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 Cu 64Mn 9Al 27 by X-rays and magnetic methods as well as small-angle neutron scattering. In the quenched state the alloy behaves as a typical spin glass. When the sample is further annealed, the formation of small particles with the stoichiometric composition Cu 2MnAl occurs. At high temperatures the alloy is a typical superparamagnet. After cooling the alloy undergoes the re-entrant phase transition `paramagnetic–ferromagnetic-spin glass'. It was shown on the example of the investigated alloy in decomposed state that the appearance of long-range ferromagnetic order in a system of superparamagnetic particles dissolved in nonmagnetic metallic matrix is due to the cooperative ordering of their magnetic moments.