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The conductivity changes associated with optical excitations and changing temperature in cobalt valence tautomer molecular thin films were investigated. Conductance switching in the presence of illumination is observed, with occasional locking in a higher conductance state, depending on the temperature, the photon energy of the illumination, and the bias voltage. Light of sufficiently short wavelengths is needed to ensure the light enhanced conductance switching, consistent with the optical absorption, but bias voltage clearly plays a role as well. The conductance switching is associated with excitations to the ligand to metal charge transfer state.
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[Fe-(pyrazine){Pd(CN)4}] (pyrazine = pz) thin films were fabricated using a layer-by-layer assembly approach, a method known to be tunable, versatile, and scalable, since thin films are better-suited for industrial applications. In this study, [Fe-(pz){Pd(CN)4}] powder was synthesized, and the results obtained from a vibrating sample magnetometer verified the presence of an abrupt hysteresis loop with widths of 45 K centered around 300 K, indicating good cooperativity. Super conducting quantum interference device magnetometry results indicated a slow spin transition with temperature but with evidence of hysteresis for thin film samples. X-ray absorption analysis provided further support of the spin crossover behavior but differs from the magnetometry because the spin state transition at the surface differs from the bulk of the thin film. X-ray photoelectron spectroscopy provided some insight into issues with the film deposition process and multiplex fitting was used to further support the claim that the surface of the film is different than the bulk of the film.
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The magnetoelectric behavior of epitaxial Fe-Ga microstructures on top of a (001)-oriented PMN-PT piezoelectric substrate is imaged with magnetic X-ray microscopy. Additionally, the micron-scale strain distribution in PMN-PT is characterized by X-ray microdiffraction and examined with respect to the results of the Fe-Ga magnetoelectric switching. The magnetic reorientation of Fe-Ga is found to be strongly correlated with size, shape, and crystallographic orientation of the microstructures. In the case of square-shaped structures, size dictates the influence of the strain distribution on both the initialization of the ground state and on the magnetic reorientation during application of voltage. On the other hand, elliptical microstructures demonstrate completely different magnetic responses depending on the relative orientation of their long axis with respect to the crystallographic directions of the PMN-PT. This study demonstrates that engineering the behavior of highly magnetostrictive epitaxial microdevices is possible. It further elucidates that voltage-induced actuation can be largely tuned to achieve the desired type of magnetic switching ranging from vortex circulation reversal, domain wall motion, to a large rotation of magnetization. Because of the outstanding properties of the investigated material system, the reported findings are expected to be of great interest for the realization of next-generation energy-efficient magnetic memory and logic devices.
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Controlling the in-plane magnetocrystalline anisotropy and interfacial exchange coupling between ferromagnetic (FM) layers plays a key role in next-generation spintronic and magnetic memory devices. In this work, we explored the effect of tuning the magnetocrystalline anisotropy of La2/3Sr1/3CoO3 (LSCO) and La2/3Sr1/3MnO3 (LSMO) layers and the corresponding effect on interfacial exchange coupling by adjusting the thickness of the LSCO layer (tLSCO). The epitaxial LSCO/LSMO bilayers were grown on (110)o-oriented NdGaO3 (NGO) substrates with a fixed LSMO (top layer) thickness of 6 nm and LSCO (bottom layer) thicknesses varying from 1 to 10 nm. Despite the small difference (â¼0.2%) in lattice mismatch between the two in-plane directions, [001]o and [11Ì 0]o, a pronounced in-plane magnetic anisotropy was observed. Soft X-ray magnetic circular dichroism hysteresis loops revealed that for tLSCO ≤ 4 nm, the easy axes for both LSCO and LSMO layers were along the [001]o direction, and the LSCO layer was characterized by magnetically active Co2+ ions that strongly coupled to the LSMO layer. No exchange bias effect was observed in the hysteresis loops. In contrast, along the [11Ì 0]o direction, the LSCO and LSMO layers displayed a small difference in their coercivity values, and a small exchange bias shift was observed. As tLSCO increased above 4 nm, the easy axis for the LSCO layer remained along the [100]o direction, but it gradually rotated to the [11Ì 0]o direction for the LSMO layer, resulting in a large negative exchange bias shift. Therefore, we provide a way to control the magnetocrystalline anisotropy and exchange bias by tuning the interfacial exchange coupling between the two FM layers.
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This work investigates the effect of copper substitution on the magnetic properties of SmCo5 thin films synthesized by molecular beam epitaxy. A series of thin films with varying concentrations of Cu were grown under otherwise identical conditions to disentangle structural and compositional effects on the magnetic behavior. The combined experimental and theoretical studies show that Cu substitution at the Co3g sites not only stabilizes the formation of the SmCo5 structure but also enhances magnetic anisotropy and coercivity. Density functional theory calculations indicate that Sm(Co4Cu3g)5 possesses a higher single-ion anisotropy as compared to pure SmCo5. In addition, X-ray magnetic circular dichroism reveals that Cu substitution causes an increasing decoupling of the Sm 4f and Co 3d moments. Scanning transmission electron microscopy confirms predominantly SmCo5 phase formation and reveals nanoscale inhomogeneities in the Cu and Co distribution. Our study based on thin film model systems and advanced characterization as well as modeling reveals novel aspects of the complex interplay of intrinsic and extrinsic contributions to magnetic hysteresis in rare-earth-based magnets, i.e., the combination of increased intrinsic anisotropy due to Cu substitution and the extrinsic effect of inhomogeneous elemental distribution of Cu and Co.
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Ultra-thin films of low damping ferromagnetic insulators with perpendicular magnetic anisotropy have been identified as critical to advancing spin-based electronics by significantly reducing the threshold for current-induced magnetization switching while enabling new types of hybrid structures or devices. Here, we have developed a new class of ultra-thin spinel structure Li0.5Al1.0Fe1.5O4 (LAFO) films on MgGa2O4 (MGO) substrates with: 1) perpendicular magnetic anisotropy; 2) low magnetic damping and 3) the absence of degraded or magnetic dead layers. These films have been integrated with epitaxial Pt spin source layers to demonstrate record low magnetization switching currents and high spin-orbit torque efficiencies. These LAFO films on MGO thus combine all of the desirable properties of ferromagnetic insulators with perpendicular magnetic anisotropy, opening new possibilities for spin based electronics.
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Magnetic skyrmions are topological spin textures that can be used as memory and logic components for advancing the next generation spintronics. In this regard, control of nanoscale skyrmions, including their sizes and densities, is of particular importance for enhancing the storage capacity of skyrmionic devices. Here, we propose a viable route for engineering ferrimagnetic skyrmions via tuning the magnetic properties of the involved ferrimagnets Fe1-xTbx. Via tuning the composition of Fe1-xTbx that alters the magnetic anisotropy and the saturation magnetization, the size of the ferrimagnetic skyrmion (ds) and the average density (ηs) can be effectively tailored in [Pt/Fe1-xTbx/Ta]10 multilayers. In particular, a stabilization of sub-50 nm skyrmions with a high density is demonstrated at room temperature. Our work provides an effective approach for designing ferrimagnetic skyrmions with the desired size and density, which could be useful for enabling high-density ferrimagnetic skyrmionics.
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The layered square-planar nickelates, Ndn+1NinO2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd6Ni5O12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd4Ni3O10, and subsequent reduction to the square-planar phase, Nd4Ni3O8. We synthesize our highest quality Nd4Ni3O10 films under compressive strain on LaAlO3 (001), while Nd4Ni3O10 on NdGaO3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd4Ni3O10 on SrTiO3 (001). Films reduced on LaAlO3 become insulating and form compressive strain-induced c-axis canting defects, while Nd4Ni3O8 films on NdGaO3 are metallic. This work provides a pathway to the synthesis of Ndn+1NinO2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.
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A chiral 3D coordination compound, [Gd2(L)2(ox)2(H2O)2], arranged around a dinuclear Gd unit has been characterized by X-ray photoemission and X-ray absorption measurements in the context of density functional theory studies. Core level photoemission of the Gd 5p multiplet splittings indicates that spin orbit coupling dominates over j-J coupling evident in the 5p core level spectra of Gd metal. Indications of spin-orbit coupling are consistent with the absence of inversion symmetry due to the ligand field. Density functional theory predicts antiferromagnet alignment of the Gd2 dimers and a band gap of the compound consistent with optical absorption.
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Electrical modulation of magnetic states in single-phase multiferroic materials, using domain-wall magnetoelectric (ME) coupling, can be enhanced substantially by controlling the population density of the ferroelectric (FE) domain walls during polarization switching. In this work, we investigate the domain-wall ME coupling in multiferroic h-YbFeO3 thin films, in which the FE domain walls induce clamped antiferromagnetic (AFM) domain walls with reduced magnetization magnitude. Simulation according to the phenomenological theory indicates that the domain-wall ME effect is dramatically enhanced when the separation between the FE domain walls shrinks below the characteristic width of the clamped AFM domain walls during the ferroelectric switching. Experimentally, we show that while the magnetization magnitude remains same for both the positive and the negative saturation polarization states, there is evidence of magnetization reduction at the coercive voltages. These results suggest that the domain-wall ME effect is viable for electrical control of magnetization.
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Future molecular microelectronics require the electronic conductivity of the device to be tunable without impairing the voltage control of the molecular electronic properties. This work reports the influence of an interface between a semiconducting polyaniline polymer or a polar poly-D-lysine molecular film and one of two valence tautomeric complexes, i.e., [CoIII(SQ)(Cat)(4-CN-py)2] â [CoII(SQ)2(4-CN-py)2] and [CoIII(SQ)(Cat)(3-tpp)2] â [CoII(SQ)2(3-tpp)2]. The electronic transitions and orbitals are identified using X-ray photoemission, X-ray absorption, inverse photoemission, and optical absorption spectroscopy measurements that are guided by density functional theory. Except for slightly modified binding energies and shifted orbital levels, the choice of the underlying substrate layer has little effect on the electronic structure. A prominent unoccupied ligand-to-metal charge transfer state exists in [CoIII(SQ)(Cat)(3-tpp)2] â [CoII(SQ)2(3-tpp)2] that is virtually insensitive to the interface between the polymer and tautomeric complexes in the CoII high-spin state.
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Applications of quantum information science (QIS) generally rely on the generation and manipulation of qubits. Still, there are ways to envision a device with a continuous readout, but without the entangled states. This concise perspective includes a discussion on an alternative to the qubit, namely the solid-state version of the Mach-Zehnder interferometer, in which the local moments and spin polarization replace light polarization. In this context, we provide some insights into the mathematics that dictates the fundamental working principles of quantum information processes that involve molecular systems with large magnetic anisotropy. Transistors based on such systems lead to the possibility of fabricating logic gates that do not require entangled states. Furthermore, some novel approaches, worthy of some consideration, exist to address the issues pertaining to the scalability of quantum devices, but face the challenge of finding the suitable materials for desired functionality that resemble what is sought from QIS devices.
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Compact domain features have been observed in spin crossover [Fe{H2B(pz)2}2(bipy)] molecular thin film systems via soft x-ray absorption spectroscopy and photoemission electron microscopy. The domains are in a mixed spin state that on average corresponds to roughly 2/3 the high spin occupation of the pure high spin state. Monte Carlo simulations support the presence of intermolecular interactions that can be described in terms of an Ising model in which interactions beyond nearest-neighbors cannot be neglected. This suggests the presence of short-range order to permit interactions between molecules beyond nearest neighbor that contribute to the formation of largely high spin state domains structure. The formation of a spin state domain structure appears to be the result of extensive cooperative effects.
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High-Tc molecular magnets have amassed much promise; however, the long-standing obstacle for its practical applications is the inaccessibility of high-temperature molecular magnets showing dynamic and nonvolatile magnetization control. In addition, its functional durability is prone to degradation in oxygen and heat. Here, we introduce a rapid prototyping and stabilizing strategy for high Tc (360 K) molecular magnets with precise spatial control in geometry. The printed molecular magnets are thermally stable up to 400 K and air-stable for over 300 days, a significant improvement in its lifetime and durability. X-ray magnetic circular dichroism and computational modeling reveal the water ligands controlling magnetic exchange interaction of molecular magnets. The molecular magnets also show dynamical and reversible tunability of magnetic exchange interactions, enabling a colossal working temperature window of 86 K (from 258 to 344 K). This study provides a pathway to flexible, lightweight, and durable molecular magnetic devices through additive manufacturing.
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From X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS), it is evident that the spin state transition behavior of Fe(II) spin crossover coordination polymer crystallites at the surface differs from the bulk. A comparison of four different coordination polymers reveals that the observed surface properties may differ from bulk for a variety of reasons. There are Fe(II) spin crossover coordination polymers with either almost complete switching of the spin state at the surface or no switching at all. Oxidation, differences in surface packing, and changes in coordination could all contribute to making the surface very different from the bulk. Some Fe(II) spin crossover coordination polymers may be sufficiently photoactive so that X-ray spectroscopies cannot discern the spin state transition.
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The [Co(SQ)2(4-CN-py)2] complex exhibits dynamical effects over a wide range of temperature. The orbital moment, determined by X-ray magnetic circular dichroism (XMCD) with decreasing applied magnetic field, indicates a nonzero critical field for net alignment of magnetic moments, an effect not seen with the spin moment of [Co(SQ)2(4-CN-py)2].
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Since the discovery of high-temperature superconductivity in copper oxide materials1, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials2. One prime materials platform has been the rare-earth nickelates and, indeed, superconductivity was recently discovered in the doped compound Nd0.8Sr0.2NiO2 (ref. 3). Undoped NdNiO2 belongs to a series of layered square-planar nickelates with chemical formula Ndn+1NinO2n+2 and is known as the 'infinite-layer' (n = ∞) nickelate. Here we report the synthesis of the quintuple-layer (n = 5) member of this series, Nd6Ni5O12, in which optimal cuprate-like electron filling (d8.8) is achieved without chemical doping. We observe a superconducting transition beginning at ~13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd6Ni5O12 interpolates between cuprate-like and infinite-layer nickelate-like behaviour. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors that can be tuned via both doping and dimensionality.
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Electrones , Superconductividad , CalorRESUMEN
All-optical switching of magnetization has great potential for use in future ultrafast and energy efficient nanoscale magnetic storage devices. So far, research has been almost exclusively focused on rare-earth based materials, which limits device tunability and scalability. Here, we show that a perpendicularly magnetized synthetic ferrimagnet composed of two distinct transition metal ferromagnetic layers, Ni3Pt and Co, can exhibit helicity independent magnetization switching. Switching occurs between two equivalent remanent states with antiparallel alignment of the Ni3Pt and Co magnetic moments and is observable over a broad temperature range. Time-resolved measurements indicate that the switching is driven by a spin-polarized current passing through the subnanometer Ir interlayer. The magnetic properties of this model system may be tuned continuously via subnanoscale changes in the constituent layer thicknesses as well as growth conditions, allowing the underlying mechanisms to be elucidated and paving the way to a new class of data storage devices.