S. I. Andronenko

Magnetic resonance studies of Co2+ ions in nanoparticles of SnO2 processed at different temperatures

Cobalt doping (⩽1%) produces ferromagnetism at room temperature in semiconducting SnO2, presumably due to oxygen vacancies and/or changes in carrier concentration. Electron paramagnetic resonance (EPR) is a sensitive technique to investigate the Co ionic states and their local environments and/or interactions. This paper reports EPR studies of Co2+ ions doped in chemically synthesized nanoparticles of SnO2 carried out at 5K. EPR spectra were recorded from 600°C prepared SnO2 with Co concentrations of 0.5%, 1%, 3%, 5%, 8%, and 12% and from 1% Co-doped SnO2 prepared at temperatures of 150, 250, 350, 450, 600, and 830°C. Each EPR spectrum in samples with cobalt doping can be simulated as an overlap of spectra due to two broad ferromagnetic resonance lines and those due to interstitially and substitutionally incorporated Co2+ ions with effective spin S=1∕2 characterized by their particular g and A tensors. It is concluded that the Co2+ ions occupy substitutional as well as interstitial sites of SnO2 and that ...

A variable temperature Fe3+ electron paramagnetic resonance study of Sn1−xFexO2 (0.00⩽x⩽0.05)

X-band (∼9.5GHz) electron paramagnetic resonance (EPR) studies of Fe3+ ions in Sn1−xFexO2 powders with 0.00⩽x⩽0.05 at various temperatures (5–300K) are reported. These samples are interesting to investigate as Fe doping (⩽5%) produces ferromagnetism in SnO2 [A. Punnooose et al., Phys. Rev. B 72, 054402 (2005)], making it a promising ferromagnetic semiconductor at room temperature. The EPR spectrum at 5K can be simulated reasonably well as the overlap of spectra due to seven magnetically inequivalent Fe3+ ions: four low-spin (S=1∕2) and three high-spin (S=5∕2) ions, characterized by different spin-Hamiltonian parameters, overlapped by three broad ferromagnetic resonance spectra. The three high-spin ions, situated substitutionally in the interior of nanodomains, are characterized by smaller zero-field splitting (ZFS) parameters D and E, so that all their energy levels are populated at 5K. On the other hand, the four low-spin ions are situated interstitially at the surfaces of nanodomains. They are character...

Cr3+ electron paramagnetic resonance study of Sn1−xCrxO2 (0.00≤x≤0.10)

This paper reports on the liquid-helium-temperature (5 K) electron paramagnetic resonance (EPR) spectra of Cr3+ ions in the nanoparticles of SnO2 synthesized at 600 °C with concentrations of 0%, 0.1%, 0.5%, 1%, 1.5%, 2.0%, 2.5%, 3.0%, 5.0%, and 10%. Each spectrum may be simulated as overlap of spectra due to four magnetically inequivalent Cr3+ centers characterized by different values of the spin-Hamiltonian parameters. Three of these centers belong to Cr3+ ions in orthorhombic sites, situated near oxygen vacancies, characterized by very large zero-field splitting parameters D and E, presumably due to the presence of nanoparticles in the samples. The fourth EPR spectrum belongs to the Cr3+ ions situated at sites with tetragonal symmetry, substituting for the Sn4+ ion, characterized by a very small value of D. In addition, there appears a ferromagnetic resonance line due to oxygen defects for samples with Cr3+ concentrations of ≤2.5%. Further, in samples with Cr3+ concentrations of ≥2.5%, there appears an ...

Variable-frequency EPR study of Mn 2þ -doped NH 4 Cl 0:9 I 0:1 single crystal at 9.6, 36, and 249.9 GHz: structural phase transition

Multifrequency electron paramagnetic resonance studies on the Mn(2+) impurity ion in a mixed single crystal NH(4)Cl(0.9)I(0.1) were carried out at 9.62 (X-band) in the range 120-295 K, at 35.87 (Q-band) at 77 and 295 K, and at 249.9 GHz (far-infrared band) at 253 K. The high-field EPR spectra at 249.9 GHz are well into the high-field limit leading to a considerable simplification of the spectra and their interpretation. Three magnetically inequivalent, but physically equivalent, Mn(2+) ions with their respective magnetic Z-axes oriented along the crystallographic [100], [010], [001] axes were observed. Simultaneous fitting of EPR line positions observed at X-, Q-, and far infra-red bands was performed using a least-squares procedure and matrix diagonalization to estimate accurately the Mn(2+) spin-Hamiltonian parameters. The temperature variation of the linewidth and peak-to-peak intensities of the EPR lines indicate the presence of lambda-transitions in the mixed NH(4)Cl(0.9)I(0.1) crystal at 242 and 228 K consistent with those observed in the pure NH(4)Cl and NH(4)I crystals, respectively. A superposition-model analysis of the spin-Hamiltonian parameters reveals that the local environment of the Mn(2+) ion is considerably reorganized to produce axially symmetric crystal fields about the respective Z-axes of the three magnetically inequivalent ions as a consequence of the vacancy created due to charge-compensation when the divalent Mn(2+) ion substitutes for a monovalent NH(4)(+) ion in the NH(4)Cl(0.9)I(0.1) crystal. This reorganization is almost the same as that observed in NH(4)Cl and NH(4)I single crystals, although the latter two are characterized by different, simple cubic and face-centered cubic, structures.

Role of dopant incorporation on the magnetic properties of Ce1-xNixO2 nanoparticles: An electron paramagnetic resonance study

Nickel doping has been found to produce weak room-temperature ferromagnetism (FM) in CeO2. The saturation magnetization (Ms) of the chemically synthesized Ce1−xNixO2 samples showed a maximum for x=0.04, above which the magnetization decreased gradually. For Ce1−xNixO2 samples with x⩾0.04, an activation process involving slow annealing of the sample to 500°C increased the Ms by more than two orders of magnitude. However, no such activation effect was observed in samples with x 0.04, and (ii) the dramatic increase in Ms in the activated Ce1−xNixO2 samples with x⩾0.04 and the absence of this behavior in samples with x<0.04. Detailed analysis by simulation of the EPR data on several as-prepared Ce1−xNixO2 samples with 0.01⩽x⩽0.10 at 5 and 300K indicates the presence of several paramagnetic species: (i) two magnetically inequivalen...

Single-crystal EPR studies of transition-metal ions in inorganic crystals at very high frequency

Electron paramagnetic resonance (EPR) single-crystal rotation studies at very high frequency (249.9 GHz) of transition metal ions with electron spins greater than one-half are reported. At 249.9 GHz, the spectra are in the high-field limit despite large zero-field splittings. This leads to a considerable simplification of the spectra, and aids in their interpretation. Single-crystal 249.9 GHz EPR spectra of Ni2+ in Ni2CdCl6· 12H2O, Mn2+ (0.2%) in ZnV2O7, and Fe3+ (2%) in CaYA104 were recorded at 253 K in an external magnetic field of up to 9.2 T, along with those at X-band and Q-band frequencies at 295 K and lower temperatures. The goniometer used at 249.9 GHz for single-crystal rotation is based on a quasi-optical design and is an integral part of a special Fabry-Pérot resonator. The values of the spin-Hamiltonian parameters were estimated from a simultaneous fitting of all of the observed line positions at several microwave frequencies recorded at various orientations of each crystal with respect to the external magnetic field with least-squares fitting in conjunction with matrix diagonalization. Estimates of zero-field splitting parameterD at room temperature are: for Ni2+, about −31 GHz (site I) and about −7 GHz (site II); for Mn2+, about 6 GHz; and for Fe3+, about 29 GHz.