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

Why are manganites so interesting ? Colossal Magnetoresistance CMR Started with

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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/ Phase Diagram of La 1-x Ca x MnO 3 Complexity in Manganites:

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)

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

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

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

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

x = 0.4  La Pr 0.40 Ca 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

Correlation between magnetic and transport properties

Dynamics of the phase separated state Relaxation measurements

Thermal cycling

ZFC Relaxation Magnetic Viscosity S(T)

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

Linear from until Numerical simulation Solid line: numerical simulation

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

Abrupt field-induced transition at low temperatures Avalanche, 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 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

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

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