Dynamic Phase Separation in Manganites
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Transcript of Dynamic Phase Separation in Manganites
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Dynamic Phase Separation in Manganites
Luis Ghivelder
IF/UFRJ – Rio de Janeiro
Main collaborator:
Francisco ParisiCNEA – Buenos Aires
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Where was this research carried out ?
Low Temperatures Laboratory, Physics InstituteFederal University of Rio de Janeiro
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Extraction Magnetometer - 9 TPPMS
VSM – 14 T SQUID - 6 T Cryogenics
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Why are manganites so interesting ?
Colossal Magnetoresistanc
e
CMR
Started with
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1140 citations !
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FM
CO
AF CAF
FI
CO
CAF
Ca x
Tem
pera
ture
(K
)
x = 1/8
3/84/8
5/8
7/8
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00
Phase Diagram of La1-xCaxMnO3
Complexity in Manganites:
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Main ingredient for understanding the Manganites
Ferromagnetic metallic
t2g
eg
Mn4+Mn3+
Antiferromagnetic Charge ordered
insulating
competition between
and
Micrometer or Nanometer scale
Phase Separation (PS)
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Qualitative (naïve) picture
AFM-COinsulating
FMmetallic
H = 0H
CMRPhase
Separation
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Pr doped manganites: Pr1-xCaxMnO3
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La5/8-xPrxCa3/8MnO3 Prototype compound
for studying Phase Separation in manganites
FM
CO
AF CAFCAF
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La5/8-xPrxCa3/8MnO3
0 50 100 150 200 250 300
0.1
1
TN
(AFM)
TCO
(CO)
TC
(FM)
x = 0.6Pr rich
x = 0.1La rich
H = 1 T
M
(B
/ Mn)
T(K)
0 50 100 150 200 250 300
0.1
1
FCW
FCC PhaseSeparation
0.6
x = 0.4
x = 0.3
x = 0.1
H = 1 T
M
(B
/ M
n)
T(K)
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x = 0.4 La0.225Pr0.40Ca0.375MnO3
0 50 100 150 200 250 300
0.1
1 FCW
FCC
H = 1 T
M(
B /
Mn
)
T(K)PM
COAFM-CO
FM
FCC curve mostly FM at low temperatures
0 50 100 150 200 250 300
0.1
1
ZFC
FCW
FCC
H = 1 T
M(
B /
Mn
)
T(K)
ZFC curve metastable frozen state at low temperatures
Magnetic Glass
TCO
TN
TCTB
TC
Blocking temperature
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Correlation between magnetic and transport properties
0 50 100 150
100
101
102
103
104
0.0
0.4
0.8
1.2
(.
cm)
T (K)
H = 1 T
ZFC FCC FCW
M ( B
/Mn)
0 50 100 150
100
101
102
103
104
0.0
0.4
0.8
1.2
(.
cm)
T (K)
H = 1 T
virgin magnetization
ZFC FCC FCW
M ( B
/Mn)
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Dynamics of the phase separated
state
Relaxation measurements
0 50 100 150 200 2500.0
0.3
0.6
0.9
M
( B
/ M
n)
H = 1 T, FCC
T (K)
0 50 100 150 200 2500.0
0.3
0.6
0.9
M
( B
/ M
n)
H = 1 T, FCC
T (K)
0 50 100 150 200 2500.0
0.3
0.6
0.92 hours
M
( B
/ M
n)
H = 1 T, FCC
T (K)
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Thermal cycling
0 20 40 60 800.0
0.3
0.6
0.9
H = 1 T, ZFC
M ( B
/Mn)
T (K)
0 20 40 60 800.0
0.3
0.6
0.9
H = 1 T, ZFC
M ( B
/Mn)
T (K)
0 20 40 60 800.0
0.3
0.6
0.9
H = 1 T, ZFC
M ( B
/Mn)
T (K)
0 20 40 60 800.0
0.3
0.6
0.9
H = 1 T, ZFC
M ( B
/Mn)
T (K)
0 20 40 60 800.0
0.3
0.6
0.9
H = 1 T, ZFC
M ( B
/Mn)
T (K)
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ZFC Relaxation
0 2000 4000 6000 8000
1.0
1.2
1.4
1.6
1.8
10 K
M/M
(0)
t (sec)
0 20 40 60 80 1000.0
0.3
0.6
0.9
1.2
ZFC, H = 1 T
M(
B)
T (K)0 20 40 60 80 100
0.0
0.3
0.6
0.9
1.2
ZFC, H = 1 T
M(
B)
T (K)0 20 40 60 80 100
0.0
0.3
0.6
0.9
1.2
ZFC, H = 1 T
M(
B)
T (K)
20 K
50 K
0 20 40 60 80 1000.0
0.3
0.6
0.9
1.2
ZFC, H = 1 T
M(
B)
T (K)0 20 40 60 80 100
0.0
0.3
0.6
0.9
1.2
ZFC, H = 1 T
M(
B)
T (K)
80 K
Magnetic Viscosity S(T)
)()1/ln()(),( 00 TMttTStTM
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Phenomenological model
Hierarchical dynamic evolution
most probable event happens before the lesser probable
one
Collective behavior evolution is described in terms of a single variable
Time evolution through a hierarchy of energy barriers, which separates the
coexisting phases
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Conventional activated dynamic functional with state-dependent energy
barriers.
T
HxU
eq
eq evxx
xx
dtdx ),(
0||
)(
)(Tx Normalized FM fraction
Proportional to the
Magnetization
EquilibriumFM fraction
Arrhenius-like activation
Diverging energy barriers
||
)(),(
xx
HUHxU
eq ),( HTxeq
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dtevtxdttx T
THxU
][)()(),,(
0
)(Txeq Linear from 0)80( Kxeq 1)20( Kxequnti
l
Numerical simulation
Solid line: numerical simulation
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Melting of the AFM-CO state
Metamagnetictransition
Alignment of the small FM fraction
Homogeneous and
irreversible FM state
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Abrupt field-induced transition at low temperaturesAvalanche, Jumps,
Steps
At very low temperatures
T = 2.5 K
Ultrasharp metamagnetic
transition
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Temperature variation of the magnetization jumps
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Magnetization jumps Relaxation
enlarged view
H = 23.6 kOe
H = 23.8 kOe
H = 24.0 kOe
H = 23.6 kOe
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Spontaneous metamagnetic transition
H = 23.6 kOe
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Open Questions
What causes these magnetization jumps ?
Why it only happens at very low temperatures ?
Martensitic scenario vs.
Thermodynamical effect
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Magnetocaloric effectHuge sample temperature rise at the magnetization
jump
heat generated when the non-FM fraction of the material is converted to the FM phase
k
0 20 40 60 80 1000.0
0.3
0.6
0.9
1.2
ZFC, H = 1 T
M(
B)
T (K)
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La5/8-xNdxCa3/8MnO3 , x = 0.5
0 20 40 60 80
0
2
4
T = 2.5 K
M ( B
/Mn)
H (kOe)
0
3
6
9
12
15
18
21
Tsa
mpl
e (K
)
T = 2.5 K
0 20 40 60 80
0
2
4
M ( B
/Mn)
H (kOe)
T = 6 K
6.0
6.5
7.0
7.5
8.0
Tsa
mpl
e (K
)
Nd based manganite
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Microscopic mechanisms promote locally a FM volume increase, which yield a local
temperature rise, and trigger the avalanche process.
Our model
The entity which is propagated is heat, not magnetic domain walls, so the roles of grain boundaries or strains
which exist between the coexisting phases are less relevant
PS and frozen metastable states are essential ingredients for the magnetization jumps
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Constructing a ZFC phase diagram
M vs. T
M vs. H
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H-T phase diagram
FMhomogeneous
AFM-COPS
dynamic
PS
frozen
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x = 0.3 La0.325Pr0.30Ca0.375MnO3
Zero field resistivity, after applying and removing Hdc
A different compound, with PS at intermediate temperatures
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Magnetic field tuned
equilibrium FM fraction
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Summary
Quenched disorder leads to the formation of inhomogeneous metastable states
ZFC process in phase separated manganites:
Dynamic nature of the phase separated state:
Equilibrium ground state is not reached in laboratory time
Large relaxation effects are observed in a certain temperature window
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References of our work
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