Charge transfer induced interfacial ferromagnetism in La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$/NdNiO${}_3$

Abstract

Charge transfer induced interfacial ferromagnetism and its impact on the exchange bias effect in La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$/NdNiO${}_3$ 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 Ni${}_3^{+}$ cations which are naturally part of the NdNiO${}_3$ layer, Ni${}_2^{+}$ 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 Mn${}_4^{+}$ ions in the La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ 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 L${}_{2,3}$ edges, the metal-insulator transition of the NdNiO${}_3$ 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 NdNiO${}_3$ layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Neel temperature of the NdNiO${}_3$ layer. Different from La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$/LaNiO${}_3$, the exchange bias observed in La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$/NdNiO${}_3$ is due to the antiferromagnetism of NdNiO${}_3$ 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.

8 pages, 5 figures