Quenching of ferromagnetism in cobalt clusters embedded in copper

Abstract

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.

Negative differential magnetization for Ni nanoparticles in Al

“…A similar result has been observed in other clusters, whatever the material, Ni or Co, and for different matrices such as AlN, ZrN, and Ag, 4,7,15 and was interpreted as being due to a magnetic dead layer of thickness t at the cluster surface. This thickness can be calculated from the diameter, D, given by XAS, the measured MMA, and the MMA for bulk, MMA bulk , through the formula…”

“…For free clusters, a magnetic moment per atom ͑MMA͒ larger than that of the bulk has been measured, 3 whereas for clusters of the same diameter embedded in matrices, a smaller MMA has been observed. 4 Very recently, negative differential magnetization reported in Fe layers deposited on W steps 5 and inverted hysteresis loops observed in a nanocrystalline/amorphous Fe 81 Nb 7 B 12 alloy 6 have been interpreted according to two-phase models and AF exchange interaction.…”

Spin-dependent transport in cluster-assembled nanostructures: influence of cluster size and matrix material

“…The matrix, on the other hand, does intervene indirectly in the MTGV experiments since the magnitude of the cluster magnetic moment strongly depends on the surrounding medium [15,16]. We believe the different degree of quenching of the cluster magnetic moment [17] to be responsible for the difference in cluster size showing a comparable MTGV for Co-doped samples of Cu or Ag matrices. Further experiments at lower temperature and higher fields are under way and are expected to show saturation also for small clusters.…”

Magnetism of Co nanocluster films

“…[10][11][12][13][14] The clusters land softly on the surface of a Si ͑100͒ substrate at room temperature simultaneously along with an atomic beam of Cu or SiO 2 from a magnetronsputtering gun. This results in the embedding of Co clusters in a Cu or SiO 2 matrix.…”

“…This results in the embedding of Co clusters in a Cu or SiO 2 matrix. 14,15 The cluster concentration is adjusted through the ratio of the cluster and atomic-beam deposition rates, as measured in situ with a quartz microbalance. The cluster-size distribution is monitored in situ by time-offlight measurements, and checked by transmission electron microscopy.…”