Magnetic dynamics of small α-Fe2O3 and NiO particles studied by neutron scattering
We have studied the magnetic dynamics in nanocrystalline samples of α-Fe2O3 (hematite) and NiO by inelastic neutron scattering. By measuring around the structural and the antiferromagnetic reflections, we have probed uniform and staggered magnetic oscillations, respectively. In the hematite particles, we observed a clear double peak in the energy distribution of the antiferromagnetic signal, in addition to a quasi-elastic peak: We interpret the double peak to represent collective magnetic excitations. Broadening of the central quasi-elastic peak with increasing temperature is interpreted as a sign of superparamagnetic relaxation. Studies of the antiferromagnetic signal from NiO also show evidence of collective magnetic excitations, but with a higher energy of the precession states than for hematite. The inelastic signal at the structural reflection of NiO presents evidence for uniform magnetic oscillations very similar to the antiferromagnetic signal, as is expected for a simple antiferromag,nét. The hematite sample did not show any signs of uniform oscillations, although these have been predicted theoretically.
PACS75.50.Tt Fine-particle systems 76.60.Es Relaxation effects 78.30.Nx Neutron inelastic scattering
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- 1.see, e.g., J.L. Dormano, D. Ficrani (Eds.): Magnetic Properties of Fine Particles ( North-Holland, Amsterdam 1992 )Google Scholar
- 2.L. Neel: Ann. Geophys. 5, 99 (1949);Google Scholar
- M. Hennion et al.: Europhys. Lett. 25, 43 (1994)Google Scholar
- M.F. Hansen et al.: Phys. Rev, Lett. 79, 4910 (1997)Google Scholar
- 5.A.H. Merrish. Canted Antiferromagnetism: Hematite ( World Scientific, Singapore 1994 )Google Scholar
- 6.F. Bodker et al.: to appear in Phys. Rev. B (2000)Google Scholar
- 9.See, e.g., G.L. Squires: Introduction to the Theory of Thermal Neutron Scattering ( Cambridge University Press, Cambridge 1978 )Google Scholar
- K.N. Clausen et al.: Physica B 241–243, 50 (1998)Google Scholar
- 11.M.F. Hansen: Magnetic Properties of Systems of Magnetic Nanoparticles ( Ph.D. thesis, Technical University of Denmark, 1998 )Google Scholar