Study of the onset of a long-range ferromagnetic order in an ensemble of small particles with giant magnetic moments

We have studied the unmixing alloy Cu64Mn9Al27 by X-rays and magnetic metho ds as well as small-angle neutron scattering. In the quenched state the all oy behaves as a typical spin glass. When the sample is further annealed, th e formation of small particles with the stoichiometric composition Cu2MnAl occurs. At high temperatures the alloy is a typical superparamagnet. After cooling the alloy undergoes the re-entrant phase transition paramagnetic-fe rromagnetiospin glass'. It was shown on the example of the investigated aho y in decomposed state that the appearance of long-range ferromagnetic order in a system of superparamagnetic particles dissolved in nonmagnetic metall ic matrix is due to the cooperative ordering of their magnetic moments. (C) 1999 Published by Elsevier Science B.V. All rights reserved.