  We present a detailed analytical theory for the plasmonic nanoring
configuration first proposed in [A. Alu, A. Salandrino, N. Engheta, Opt. Expr.
14, 1557 (2006)], which is shown to provide negative magnetic permeability and
negative index of refraction at infrared and optical frequencies. We show
analytically how the nanoring configuration may provide superior performance
when compared to some other solutions for optical negative index materials,
offering a more 'pure' magnetic response at these high frequencies, which is
necessary for lowering the effects of radiation losses and absorption.
Sensitivity to losses and the bandwidth of operation of this magnetic inclusion
are also investigated in details and compared with other available setups.
