The influence of surface on magnetic relaxation has been investigated for a series of ferrihydrite particles (d approx 5 nm) prepared over a range of surface chemisorbed SiO_4~4-.As the SiO_4~4- content increases, the superparamagnetic transition temperature (T_B) decreases from 100 K to 40 K and the transition sharpens. It indicates that the SiO_4~4- species compensate the surface unpaired spins and create a nonmagnetic medium between the particles, thereby reducing the magnetic couplin between the surface iron atoms from neighboring particles.
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