Transcript Slide 1

Dynamic Phase Separation in Manganites
Luis Ghivelder
IF/UFRJ – Rio de Janeiro
Main collaborator:
Francisco Parisi
CNEA – Buenos Aires
Where was this research carried out ?
Low Temperatures Laboratory, Physics Institute
Federal University of Rio de Janeiro
Extraction Magnetometer - 9 T
PPMS
VSM – 14 T
SQUID - 6 T
Cryogenics
MR (%)
Resistivity (a.u.)
Why are manganites so interesting ?
La0.67Ca0.33MnO3
Started with
0.06
H=0
2T
0.04
0.02
9T
CMR
60
9T
40
2T
20
160
Colossal
Magnetoresistance
200
240
T (K)
280
1140 citations !
Complexity in Manganites:
Phase Diagram of La1-xCaxMnO3
5/8
3/8
Temperature (K)
4/8
x = 1/8
7/8
CO
FM
FI
CAF
AF
CO
0
0.0
0.1
0.2
0.3
0.4
0.5
Ca x
0.6
0.7
CAF
0.8
0.9
1.0
Main ingredient for understanding the Manganites
competition between
Ferromagnetic metallic
eg
t2g
Antiferromagnetic
Charge ordered insulating
and
Mn3+ Mn4+
Phase Separation (PS)
Micrometer or Nanometer scale
Qualitative (naïve) picture
FM
metallic
AFM-CO
insulating
H
H = 0
Phase Separation
CMR
Pr doped manganites: Pr1-xCaxMnO3
Prototype compound
for studying
Phase Separation in manganites
La5/8-xPrxCa3/8MnO3
CO
FM
CAF
AF
CAF
La5/8-xPrxCa3/8MnO3
H=1T
x = 0.1
x =La0.1
rich
x = 0.4
M(B / Mn)
B
1
0.1
Phase
T (FM)
Separation
C
FCC
T
x = 0.6
Pr rich
(CO)
x =CO0.3
T (AFM)
N
FCW
0.6
0
50
100
150
T(K)
200
250
300
x = 0.4  La0.225Pr0.40Ca0.375MnO3
FCC
curve
 mostly
FM atstate
low temperatures
ZFC curve
metastable
frozen
at low temperatures
H=1T
TC
TB
TCO
TC
M(B / Mn)
Blocking
temperature
FCW
1
TN
0.1
ZFC
FCC
FCC
0
50
FM
100
150
200
250
T(K)
AFM-CO
CO
Magnetic Glass
300
PM
Correlation between magnetic and transport properties
H=1T
M (B /Mn)
1.2
ZFC
FCC
FCW
0.8
virgin
magnetization
0.4
 (.cm)
0.0
10
4
10
3
10
2
10
1
10
0
0
50
100
T (K)
150
Dynamics of the
phase separated state
Relaxation
measurements
H = 1 T, FCC
M (B / Mn)
0.9
2 hours
0.6
0.3
0.0
0
50
100
150
T (K)
200
250
Thermal cycling
H = 1 T, ZFC
M (B /Mn)
0.9
0.6
0.3
0.0
0
20
40
T (K)
60
80
ZFC Relaxation
1.8
20 K
ZFC, H = 1 T
1.2
1.6
M/M(0)
0.6
1.4
50 K
1.2
10 K
1.0
80 K
0.3
0.0
0
20
40
60
80
0
100
T (K)
2000
4000
6000
8000
t (sec)
1.5
S (a.u.)
M(B)
0.9
M (T , t )  S (T ) ln(t / t0  1)  M 0 (T )
1.0
Magnetic Viscosity S(T)
0.5
0.0
0
20
40
60
T (K)
80
100
Phenomenological model
Collective behavior
Hierarchical dynamic
evolution
evolution is described
in terms of a single variable
most probable event happens
before the lesser probable one
Time evolution through a hierarchy of
energy barriers, which separates the
coexisting phases
Normalized
FM fraction
x (T )
Proportional to the
Magnetization
Conventional activated dynamic functional
with state-dependent energy barriers.
( xeq  x)
dx

v0e
dt | xeq  x |
xeq (T , H )
Equilibrium
FM fraction

U ( x, H )
T
Arrhenius-like
activation
Diverging energy
barriers
U (H )
U ( x, H ) 
| xeq  x |
Linear from
xeq (T )
xeq (80K )  0 until xeq (20K )  1
Numerical simulation x(t  dt)  x(t )  [v e
0
0.3
FM fraction x (T)
equilibrium
FM fraction
FCW
0.2
FCC
0.1
ZFC
0.0
0
30
60
T (K)
90

U ( x , H ,T )
T
Solid line:
numerical
simulation
]dt
Melting of the AFM-CO state
4
Homogeneous
and irreversible
FM state
T=6K
M(B / Mn)
3
Metamagnetic
transition
2
1
0
0
10
20
30
H (kOe)
40
50
60
Alignment of the
small FM fraction
Abrupt field-induced transition at low temperatures
Avalanche, Jumps, Steps
M (B/Mn)
4
At very low
temperatures
3
2
1
T = 2.5 K
0
C (mJ/molK)
23
Ultrasharp
metamagnetic
transition
22
21
20
0
10
20
30
H (kOe)
40
50
Temperature variation of the magnetization jumps
M ( / Mn)
3
2
2K
3K
4K
5K
6K
1
20
25
H (kOe)
30
35
Magnetization jumps
Relaxation
T = 2.5 K
T = 2.5 K
2
3
1
20
0.44
T = 2.5 K
22
24
1
H = 23.6 kOe
enlarged view
2
H (kOe)
26
M (/Mn)
M (/Mn)
M (/Mn)
3
28
H = 24.0 kOe
0.40
30
H = 23.8 kOe
H = 23.6 kOe
2
4
0.36
t (hour)
6
6.4
6.8
t (hour)
7.2
Spontaneous metamagnetic transition
T = 2.5 K
M (/Mn)
3
H = 23.6 kOe
2
1
7.15
7.20
7.25
7.30
t (hour)
7.35
Open Questions
Why it only happens at very low temperatures ?
What causes these magnetization jumps ?
Martensitic scenario
vs.
Thermodynamical effect
Magnetocaloric effect
Huge sample temperature rise at the magnetization jump
ZFC, H = 1 T
1.2
25
0.9
T(K)
M(B)
20
0.6
0.3
15
0.0
0
20
10
40
60
80
100
T (K)
5
0
14
16
18
20
22
24
26
H (KOe)
k
heat generated when the non-FM fraction of the material
is converted to the FM phase
Nd based manganite La5/8-xNdxCa3/8MnO3 , x = 0.5
2.5 KK
TT== 2.5
21
4
12
2
9
8.0
T = 66 K
0
20
40
H (kOe)
60
7.5
3
80
0
7.0
2
6.5
6.0
0
0
20
40
H (kOe)
60
80
Tsample (K)
4
0
M (B/Mn)
M (B/Mn)
15
Tsample (K)
18
Our model
Microscopic mechanisms promote locally a FM
volume increase, which yield a local temperature
rise, and trigger the avalanche process.
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
Constructing a ZFC phase diagram
M vs. T
M (B/Mn)
3
2
1
0
0
30
60
90
120
8 kOe
10
11
12
13
14
15
16
17
183
19
20
22
2
24
26
28
150
1
M vs. H
2.5 K
8K
15 K
40 K
60 K
70 K
90 K
106 K
140 K
M (B/Mn)
4
T (K)
0
0
20
40
H (kOe)
60
H-T phase diagram
FM
homogeneous
PS
PS
dynamic
AFM-CO
A different compound, with PS at intermediate temperatures
x = 0.3  La0.325Pr0.30Ca0.375MnO3
H=0
0.8
0.6
5
0.4
Hth
0.2
M (B/Mn)
0.0
H = 60 Oe
0.1
0.01
SR [d(ln)/dt]
0
H = 4 kOe
1.002
-5
R/R(0)
 (.cm)
Zero field resistivity,
after applying and removing Hdc
-10
-15
1E-3
0
50
100
150
T (K)
200
250
H = 2 kOe
0.998
H = 0.5 kOe
H=0
0.996
0
T = 110 K
300
-20
H = 3 kOe
1.000
0
2
4
2
4
6
H (kOe)
6
t (min)
8
8
10
10
FM fraction
x
1.0
0.8
0.6
Magnetic field
tuned equilibrium
FM fraction
0.4
0.2
equilibrium FM fraction
measured FM fraction
0.0
SM [d(lnM)/dt]
0.15
0.10
0.05
0.00
20
40
60
T (K)
80
100
120
Summary
ZFC process in phase separated manganites:
Quenched disorder leads to the formation of
inhomogeneous metastable states
Dynamic nature of the phase separated state:
Large relaxation effects are observed in a
certain temperature window
Equilibrium ground state is not reached in
laboratory time
References of our work