Exchange bias in Ba0.4Sr0.6TiO3/La0.7Sr0.3MnO3 heterostructures

This work relates to the integration of the two-layer stack of the proposed multiferroic structure onto silicon substrates. Ba1-xSrxTiO3 is an excellent material for room-temperature voltage-tunable dielectric applications due to its high (ε=6000) dielectric constant. In this study we choose a composition of Ba0.4Sr0.6TiO3 (BST), which is cubic and paraelectric at 300K, and transforms to a ferroelectric tetragonal phase upon cooling through the Curie temperature (TC) at 200K. The main focus of the present work is to study what happens when BST is placed in contact with a room temperature ferromagnetic layer such as La0.7Sr0.3MnO3 (LSMO). In this study, the magnetic properties of a BST (200nm)/LSMO (63nm) heterostructure was compared to that of a single LSMO layer (63nm). Both films were deposited onto MgO/TiN buffered Si (100) using pulsed laser deposition (PLD) and a domain matching epitaxy (DME) paradigm. X-ray diffraction (XRD) measurements showed that these films were of single phase and epitaxial in nature, with an unrelaxed lattice strain of ∼0.2% that was predominately composed of thermal and defect-induced strain. The magnetic measurements showed that the Curie temperature (TC) of LSMO remained unchanged at 350K when the BST was in contact with the LSMO layer. Interestingly, at 4K both the coercive field (Hc) and the exchange bias (HEB) of the BST/LSMO heterostructure as compared to the lone LSMO film increased significantly from 400 to 800 Oe and from 155 to 305 Oe, respectively. These differences were found to disappear above 200 K, the ferroelectric TC of the BST over-layer. This strongly suggests that the observed changes in the magnetic behavior of the heterostructure was the result of stress and/or charge redistributions that resulted when the BST layer transformed from the cubic (paraelectric) to tetragonal (ferroelectric) phase at low temperature.