Dependence of the coercive field on the Cu overlayer thicknessi n thin Co/Cu(001) and Ni/Cu(001) fcc epitaxial films
The effect of nonmagnetic overlayers on magnetic thin films has been a topic of intense research due to the crucial role of the interface in determining the magnetic properties of the system, namely the magnetic anisotropy, coercivity or the critical behaviour. While these effects are most prominent in the low coverage regime, where surface effects often dominate, we have also observed large changes in structural (strain) and magnetic properties of thin Ni films with the Cu overlayer well beyond the thickness corresponding to the film coverage, and in particular a large variation in the Ni coercivity. Although not an intrinsic property of magnetic materials, the coercive field is an important parameter in that its value depends sensitively on most parameters that influence the magnetic properties of the system. Technologically it is an important parameter that must be tuned to suit particular applications. In thin epitaxial films, the main contribution to the coercive field comes from film imperfections, such as roughness and misfit dislocations.
Magneto-optic Kerr effect (MOKE) magnetometry was used to study the magnetic easy axis and coercive field Hc. PMA was observed for all the Ni/Cu(001) structures, while a large increase in the coercive field with increasing tCu was found (the absolute values changed slightly for independent sets of samples). The variation of Hc with the Cu overlayer thickness is presented in Fig. 1. The values shown correspond to the statistical average of a series of measurements (of the order of 10). This marked variation in coercivity clearly indicates that structural changes take place with increasing Cu capping layer thickness.
Figure 1: Variation of Hc with Cu overlayer thickness.
This is attributed to an increase in the magnetoelastic energy for the case of the perpendicularly magnetised Ni, whereas for the case of Co the Néel models for the coercive field fail to describe the observed variation. The experiment described here may prove to be a useful way of assessing the effect of strain on the magnetic anisotropy of thin epitaxial films.
C.A.F. Vaz, J.A.C. Bland, JAP 89 (2001) 7374 C.A.F. Vaz et al., JMMM 226-230 (2001) 1618