial stateDependence on the initial state

-6 -4 -2 0 2

1

10

100

1000

O.C.

2

4

6

R (k

Ω)

t (s)

T (K

)

Initial FM-CD phase concentration bigger than 10%

Colossal and fast magnetoresistance Colossal and fast magnetoresistance variationvariation High Temperature resultsHigh Temperature results

0 10 20 30 40 50

0

20

40

60

80

100

120

140

2

3

4

5

6

R (

kΩ)

t (s)

T (

K)

At high temperature, no magnetic avalanche occursBut we still have some resistivity jumps

Resistivity avalanchesResistivity avalanchesPercolationPercolation

•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

ConclusionsConclusions

Completely new experiments:Completely new experiments: SAW +HFEPR +spin dynamicsSAW +HFEPR +spin dynamics Magnetic deflagration is observed in manganites.Magnetic deflagration is observed in manganites. Resistivity avalanches are associated to Resistivity avalanches are associated to

percolation of conducting paths (new ingredient).percolation of conducting paths (new ingredient).

ReferencesReferences 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) (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 , Phys. Rev. B 7777, , 012403 (2008) 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. F. Macià, A. Hernández-Mínguez, G. Abril, J. M. Hernandez, A. García-Santiago, J. Tejada, F. Parisi, and P. V. Santos, Phys. Rev. B V. Santos, Phys. Rev. B 7676, 174424 (2007) , 174424 (2007)

F. Macià, G. Abril , N. Domingo, J. M. Hernandez, J. Tejada, and S. Hill, F. Macià, G. Abril , N. Domingo, J. M. Hernandez, J. Tejada, and S. Hill, Europhys. Lett. 8282 37005 (2008) 37005 (2008)

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