Magnetic and electronic properties of oxides heterostructures probed with x-ray spectroscopy.

Fundamental investigation of interfacial properties in complex oxide heterostructures is crucial to boost the development of modern electronics. Coupled degrees of freedom in transition metal oxides (TMOs) provide a rich playground to tailor precisely properties of artificial materials. Yet, entangling complex correlated behaviour is a very challenging task, although it is necessary for heterostructures application.
A variety of x-rays techniques allowed us to build a connection between magnetic prop- erties and the electronic and structural behaviour of TMO as a function of the proximity layer. We applied x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and x-ray linear dichroism (XLD) to probe the magnetic and orbital properties of Mn in ultra-thin La0.7Sr0.3MnO3 in proximity to SrRuO3 in comparison to the interface with SrTiO3. We have revealed the large impact of proximity and dimensionality, resulting in the magnetic dead layer in La0.7Sr0.3MnO3/SrRuO3 bilayers being drastically diminished. This restoration of magnetism in the ultra-thin La0.7Sr0.3MnO3 layer could be evoked by strong hybridization between La0.7Sr0.3MnO3 and SrRuO3, hindering the quantum confinement effect. The orbital rearrangement at the La0.7Sr0.3MnO3/SrRuO3 interface, its origin and consequences are discussed. In addition to the strong magnetic and electronic coupling to SrRuO3, the preservation of the mixed-valence and the remaining dx2−y2 occupation still allows for a strong double exchange coupling in the plane. Based on quantitative analysis and theoretical simulation of the x-ray spectra, the magnetic stability mechanisms in La0.7Sr0.3MnO3/SrRuO3 are discussed.
Further, we explored phenomena of emerged ferromagnetism in NdNiO3 using x- ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). When interfaced with La0.7Sr0.3MnO3 we found a net ferromagnetic moment on NdNiO3. Reduction of the layer averaged magnetic moment between 5uc and 10uc NdNiO3 on La0.7Sr0.3MnO3 demonstrated the interfacial nature of the effect. We have found a larger charge transfer between manganite and nickelate layers in compressively strained bilayers. In addition, compressive strain promotes ferromagnetism in the NdNiO3 layer due to superexchange between Mn4+-Ni2+ at the interface. On the other hand, films under small tensile strain were demonstrated to have almost no sizeable Mn4+-Ni2+ charge transfer.
Resonant x-ray reflectivity reveals a complex coupling with antiferromagnetic components on NdNiO3. It gradually decreases size of the magnetic moment causing still measurable net ferromagnetic moment on NdNiO3. It may originate from antiferromagnetic superexchange between Ni2+/3+ and Mn3+/4+ planes.
When the manganite layer is insulating (La0.88 Sr0.12 MnO3 ), NdNiO3 ferromagnetic ordering is almost quenched. Indeed, the exchange between two insulating layers is blocked suppressing the appearance of ferromagnetism in the NNO layer. At the interface with La0.88Sr0.12MnO3, Ni is in a weakly ferromagnetic or frustrated magnetic phase, in contrast to the Ni in NdNiO3/La0.7Sr0.3MnO3. By employing resonant x-ray reflectivity we have found different magnetic reconstructions at the NdNiO3/La1−xSrxMnO3 interface.
Summarizing our investigation discovers the role of the proximity layer in TMO heterostructure. By presenting novel experimental data we found complex magnetic behaviour when uniting two dissimilar TMO utilizing x-ray resonant absorption spectroscopy and resonant x-ray reflectivity. We have explored switching between non-magnetic, ferromagnetic and antiferromagnetic phases revealing fine control over the magnetism in bilayers.