Charge-transfer-induced interfacial ferromagnetism in La_0.7Sr_0.3MnO₃/NdNiO₃

Charge-transfer-induced interfacial ferromagnetism and its impact on the exchange bias effect in La0.7Sr0.3MnO3/NdNiO3 correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni3+ cations which are naturally part of the NdNiO3 layer, Ni2+ ions are formed at the interface due to a charge-transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn4+ ions in the La0.7Sr0.3MnO3 layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line-shape changes of the x-ray absorption at the Ni L2,3 edges, the metal-insulator transition of the NdNiO3 layer is resolved in an element-specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of the NdNiO3 layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Néel temperature of the NdNiO3 layer. Different from La0.7Sr0.3MnO3/LaNiO3, the exchange bias observed in La0.7Sr0.3MnO3/NdNiO3 is due to the antiferromagnetism of NdNiO3 and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge-transfer mechanism.