In this work, we investigate magnetic responses in various Ag-SiO2-Ag nanosandwich structures at visible wavelengths. The two electric resonant modes corresponding to the in-phase (symmetric) and anti-phase (asymmetric) electric dipole on the top and the bottom nanopillars are observed by the finite difference time domain (FDTD) simulation. In the asymmetric resonant mode, the phases of electric fields oscillating in the top and bottom pillars have opposite directions, leading to a virtual current loop that induces the magnetic field reversal. The nanosandwich structure produces a large enhancement of the magnetic field as the thickness of SiO2 nanopillar is much smaller than wavelength. By increasing the diameter of nanopillars from 150 nm to 250 nm, the inverse magnetic response wavelength shifts from 532 nm to 690 nm. On account of the magnetic field reversal caused by the anti-phase electric dipole coupling, the real part of the equivalent permeability of the film is negative. Therefore, the wavelength range associated with the intensity of inverse magnetic response is tunable by varying the size of Ag-SiO2-Ag nanosandwich structure. The equivalent electromagnetic parameters of the Ag-SiO2-Ag nanosandwich thin film prepared by glancing angle deposition are derived from the transmission and the reflection coefficients measured by walk-off interferometers. The measured results indicate that film exhibit double negative properties and lead to negative values of the real parts of equivalent refractive indices -0.854, -1.179, and -1.492 for λ = 532 nm, 639 nm, and 690 nm, respectively. Furthermore, the real part of permeability is negatively enhanced to be -4.771 and the maximum value of figures of merit (FOM) recorded being 6.543 for p-polarized light at λ = 690 nm. Finally, we analyze the admittance loci for our nanosandwich thin film. This analysis can be applied to interpret extraordinary optical properties such as negative index of refraction from Ag-SiO2-Ag nanosandwich films.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.