Charge transfer-induced magnetic exchange bias and electron localization in (111)- and (001)-oriented LaNiO₃/LaMnO₃ superlattices

High-quality lattice-matched LaNiO₃/LaMnO₃ superlattices with monolayer terrace structure have been grown on both (111)- and (001)-oriented SrTiO₃ substrates by pulsed laser deposition. In contrast to the previously reported experiments, a magnetic exchange bias is observed that reproducibly occurs in both (111)- and (001)-oriented superlattices with the thin single layers of 5 and 7 unit cells, respectively. The exchange bias is theoretically explained by charge transfer-induced magnetic moments at Ni atoms. Furthermore, magnetization data at low temperature suggest two magnetic phases in the superlattices, with Néel temperature around 10 K. Electrical transport measurements reveal a metal-insulator transition with strong localization of electrons in the superlattices with the thin LaNiO₃ layers of 4 unit cells, in which the electrical transport is dominated by two-dimensional variable range hopping.