Low-energy magnetic excitations in Co/CoO core/shell nanoparticles

• Published in: Physical review. B, Condensed matter and materials physics Condensed matter and materials physics, 2011-05, Vol.83 (17), Article 174414
• Main Authors: Feygenson, M., Teng, X., Inderhees, S. E., Yiu, Y., Du, W., Han, W., Wen, J., Xu, Z., Podlesnyak, A. A., Niedziela, J. L., Hagen, M., Qiu, Y., Brown, C. M., Zhang, L., Aronson, M. C.
• Format: Article
• Language: English
• Subjects: COBALT; COBALT OXIDES; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; EXCITATION; INELASTIC SCATTERING; MONOCRYSTALS; NANOSTRUCTURES; NEEL TEMPERATURE; NEUTRON DIFFRACTION; ORDER PARAMETERS; QUASI-ELASTIC SCATTERING; VECTORS
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.83.174414

We have used inelastic neutron scattering measurements to study the magnetic excitations of Co core/CoO shell nanoparticles for energies from 0 to 50 meV. Above the blocking temperature T{sub B}, broad quasielastic scattering is observed, corresponding to the reorientation of the Co core moments and to paramagnetic CoO scattering. Below T{sub B}, two nearly dispersionless inelastic peaks are found, whose energies increase with decreasing temperature as order parameters, controlled by the nanoparticle Neel temperature T{sub N} = 235 K, and saturating as T {yields} 0 at 2.7 and 6.7 meV, respectively. Similar excitations were observed in a powdered single crystal of CoO, indicating that both are intrinsic excitations of CoO, resulting from the exchange splitting of single-ion states for T-T{sub N}. Pronounced finite-size effects are observed for the scattering from the CoO nanoparticle shells, whose thicknesses range from 1.7 to 4.5 nm. These include an enhanced excitation linewidth, as well as a response that is not only spread over a much wider range of wave vectors, but is also significantly more intense in the nanoparticles than in bulk CoO.