Exchange biasing of single-domain Ni nanoparticles spontaneously grown in an antiferromagnetic MnO matrix

Exchange biased composites of ferromagnetic single-domain Ni nanoparticles embedded within and on the surface of large grains of MnO have been prepared by the reduction of NixMn3-xO4 monoliths in flowing 5% H-2/N-2. The Ni precipitates are 20-30 nm in extent, and the majority are completely encased within MnO. The manner in which the Ni nanoparticles are spontaneously formed-starting from a single homogeneous phase-ensures a high degree of dispersion of the Ni and large Ni/MnO interface/volume ratios. The interfaces between ferromagnetic Ni and antiferromagnetic MnO result in magnetic exchange bias below the Neel temperature of MnO. A threefold change in exchange bias fields (from 40 to 120 Oe at 5 K) is observed, depending on the starting Ni content x in the precursor NixMn3-xO4, with smaller x values - fewer or smaller Ni nanoparticles - resulting in larger exchange fields. The exchange bias leads to significant hardening of the magnetization, with coercive fields scaling nearly linearly with the exchange field. The strategies outlined here suggest new and simple routes for creating highly interfacial, functional biphasic composites of nanoparticles embedded within or supported on an oxide matrix.