Nanoparticles of antiferromagnetically ordered materials acquire the uncompensated magnetic moment caused by defects and surface effects. A bright example of such a nanoantiferromagnet is nanoferrihydrite consisting of particles 2−5 nm in size, the magnetic moment of which amounts to hundreds of Bohr magnetons per particle. We present a brief review of the studies on magnetic properties of ferrihydrite produced by bacteria. Special attention is focused on the aspects of possible biomedical applications of this material, i.e., the particle elimination, toxicity, and possible use for targeted drug delivery.
A cold finger cell
intended for the wax deposition measurements
was fabricated and integrated into an NMR imaging probe for the noninvasive
study of wax precipitation processes in situ. The cell was first tested
with a model system; then, a series of experiments with different
thermal gradients applied to the cell were performed for a waxy crude
oil. NMR imaging of the operating cell revealed the formation of a
deposit with the morphology and dynamics strongly correlating with
the temperature regime. At higher temperatures of cold finger, the
incipient wax gel ages uniformly, giving rise to the hard and thin
inner layer of deposit accompanied by a branched loosely consolidated
outer layer. Conversely, the lower temperatures facilitate formation
of a thick deposit which no longer ages uniformly and slow down the
diffusion-controlled growth of the branched deposit structure. The
results obtained are consistent with the majority of the data previously
reported. Thus, gelation of the wax at temperatures below the cloud
point and subsequent thermal-driven diffusion processes are considered
to be the dominant mechanisms of the deposit formation. The counter
diffusion and Ostwald ripening aging concepts were found to be relevant
in the case of the cold finger study and account for the phenomena
observed in this work. The information obtained via NMR imaging is
highly complementary to the results obtained by other techniques that
can aid in understanding the essential processes behind the wax precipitation
phenomena. The approach developed can be effectively extended to study
any thermal-driven phase separation process.
High‐resolution 77Se NMR spectra in single crystals of cesium trihydrogenselenite are measured at 14.09 kG and different temperatures using the cross‐polarization method. The chemical shift tensors of all structurally non‐equivalent selenium nuclei are determined from the angular dependences of spectra separate lines in paraelectric and antiferroelectric phases. It is established that chemical shift tensor parameters are closely connected with SeO3 group geometry reflecting its distortion degree. The phase transition to the ordered phase is accompanied by increasing of SeO3 group distortion due to the ordering of protons on hydrogen bonds. The marked anisotropy of the 77Se NMR separate linewidths is founded. An analysis of the theoretical and experimental angular dependences of line second moments shows that the observed anomalous broadening is caused by indirect magnetic interaction between 77Se and 133Cs nuclei of the Cs+—HSeO3− fragment in crystals. The principal values of the indirect dipolar interaction tensor of this selenium nucleus are determined in the paraelectric phase.
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