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The topological Hall effect (THE), a quantum phenomenon arising from the emergent magnetic field generated by a topological spin texture, is a key method for detecting non-coplanar spin structures like skyrmions in magnetic materials. Here, we investigate a bilayer structure of Pt and the conducting ferrimagnet NiCo2O4 (NCO) of perpendicular magnetic anisotropy and demonstrate a giant THE across a temperature range of 2–350 K. The absence of THE in a single-layer Pt and NCO, as well as in Pt/Cu/NCO, suggests its interfacial origin. The maximum THE occurring just before the NCO coercive field indicates its connection to magnetic nucleation centers, which are topologically equivalent to skyrmions. The large normalized THE, based on the emergent-field model, points to a high population density of small magnetic nucleation centers. This aligns with the seemingly unresolvable domain structures by the employed techniques during magnetization reversal, even though clear domain structures are detected after zero-field cooling. These results establish heavy metal/NCO as a promising system for exploring topological spin structures. 

Abstract From a comparison of the known molecular stoichiometry and x-ray photoemission spectroscopy, it is evident that the Fe(III) spin crossover salt [Fe(qsal)₂Ni(dmit)₂] has a preferential surface termination with the Ni(dmit)₂moiety, where qsal = N(8quinolyl)salicylaldimine, and dmit²⁻= 1,3-dithiol-2-thione-4,5-dithiolato. This preferential surface termination leads to a significant surface to bulk core level shift for the Ni 2p x-ray photoemission core level, not seen in the corresponding Fe 2p core level spectra. A similar surface to bulk core level shift is seen in Pd 3d in the related [Fe(qsal)₂]₂Pd(dmit)₂. Inverse photoemission spectroscopy, compared with the x-ray absorption spectra at the Ni-L3,2 edge provides some indication of the density of states resulting from the dmit²⁻= 1,3-dithiol-2-thione-4,5-dithiolato ligand unoccupied molecular orbitals and thus supports the evidence regarding surface termination in the Ni(dmit)₂moiety. 

Abstract X-ray photoelectron spectroscopy (XPS) shows that dramatic changes in the core level binding energies can provide strong indications of transitions between more dielectric and more metallic CoFe₂O₄and NiCo₂O₄thin films. These significant variations in the XPS core level binding energies are possible with a combination of annealing and oxygen exposure; however, the behaviors of the CoFe₂O₄and NiCo₂O₄thin films are very different. The XPS Co and Fe 2p_3/2core levels for the CoFe₂O₄thin film at room temperature show large photovoltaic surface charging, leading to binding energy shifts, characteristic of a highly dielectric (or insulating) surface at room temperature. The photovoltaic charging, observed in the XPS binding energies of the Co and Fe 2p_3/2core levels, decreases with increasing temperature. The XPS core level binding energies of CoFe₂O₄thin film saturated at lower apparent binding energies above 455 K. This result shows that the prepared CoFe₂O₄thin film can be dielectric at room temperature but become more metallic at elevated temperatures. The dielectric nature of the CoFe₂O₄thin film was restored only when the film was annealed in sufficient oxygen, indicating that oxygen vacancies play an important role in the transition of the film from dielectric (or insulating) to metallic. In contrast, the XPS studies of initially metallic NiCo₂O₄thin film demonstrated that annealing NiCo₂O₄thin film led to a more dielectric or insulating film. The original more metallic character of the NiCo₂O₄film was restored when the NiCo₂O₄was annealed in sufficient oxygen. Effective activation energies are estimated for the carriers from a modified Arrhenius-type model applied to the core level binding energy changes of the CoFe₂O₄and NiCo₂O₄thin films, as a function of temperature. The origin of the carriers, however, is not uniquely identified. This work illustrates routes to regulate the surface metal-to-insulator transition of dielectric oxides, especially in the case of insulating NiCo₂O₄thin film that can undergo reversible metal-to-insulator transition with temperature. 

Abstract In this study, the structure and transport properties of two polymorphs, nanoparticles and nanorods, of the iron(II) triazole [Fe(Htrz)₂(trz)](BF₄) spin crossover complex were compared. Conductive atomic force microscopy was used to map the electrical conductivity of individual nanoparticles and nanorods. The [Fe(Htrz)₂(trz)](BF₄) nanorods showed significantly higher conductivity compared to nanoparticles. This difference in electrical conductivity is partially associated to the different Fe–N bond lengths in each of the polymorphs, with an inverse relationship between Fe–N bond length and conductivity. Transport measurements were done on the nanorods for both high spin (at 380 K) and low spin (at 320 K) states under dark and illuminated conditions. The conductance is highest for the low spin state under dark conditions. In illumination, the conductance change is much diminished. 

The spin crossover complex Fe(phen)2(NCS)2 and its composite, Fe(phen)2(NCS)2, combined with the conducting polymer polyaniline (PANI) plus varying concentrations of iron magnetite (Fe3O4) nanoparticles were studied. A cooperative effect is evident from the hysteresis width in the plot of magnetic susceptibility multiplied by temperature versus temperature (χmT versus T) for Fe(phen)2(NCS)2 with PANI plus varying concentrations of Fe3O4 nanoparticles. The hysteresis width in the composites vary no more than 2 K with respect to the pristine Fe(phen)2(NCS)2 spin crossover crystallites despite the fact that there exists a high degree of miscibility of the Fe(phen)2(NCS)2 spin crossover complex with the PANI. The Fe3O4 nanoparticles in the Fe(phen)2(NCS)2 plus PANI composite tend to agglomerate at higher concentrations regardless of the spin state of Fe(phen)2(NCS)2. Of note is that the Fe3O4 nanoparticles are shown to be antiferromagnetically coupled with the Fe(phen)2(NCS)2 when Fe(phen)2(NCS)2 is in the high spin state. 

The bistability and the conductivity changes associated with optical excitations in cobalt valence tautomer molecular thin films were investigated. 

Here, we examine the conductance changes associated with the change in spin state in a variety of different structures, using the example of the spin crossover complex [Fe(H2B(pz)2)2(bipy)] (pz = (pyrazol-1-yl)-borate and bipy = 2,2′-bipyridine) and [Fe(Htrz)2(trz)](BF4)] (Htrz = 1H-1,2,4-triazole) thin films. This conductance change is highly variable depending on the mechanism driving the change in spin state, the substrate, and the device geometry. Simply stated, the choice of spin crossover complex used to build a device is not the only factor in determining the change in conductance with the change in spin state. 

Abstract In an effort to reconcile the various interpretations for the cation components of the 2p_3/2observed in x-ray photoelectron spectroscopy (XPS) of several spinel oxide materials, the XPS spectra of both spinel alloy nanoparticles and crystalline thin films are compared. We observed that different components of the 2p_3/2core level XPS spectra, of these inverse spinel thin films, are distinctly surface and bulk weighted, indicating surface-to-bulk core level shifts in the binding energies. Surface-to-bulk core level shifts in binding energies of Ni and Fe 2p_3/2core levels of NiFe₂O₄thin film are observed in angle-resolved XPS. The ratio between surface-weighted components and bulk-weighted components of the Ni and Fe core levels shows appreciable dependency on photoemission angle, with respect to surface normal. XPS showed that the ferrite nanoparticles Ni_xCo_1−xFe₂O₄(x= 0.2, 0.5, 0.8, 1) resemble the surface of the NiFe₂O₄thin film. Surface-to-bulk core level shifts are also observed in CoFe₂O₄and NiCo₂O₄thin films but not as significantly as in NiFe₂O₄thin film. Estimates of surface stoichiometry of some spinel oxide nanoparticles and thin films suggested that the apportionment between cationic species present could be farther from expectations for thin films as compared to what is seen with nanoparticles. 

Spin crossover complexes are a route toward designing molecular devices with a facile readout due to the change in conductance that accompanies the change in spin state. Because substrate effects are important for any molecular device, there are increased efforts to characterize the influence of the substrate on the spin state transition. Several classes of spin crossover molecules deposited on different types of surface, including metallic and non-metallic substrates, are comprehensively reviewed here. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules. Most metallic substrates, such as Au and Cu, tend to suppress changes in spin state and affect the spin state switching process due to the interaction at the molecule–substrate interface that lock spin crossover molecules in a particular spin state or mixed spin state. Of course, the influence of the substrate on a spin crossover thin film depends on the molecular film thickness and perhaps the method used to deposit the molecular film.