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Resonant magnetic x-ray scattering from dense self-assemblies of 9-nm diameter epsilon-Co and hcp-Co particles is reported. For lower anisotropy epsilon-Co we find remanent magnetic scattering that is significantly enhanced, indicating preferred inter-particle moment orientations of both antiferromagnetic and ferromagnetic character. This interaction-mediated collective behavior is consistent with dipolar fields and exists well above the isolated particles blocking temperature where thermal activation is operative, suggesting that magnetostatic super-spin waves exist in such systems.

Nanostructured metal-nitrides have attracted tremendous interest as a new generation of catalysts for electroreduction of CO2, but these structures have limited activity and stability in the reduction condition. Herein, we report a method of fabricating FeN/Fe3N nanoparticles with FeN/Fe3N interface exposed on the NP surface for efficient electrochemical CO2 reduction reaction (CO2RR). The FeN/Fe3N interface is populated with Fe−N4 and Fe−N2 coordination sites respectively that show the desired catalysis synergy to enhance the reduction of CO2 to CO. The CO Faraday efficiency reaches 98% at −0.4 V vs. reversible hydrogen electrode, and the FE stays stable from −0.4 to −0.9 V during the 100 h electrolysis time period. This FeN/Fe3N synergy arises from electron transfer from Fe3N to FeN and the preferred CO2 adsorption and reduction to *COOH on FeN. Our study demonstrates a reliable interface control strategy to improve catalytic efficiency of the Fe–N structure for CO2RR. Understanding and controlling chemical environment of metal-N-catalysts is of great importance. In this work, the authors reveal FeN/Fe3N interface with Fe-N4 and Fe-N2 coordination sites for enhanced electrochemical CO2 reduction to CO. [ABSTRACT FROM AUTHOR]

[Fe.sub.3][O.sub.4] (9 nm) grown epitaxially on one or several facets of a seed Au particle (8 nm) leads to two distinct composite nanoparticle structures dubbed 'dumbbell-' and 'flower-like.' These particles exhibit novel magnetic behavior such as large magnetic anisotropy compared to the single component [Fe.sub.3][O.sub.4]. We have studied the static and dynamic magnetic properties using temperature and field-dependent dc magnetization and RF transverse susceptibility. While the magnetic properties of dumbbell-like particles are relatively conventional, field-cooled M-H loops of flower-shaped particles as well as the transverse susceptibility show large shifts along the field axis indicative of exchange bias. Index Terms--Exchange bias, magnetic anisotropy, magnetic nanoparticles, transverse susceptibility.

We have proposed a DNA detection system that relies on superparamagnetic nanoparticles (Co and [Fe.sub.3][O.sub.4]) as biological labels and submicrometer spin valves as sensors. In sensor operation, the magnetic labels are subjected to annealing at 95 [degrees]C-98 [degrees]C, at which DNA denatures, and the spin valve sensor surface is exposed to various DNA containing buffer solutions. We have performed stability studies to validate that the components of our system will survive normal sensor operation. We demonstrate that our proposed biological labels are magnetically stable after five thermal cycles of 5 min (with [T.sub.max] = 98 [degrees]C). By tracking the magnetic diameter ([D.sub.m]) of the nanoparticles, obtained from a Langevin function fitting method, we observe that [D.sub.m] decreases by about 20% after the fifth temperature cycle. Compliance of the submicrometer spin valve sensor with a width of 300 nm and an magnetoresistance (MR) ratio of ~7 % is studied by immersion cycling in buffer solutions (pH = 7.5-7.9) with a high DNA concentration. The spin valve sensor maintains its characteristic MR ratio after four 30 min exposures to DNA solution and continues to perform with a ~ 7 % MR after 24 h in solution. Notably, the spin valve sensor demonstrates this survivability with an ultra thin passivation layer (~ 4 nm). Thus, the biochemical stability of these components suggests that our DNA detection system is compliant with standard biological sensor operation. Index Terms--Biomagnetics, magnetic DNA microarray, magnetic particles, magnetic sensors, spin valves.

We present an analytical model for detection of multiple magnetic nanoparticles (NP) as biomolecular labels by spin valve (SV) sensors, aiming to establish the relationship between the SV sensor signal and the number of magnetic labels. The model is based on the assumptions of equivalent average field of magnetic NPs and the coherent magnetization rotation of SVs free layer. Using the model, we have calculated the sensor signals of multiple NPs uniformly or randomly distributed over a SV sensor at various aspect ratios of the NP array. Satisfactory signal linearity at low particle number or high aspect ratio has been found. The model also reveals that the SV sensors could be made insensitive to the random configuration of NPs and only sensitive to the number of NPs. This feature is desired for quantitative bio-detection. To check the validity of the model, we performed experiments on a monolayer of 16-um [Fe.sub.3][O.sub.4] NPs coated on 0.3-[micro]m-wide SV sensors. We found that the measured signals could be well described by the analytical model Index Terms--Biosensor, magnetic nanoparticle, multiple nanoparticle detection, spin valve.

Thermal treatment of self-assembled FePt nanoparticles reduces the interparticle distances, resulting in dramatic changes in the type and strength of interparticle interactions. Consequently, magnetic properties such as the hysteresis and magnetization reversal mechanisms are strongly affected. It is suggested that controlled annealing of self-assembled nanoparticles may offer a novel approach for producing hard magnetic nanocomposites. Index Terms--Exchange coupling, FePt nanoparticles, magnetization reversal, self-assembly.