57Fe Mössbauer spectroscopy and magnetization study of YFexAl12−x (4.4⩽x⩽5)

YFe_xAl_12−x in the composition range 4.4⩽x⩽5 was prepared by induction melting followed by annealing in vacuum at 1270 K. Magnetization data below 150 K show complex magnetic behaviour dependent on applied field, composition and temperature. The transition temperature T_c, corresponding to the main maximum of the magnetization vs. temperature curves and below which magnetic interactions are observed for a significant fraction of the Fe atoms in the Mössbauer spectra, decreases from 180 K for x=4 down to 100 K for 4.2⩽x⩽4.7 and rises again up to 160 K for x=5. The analysis of the spectra obtained at 5 K is consistent with full occupation of the 8f sites by Fe atoms and sharing of the 8j sites by Fe and Al as deduced from the Rietveld analysis of X-ray powder diffraction data. The Mössbauer spectra further show a dependence of magnetic hyperfine fields and isomer shifts on the crystallographic site and on the number of the Fe nearest neighbours similar to that observed in UFe_xAl_12−x (4⩽x⩽6) and RFe_xAl_12−x (R=Y, Lu, x=4, 4.2). The magnetic properties of the UFe_xAl_12−x and YFe_xAl_12−x series are compared and the magnetic interactions between the different Fe sublattices are discussed.     

Microscopic magnetic properties of (V1−xTix)2O3 near the phase boundary of the metal–insulator transition

Magnetic susceptibility (χ) and ⁵¹V NMR have been measured in (V_1−xTi_x)₂O₃ near the phase boundary of the metal–insulator transition. It is established that the transition from antiferromagnetic insulating (AFI) to antiferromagnetic metallic phases near x_c≈0.05 is not quantum critical, but is discontinuous with a jump of the transition temperature. In the AFI phase at 4.2 K, we observed the satellite in the zero-field ⁵¹V NMR spectrum around 181 MHz in addition to the ‘host’ resonance around 203 MHz. The satellite is also observable in the paramagnetic metallic phase of the x=0.055 sample. We associated the satellite with the V sites near Ti, which are in the V³⁺-like oxidation state, but has different temperature dependence of the NMR shift from that of the host V site. The host d-spin susceptibility for x=0.055 decreases below ∼60 K, but remains finite in the low-temperature limit.     

Internal magnetic fields in submicron particles of barium hexaferrite detected by 57Fe NMR

⁵⁷Fe nuclear magnetic resonance (NMR) spectra of hexaferrite BaFe₁₂O₁₉ powder samples prepared by glass crystallization method were measured at 4.2 K and analyzed in comparison to spectra of single crystals. Samples with various mean particle dimensions were tested. NMR spectral lines corresponding to individual iron sublattices showed pronounced frequency shifts of their positions and a significant line broadening compared to single crystals. The significant contribution to the line shifts and line shape had a uniform macroscopic origin giving identical absolute value of shifts and the same line shapes for all measured lines of a particular sample. Estimations of demagnetization fields based on mean particle dimensions reasonably corresponded to the observed frequency shifts for particle mean diameter 67 nm, or had a higher value for a sample with mean diameter of 340 nm, for which a presence of domain walls was detected by NMR. In the spectrum of a sample with the smallest particles (～16 nm), an additional contribution having broader lines and faster spin-spin relaxations was found. It could be assigned to weaker exchange interactions or deviations of magnetic moment directions from the hexagonal axis in a surface layer.     

A comparative study of different concentrations of pure Zn powder effects on synthesis,structure, magnetic and microwave-absorbing properties in mechanically-alloyed Ni–Zn ferrite

In this study, a powder mixture of Zn, Fe₂O₃ and NiO was used to produce different compositions of Ni_1−xZn_xFe₂O₄ (x=0.36, 0.5 and 0.64) nanopowders. High-energy ball milling with a subsequent heat treatment method was carried out. The XRD results indicated that for the content of Zn, x=0.64 a single phase of Ni–Zn ferrite was produced after 30 h milling while for the contents of Zn, x=0.36 and 0.5, the desired ferrite was formed after sintering the 30 h-milled powders at 500 °C. The average crystallite size decreased with increase in the Zn content. A DC electrical resistivity of the Ni–Zn ferrite, however, decreased with increase in the Zn content, its value was much higher than those samples prepared by the conventional ceramic route by using ZnO instead of Zn. This is attributed to smaller grains size which were obtained by using Zn. The FT-IR results suggested two absorption bands for octahedral and tetrahedral sites in the range of 350–700 cm⁻¹. The VSM results revealed that by increasing the Zn content from 0.36 to 0.5, a saturation magnetization reached its maximum value; afterwards, a decrease was observed for Zn with x=0.64. Finally, magnetic permeability and dielectric permittivity were studied by using vector network analyzer to explore microwave-absorbing properties in X-band frequency. The minimum reflection loss value obtained for Ni_0.5Zn_0.5Fe₂O₄ samples, about −34 dB at 9.7 GHz, making them the best candidates for high frequency applications.     

Band overlap via chemical pressure control in double perovskite (Sr2−xCax)FeMoO6 (0 ⩽ x ⩽ 2.0) with TMR effect

The chemical pressure control in (Sr_2−xCa_x)FeMoO₆ (0 ⩽ x ⩽ 2.0) with double perovskite structure has been investigated systematically. We have performed first-principles total energy and electronic structure calculations for x = 0 and x = 2.0. The increasing Ca content in (Sr_2−xCa_x)FeMoO₆ samples increases the magnetic moment close to the theoretical value due to reduction of Fe/Mo anti-site disorder. An increasing Ca content results in increasing (Fe²⁺ + Mo⁶⁺)/(Fe³⁺ + Mo⁵⁺) band overlap rather than bandwidth changes. This is explained from simple ionic size arguments and is supported by X-ray absorption near edge structure (XANES) spectra and band structure calculations.     

Superconductivity,magnetism and crystal chemistry of Ba1−xKxFe2As2

BaFe₂As₂ is the parent compound of the ‘122’ iron arsenide superconductors and crystallizes with the tetragonal ThCr₂Si₂-type structure, space group I4/mmm. A spin-density-wave transition at 140 K is accompanied by a symmetry reduction to space group Fmmm and simultaneously by antiferromagnetic ordering. Hole-doping induces superconductivity in Ba_1?xK_xFe₂As₂ with a maximum T_c of 38 K at x ≈ 0.4. The upper critical fields approach 75 T with rather small anisotropy of H_c2. At low potassium concentrations (x ? 0.2), superconductivity apparently co-exists with the orthorhombically distorted and magnetically ordered phase. At doping levels x ? 0.3, the structural distortion and antiferromagnetic ordering is completely suppressed and the T_c is maximized. No magnetically ordered domains could be detected in optimally doped Ba_1?xK_xFe₂As₂ (x ? 0.3) by ⁵⁷Fe Mössbauer spectroscopy in contrast μSR results obtained with single crystals. The magnetic hyperfine interactions investigated by ⁵⁷Fe Mössbauer spectroscopy are discussed and compared to the ZrCuSiAs-type materials.     

Feasibility of arsenic and antimony NMR spectroscopy in solids: An investigation of some group 15 compounds

The feasibility of obtaining ⁷⁵As and ^121/123Sb NMR spectra for solids at high and moderate magnetic field strengths is explored. Arsenic-75 nuclear quadrupolar coupling constants and chemical shifts have been measured for arsenobetaine bromide and tetraphenylarsonium bromide. Similarly, ^121/123Sb NMR parameters have been measured for tetraphenylstibonium bromide and potassium hexahydroxoantimonate. The predicted pseudo-tetrahedral symmetry at arsenic and the known trigonal bipyramidal symmetry at antimony in their respective tetraphenyl-bromide “salts” are reflected in the measured ⁷⁵As and ¹²¹Sb nuclear quadrupole coupling constants, C_Q(⁷⁵As)=7.8 MHz and C_Q(¹²¹Sb)=159 MHz, respectively. Results of density functional theory quantum chemistry calculations for isolated molecules using ADF and first-principles calculations using CASTEP, a gauge-including projector augmented wave method to deal with the periodic nature of solids, are compared with experiment. Although the experiments can be time consuming, measurements of ⁷⁵As and ¹²¹Sb NMR spectra (at 154 and 215 MHz, respectively, i.e., at B₀=21.14 T) with linewidths in excess of 1 MHz are feasible using uniform broadband excitation shaped pulse techniques (e.g., WURST and WURST-QCPMG).     

Proton diffusion in the superprotonic phase of [(NH4)1 − xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation

Proton diffusion in [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 < x < 1) has been studied by means of ¹H spin-lattice relaxation times, T₁. The relaxation times were measured at 200.13 MHz in the range of 296–490 K and at 19.65 MHz in the range of 300–470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 ≤ x ≤ 0.54). Rb₃H(SO₄)₂ does not obey this trend. The results of NMR well explain the macroscopic proton conductivity.     

Mössbauer and X-ray studies for Ni0.2ZnxMg0.8−xFe2O4 ferrites

The spinel ferrites of Ni_0.2Zn_xMg_0.8−xFe₂O₄, 0⩽x⩾0.8, were studied at room temperature using X-ray diffraction and Mössbauer patterns. The analysis of the X-ray diffraction patterns proved that the samples have a single phase cubic spinel structure. The calculated values of the theoretical, true and average lattice constants, tetrahedral bond, tetrahedral edge and unshared octahedral edge were found to increase while the shared octahedral edge and octahedral bond decrease as the Zn²⁺ ion substitution increases. Mössbauer studies showed that the samples for x=0, 0.2 and 0.4 are magnetic and show rather broad lines, while for x=0.6 and 0.8 are paramagnetic. The hyperfine parameters of the tetrahedral and octahedral sites were determined as functions of composition x. The cation distributions were deduced and supported by X-ray studies. The B-site pattern was composite and has been fitted into multicomponents and the deduced hyperfine parameters have been discussed as a function of x.     

Effect of Fe doping on the room temperature ferromagnetism in chemically synthesized (In1−xFex)2O3 (0 ⩽ x ⩽ 0.07) magnetic semiconductors

Observation of room temperature ferromagnetism in Fe doped In₂O₃ samples (In_1−xFe_x)₂O₃ (0 ⩽ x ⩽ 0.07) prepared by co-precipitation technique is reported. Lattice parameter obtained from powder X software shows distinct shrinkage of the lattice constant indicating an actual incorporation of Fe ions into the In₂O₃ lattice. X-ray diffraction data measurements show that the entire sample exhibits single phase polycrystalline behavior. SEM micrographs showed the prepared powder was in the range 25–36 nm. SEM EDS mapping showed the presence of Fe and In ions in the Fe doped In₂O₃ sample. The highest remanence magnetization moment (6.624 × 10⁻⁴ emu/g) is reached in the sample with x = 0.03.     

Nuclear polarization by optical pumping in InP:Fe above liquid nitrogen temperature

Hyperpolarized nuclear spins are observed in optically pumped iron-doped InP from 70 K to 140 K. ³¹P NMR was carried out at 9.28 T (159.8 MHz) during optical excitation with circularly polarized light, using a laser diode (λ∼830 nm) as a source. The enhancement of the nuclear spin polarization by optical pumping at 70 K is estimated to be about 34 for those nuclei in the region of the sample absorbing light. This enhancement decreases with increasing temperature. As the direction of the enhanced nuclear spin polarization is found parallel or antiparallel to the travelling direction of the σ⁺ or σ⁻, the contact hyperfine interaction is dominant compared to the dipolar hyperfine interaction.     

7Li and 51V NMR study on Li+ ionic diffusion in lithium intercalated LixV2O5

Li_xV₂O₅ (0.4 < x < 1.4) prepared by solid-state reaction were studied by ⁷Li and ⁵¹V NMR spectroscopy. ⁷Li NMR spectra showed a narrowing of the line width in relation to Li⁺ionic diffusion. Analysis of Li_xV₂O₅ using a Debye-type relaxation model showed a low activation energy ∼0.07 eV in the sample of x = 0.4 below room temperature, and revealed a Li⁺ionic diffusion with larger activation energy ∼0.5 eV above 450 K in lithium-rich samples. The latter is ascribed to the existence of a multi-phase system comprising stable ɛ- and γ-phases, resulting from complicated phase transitions at high temperature. These shapes and shifts enable the classification of the β-, ɛ-, δ-, and γ-phases. The ionic diffusion of Li⁺ ions is discussed in relation to the complicated phase transitions.     

Electrical conductivity of (Ba0.3Sr0.2La0.5)(In1−xFex)O3−δ

We have synthesized the perovskite oxides of the (Ba_0.3Sr_0.2La_0.5)(In_1−xFe_x)O_3−δ system and measured the total electrical conductivity as a function of temperature and oxygen partial pressure. It was found that the single-phase composition region extended from x = 0.0 to x = 1.0, and that the Fe valence increased from 3.06 to 3.50 in that region. The electrical conductivity was semiconducting from x = 0.0 to x = 0.40 and metallic from x = 0.50 to x = 1.0. The total electrical conductivity at 800 °C also increased with the Fe content and achieved a maximum value of 140 (S/cm) at x = 1.0. From the dependence of the electrical conductivity on the oxygen partial pressure, we conclude that above x = 0.50, the majority carriers are holes. The estimated hole conductivity increased exponentially with the amount of Fe⁴⁺ cation present. The oxide ion conductivity was dependent on the oxygen vacancy content.     

1H NMR study of proton dynamics in (NH4)3H(SO4)2

Proton dynamics in (NH₄)₃H(SO₄)₂ has been studied by means of ¹H solid-state NMR. The ¹H magic-angle-spinning (MAS) NMR spectra were traced at room temperature (RT) and at Larmor frequency of 400.13 MHz. ¹H static NMR spectra were measured at 200.13 MHz in the range of 135–490 K. ¹H spin-lattice relaxation times, T₁, were measured at 200.13 and 19.65 MHz in the ranges of 135–490 and 153–456 K, respectively. The ¹H chemical shift for the acidic proton (14.7 ppm) indicates strong hydrogen bonds. In phase III, NH₄⁺ reorientation takes place; one type of NH₄⁺ ions reorients with an activation energy (E_a) of 14 kJ mol^− 1 and the inverse of a frequency factor (τ₀) of 0.85 × 10^− 14 s. In phase II, a very fast local and anisotropic motion of the acidic protons takes place. NH₄⁺ ions start to diffuse translationally, and no proton exchange is observed between NH₄⁺ ions and the acidic protons. In phase I, both NH₄⁺ ions and the acidic protons diffuse translationally. The acidic protons diffuse with parameters of E_a = 27 kJ mol^− 1 and τ₀ = 4.2 × 10^− 13 s. The translational diffusion of the acidic protons is responsible for the macroscopic proton conductivity, as the NH₄⁺ translational diffusion is slow and proton exchange between NH₄⁺ ions and the acidic protons is negligible.     

Electron spin resonance and cation distribution studies of the Co0.6Zn0.4MnxFe2−xO4 ferrite system

The ESR spectra of the ferrite system Co_0.6Zn_0.4Mn_xFe_2−xO₄ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) were obtained at room temperature. The experimental values of the magnetic moment (μ_exp) were estimated from the ESR spectra and the cation distribution was consequently established from the values of μ_exp. The systematic decrease in ESR line width observed in our present study was attributed to the decrease of Fe²⁺ concentration with increasing Mn content. The resonance field decreases and reaches a minimum at high values of Mn content whereas the magnetic moment reaches a maximum at these values. The IR spectra were recorded in the range 200–1200 cm⁻¹. The bands at 569 (ν₁) and 389 cm⁻¹ were assigned to the tetrahedral and octahedral complexes, respectively. The band at 441 cm⁻¹ is due to the Mn–O bond vibration. The theoretical lattice parameter was calculated and was found to be larger than the experimental one a_exp due to the presence of Mn⁴⁺ ions.     

Optical delay with spectral hole burning in Doppler-broadened cesium vapor

The full frequency dependence of the optical delay in the Cs D₁ (6 ²S_1/2 ? 6 ²P_1/2) line has been observed, including all four hyperfine split components. Pulse delays of 1.6 ns to 24.1 ns are obtained by scanning across the hyperfine splitting associated with the lower ²S_1/2 state. Optical control of pulse delays in cesium vapor was demonstrated by pumping the D₂ (6 ²S_1/2 ? 6 ²P_3/2) transition and observing resulting holes in the D₁ delay spectrum. For a pump at four times the saturation intensity, the pulse delays are reduced by a maximum of 78% in a narrow region of 110 MHz. The frequency dependence of the delays of the probe laser in the vicinity of the spectral holes agrees with a Kramers–Kronig model prediction.     

NMR study of local magnetic properties of beryllium substituted copper ferrite

The spin echo NMR spectra of⁵⁷Fe have been taken at 77 K to describe the local magnetic properties of beryllum substituted copper ferrite Cu_1−xBe_xFe₂O₄ for 0≤x≤0.2. From the spectra the concentration dependence of hyperfine magnetic fields for tetrahedral and octahedral sites have been derived. The results show that statistical distribution of beryllium atoms over magnetic sublattices takes place and suggest that only below x=0.2 solid solutions of beryllium copper ferrites can exist.     

Conductivity and oxygen permeability of a novel oxide Pr2Ni0.8 − x Cu0.2FexO4 and its application to partial oxidation of CH4

Iron-doped Pr₂Ni_0.8Cu_0.2O₄ was studied as a new mixed electronic and oxide-ionic conductor for use as an oxygen-permeating membrane. An X-ray diffraction analysis suggested that a single phase K₂NiF₄-type structure was obtained in the composition range from x = 0 to 0.05 in Pr₂Ni_0.8 − xCu_0.2Fe_xO₄. It is considered that the doped Fe is partially substituted at the Ni position in Pr₂NiO₄. The prepared Pr₂NiO₄-based oxide exhibited a dominant hole conduction in the P_O2 range from 1 to 10^− 21 atm. The electrical conductivity of Pr₂Ni_0.8−xCu_0.2Fe_xO₄ is as high as 10² S cm^− 1 in the temperature range of 873–1223 K and it gradually decreased with the increasing amount of Fe substituted for Ni. The oxygen permeation rate was significantly enhanced by the Fe doping and it was found that the highest oxygen permeation rate (60 μmol min^− 1 cm^− 2) from air to He was achieved for x = 0.05 in Pr₂Ni_0.8 − xCu_0.2Fe_xO₄. Since the chemical stability of the Pr₂NiO₄-based oxide is high, Pr₂Ni_0.75Cu_0.2Fe_0.05O₄ can be used as the oxygen-separating membrane for the partial oxidation of CH₄. It was observed that the oxygen permeation rate was significantly improved by changing from He to CH₄ and the observed permeation rate reached a value of 225 μmol min^− 1 cm^− 2 at 1273 K for the CH₄ partial oxidation.     

Quantification of local oxygen defects around Yttrium ions for yttria-doped ceria–zirconia ternary system

Local coordination structure around Yttrium ions in CeO₂–Y₂O₃ binary and [(CeO₂)_x(ZrO₂)_1?x]_0.8(YO_1.5)_0.2 (x = 0.0 ~ 1.0) ternary system has been investigated by ⁸⁹Y MAS-NMR. NMR spectra are found to be consisted of multiple peaks that can be assigned to 6-, 7- and 8-oxygen coordinated Yttrium ions. Compositional dependence of the spectrum was observed and compared with the previous results for ZrO₂–Y₂O₃ binary system. The present investigation suggested the degree of localization of the oxygen vacancy around the cation is in the order of Zr⁴⁺ > Y³⁺ > Ce⁴⁺. The degree of the oxygen vacancy preference for each cation was quantitatively determined for CeO₂–ZrO₂–Y₂O₃ ternary system the first time.     

Suppression of superconductivity in Zn-doped SmFeAsO0.8F0.2 system

A series of SmFe_1?xZn_xAsO_0.8F_0.2 samples with x = 0, 0.05, 0.1, 0.2 and 0.4 have been successfully synthesized using a solid state method. The lattice parameters are found to increase with increasing Zn doping content. The superconductivity has been definitely suppressed by Zn doping at Fe site with the transition temperature T_c being reduced from 52.5 K to 23.3 K for the sample of x = 0.05, and to 18.2 K for the sample of x = 0.1. For the samples with x > 0.1, the superconducting transition vanishes, and, at the meantime, the spin-density-wave anomaly recovers at 140 K. The metal to semiconductor transition is also observed in the SmFe_1?xZn_xAsO_0.8F_0.2 system. The behavior of SmFe_1?xZn_xAsO_0.8F_0.2 is very different from that of R_EFeAsO (R_E = rare earth metal), which reveals a very strong electron correlation in SmFe_1?xZn_xAsO_0.8F_0.2.