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Dynamic Phase Separation in Manganites 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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Extraction Magnetometer - 9 T PPMS VSM – 14 T SQUID - 6 T Cryogenics

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Why are manganites so interesting ? Colossal Magnetoresistance CMR Started with

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1140 citations !

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FM CO AF CAF FI CO CAF Ca x Temperature (K) x = 1/8 3/8 4/8 5/8 7/8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 Phase Diagram of La 1-x Ca x MnO 3 Complexity in Manganites:

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Main ingredient for understanding the Manganites Ferromagnetic metallic t 2g egeg Mn 4+ Mn 3+ Antiferromagnetic Charge ordered insulating competition between and Micrometer or Nanometer scale Phase Separation (PS)

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Qualitative (naïve) picture AFM-CO insulating FM metallic H = 0 H CMR Phase Separation

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Pr doped manganites: Pr 1-x Ca x MnO 3

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La 5/8-x Pr x Ca 3/8 MnO 3 Prototype compound for studying Phase Separation in manganites FM CO AF CAF

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La 5/8-x Pr x Ca 3/8 MnO 3

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x = 0.4 La 0.225 Pr 0.40 Ca 0.375 MnO 3 PM CO AFM-CO FM FCC curve mostly FM at low temperatures ZFC curve metastable frozen state at low temperatures Magnetic Glass T CO TNTN TCTC TBTB TCTC Blocking temperature

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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 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. Normalized FM fraction Proportional to the Magnetization Equilibrium FM fraction Arrhenius-like activation Diverging energy barriers

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Linear from until Numerical simulation Solid line: numerical simulation

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Melting of the AFM-CO state Metamagnetic transition Alignment of the small FM fraction Homogeneous and irreversible FM state

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Abrupt field-induced transition at low temperatures Avalanche, 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 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 k

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La 5/8-x Nd x Ca 3/8 MnO 3, x = 0.5 T = 2.5 K T = 6 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 FM homogeneous AFM-CO PS dynamic PS frozen

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x = 0.3 La 0.325 Pr 0.30 Ca 0.375 MnO 3 Zero field resistivity, after applying and removing H dc 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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