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Register a new userLocal polarization and valence distribution in LaNiO3/LaMnO3 heterostructures
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The inside of the electrical double layer at perovskite oxide heterointerfaces is examined. Here, we report the local polarization and valence distribution in LaNiO$_3$/LaMnO$_3$ and LaMnO$_3$/LaNiO$_3$ bilayers on a SrTiO$_3$ (001) substrate. Simultaneous measurements of two aspects of the structure are realized by using Bayesian inference based on resonant- and nonresonant-surface X-ray diffraction data. The results show that the average Mn valences are Mn$^{3.12+}$ and Mn$^{3.19+}$ for the two samples. The intensity of their local electric field is $sim$1~GV/m and the direction of the field points from LaMnO$_3$ to LaNiO$_3$.
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(LaNiO3)n/(LaMnO3)2 superlattices were grown using ozone-assisted molecular beam epitaxy, where LaNiO3 is a paramagnetic metal and LaMnO3 is an antiferromagnetic insulator. The superlattices exhibit excellent crystallinity and interfacial roughness of less than 1 unit cell. X-ray spectroscopy and dichroism measurements indicate that electrons are transferred from the LaMnO3 to the LaNiO3, inducing magnetism in LaNiO3. Magnetotransport measurements reveal a transition from metallic to insulating behavior as the LaNiO3 layer thickness is reduced from 5 unit cells to 2 unit cells and suggest a modulated magnetic structure within LaNiO3.
We studied switchable photovoltaic and photo-diode characteristics of Pt (Bi0.9Sm0.1)(Fe0.97Hf0.03)O3 LaNiO3 (Pt BSFHO LNO) heterostructures integrated on Si (100). The directions of photocurrent (JSC) and rectification are found to be reversibly switchable after applying external poling voltages. In pristine state, metal-ferroelectric-metal capacitor Pt BSFHO LNO shows JSC 32 microAmp cm2 and VOC 0.04 V, which increase to maximum value of JSC 303 ( 206) microAmp cm2 and VOC 0.32 (0.26) V after upward (downward) poling at 8 V. We believe that Schottky barrier modulation by polarization flipping at Pt BSFHO interface could be a main driving force behind switchable photovoltaic and rectifying diode characteristics of Pt BSFHO LNO heterostructures.
Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy we report electronic structure measurements of (111)-oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial films. For thin films we observe a 3D Fermi surface with an electron pocket at the Brillouin zone center and hole pockets at the zone vertices. Superlattices with thick nickelate layers present a similar electronic structure. However, as the thickness of the LaNiO3 is reduced the superlattices become insulating. These heterostructures do not show a marked redistribution of spectral weight in momentum space but exhibit a pseudogap of 50 meV.
Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO3/SrTiO3 (001) heterostructures. Using a combination of element-specific X-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO3. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron over-accumulation. In turn, by controlling the doping of the LaMnO3, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO3 films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.
The element-specific technique of x-ray magnetic circular dichroism (XMCD) is used to directly determine the magnitude and character of the valence band orbital magnetic moments in (III,Mn)As ferromagnetic semiconductors. A distinct dichroism is observed at the As K absorption edge, yielding an As 4p orbital magnetic moment of around -0.1 Bohr magnetons per valence band hole. This is strongly influenced by strain, indicating its crucial influence on the magnetic anisotropy. The dichroism at the Ga K edge is much weaker. The K edge XMCD signals for Mn and As both have positive sign, which indicates the important contribution of Mn 4p states to the Mn K edge spectra.
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