Magnetic properties of the metastable phases formed in evaporated Co/Nb multilayers

A series of Co/Nb multilayers with a fixed Nb layer thickness of 7.2 nm and a constant Co layer thickness ranging from 1.2 to 7.2 nm, respectively, are prepared by alternate vapor deposition in high vacuum. The magnetic properties and microstructures of the films and their dependencies on the ferromagnetic layer thickness are investigated. It is found that the average magnetic moment of Co atoms decreases with decreasing Co layer thickness, and an abrupt decrease of the moment for samples with thinner Co layer attributes to the formation of magnetically "dead" layer, i.e. a metastable solid solution, Co7Nb6 phase formed at Co/Nb interfaces. The large number of nearest Nb neighbors per Co atom may account for the nonmagnetic behavior of the Co7Nb6 phase, while the formation of the other metastable phase Co5Nb4 and stable HCP Co phase corresponds to the increasing magnetization for samples with thicker Co layer. (C) 2003 Elsevier B.V. All rights reserved.