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 intravalley 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 $H_1<H_{\mathrm{dc}}<H_2,$ the response is mainly due to the interstitial vortices and $\alpha$ drops to smaller values as expected for this metallic like regime. Strikingly, for $H_2<H_{\mathrm{dc}}<H_3,$ we observe that $\alpha$ reduces further down. However, for $H_3<H_{\mathrm{dc}}<H_4,$ 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 $\alpha {(H}_{\mathrm{dc}})$ dependence based on the different flux-line lattice configurations.