Large magnetic exchange anisotropy at a heterointerface composed of nanostructured BiFeO3 and NiO.

Magnetic interfaces created by the formation of a nanocomposite of ferromagnetic BiFeO₃ nanoparticles of mean size 11 nm and antiferromagnetic network-like nanostructured NiO are studied. The effective radius of the ferromagnetic region is smaller than the actual nanoparticle size, thus confirming the involvement of a large fraction of surface spins in the interfacial magnetism. The exchange coupling between the BiFeO₃ nanoparticles and the network-like nanostructured NiO leads to an interfacial blocking phase having an average blocking temperature of about 80 K. The temperature dependent saturation magnetization follows the Bloch law at high (150–300 K) temperature and exponential behaviour in intermediate (25–150 K) and low (5–25 K) temperature regimes. Exchange bias appears below the irreversible temperature of about 300 K, well below the Néel temperature of NiO. The composition dependences of the exchange bias field (H_EB) and the coercivity (H_C) reveal maximum values of H_EB = 1550 Oe and H_C = 1730 Oe at 5 K and a cooling field of 50 kOe for a BiFeO₃ : NiO :: 50 : 50 concentration ratio. The exchange bias field decreases exponentially with increase of the temperature. Strong interfacial coupling due to uncompensated surface spins leads to a remarkable enhancement of the magnetic anisotropy at the BiFeO₃/NiO interface. [ABSTRACT FROM AUTHOR]