Luis Ghivelder

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Luis Ghivelder
IF/UFRJ Rio de Janeiro
Main collaborator Francisco Parisi CNEA
Buenos Aires
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Where was this research carried out ?
Low Temperatures Laboratory, Physics
Institute Federal University of Rio de Janeiro
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Why are manganites so interesting ?
Started with
Colossal Magnetoresistance
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1140 citations !
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Complexity in Manganites
Phase Diagram of La1-xCaxMnO3
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Main ingredient for understanding the Manganites
competition between
Ferromagnetic metallic
Antiferromagnetic Charge ordered insulating
and
Phase Separation (PS)
Micrometer or Nanometer scale
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Qualitative (naïve) picture
Phase 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
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La5/8-xPrxCa3/8MnO3
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x 0.4 ? La0.225Pr0.40Ca0.375MnO3
FCC curve ? mostly FM at low temperatures
ZFC curve ? metastable frozen state at low
temperatures
Blocking temperature
TC
TCO
TN
Magnetic Glass
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Correlation between magnetic and transport
properties
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Dynamics of the phase separated state
Relaxation measurements
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Thermal cycling
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ZFC Relaxation
Magnetic Viscosity S(T)
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Phenomenological model
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.
Diverging energy barriers
Arrhenius-like activation
Equilibrium FM fraction
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Linear from
until
Numerical simulation
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Melting of the AFM-CO state
Homogeneous and irreversible FM state
Metamagnetic transition
Alignment of the small FM fraction
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Abrupt field-induced transition at low
temperatures
Avalanche, Jumps, Steps
At very low temperatures
Ultrasharp metamagnetic transition
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Temperature variation of the magnetization jumps
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Magnetization jumps Relaxation
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Spontaneous metamagnetic transition
H 23.6 kOe
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Open Questions
Why it only happens at very low temperatures ?
What causes these magnetization jumps ?
Martensitic scenario vs. Thermodynamical effect
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Magnetocaloric effect
Huge sample temperature rise at the magnetization
jump
heat generated when the non-FM fraction of the
material is converted to the FM phase
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Nd based manganite
La5/8-xNdxCa3/8MnO3 , x 0.5
T 2.5 K
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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
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Constructing a ZFC phase diagram
M vs. T
M vs. H
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H-T phase diagram
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A different compound, with PS at intermediate
temperatures
x 0.3 ? La0.325Pr0.30Ca0.375MnO3
Zero field resistivity, after applying and
removing Hdc
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Magnetic field tuned equilibrium FM fraction
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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
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References of our work
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