  Composite wires with a three-layered structure are known to show a
particularly large magnetoimpedance effect. The wires consist of a highly
conductive core, an insulating layer and an outer ferromagnetic shell. In order
to understand the origin of the effect a theory based on a coupling of the
Maxwell equations to the Landau-Lifschitz-Gilbert equation is suggested. The
theory is phenomenological in the sense that it does not account for a domain
structure. However, theoretical results nicely reproduce those obtained in
various measurements. Furthermore, an upper limit of the magnetoimpedance ratio
for a given combination of materials can be determined.
