Crystal Growth and Physical Property Characterization of Complex Perovskite Oxides

Bulk EuTiO₃ [europium titanate], a quantum paraelectric antiferromagnet, is shown to exhibit multiferroic behavior in strained thin film form, which highlights the spin-phonon coupling in this system. We have investigated the structural, elastic, magnetic, thermal and transport properties of single crystals of EuTiO₃ as well as doped system EuTi_1-xB_xO₃ (B = Zr, Nb) [Zr and Nb doped europium titanate] utilizing various experimental techniques and theoretical calculations.

The cubic to tetragonal structural transition in pure EuTiO₃ is characterized by a pronounced step-like softening of the elastic moduli near 288 K [kelvin], which resembles that of SrTiO₃ [strontium titanate] and can be adequately modeled using the Landau free energy model employing the same coupling between strain and octahedral tilting order parameter as previously used to model SrTiO₃. Zr doping pushes the antiferromagnetic transition to lower temperatures while maintaining the insulating behavior. Intriguingly, Nb doping shifts the structural phase transition in EuTiO₃ to higher temperatures, accompanied by a weakening of octahedral tilting. Furthermore, Nb doping destabilizes the antiferromagnetic ground state of the parent compound and long range ferromagnetic order is observed in the samples containing more than 5% Nb, which is most likely accounted for in terms of the ferromagnetic interaction between localized Eu 4f spins mediated by itinerant electrons introduced by Nb doping.

We have also investigated the magnetoelastic coupling effect in 5d Re-based double perovskites Ba₂FeReO₆ [barium two iron rhenium oxygen six] and Ca₂FeReO₆ [calcium two iron rhenium oxygen six]. A large softening in C₄₄ [shear modulus] over a wide temperature range is observed for Ba₂FeReO₆ below its Curie temperature, which is indicative of a structural distortion. For Ca₂FeReO₆, both the longitudinal and shear modulus show a softening starting at 160 K. In addition, magnetoelastic coupling constants have been estimated from elastic constants and magnetostriction data, which provide direct evidence of pronounced coupling between magnetism and the lattice degrees of freedom present in both compounds.