Irreversible magnetization in nickel nanoparticles

We report magnetic studies on nickel nanoparticle films of average particle size of 10 nm. Magnetization as a function of field and temperature show t hat the system behaves like a random magnet with a strongly field-dependent irreversible temperature, below which the magnetization relax logarithmica lly with time. The effective barrier extrapolated increases strongly with t emperature for a given field. The time dependence suggests the dominant dip ole-dipole interaction in this magnetic nanoparticle system. (C) 2001 Elsev ier Science B.V. All rights reserved.