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Abstract: The study of transition metal clusters exhibiting fast electron hopping or delocalization remains challenging, because intermetallic communications mediated through bridging ligands are normally weak. Herein, we report the synthesis of a nanosized complex, [Fe(Tp)(CN)3]8[Fe(H2O)(DMSO)]6 (abbreviated as [Fe14], Tp−, hydrotris(pyrazolyl)borate; DMSO, dimethyl sulfoxide), which has a fluctuating valence due to two mobile d-electrons in its atomic layer shell. The rate of electron transfer of [Fe14] complex demonstrates the Arrhenius-type temperature dependence in the nanosized spheric surface, wherein high-spin centers are ferromagnetically coupled, producing an S = 14 ground state. The electron-hopping rate at room temperature is faster than the time scale of Mössbauer measurements (<~10−8 s). Partial reduction of N-terminal high spin FeIII sites and electron mediation ability of CN ligands lead to the observation of both an extensive electron transfer and magnetic coupling properties in a precisely atomic layered shell structure of a nanosized [Fe14] complex.

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Abstract: Mechanically-exfoliated two-dimensional ferromagnetic materials (2D FMs) were discovered to possess long-range ferromagnetic order and topologically nontrivial skyrmions in few-layers. However, owing to the dimensionality effect, such few-layer systems usually exhibit much lower Curie temperature (TC) compared to their bulk counterparts. It is therefore of great interest to explore effective approaches to enhance their TC, particularly in wafer-scale for practical applications. Here, we report an interfacial proximity-induced high-TC 2D FM Fe3GeTe2 (FGT) via A-type antiferromagnetic material CrSb (CS) which strongly couples to FGT. A superlattice structure of (FGT/CS)n, where n stands for the period of FGT/CS heterostructure, has been successfully produced with sharp interfaces by molecular-beam epitaxy on 2-inch wafers. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally discovered that TC of 4-layer Fe3GeTe2 can be significantly enhanced from 140 K to 230 K because of the interfacial ferromagnetic coupling. In the meanwhile, an inverse proximity effect occurs in the FGT/CS interface, driving the interfacial antiferromagnetic CrSb into a ferrimagnetic state as evidenced by a double-switching behavior in hysteresis loops and the XMCD spectra. Density functional theory calculations show that the Fe-Te/Cr-Sb interface is strongly FM coupled and doping of the spin-polarized electrons by the interfacial Cr layer gives rise to the TC enhancement of the Fe3GeTe2 films, in accordance with our XMCD measurements. Strikingly, by introducing rich Fe in 4-layer FGT/CS superlattice, TC can be further enhanced to near room temperature. Our results provide a feasible approach in enhancing the magnetic order of few-layer 2D FMs in wafer-scale and render opportunities for realizing realistic ultra-thin spintronic devices.

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Abstract: A detailed magnetic phase diagram of FeGe membranes with fields applied out-of-plane is determined using small-angle soft x-ray elastic scattering. In addition to the well-established skyrmion and helical phase, we identify an additional ordered phase which partly coexists with the helical phase in a pocket beneath the skyrmion phase. Furthermore, an evolution of the modulation wave vector, q , is observed when traversing the magnetic phase pocket. For the ordinary and the additional helical phase, q only varies with normalized field but not with temperature—in sharp contrast to the skyrmion phase, for which q is both field and temperature dependent. This study of the helical phases in thin FeGe membranes lays the foundation for their use in future magnetic storage and logic devices.

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Abstract: B2-ordered FeRh undergoes a first-order metamagnetic transition from an antiferromagnet (AF) to a ferromagnet (FM) upon heating. Thin films of B2-ordered FeRh are grown using DC magnetron sputtering and characterized for their behaviour at GHz frequencies and their dynamic behaviour over hour timescales. Ferromagnetic resonance investigations reveal a change in the spectroscopic splitting factor, g, through the range of the transition probed here. By introducing a model that describes the development of the two magnetic phases through the transition, this change in g is shown to be consistent with the development of an exchange coupling across the magnetic phase boundary that induces a non-zero magnetic moment in the AF phase as the result of a thickness dependent phase transition in the AF layer. The influence of such a phase transition is also seen in the extracted value of the Gilbert damping parameter in this experiment. Spin-wave resonance measurements are then performed to try and conclusively measure the exchange coupling between the two magnetic states in FeRh. Measuring the exchange stiffness through the transition reveals that the AF phase has a non-zero exchange energy that varies through the measurement range probed here. The behaviour of the exchange stiffness in the AF layer is attributed to a combination of both the onset of the exchange coupling and the presence of evanescent spin-waves, both of which are consequences of the thickness dependent phase transition in the AF layer. It was then shown that the structure of both magnetic phases could be measured directly with X-Ray Magnetic Dichroism using both linearly and circularly polarized light. The objects measured in these experiments are then characterized for their dynamic properties using X-Ray Photon Correlation Spectroscopy. These studies reveal reveal that the dynamic behaviour of the system is dependent on the type of magnetic dichroism used to probe it. This study also shows it is possible use x-ray magnetic linear dichroism to directly measure the structural and dynamic behaviour of AF materials.

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Abstract: The design of active and durable catalysts for the H2O/O2 interconversion is one of the major challenges of electrocatalysis for renewable energy. The oxygen evolution reaction (OER) is catalyzed by SrRuO3 with low potentials (ca. 1.35 VRHE), but the catalyst’s durability is insufficient. Here we show that Na doping enhances both activity and durability in acid media. DFT reveals that whereas SrRuO3 binds reaction intermediates too strongly, Na doping of ~0.125 leads to nearly optimal OER activity. Na doping increases the oxidation state of Ru, thereby displacing positively O p-band and Ru d-band centers, weakening Ru-adsorbate bonds. The enhanced durability of Na-doped perovskites is concomitant with the stabilization of Ru centers with slightly higher oxidation states, higher dissolution potentials, lower surface energy and less distorted RuO6 octahedra. These results illustrate how high OER activity and durability can be simultaneously engineered by chemical doping of perovskites.