Particles (a few nanometers in diameter) of some ferrites seem to have a magnetically anomalous surface layer about a nanometer thick. This layer also appears to be characterized by significant relaxation (expansion) arising from the different termination properties of the long range coulombic attractions and the short range repulsion between ions. This and the magnetoelasticity of the lattice may, at least in part, account for the magnetic anomaly.

Some papers on the subject:

• Nunes, A.C., Majkrzak, C.F. and Berkowitz, A.E., Polarized Neutron Study of the Magnetization Density Distribution Within a CoFe2O4 Colloidal Particle, Proceedings of the Third International Conference on Magnetic Fluids, Bangor, UK, (June 28-30, 1983). Jour. Magn. and Magn. Materials 35 (1983) 59-63.

• Nunes, A.C. and Yu, Z-Ch., Fractionation of a Water Based Ferrofluid, Jour. Magn. and Magn. Materials 65 (1987) 265-268.

• Briggs, E. and Nunes, A.C., X-Ray Powder Spectroscopy to Determine Easy Axis in Colloidal Magnetic Particles, J. Appl. Cryst. 20 (1987) 530-532.

• Nunes, A.C., Effect of Magnetic Polydispersion in Superparamagnetic Colloids on Neutron Scattering Line-Shapes, J. Appl. Cryst., 21 (1988) 129-135.

• Nunes, A.C., Hall, E.L. and Berkowitz, A.E., High Resolution Transmission Electron Microscopy and X-Ray Study of Colloidal Cobalt Ferrite Particles, J. Appl. Phys., 63 #10 (1988) 5181-5183.

• Nunes, A.C. and Yu, Z-Ch., Size and Quality Dispersion of a Single Preparation of Colloidal Magnetic Particles, Jour. Magn. Magn. Materials, 78 (1989) 241-246.

• I.I. Yaacob, A.C. Nunes, A. Bose, and D.O. Shah, Synthesis and Characterization of Magnetic Nanoparticles in Spontaneously Generated Vesicles, J. Colloid Int. Sci., 168 (1994) 289-301.

• D. Lin, A.C. Nunes, C.F. Majkrzak, and A.E. Berkowitz, Polarized Neutron Study of the Magnetization Density Distribution within a CoFe2O4 Colloidal Particle II, J. Magn. Magn. Materials, 145 (1995) 343-348.

• I.I. Yaacob, A.C. Nunes, and A. Bose, Magnetic Nanoparticles Produced in Spontaneous Cat-anionic Vesicles - Room Temperature synthesis and Characterization, J. Colloid Int. Sci., 171 (1995) 73-84.

• A.C. Nunes and D. Lin, Effects of Surface Relaxation on Powder Diffraction Patterns of Very fine Particles, J. Appl. Cryst. 28 (1995) 274-278.

• A.C. Nunes and L. Yang, Calculated Ferrite Nanocrystal Relaxation and its Magnetic Implications, Surface Science 399 (1998) 225-333.