Room temperature ferromagnetism and ferroelectricity in strained multiferroic BiFeO3 thin films on La0.7Sr0.3MnO3/SiO2/Si substrates

A novel ferromagnetic state coexisting with ferroelectric ordering at room temperature in strained BiFeO3 (BFO) thin films grown using a sputtering technique on La0.7Sr0.3MnO3/SiO2/Si(100) (LSMO/SOS) substrates is reported. The properties of BFO films with different thicknesses (tBFO = 15, 50, 70, 120, and 140 nm) on 40 nm LSMO layers are explored. [012] out-of-plane highly textured BFO/LSMO stacks grew with rhombohedral structures. LSMO layers are nanostructured in nature, constituted by ferromagnetic single-domain nanoregions induced by the constrain of the SiO2 surface, with TC ∼200 K and high coercive field (HC) of ∼1100 Oe at 2.5 K. BFO films grew epitaxially nanostructured on LSMO, exhibiting ∼4 nm spherical nanoregions. The BFO layers show typical antiferromagnetic behavior (in a greater volume fraction) when made thicker (tBFO > 70 nm). The thinner films (tBFO < 50 nm) display ferromagnetic behavior with TC > 400 K, HC ∼ 1600 Oe for 15 nm and ∼1830 Oe for 50 nm. We propose that such ferromagnetic behavior is originated by the establishment of a new magnetic configuration in the Fe3+OFe3+ sublattice of the BFO structure, induced by strong hybridization at the interface as consequence of superexchange coupling interactions with the ferromagnetic Mn3+OMn3+/Mn4+ sublattice of LSMO. All BFO layers show excellent ferroelectric and piezoelectric properties (coercive field ∼ 740 kV/cm, and d33 = 23 p.m./V for 50 nm; ∼200 kV/cm and 55 p.m./V for 140 nm), exhibiting 180° and 109° DWs structures depending on the thickness. Such multiferroic properties predict the potential realization of new magneto-electronic devices integrated with Si technology.