Tunable synchronous sign reversal of ordinary and anomalous Hall effects in cluster-assembled nanomagnets

The sign reversal of Hall effects has attracted considerable research interest in both fundamental physics and device applications. Here, we utilize the low-energy condensation-type deposition technique to fabricate cluster-assembled NiCo nanomagnets to maximize surface effects, attempting to establish an optimal platform for exploring nontrivial magnetotransport. As the cluster size decreases, both the ordinary and anomalous Hall effects exhibit sign reversal behavior, especially completely reversed behavior over the entire temperature range below the characteristic size of 16.09 nm. Based on scaling laws and theoretical analysis, we demonstrate that the synchronous sign reversal of both Hall effects originates from the switching of dominance between bulk and surface effects. The size-dependent magnetoresistance and magnetic properties also change significantly at the characteristic size, further confirming the evolution of the two effects. This study provides an effective route to engineer Hall effects in nanomagnets, which is beneficial for improving the performance and functionality of spintronic devices.