Spinel ferrites: old materials bring new opportunities for spintronics

TL;DR

This paper demonstrates that ultrathin epitaxial NiFe2O4 films exhibit enhanced magnetic and resistive properties, enabling their use in advanced spintronic devices like magnetic tunnel junctions and spin-filters, thus opening new avenues for monolithic spintronics architectures.

Contribution

It introduces the stabilization of novel ferrite phases in ultrathin NiFe2O4 films with properties different from bulk, and their application in spintronics devices.

Findings

NiFe2O4 films have twice the saturation magnetization of bulk.

Resistivity of NiFe2O4 can be tuned by growth conditions.

NiFe2O4 can serve as a conductive electrode or spin-filter in tunnel junctions.

Abstract

Over the past few years, intensive studies of ultrathin epitaxial films of perovskite oxides have often revealed exciting properties like giant magnetoresistive tunnelling and electric field effects. Spinel oxides appear as even more versatile due to their more complex structure and the resulting many degrees of freedom. Here we show that the epitaxial growth of nanometric NiFe2O4 films onto perovskite substrates allows the stabilization of novel ferrite phases with properties dramatically differing from bulk ones. Indeed, NiFe2O4 films few nanometres thick have a saturation magnetization at least twice that of the bulk compound and their resistivity can be tuned by orders of magnitude, depending on the growth conditions. By integrating such thin NiFe2O4 layers into spin-dependent tunnelling heterostructures, we demonstrate that this versatile material can be useful for spintronics, either as a conductive electrode in magnetic tunnel junctions or as a spin-filtering insulating barrier in the little explored type of tunnel junction called spin-filter. Our findings are thus opening the way for the realisation of monolithic spintronics architectures integrating several layers of a single material, where the layers are functionalised in a controlled manner.