Dynamic Phase Separation in Manganites

Luis Ghivelder

IF/UFRJ – Rio de Janeiro

Main collaborator:

Francisco ParisiCNEA – Buenos Aires

Where was this research carried out ?

Low Temperatures Laboratory, Physics InstituteFederal University of Rio de Janeiro

Extraction Magnetometer - 9 TPPMS

VSM – 14 T SQUID - 6 T Cryogenics

Why are manganites so interesting ?

Colossal Magnetoresistanc

e

CMR

Started with

1140 citations !

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:

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)

Qualitative (naïve) picture

AFM-COinsulating

FMmetallic

H = 0H

CMRPhase

Separation

Pr doped manganites: Pr1-xCaxMnO3

La5/8-xPrxCa3/8MnO3 Prototype compound

for studying Phase Separation in manganites

FM

CO

AF CAFCAF

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)

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

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)

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)

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)

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

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

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

dtevtxdttx T

THxU

][)()(),,(

0

)(Txeq Linear from 0)80( Kxeq 1)20( Kxequnti

l

Numerical simulation

Solid line: numerical simulation

Melting of the AFM-CO state

Metamagnetictransition

Alignment of the small FM fraction

Homogeneous and

irreversible FM state

Abrupt field-induced transition at low temperaturesAvalanche, Jumps,

Steps

At very low temperatures

T = 2.5 K

Ultrasharp metamagnetic

transition

Temperature variation of the magnetization jumps

Magnetization jumps Relaxation

enlarged view

H = 23.6 kOe

H = 23.8 kOe

H = 24.0 kOe

H = 23.6 kOe

Spontaneous metamagnetic transition

H = 23.6 kOe

Open Questions

What causes these magnetization jumps ?

Why it only happens at very low temperatures ?

Martensitic scenario vs.

Thermodynamical effect

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)

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

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

Constructing a ZFC phase diagram

M vs. T

M vs. H

H-T phase diagram

FMhomogeneous

AFM-COPS

dynamic

PS

frozen

x = 0.3 La0.325Pr0.30Ca0.375MnO3

Zero field resistivity, after applying and removing Hdc

A different compound, with PS at intermediate temperatures

Magnetic field tuned

equilibrium FM fraction

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

References of our work