Thickness independence of true phase transition temperatures in barium strontium titanate films

Abstract

The functional properties of two types of barium strontium titanate ͑BST͒ thin film capacitor structures were studied: one set of structures was made using pulsed-laser deposition ͑PLD͒ and the other using chemical solution deposition. While initial observations on PLD films looking at the behavior of T m ͑the temperature at which the maximum dielectric constant was observed͒ and T c * ͑from Curie-Weiss analysis͒ suggested that the paraelectric-ferroelectric phase transition was progressively depressed in temperature as BST film thickness was reduced, further work suggested that this was not the case. Rather, it appears that the temperatures at which phase transitions occur in the thin films are independent of film thickness. Further, the fact that in many cases three transitions are observable, suggests that the sequence of symmetry transitions that occur in the thin films are the same as in bulk single crystals. This new observation could have implications for the validity of the theoretically produced thin film phase diagrams derived by Pertsev et al. ͓Phys. Rev. Lett. 80, 1988 ͑1998͔͒ and extended by Ban and Alpay ͓J. Appl. Phys. 91, 9288 ͑2002͔͒. In addition, the fact that T m measured for virgin films does not correlate well with the inherent phase transition behavior, suggests that the use of T m alone to infer information about the thermodynamics of thin film capacitor behavior, may not be sufficient.

Microstructure and dielectric relaxor properties for Ba0.5Sr0.5TiO3/La0.67Sr0.33MnO3 heterostructure

“…21 Moreover, the value of ␦ is calculated as 151 from the fitting curve, which is close to that of the Au/ Ba 0.5 Sr 0.5 TiO 3 /La 0.5 Sr 0.5 CoO 3 heterostructure. 22 The inset of Fig. 4 shows the Curie-Weiss plots ͓ ͑T͒ −1 against T͔ deduced from ͑E=0͒ vs T. By fitting the experimental data, the Curie-Weiss constant is estimated as 2.23ϫ 10 5 K, which agrees well with other values of BST films of the displacive type.…”

Thickness dependence of intrinsic dielectric response and apparent interfacial capacitance in ferroelectric thin films

“…The reduction of T m in thinner films was observed in SrRuO 3 /Ba 0.7 Sr 0.3 TiO 3 / SrRuO 3 , Pt/Ba 0.7 Sr 0.3 TiO 3 / Pt, and Au/ Ba 0.5 Sr 0.5 TiO 3 /La 0.5 Sr 0.5 CoO 3 thin-film capacitors. 11,15,23 At the same time, the dielectric-peak position T m was found to shift to higher temperatures with decreasing thickness in Pt/ Ba 0.7 Sr 0.3 TiO 3 / SrRuO 3 and Au/ Ba 0.5 Sr 0.5 TiO 3 / SrRuO 3 capacitors. 15,22 The competition between the strain and depolarizingfield effects also governs the thickness dependence of the film polarization and permittivity at t Ͼ t c .…”

“…Indeed, several groups reported on the observation of a thickness-dependent phase transition temperature. 11,15,22,23 Moreover, the intrinsic film permittivity is expected to vary with thickness at least for the following two reasons. First, the dielectric response of epitaxial films depends on the substrate-induced lattice strains, 24 which relax as the thickness increases due to the generation of misfit dislocations.…”

Ferroelectric capped magnetization in multiferroic PZT/LSMO tunnel junctions

“…PZT has a track record of good ferroelectricity down to 2.4 nm, and it is popular among researchers for studies based on ultra-thin films. 16 Among strongly correlated materials, strontium-doped lanthanum manganite (LSMO) has shown colossal magneto-resistance phenomena under application of small external magnetic fields. Exchange among the various valence states of Mn-cations leads to complex magnetization properties and transport behavior of LSMO, modulated by external hole/electron doping and electric/magnetic field application.…”