Magnetic Properties of Mixed-Metal Oxide Nanoparticles
Mixed-metal transition metal oxides (TMOs) present the opportunity to alter electronic and magnetic properties through choice of composition, and to design new systems with unique materials characteristics. TMO magnetic nanoparticles are of interest because their reduced coordination and size can produce systems with exceptional magnetic properties.
We combine these areas in a study of the magnetic properties of M'xM1-xO nanoparticles (M' = Zn2+, Cu2+; M = Ni2+, Mn2+, Co2+) in which M' is held as an octahedrally-coordinated “guest” in a MO rocksalt structure. For example, in ZnxNi1-xO (0 ≤ x ≤ 0.3), the guest zinc is formally M2+, its stable oxidation state in solid state oxides, but the octahedral site is unusual for the metal, which is typically found in tetrahedral coordination as, for example, in ZnO.
Preliminary results (see Fig.) indicate that zinc destroys the antiferromagnetic coupling without changing the Ni2+3d8 character. Thermal diffusion and soft (sol-gel) chemical synthetic methods are used to fabricate the nanoparticles, which are then characterized for compositional, morphological, electronic and magnetic properties. A range of techniques, including AES, SIMS, XRD, EM, XPS and SQUID magnetometry are used in the nanoparticle characterization.
Ni 2p XPS for ZnxNi1-xO with peak fitting of the 2p3/2transitions. Note that the 40% ZnO sample has phase separated. In the charge-transfer model of the electronic structure, peaks 1 and 2 result from the 2p53d9L final state and 3 is a composite of 2p53d8 and other final states. The relative intensity of peak 2 contains information on the coupling of Ni2+ ions with metals in next-nearest neighbor positions.