Field-dependent Vortex Pinning Strength in a Periodic Array of Antidots

We explore the dynamic response of vortex lines in a Pb thin film with a periodic array of antidots by means of ac-susceptibility measurements. For low drive field amplitudes, within the Campbell regime, vortex motion is of intra-valley type and the penetration depth is related to the curvature of the pinning potential well, $\alpha$. For dc-fields below the first matching field $H_1$, $\alpha$ reaches its highest value associated with a Mott Insulator-like phase where vortex lines are strongly localized at the pinning sites. For $H_1<H_{dc}<H_2$, the response is mainly due to the interstitial vortices and $\alpha$ drops to smaller values as expected for this metallic-like regime. Strikingly, for $H_2<H_{dc}<H_3$, we observe that $\alpha$ reduces further down. However, for $H_3<H_{dc}<H_4$, a reentrance in the pinning strength is observed, due to a specific configuration of the flux line lattice which strongly restricts the mobility of vortices. We present a possible explanation for the measured $\alpha(H_{dc})$ dependence based on the different flux line lattice configurations.