Dynamic Phase Separation in Manganites

1. Dynamic Phase Separation in Manganites Luis Ghivelder IF/UFRJ – Rio de Janeiro Main collaborator: Francisco Parisi CNEA – Buenos Aires

2. Where was this research carried out ? Low Temperatures Laboratory, Physics Institute Federal University of Rio de Janeiro

3. Extraction Magnetometer - 9 T PPMS VSM – 14 T SQUID - 6 T Cryogenics

4. CMR Why are manganites so interesting ? Started with Colossal Magnetoresistance

5. 1140 citations !

6. 5/8 3/8 4/8 x = 1/8 7/8 CO Temperature (K) FM FI CAF AF CAF CO 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ca x Complexity in Manganites: Phase Diagram of La1-xCaxMnO3

7. eg t2g Mn4+ Mn3+ Main ingredient for understanding the Manganites competition between Ferromagnetic metallic Antiferromagnetic Charge ordered insulating and Phase Separation (PS) Micrometer or Nanometer scale

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

9. Pr doped manganites: Pr1-xCaxMnO3

10. La5/8-xPrxCa3/8MnO3 CO FM CAF AF CAF Prototype compound for studying Phase Separation in manganites

11. La5/8-xPrxCa3/8MnO3

12. TB TC PM FM AFM-CO CO 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

13. Correlation between magnetic and transport properties

14. Dynamics of the phase separated state Relaxation measurements

15. Thermal cycling

16. ZFC Relaxation Magnetic Viscosity S(T)

17. evolution is described in terms of a single variable Collective behavior Hierarchical dynamic evolution most probable event happens before the lesser probable one Phenomenological model Time evolution through a hierarchy of energy barriers, which separates the coexisting phases

18. Proportional to the Magnetization Normalized FM fraction Conventional activated dynamic functional with state-dependent energy barriers. Diverging energy barriers Arrhenius-likeactivation Equilibrium FM fraction

19. Solid line: numerical simulation until Linear from Numerical simulation

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

23. T = 2.5 K Abrupt field-induced transition at low temperatures Avalanche, Jumps, Steps At very low temperatures Ultrasharp metamagnetic transition

24. Temperature variation of the magnetization jumps

25. H = 23.6 kOe enlarged view H = 24.0 kOe H = 23.8 kOe H = 23.6 kOe Magnetization jumps Relaxation

26. Spontaneous metamagnetic transition H = 23.6 kOe

27. Open Questions Why it only happens at very low temperatures ? What causes these magnetization jumps ? Martensitic scenario vs. Thermodynamical effect

28. k 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

29. T = 6 K Nd based manganite La5/8-xNdxCa3/8MnO3, x = 0.5 T = 2.5 K

30. 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

31. Constructing a ZFC phase diagram M vs. T M vs. H

32. FM homogeneous PS AFM-CO PS frozen dynamic H-T phase diagram

33. A different compound, with PS at intermediate temperatures x = 0.3 La0.325Pr0.30Ca0.375MnO3 Zero field resistivity, after applying and removing Hdc

34. Magnetic field tuned equilibrium FM fraction

35. 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

36. References of our work