Quenching of ferromagnetism in cobalt clusters embedded in copper

Magnetic x-ray circular dichroism has been used to measure the room-temperature magnetization of cobalt clusters embedded in a copper matrix. It is found that the magnetization depends on the average cluster size and concentration but, in all cases, is significantly less than that of a thin cobalt film even at magnetic fields of 4 T. Various mechanisms for this behaviour are discussed including the possibility that there are significant cluster - matrix and cluster - cluster exchange interactions. Anomalously large values of the ratio of the average orbital to spin moment, measured for some samples, may be a signature of antiferromagnetic exchange coupling. Antiferromagnetic coupling of clusters would necessitate a stronger interaction than can be predicted with RKKY theory and the possibility that this coupling is a `superexchange' mechanism is discussed.