c-axis oriented epitaxial Ba0.25Sr0.75TiO3 films display Curie-Weiss behavior

Thin films of ferroelectrics have inferior dielectric properties, including microwave losses, compared to bulk material and generally do not display a proper Curie-Weiss behavior. This study shows that the film properties can be improved considerably, with a Curie-Weiss behavior. by choosing lattice matched electrodes and proper stoichiometry. A 700 nm thick Ba0.25Sr0.75TiO3 layer was inserted. by laser ablation, between two epitaxial metallic oxide (200 nm) SrRuO3 electrodes. Because of compressive stress in the plane of the substrate, the e-axis of the unit cell in the Ba0.25Sr0.75TiO3 layer was normal to the substrate plane. Grains were of the order of 100-200 nm (with small misorientation angles in a x b plane) as deter-mined by X-rays and AFM. The positions of pronounced maxima in the temperature dependence of the permittivity depended on external bias voltage applied between the SrRuO3 electrodes to the dielectric film. The measured epsilon(T) curves agreed well with existing theoretical models at temperatures below and above the ferroelectric phase transition point. At T approximate to 200 K epsilon/epsilon(0) for the Ba0.25Sr0.75TiO3 layer was suppressed up to 85% (from 4400 down to 560) when +/- 2.5 V bias voltage was applied to the metallic oxide electrodes. Well saturated polarization-vs.-voltage hysteresis loops were measured for the Ba0.25Sr0.75TiO3 layer in the temperature interval 4.2-200 K. Because of depolarization effects. the polarization of the Ba0.25Sr0.75TiO3 layer was suppressed at positive voltage applied between the electrodes, as compared with a negative one. (C) 2002 Elsevier Science B.V. All rights reserved.