Engineering exchange bias at the interface of self-polarized ultrathin ferroelectric BaTiO3 and ferromagnetic La0.67Sr0.33MnO3

Abstract Atomic-scale interfaces of 3d transition-metal oxides are a fascinating landscape of atomic-scale interaction of spins, charge transfers, lattice distortions, and orbital reconstructions. Emergent physical phenomena originating at such interfaces have huge potential for next-generation nanoscale spintronic devices. Among them, exchange bias (EB) has been extensively studied owing to its importance for device applications such as high-density memory and magnetic sensor. Here, at the interface of epitaxially grown ferromagnetic La0.67Sr0.33MnO3 (LSMO) – ferroelectric BaTiO3 (BTO) bilayer thin films on SrTiO3 (STO) substrates, we demonstrate EB coupling (exchange bias shift, HE ~50 Oe at 2 K) without any conventional antiferromagnet. Such EB coupling is observed only for LSMO and BTO layers of <10 nm thickness. Importantly, we find that the EB coupling follows a conventional training effect, bias field dependency, and temperature dependency. By undertaking X-ray magnetic circular dichroism measurements of the epitaxial heterostructures, it is observed that Ti in the BTO exhibits magnetization at 2 K which can be reversibly switched between two distinct magnetization states on switching the magnetic field direction. High resolution electron microscopy and ab initio calculations show that the displacement of Ti4+ in the highly strained ultrathin BTO displaces the interfacial Mn4+ from its centrosymmetric position in LSMO, changing its d-orbital occupancy to favour an antiferromagnetic spin configuration. This in turn leads to an interfacial EB effect from ferroelectric polarization. This work represents an important step towards a new class of memory devices by controlled ferroic polarization and EB coupling in the same device.