Journal
PHYSICAL REVIEW B
Volume 83, Issue 17, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.83.174414
Keywords
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Funding
- US Department of Energy, Office of Basic Energy Sciences [DE-AC02-98CH1886, DE-AC02-98CH10886]
- NSF [DMR-0454672]
- Scientific User Facilities Division, Department of Energy, Office of Basic Energy Sciences
- University of New Hampshire
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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-B, broad quasielastic scattering is observed, corresponding to the reorientation of the Co core moments and to paramagnetic CoO scattering. Below T-B, two nearly dispersionless inelastic peaks are found, whose energies increase with decreasing temperature as order parameters, controlled by the nanoparticle Neel temperature T-N = 235 K, and saturating as T -> 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 <= TN. 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.
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