Phase transitions in Fe doped VO2

Resistivity measurements were performed on single crystals of V_1-χFe_χO₂ (0 ? χ < 0.03). Anomalies in the ln σ vs T^-1 curve indicate the presence of different structural phases. The results of these experiments and previous ⁵⁷Fe Mössbauer data are discussed in terms of a model recently proposed by Pouget et al. for the structural phases in the system V_1-χCr_χO₂     

多铁材料MnSb2O6中自旋涨落的ESR研究

具有手性晶体结构的MnSb₂O₆其基态为螺旋磁序,对外磁场有着响应丰富的铁电性.本文通过助熔剂法制备了高质量MnSb₂O₆单晶.电子自旋共振谱(ESR)的结果表明其共振场具有类似铁磁材料的各向异性温度依赖关系.这一结果表明MnSb₂O₆基态的螺旋磁序在外磁场中形成了随磁场方向转动的圆锥磁序相(conical phase).对共振峰半高宽的进一步拟合得到一个意外小的临界指数,这表明MnSb₂O₆中的磁矩具有二维特征并且存在着较强的竞争相互作用.     

Electron spin resonance in the linear chain compound RbFeS2

ESR measurements have been performed in the linear chain entiferromagnet RbFeS₂ in the temperature range 100–380 K. Both the resonance field and the linewidth are found to change steadily as the temperature is increased, without any discontinuity or singularity at the transition temperature T_N = 185 K. The experimental results are compared with available data on other one-dimensional compounds.     

Electron spin resonance of diluted solid solutions of MnO2 in Y2O3

Electron spin resonance spectra of Mn²⁺ in diluted solid solutions of MnO₂ in Y₂O₃ have been studied at room temperature for Mn concentrations between 0.20 and 2.00 mol%. Isolated Mn²⁺ ions in sites with two different symmetries were observed, as well as Mn²⁺ ions coupled by the exchange interaction. The relative concentration of isolated to coupled Mn²⁺ ions decreases with increasing manganese concentration. The results are consistent with the assumption that the manganese ions occupy preferentially the C₂ symmetry sites. A theoretical calculation based on this model yields an effective range of the exchange interaction between Mn²⁺ ions of 0.53 nm, of the same order as that of Mn²⁺ ions in CaO.     

Electrical and seebeck effect measurements in Nb doped VO2

The resistance of pure and Nb doped VO₂ and the Seebeck coefficient of Nb doped VO₂ have been measured in the temperature range of 78–360 K. A simple analysis of the results shows that above 140 K and below the transition temperature the effective density of states in the conduction band of VO₂ is of the order of (but larger than) one state per vanadium atom. This high effective density of states is consistent with the large effective mass (and low mobility) of electrons in this material. It is shown also that in this range, the temperature dependence of the electronic mobility in VO₂ is T^?γ where γ ? 2. Additional results are discussed in the text.     

Low frequency dielectric relaxation in lightly doped VO2 single crystals

The relative effects of intrinsic and extrinsic defects on the dielectric relaxation of VO₂ crystals have been investigated by measurement of the dielectric parameters of undoped crystals and crystals doped with Ti, Cr and Al. Measurements have been made in the temperature range 77–250 K and the frequency range 50–100 kHz. The dielectric data is described by a Cole-Cole distribution function with a distribution parameter α ? 0.45 which decreases with increasing temperature. However, the distribution of activation energies g(E) derived from α is almost independent of temperature. The overall dielectric relaxation behaviour is determined primarily by the intrinsic defect structure of VO₂, and the effect of impurities is observed only in changes in the low frequency limiting (static) value of the dielectric constant. The same transport mechanism is found to determine the dc conductivity and the dielectric relaxation and evidence is presented that the dielectric relaxation is of dipolar origin.     

Photoluminescence of chromium doped CdIn2S4

Well-resolved photoluminescence spectra of CdIn₂S₄: Cr³⁺ recorded at 77K and 2K have been analysed. The R-lines from the doublet ²E-⁴A₂ transition and their associated vibronic spectra have been assigned. These results yield a revised value for the crystal field parameter of B = 618 cm^-1     

Electron spin resonance study of the spin dynamics in the impurity doped one-dimensional magnetic crystal (CH3)4NMnCl3:Cu2+

We have studied the spin dynamics in the one dimensional magnetic crystal, tetramethyl manganese trichloride (TMMC) doped with different concentrations of the Cu²⁺ ion, using electron spin resonance. We have observed the line widths to increase with Cu²⁺ impurity and the line shapes to deviate from the one-dimensional character.     

Electron spin resonance studies on γ-irradiated KMnO4

Electron spin resonance (ESR) studies have been carried out on the radicals produced by the γ-irradiation of single crystals of KMnO₄.Analysis of the data yields at least two paramagnetic species. The first radical is produced by room temperature irradiation and gives a single line for each of two inequivalent magnetic sites. There is no resolvable hyperfine structure and the g tensor for the species is given by, g_b = g_μ′ = 2·0176, g_x′ = 2·0020, g_z′ = 2·0112. The spectrum is tentatively assigned to the O₃^? radical although MnO₂ and MnO₄ are also possibilities.The second radical is produced by irradiation at 77°K and the spectrum disappears irreversibly at slightly higher temperatures. Analysis of the very complicated spectra yields, g_b = 1·9617, g_x = 1·9452, g_u = 1·9940, A_b = 386 MHz, A₂ = 87 MHz, A₃ = 20 MHz. This spectrum is assigned to the MnO₄^?2 radical. The relationship of the radicals observed to the effect of high energy radiation on the decomposition of KMnO₄ in the light of current solid state theory is discussed.     

Electron spin resonance study of Mo centers in YAlO3

Electron spin resonance measurements of Ce-doped YAlO₃ single crystals (40–50 ppm of Ce) grown by Czochralski method from Molybdenum crucibles evidenced two kinds of Mo³⁺ centers. The ESR spectra of both centers were described by spin Hamiltonian with orthorhombic symmetry and an effective electron spin . The experimental g-factors as well as four magnetically nonequivalent positions found for each center support a hypothesis that Mo impurity ion is located at Al site. Under high-temperature annealing and UV irradiation the Mo centers show the change of charge state, indicating, that they are probably involved in the charge carrier capture in the energy transfer and storage processes in this scintillator material.     

Magnon modes and impurity spin resonance in FeBr2 doped with 1% Mn2+

We report results of far infrared magneto-absorption experiments in FeBr₂ doped with 1% MnBr₂. Using radiations from several carcinotrons covering the frequency range 77–600 GHz, we observe both the spectra corresponding to the localized Mn²⁺ impurity modes and the uniform magnon modes of the host crystal. The excitation energy gap of the A.F. magnons at zero field is E_o = 500 ± 2 GHz.     

The phase transition in VO2

The electronic d-states in VO₂ are investigated by a semi-empirical MO LCAO method. The calculations of clusters for the crystal structures above and below the phase transition point show that the energy gap is mainly formed by the crystal distortions. The energies calculated agree well with the ESCA experimental estimations and explain the large gap values obtained from the optical data.     

Coulomb interactions in metallic VO2

The intra-cationic coulomb energy in the metallic phase of VO₂ is estimated using a model in which the observed spin susceptibility and its temperature dependence arise from correlation modified exchange enhancement of the spin susceptibility of the d - subbands.     

Electron spin resonance and optical identification of Fe4+-V0 in SrTiO3

The electron spin resonance of the minority defect Fe⁴⁺-V₀ has been observed in SrTiO₃. It is the first pair center identified with a two-fold positive charge with respect to the lattice. For Fe⁴⁺ it occurs in the unusual high spin S = 2 state. Optically-induced conversion to the neighbor Fe³⁺-V₀ charge state served to identify the acceptor transfer bands, showing the potentiality of the conversion-rate method.     

The metal-insulator transition in VO2

A model of the phase transition in VO₂ is presented which is based on phonon softening effects and gives quantitative agreement with experiment.     

Electron spin resonance spectroscopy of the organic conductor: (TMTSF)2PF6

The ESR g-factor, linewidth and spin susceptibility of (TMTSF)₂PF₆ are each found to have a distinct temperature dependence. The anisotropy of g and of the linewidth for static magnetic fields in the plane perpendicular to the highly conducting axis suggest that spin-phonon scattering is the dominant relaxation mechanism. The metal-insulator transition at 19 K is reflected particularly clearly in the magnetic properties.     

One-electron dispersion relations in metallic VO2

One-electron dispersion relations are presented for those bands in VO₂ involved in the metallic conduction which characterises the rutile phase. The density of states at the Fermi energy (0.56 eV) is found to be 2.89 eV^-1 per cation, and the metallic state to be stable against Mott-insulation.     

Electron spin resonance study of bulk and nanosized La0.875Sr0.125MnO3

The structural, transport and electron spin resonance properties of bulk and nanosized La_0.875Sr_0.125MnO₃ prepared by a sol-gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T_C. In addition to a sharp peak in the vicinity of T_C, another sharp peak close to is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.     

Electron spin resonance study of the Ba-doping manganite Nd0.5Sr0.5MnO3

We have performed magnetization measurements and electron spin resonance （ESR） on polycrystalline manganites of Nd0.5Sr0.5-xBaxMnO3 （x = 0.1）. Phase separation and phase transitions are observed from the susceptibility and the ESR spectra data. Between 260 K （～ Tc） and 185 K （～ TN）, the system coexists of the paramagnetic phase and the ferromagnetic （FM） phase. Between 185 K and 140 K, the system coexists of the FM phase and the antiferromagnetic （AFM） phase. These results indicate that the system has a very complex magnetic state due to the origin of the instability stemming from manganite Nd0.5Sr0.4Ba0.1MnO3 by partially substituting the larger Ba^2＋ ions for the smaller Sr^2＋ ions.     

Electron spin resonance study of Fe doping effect in La0.67Ca0.33MnO3

Electron spin resonance (ESR) study was carried out on La_0.67Ca_0.33Mn_1−xFe_xO₃ (x=0.0, 0.04) samples. The temperature dependence of the ESR spectra indicates the presence of phase separation above and below T_C in x=0.0 and 0.04 sample, respectively. The increase of the g-value in the high-temperature region indicates the existence of local spin correlations even in the paramagnetic state. The activation energy obtained from both the temperature dependence of the ESR intensity and linewidth exhibits a smaller value in the Fe-doped sample. Our study suggests that the ferromagnetic spin correlations would be significantly weakened by a slight doping of Fe ions on Mn sites.