  We have studied the magnetic and power absorption properties of three samples
of CoFe2O4 nanoparticles with sizes from 5 to 12 nm prepared by thermal
decomposition of Fe (acac)3 and Co(acac)2 at high temperatures. The blocking
temperatures TB estimated from magnetization M(T) curves spanned the range 180
< TB < 320 K, reflecting the large magnetocrystalline anisotropy of these
nanoparticles. Accordingly, high coercive fields HC \approx 1.4 - 1.7 T were
observed at low temperatures. Specific Power Absorption (SPA) experiments
carried out in ac magnetic fields indicated that, besides particle volume, the
effective magnetic anisotropy is a key parameter determining the absorption
efficiency. SPA values as high as 98 W/g were obtained for nanoparticles with
average size of \approx12 nm.
