Spin exchange dynamics in 4H SiC monocrystals with different nitrogen donor concentrations

4H silicon carbide (SiC) polytype is preferred over other SiC polytypes for high-power, high-voltage, and high-frequency applications due to its superior electrical, thermal, and structural characteristics. In this manuscript, we provide a comprehensive study of the spin coupling dynamic between conduction electrons and nitrogen (N) donors in monocrystalline 4H SiC with various concentrations of uncompensated N donors from 10^17 cm^-3 to 5x10^19 cm^-3 by continuous wave, pulsed EPR, and microwave perturbation techniques at T=4.2-300 K. At low temperatures two triplets due to N donors in cubic (Nk) hexagonal (Nh) positions and triplet arisen from spin-interaction between Nh and Nk were observed in 4H SiC having Nd-Na=10^17 cm^-3. A single S-line (S=1/2) dominates the EPR spectra in all investigated 4H SiC monocrystals at high temperatures. It was established that this line occurs due to the exchange coupling of localized electrons (dominate at low temperatures) and non-localized electrons (dominate at high temperatures). The localized electrons were attributed to Nh for Nd-Na=10^17 cm^-3 and Nk donors for Nd-Na>=5x10^18 cm^-3. We have concluded that the conduction electrons in 4H SiC monocrystals are characterized by gpar=2.0053(3) gper=2.0011(3) for Nd-Na<=5x10^18 cm^-3 and gpar=2.0057(3) and gper=2.0019(3) for Nd-Na=5x10^19 cm^-3. Using the theoretical fitting of the temperature variation of S-line EPR linewidth in 4H SiC having Nd-Na<5x10^18 cm-^3, the energy levels of 57-65 meV that correlate with the valley-orbit splitting values for Nk donors in 4H SiC monocrystals were obtained

Mössbauer and SQUID Characterization of Iron in Human Tissue: Case of Globus Pallidus

Lyophilized samples of human brain tissue from the region of Globus Pallidus were investigated in powder form by $\text{}^{57}Fe$ Mössbauer spectroscopy and SQUID magnetometry. The obtained hyperfine parameters including isomer shift and quadrupole splitting exhibit a ferritin-like behaviour with high-spin Fe(III) sites. Three types of features were derived from temperature development of magnetic susceptibility: dominating diamagnetism, dominating paramagnetism, and intermediate states. The magnetic characteristics of the samples do not correlate with age of the donors

Fe2O3/SiO2Fe_2O_3/SiO_2 Hybrid Nanocomposites Studied Mainly by Mössbauer Spectroscopy

Magnetic nanocomposites exhibit promising applications in many areas, for example optics, electronics, biology, medicine, etc. The main goal of this study was to synthesize magnetic ε-$Fe_2O_3$ nanoparticles embedded in amorphous $SiO_2$. These materials were prepared by the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 and 30 wt.%. The structure and properties of the final product were analysed by the Mössbauer spectrometry as well as by X-ray diffraction, scanning electron microscopy, and high resolution transmission electron microscopy. They strongly depend on the initial conditions of preparation

Magnetic Hyperfine Fields of Nanoperm Alloys

Magnetic hyperfine fields of $Fe_{90}Zr_7B_3$ Nanoperm nanocrystalline alloy are characterized by $\text{}^{57}Fe$ Mössbauer spectrometry and $P^{57}Fe$ NMR as well as by magnetic force microscopy. $\text{}^{57}Fe$ NMR enables to distinguish a broad signal of iron atoms located in a residual amorphous matrix from a narrow one which belongs to Fe in nanograins. The former coincides with the distribution of hyperfine fields obtained from $\text{}^{57}Fe$ Mössbauer spectroscopy. In addition, it is possible to make a distinction between NMR signals of the Fe nanograins located in magnetic domains from that of the nanograins positioned in domain walls. This is confirmed by magnetic force microscopy where appearance of maze-domains is observed

Fe 2

Magnetic nanocomposites exhibit promising applications in many areas, for example optics, electronics, biology, medicine, etc. The main goal of this study was to synthesize magnetic ε-$Fe_2O_3$ nanoparticles embedded in amorphous $SiO_2$. These materials were prepared by the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 and 30 wt.%. The structure and properties of the final product were analysed by the Mössbauer spectrometry as well as by X-ray diffraction, scanning electron microscopy, and high resolution transmission electron microscopy. They strongly depend on the initial conditions of preparation

Mössbauer studies of magnetic Fe2O3/SiO2 nanocomposites

Suppl. E (2006): Proceedings of the International Colloquium "Mössbauer Spectroscopy in Materials Science" (June 11–15, 2006, Kočovce, Slovak Republic)A large variety of glass and glass ceramics may be obtained by sol-gel process from hydrolysis of tetraethoxysilane. The transformation involves hydrolysis and polycondensation reactions leading to the growth of clusters that eventually collide together to form a gel. The structure and properties of the final product have been found to be strongly dependent on the initial conditions of preparation. Silica nanocomposites based on Fe2O3/SiO2 were prepared with the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 to 30wt.%

Mössbauer studies on ultraporous Fe-Oxide/SiO2 aerogel

Magnetic aerogels with very low volume density of ∼0.2 g/cm3 were prepared by sol-gel method and supercritical drying. The resulting materials were monolithic and displayed high surface area. By X-ray diffraction and Mössbauer spectroscopy the crystalline phase formed inside the mesopores of the SiO2 matrix was identified as a spinel iron oxide. Comparison of the magnetic measurements with Mössbauer spectra at various temperatures contributed to the elucidation of the magnetic state of this nanocomposite system with restricted magnetic interactions, in particular its transition to a superparamagnetic state