ENGINEERING OF SELF-ASSEMBLED MULTIFERROIC NANOSTRUCTURES IN PbTiO3-CoFe2O4 THIN FILMS

Abstract

Multiferroic materials which display a coexistence of ferroelectric and ferromagnetic properties attract considerable interest for their potential for novel device applications as well as for the interesting physics and materials science underlying their functional responses. In multiferroic composite, electromagnetic coupling facilitate elastic interaction between ferroelectric and ferromagnetic components via piezoeffect and magnetostriction. The major goal of our research is designing the transverse epitaxial multiferroic nanostructures with controlled morphologies. The PbTiO3-CoFe2O4 system was selected for this study because of the (i) large spontaneous strain associated with the ferroelectric phase transition in PbTiO3 (6.5%), which should create strong elastic interactions between the two phases accompanying the piezoelectric effect, and (ii) large magnetostriction of ferrimagnetic CoFe2O4. We successfully fabricate self-assembling multiferroic nanostucture films of CoFe2O4-PbTiO3 by PLD deposition on SrTiO3 substrates of different orientations. X-ray and TEM characterization show that all films have columnar architecture and 3D epitaxial relationships between phases and each phase and substrates. The morphology of nanostructures has been controlled by changing orientation of a substrate. It has been shown that it is possible to obtain the ferromagnetic (CoFe2O4) rods with a diameter about 10-20 nm in the ferroelectric PbTiO3 matrix in (001) films of composition 0.67PbTiO3-0.33CoFe2O4, and vise versa: ferroelectric rods in ferrimagnetic matrix in (111) films of composition 0.33PbTiO3-0.67CoFe2O4. The lamellate morphology with a specific crystallographic orientation of lamellae corresponding to {111} planes has been obtained in (110) films. The measurements of lattice parameters of the constitutive phases at different temperature allows us to determine the level of internal stresses due to misfit between phases. The measurements of piezo- and magnetic responses of the films prove that the films are ferroelectric and ferromagnetic simultaneously. The piezo- and magnetic responses are considerable suppressed due to mutual constraints between phases. This suppression indicates the strong elastic interactions between the phases which allows us to suggest the strong electro-magnetic coupling in the films. Combining theoretical and experimental studies of self-assembled multiferroic nanostructures in epitaxial films has revealed that the elastic interactions caused by epitaxial stresses play the dominate role in defining the morphology of the nanostructures and their magnetic and electric responses.