Dielectric response to the spin state transition in LaCoO3-d Ceramics Engineering Materials Ceramics and Composites Laboratory Rainer Schmidt 8th July 2008

Dielectric response to the spin state transition in LaCoO3-d Ceramics Outline Dielectric response to the spin state transition in LaCoO3-d Ceramics Crystal Structure Electronic Configuration and Magnetic Properties Impedance Spectroscopy of LaCoO3-d Ceramics Magneto-Electric Coupling Conclusions

contain oxygen vacancies Crystal Structure LaCoO3-d : Tilted Perovskite Structure La3+:1.36 Å Shannon, Acta Cryst.A 32 (1976) p7518 O2-: 1.4 Å Co3+: 0.545 Å (LS) 0.61 Å (HS) Ideal Perovskite Tolerance factor: LaCoO3-d was claimed to contain oxygen vacancies Radaelli & Cheong, Phys.Rev.B 66 (2002) p094408

Tilted Perovskite Structure of LaCoO3-d Crystal Structure Tilted Perovskite Structure of LaCoO3-d Looking down the b axis Looking down the c axis c b a Rhombohedral Unit Cell b a Space group: R-3c (167) Glazier notation: a - a - a - Alexandrov notation: f f f Koehler & Wollan, J.Phys.Chem.Solids 2 (1957) p100 Goodenough, J.Phys.Chem.Solids 6 (1958) p287 Thornton et al. J.Solid State Chem. 61 (1986) p301

Co3+ : 3d6 configuration in an octahedral coordination Electronic Configuration and Magnetic Properties Co3+ : 3d6 configuration in an octahedral coordination Co3+ : 3d6 eg 3d Crystal field split - Hund’s coupling ~ 7 meV = 80 K t2g LS HS IS S = 0 S = 2 S = 1 Pauli’s rule: Two electrons can not occupy the same quantum state Hund’s rule: Two Electrons prefer to half-occupy two degenerate orbitals and paralell spin, rather than to fully occupy one orbital with anti-paralell spin

Intermediate Spin State Model Electronic Configuration and Magnetic Properties High Spin State Model Intermediate Spin State Model Heikes et al. Physica (Amsterdam) 30 (1964) p1600 Jonker, J.Appl.Phys. 37 (1966) p1424 Raccah & Goodenough, Phys.Rev. 155 (1967) p932 Korotin et al., Phys.Rev.B 54 (1996) p5309 Radaelli & Cheong, Phys.Rev.B 66 (2002) p094408 Louca & Sarrao, Phys.Rev.Lett. 91 (2003) p155501 Ishikawa et al. Phys.Rev.Lett. 93 (2004) p136401 Haverkort et al., Phys.Rev.Lett. 97 (2006) p176405 Podlesnyak et al., Phys.Rev.Lett. 97 (2006) p247208 Klie et al., Phys.Rev.Lett. 99 (2007) p047203

LaCoO3-d Paramagnetism "Curie-Tail" due to a magnetic defect Electronic Configuration and Magnetic Properties LaCoO3-d Paramagnetism "Curie-Tail" due to a magnetic defect structure called magnetic polarons or excitons 1/c Yamaguchi et al., Phys.Rev.B 53 (1996) R2926 Giblin et al. Europhys.Lett. 70 (2005) p677

Impedance Spectroscopy 2. Impedance Spectroscopy Rainer Schmidt Impedance Spectroscopy Application of an Alternating Voltage Signal to a Sample: Measurement of the Alternating Current Response: U(w,t )=U0 cos(w t ) I(w,t ) = I0 cos(w t +d ) Time Dependent Definition of the Impedance: Time Independent Impedance: U(w,t ) U0 cos(w t ) I(w,t ) I0 cos(w t + d ) Z(w,t ) (id ) Z* Complex Relationship

Capacitance vs Frequency Plots Equivalent Circuit Fitting of Impedance Spectra Capacitance vs Frequency Plots bad fit Two plateaus indicate GB and bulk relaxations

Equivalent Circuit Fitting of Impedance Spectra M '' & Z '' vs Frequency Plots

- Z '' vs Z ' Plots Low frequency Z’: Equivalent Circuit Fitting of Impedance Spectra - Z '' vs Z ' Plots Low frequency Z’:

Z '' / Z '' (max) vs Frequency Plots at Various Temperatures Equivalent Circuit Fitting of Impedance Spectra Z '' / Z '' (max) vs Frequency Plots at Various Temperatures

Strong Magneto-Electric Coupling of Magnetic Polarons No Magneto-Electric Coupling at the Spin-State Transition Ts Ts

Equivalent Circuit Fitting of Impedance Spectra C1, C2, R1, R2 vs Temperature Grain Boundary 2 Grain Boundary 1 Bulk

Conclusions No Clear Magneto-Electric Coupling at the Spin State Transition in LaCoO3-d Stronger Magneto-Electric Coupling with the Magnetic Polaron Defect Structure The GB Relaxation Splits at the Spin State Transition The Second GB Relaxation Shows Typical GB Capacitance and is not an Electrode Interface Effect

Acknowledgments University of Durham, Department of Physics Ian Terry, Sean Giblin University of Durham, Department of Physics Chris Leighton University of Minneapolis, Department of Chemical Engineering and Materials Science