  In this paper we discuss effective material parameter description of new
nanostructures designed to perform as artificial magnetic materials for vis-
ible light. Among these structures there are various split-ring resonators,
dual-bar structures, fishnet layers and other geometries. Artificial magnetic
response in these structures appears due to weak spatial dispersion effects,
and it is important to study the conditions under which the magnetic re- sponse
can be adequately measured with effective permeability tensor. On the examples
of dual bars and split rings we show that this is possible only under some
quite restrictive conditions. In the general case, more compli- cated
constitutive relations with more effective material parameters need to be
developed.
