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Abstract Surface segregation is a ubiquitous phenomenon driven by minimization of the total free energy. In this paper we study surface segregation in multicomponent magnetic Bismuth ferrite nanoparticles alloyed with varying amounts of Dysprosium, Zinc and Titanium. We employ surface and bulk sensitive spectroscopic probes to unravel a significant surface segregation of Bismuth oxide and Titanium oxide. High coercive fields of BiFe_0.95Ti_0.05O₃(BFTO) and BiFe_0.96(Zn, Ti)_0.02O₃(BFZTO) at room temperature reveal that they have a strong exchange bias. This suggests that the Titanium oxide is magnetically active, and there is a Ti inducedd^oferromagnetism in action between these nanoparticles. We show, with the addition of Dy₂O₃, the Ti inducedd^oferromagnetism is suppressed making (BDFZTO) superparamagnetic. We observe that all three differently alloyed Bismuth ferrite nanoparticles show a non-saturating paramagnetic background. 

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 Evidence of chirality was observed at the Fe metal center in Fe(III) spin crossover coordination salts [Fe(qsal)₂][Ni(dmit)₂] and [Fe(qsal)₂](TCNQ)₂from x-ray absorption (XAS) spectroscopy at the Fe 2p_3/2core threshold. Based on the circularly polarized XAS data, the x-ray natural circular dichroism for [Fe(qsal)₂][Ni(dmit)₂] and [Fe(qsal)₂](TCNQ)₂is far stronger than seen for [Fe(qsal)₂]Cl suggesting this natural circular dichroism signature is a ligand effect rather than a result of just a loss of octahedral symmetry on the Fe core. The larger the chiral effects in the Fe 2p core to bound XAS, the greater the perturbation of the Fe 2p_3/2to 2p_1/2spin–orbit splitting seen in the XAS spectra. 

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. 

Abstract We study the effect of strain on the magnetic properties and magnetization configurations in nanogranular Fe_xGe ${}_{1-x}$films ( $x=0.53\pm 0.05$) with and without B20 FeGe nanocrystals surrounded by an amorphous structure. Relaxed films on amorphous silicon nitride membranes reveal a disordered skyrmion phase while films near and on top of a rigid substrate favor ferromagnetism and an anisotropic hybridization of Fedlevels and spin-polarized Gespband states. The weakly coupled topological states emerge at room temperature and become more abundant at cryogenic temperatures without showing indications of pinning at defects or confinement to individual grains. These results demonstrate the possibility to control magnetic exchange and topological magnetism by strain and inform magnetoelasticity-mediated voltage control of topological phases in amorphous quantum materials. 

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. 

To investigate the paramagnetic correlation lengths by XMCD, in a polyaniline plus a spin crossover molecular complex, dilute concentrations of magnetic nickel cobaltite (NiCo₂O₄) nanoparticles were added to the bimolecular mixture.