Carlos Antonio Fernandes Vaz

We report the evolution of the antiferromagnetic domain structure of epitaxial La 0.45 Sr 0.55 MnO 3 (LSMO) ultrathin films with thickness in the range from 5–50 unit cells (uc), using x-ray photoemission electron microscopy (XPEEM). While the 5 uc thick LSMO shows no magnetic contrast down to ∼ 80 K , thicker films display a multidomain antiferromagnetic configuration with a nonmonotonic variation in the characteristic domain size from 0.3 µ m at 10 uc, to 3–5 µ m at 15–16 uc and 1 µ m for 50 uc. Post-growth annealing is found not to impact strongly the magnetic domain state. In contrast, for a 15 uc film grown on a substrate characterized by large atomic steps ( ∼ 1 µ m ) , we observe much larger antiferromagnetic domains and the presence of net magnetic moment in the form of stripes with alternating contrast, assigned to the signal from the top uncompensated spins of the A-type antiferromagnetic state of LSMO. From the combined antiferromagnetic domain structure and the net magnetic moment contrast, we determine the exact orientation of the Néel vector, including at domain walls. We describe the antiferromagnetic domain size distribution in terms of interface and bulk contributions to the density of defects that pin the antiferromagnetic domain walls and which determine the equilibrium domain configuration. Our results demonstrate the impact of thickness and defects on the antiferromagnetic domain size and constitute a stepping stone in controlling the antiferromagnetic domain state required for oxide antiferromagnetic device applications.

Soft x-ray ptychography is becoming a key synchrotron microscopy technique in the fields of condensed matter physics, chemistry, and environmental and life sciences. Its attractiveness across broad disciplinary fields is owed to the favorable combination of high spatial resolution and strong contrast mechanisms. The SOft X-ray Ptychography Highly Integrated Endstation (SOPHIE) at the Swiss Light Source (SLS) was developed to accommodate soft x-ray ptychography experiments requiring high spatial resolution, in addition to high chemical and ferroic sensitivities. An introduction to soft x-ray ptychography with SOPHIE aimed at prospective users is provided. Furthermore, an overview of the instrumentation of SOPHIE is given along with an example of the imaging capabilities, which demonstrate the achievement of a sub-10 nm spatial resolution at a photon energy of 706 eV.

Controlling the correlations and electronic reconstruction at the interface of transition metal oxide heterostructures provides a new pathway for tuning their unique physical properties. Here, we investigate the effects of interfacial nonstoichiometry and vertical phase separation on the magnetic properties and proximity-induced magnetism of epitaxial La _0.7 Sr _0.3 MnO ₃ (LSMO)/SrTiO ₃ (001) oxide heterostructures. We also reinvestigate the recently observed inverse hysteresis behavior reported for this system, which we find emanates from the remanent field of the superconducting solenoid and not from antiferromagnetic intra-layer exchange coupling in low coercivity LSMO thin films. Combined atomically resolved electron energy loss spectroscopy, element-specific X-ray magnetic circular dichroism, and interface-sensitive polarized soft X-ray resonant magnetic reflectivity show the formation of a Mn ³⁺ -enriched interfacial LSMO layer, of a Ti ³⁺ -derived magnetic interface layer coupled ferromagnetically to La _0.7 Sr _0.3 MnO ₃ , together with a small density of O-vacancies at the interface. These results not only advance the understanding of the magnetism and spin structure of correlated oxide interfaces but also hold promise for practical applications, especially in devices where the performance relies on the control and influence of spin polarization currents by the interfacial spin structure.

The (electro)chemical degradation at the interface between Li₆PS₅Cl (LPSC) and LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) is systematically investigated using nondestructive synchrotron X-ray absorption spectroscopy and X-ray photoemission electron microscopy. These measurements were surface chemical depth profiling (from 2 to several hundred nanometers) and high-resolution elemental imaging of both LPSC and NCM622 particles. This analysis was complemented by galvanostatic cycling, impedance spectroscopy, and operando cell pressure characterization. Several correlations between interphase evolution and cell electrochemical performance are clarified, while some inconsistencies are rationalized and discussed. First, the intrinsic LPSC electrochemical oxidation mechanisms were studied using an LPSC:C65 working electrode (WE). The results showed that increased cell resistance during the first charge stemmed from polysulfide byproducts and particle contact loss due to LPSC volume shrinkage at the interface. Second, when using an NCM622:LPSC WE, species, such as SO₃^2-, SO₄^2-, and PO₄^3-, were detected on both LPSC and NCM622 particles, while electrochemically inactive reduced transition metals were observed only at NCM622 surfaces. These species, initially present at open-circuit potential, increased after the first charge due to the chemical reactions between LPSC and NCM622 surface lattice oxygen. The estimated interphase thickness on the LPSC and NCM622 surfaces over the cycling remains below ∼3 nm. Our findings highlight that the formation of an electrochemically inactive NCM622 surface is a primary cause of impedance rise during the first charge, along with the formation of LPSC byproducts and contact loss. However, the continuous increase in cell resistance could not be attributed to further interphase growth after the first charge. We hypothesize that this may result from slow and progressive LPSC polymerization reactions (e.g., Li₂P₂S₆ and P₂S₅) and structural changes at the NCM622 surface.

Epitaxial strain in thin films can yield surprising magnetic and electronic properties not accessible in bulk. One materials system destined to be explored in this direction are orthoferrites with two intertwined spin systems where strain is predicted to have a significant impact on magnetic and polar properties by modifying the strength of the rare earth (R)‐Fe interaction. Here the impact of epitaxial strain is reported on the linear magneto‐electric DyFeO₃, a canted bulk antiferromagnet with a high Néel temperature (645 K) exhibiting a Dy‐induced spin reorientation (SR) transition at ≈50 K and antiferromagnetic (AFM) ordering of the Dy spins at 4 K. An increase in the spin transition of >20 K is found and a strictly linear, abnormal temperature magnetic response under an applied magnetic field between 100 and 400 K for [010]‐oriented DyFeO₃ thin films with an in‐plane compressive strain between 2% and 3.5%. At room temperature and above, it is found that application of ≈0.06 T causes a spin‐flop of the Dy spins coupled to the AFM Fe spin lattice, whereby the Dy spins change from an AFM alignment to ferromagnetic (FM). The spin‐flop field gives a lower energy bound on the Dy‐Fe exchange interaction of ≈15 µeV.

We present an experimental study of the magnetoelectric coupling in the Fe-Ga/Pb[(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.31 thin-film multiferroic composite using x-ray magnetic circular dichroism and ferromagnetic resonance (FMR). Our measurements show evidence for a charge-mediated coupling mechanism, suggested by the asymmetric magnetic remanence (Mrem) behavior under opposite electric fields (±E) and the asymmetric resonance field (Hr) in the FMR measurements. Also, the FMR measurements reveal a perpendicular magnetic anisotropy that can be related to an interface charge effect and it is tunable by the E field. Ab initio calculations support the existence of a charge-mediated coupling at the Fe–Ga/PMN-PT interface.

The (electro-)chemical degradation at the interface between Li6PS5Cl (LPSC) and LiNi0.6Co0.2Mn0.2O2 (NCM622) is systematically investigated using non-destructive synchrotron X-ray absorption spectroscopy (XAS) and X-ray photoemission electron microscopy (XPEEM). These techniques provide surface chemical depth profiling (from 2 nm to several hundred nm) and high-resolution elemental imaging of both LPSC and NCM622 particles. This analysis was complemented by galvanostatic cycling, impedance spectroscopy, and operando cell pressure measurements. Several correlations between interphase evolution and cell electrochemical performance are clarified, while some inconsistencies are rationalized and discussed. Firstly, the intrinsic LPSC electrochemical oxidation mechanisms were studied using an LPSC:C65 working electrode. The results showed that increased cell resistance during the first charge stemmed from polysulfide by-products and particle contact loss due to LPSC volume shrinkage at the interface. Secondly, when using an NCM622:LPSC working electrode, species such as sulfites, sulfates and phosphates, were detected on both LPSC and NCM622 particles, while electrochemically inactive reduced transition metals (TMs) were observed only at NCM622 surfaces. These species, initially present at open circuit potential, increased after the first charge, due to the chemical reactions between LPSC and NCM622 surface lattice oxygen. The estimated interphase thickness on the LPSC and NCM622 surface over the cycling remains below ⁓3 nm. Our findings highlight that the formation of an electrochemically inactive NCM622 surface is a primary cause of impedance rise during the first charge, along with the formation of LPSC by-products and contact loss. However, the continuous increase in cell resistance could not be attributed to further interphase growth after the first charge. We hypothesize that this may result from slow and progressive LPSC polymerization reactions (e.g., form Li2PS6 and P2S5) and structural changes at the NCM622 surface.

The evolution of the magnetic ground state of ultrathin 0–10 unit cells (uc) thick La0.8Sr0.2MnO3 films interfaced to an antiferromagnetic La0.45Sr0.55MnO3/SrTiO3(001) buffer layer was investigated with x-ray photoemission electron microscopy. For 0–3 uc La0.8Sr0.2MnO3, we observe antiferromagnetic domains but no ferromagnetic contrast, showing that nominally ferromagnetic La0.8Sr0.2MnO3 adopts the antiferromagnetic ground state of the buffer layer. For larger thicknesses, ferromagnetic domains emerge, confirming that the additional layers revert to the ferromagnetic ground state. We also observe a drastic increase in the complexity of the domain configuration between 3 and 5 uc, which we attribute to competing magnetic and electronic ground states in the system. We attribute the interfacial modified magnetic ground state to charge sharing at the interface due to the chemical potential mismatch, which leads to hole doping at the La0.8Sr0.2MnO3 interface. The present work sheds light on the impact of charge sharing at the interface of complex oxide materials, in particular on the magnetic and electronic states, and presents a strategy for modulating the electronic ground state properties at metallic interfaces.