Interface-Induced Magnetization and Exchange Bias in LSMO/BFO Multiferroic Heterostructures

Interface-induced magnetization in BiFeO3 (BFO) at the La0.67Sr0.33MnO3 (LSMO)/BFO interface is an important factor in determining the magnetoelectric coupling, exchange bias, and other phenomena in LSMO/BFO heterostructures. Using spin-dependent polarized neutron reflectivity (PNR), we report experimental evidence for the existence of a ferromagnetic (FM) interfacial BFO layer of a relatively large thickness of similar to 8-15 unit cells (u.c.) at the LSMO/BFO interface. The interfacial FM BFO layer in different LSMO/BFO heterostructures is coupled both FM and antiferromagnetically (AFM), with the FM LSMO layer at the LSMO/BFO interface. We find an enhanced magnetization of 165 emu/cc (FM coupled) to -135 emu/cc (AFM coupled) for the interfacial BFO layer. We also observed an exchange bias in LSMO/BFO heterostructures, which was independently confirmed using PNR measurements by investigating the magnetization of the system under field cooling in a magnetic field of +/- 500 Oe. Our results further suggest that while interface roughness favors the formation of an interfacial ferromagnetic BFO layer with a higher thickness and order temperature, strain plays an important role in increasing the exchange bias blocking temperature in the LSMO/BFO heterostructure.