Non-monotonic magnetic friction from collective rotor dynamics.

Amontons' law postulates a monotonic relationship between frictional force and the normal load applied to a sliding contact. This empirical rule, however, fails in systems where internal degrees of freedom-such as structural or electronic order-play a central role. Here, we demonstrate that friction can emerge entirely from magnetically driven configurational dynamics. Using a two-dimensional array of rotatable magnetic moments sliding over a commensurate magnetic substrate, we observe a pronounced non-monotonic dependence of friction on the interlayer separation, and thus on the effective load. The friction peaks at an intermediate distance where competing ferromagnetic and antiferromagnetic interactions induce dynamical frustration and hysteretic torque cycles during sliding. Molecular dynamics simulations and a simplified two-sublattice model confirm that energy dissipation is governed by collective magnetic reorientations and their hysteresis. Our results establish scale-free sliding-induced changes in interfacial collective magnetic order, which has a strong impact on friction, and thus open new possibilities for contactless friction control, magnetic sensing and the design of reconfigurable, wear-free frictional interfaces and metamaterials.