Том 48, № 5 (2017)

High-frequency electron spin resonance (ESR) measurements were carried out on a single crystal β-Cu₂V₂O₇, where Cu–O edge-sharing chains are connected by non-magnetic VO₄ tetrahedrons. Due to sizable anisotropies, analyses of bulk magnetic properties alone have not yielded any definite conclusion and the modeling of the magnetic lattice is controversial. By means of high-frequency ESR with operating frequencies of 135–405 GHz and at a temperature range of 4.2–50 K, the large g value anisotropy is determined, which removes the ambiguity of susceptibility fitting. Moreover, non-negligible exchange anisotropy is evaluated by analyzing high-field magnetization and antiferromagnetic resonance in antiferromagnetic state appearing below T_N = 26 K. Based on these microscopic information, we found that alternating-chain model is the most reasonable candidate for explaining properties of the crystal. The present study shows the importance of combining macroscopic and microscopic probes in analyzing magnetic network of complex magnetic materials.

Iron-doped TiO₂ nanoparticles with iron content in the range of 0.005 < Fe/Ti < 0.3 were prepared using the flame spray pyrolysis method and investigated with CW X-band electron paramagnetic resonance (EPR), X-ray diffraction, and Fourier transform infrared spectroscopy. This allowed for the clarification of the internal organization of Fe–TiO₂ nanoparticles. Different types of Fe(III) centers were distinguished in the samples: isolated high-spin paramagnetic Fe(III) ions (S = 5/2) in rhombic ligand fields state at 0.005 < Fe/Ti < 0.05, and Fe(III) ferromagnetic clusters at Fe/Ti < 0.1. All Fe-doped samples had rather high activity for the photocatalytic mineralization of oxalic acid under visible light illumination (λ > 400 nm) at 25 °C. Correlations were made between EPR and photocatalytic activity results. The specific surface area [S] data allowed us to deduce that the isolated Fe(III) centers were responsible for the photomineralisation of oxalic acid, while the Fe(III) ferromagnetic aggregates decreased the total efficiency of the system.

We present the results of experiments on dynamic nuclear polarization and relaxation of ⁷⁵As in silicon crystals. Experiments are performed in strong magnetic fields of 4.6 T and temperatures below 1 K. At these conditions donor electron spins are fully polarized, and the allowed and forbidden electron spin resonance transitions are well resolved. We demonstrate effective nuclear polarization of ⁷⁵As nuclei via the Overhauser effect on the time scale of several hundred seconds. Excitation of the forbidden transitions leads to a polarization through the solid effect. The relaxation rate of donor nuclei has strong temperature dependence characteristic of Orbach process.

The layout of radio-frequency received coils is related to signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI). In this paper, different structures of four-channel received coil arrays for vertical-field MRI are constructed and optimized by establishing the relationship between coil geometry and SNR to achieve a high SNR and a uniform SNR distribution in the region of interest (ROI). Then, the SNR distributions of three optimized configurations, including rectangular loops, non-definite shape surface coils, and solenoid loops as the main unit, are simulated and compared. The four-channel coil of solenoid loops as the main unit has been found to have the best performance with the highest mean SNR in the ROI when imaging without acceleration. In addition, g-factor and 2D SENSE SNR in yoz-plane are simply analyzed, which show a sharp decrease in SNR for all the coils. Finally, all the coils are manufactured and operated at a 0.5 T permanent magnet MRI system with phantom and joint imaging experiments. Using pixel-by-pixel manner to evaluate SNR map, the experimental results are consistent with the simulation results, while parallel imaging experiment results show that the major consideration in low field MRI is the improvement of SNR value and uniformity rather than that of the imaging speed. As different constructions of four-channel received coils are investigated, we have found the most effective configuration with high and uniform SNR for vertical-field MRI.