Role of interfacial layer on exchange-coupled magnetic properties of bi-magnetic nanostructures: An experimental and theoretical approach

Abstract

Bi-magnetic hetero-nanostructures exhibiting significant exchange bias effects are highly desirable for technological advancement in spin valves, memory devices, data storage, biomedicine, and hyperthermia-based cancer treatment. As the interfacial spin interaction and the resulting anisotropy play a central role in these phenomena, investigating the tailored physical properties of hetero-structures of magnetically contrasting nanoparticles in different architectures is extremely useful. In this work, we report the formation of microspheres containing nano-aggregates of ferrimagnetic Fe3O4 and antiferromagnetic NiO phases. We demonstrate that this kind of bi-magnetic hetero-structures provide a conducive atmosphere for strong interfacial coupling giving rise to an extensive exchange bias effect (∼1.6 kOe). In addition, we also observe a substantial enhancement of the coercive field, indicating the onset of an additional unidirectional anisotropy under the field-cooled condition. Our first-principle density functional theory calculations reveal the role of cation intermixing at the interface of the two phases, being the source of an additional higher magneto-crystalline anisotropy than the original phases. This cation intermixing at the interface also causes a narrowed energy band, which was demonstrated by UV–Vis spectroscopy and first principle calculations. Having these technologically essential features with a tunable band gap, the embedded hetero-nanostructures are envisioned to cater to the requirement of high-density data storage and memory devices.