Synthesis and characterization of the xZnO-(1−x)α-Fe2O3 nanoparticles system

The xZnO-(1−x)α-Fe₂O₃ nanoparticles system has been obtained by mechanochemical activation for x=0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 h. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy. The Rietveld structure of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x=0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 h. Similar results were obtained for x=0.3, while for x=0.5 the zinc ferrite phase occurred at 2 h and entirely dominated the spectrum at 24 h milling time. The Mössbauer spectra corresponding to x=0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mössbauer spectrum for 12 h of milling had to be fitted with two sextets. For x=0.3 and 12 milling hours, the Mössbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristic to zinc ferrite, ZnFe₂O₄. This doublet clearly dominates the Mössbauer spectrum for x=0.5 and 24 h of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Crystal field investigation on the magnetic properties of Yb in Yb(CF3SO3)3·9H2O

Ytterbium tri-fluoromethanesulfonate (YbTFMS) single crystals are prepared from the slow evaporation of the aqueous solution of YbTFMS and the principal magnetic susceptibility perpendicular to the c-axis of the hexagonal crystal (χ_⊥) is measured from 300 K down to 13 K. Principal magnetic anisotropy Δχ(=χ_∥−χ_⊥) is measured from 300 K down to 80 K which provides principal magnetic susceptibility parallel to the c-axis (χ_∥) down to 80 K. Very good theoretical simulation of the observed magnetic properties of YbTFMS has been obtained using one electron crystal field (CF) analysis having C_3h site symmetry. No signature of ordering effect in the observed magnetic data is noticed down to the lowest temperature (13 K) attained, indicating the inter-ionic interaction to be of predominantly dipolar type. The calculated g-values are found to be g_∥=2.67 and g_⊥=2.51, respectively. CF analysis provides the electronic specific heat which gives two Schottky anomalies in its thermal variation down to ∼13 K. The temperature dependences of quadrupole splitting and hyperfine heat capacity are studied from the necessary information obtained from the CF analysis.     

Magnetism of ErNi2B2C investigated with a two-ion model for rare-earth antiferromagnets

A two-ion model for rare-earth antiferromagnets was employed in the present work to study the magnetic and thermodynamic properties of ErNi₂B₂C, and derive an analytic formula for the Néel temperature of the compound with the quantum perturbation theory. To apply the theory, four new base functions had to be composed due to the special forms of its crystal-field (CF) wavefunctions. Our theoretical analysis suggested that the magnetic behavior of the compound be governed by these low-lying CF levels in the temperature region at least up to 100 K.     

Magnetic and thermodynamic properties of DyFe2Si2 further investigated with crystal-field theory and two-ion model

We report our theoretical studies on the magnetic and thermodynamic properties of DyFe₂Si₂ using the crystal-field parameters (CFPs) proposed by Chatterji et al. recently [1]. Our calculated magnetization and magnetic specific heat curves obtained with the above CFPs based on the two-ion model we developed recently show reasonable agreement with the experimental data.     

Specific heat and magnetic susceptibility of single-crystalline ZnCr2−xAlxSe4 (x=0.15, 0.23)

A correlation between the second critical field H_c2 of the helix to paramagnetic transition and the magnetic specific heat C-peak was found in ZnCr_2−xAl_xSe₄ spinel single crystals with x=0.15, 0.23. The specific heat peak is anomalously sharp for all finite magnetic fields used here and this points to a first order magneto-structural transition (from cubic to tetragonal symmetry). The C(T)-peak is increasingly suppressed as the external field increases. Approaching the Neel temperature T_N, a broad ac-magnetic susceptibility peak is observed for zero dc-magnetic field. That peak does not show an energy loss and thus points towards a return to a second order type of transition. The magnetic contribution to the specific heat displays a sharp peak at T_N and is maximal at the spin fluctuation temperature T_sf=34 K. T_sf is related to the maximum of the magnetic susceptibility at T_m=40 K (at 50 kOe) in the spin fluctuation region, as evidenced by the entropy exceeding 90% of the entropy calculated classically for the complete alignment of the Cr spins, (2−x)R ln(2S+1). The X-ray photoelectron spectroscopy (XPS) data indicate that Al-substitution does not affect Cr³⁺ 3d³ electronic configuration.     

Mössbauer study of Na0.82Co0.99Fe0.01O2

We studied by Mössbauer spectroscopy the Na_0.82CoO₂ compound using 1% ⁵⁷Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe³⁺ and Fe⁴⁺. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co⁺³, Co⁺⁴ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe⁴⁺ and all of Fe³⁺, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.     

Hydrothermal synthesis and structural characterization of (1−x)α-Fe2O3-xSnO2 nanoparticles

Structural and morphological characteristics of (1−x)α-Fe₂O₃-xSnO₂ (x=0.0-1.0) nanoparticles obtained under hydrothermal conditions have been investigated by X-ray diffraction (XRD), transmission Mössbauer spectroscopy, scanning and transmission electron microscopy as well as energy dispersive X-ray analysis. On the basis of the Rietveld structure refinements of the XRD spectra at low tin concentrations, it was found that Sn⁴⁺ ions partially substitute for Fe³⁺ at the octahedral sites and also occupy the interstitial octahedral sites which are vacant in α-Fe₂O₃ corundum structure. A phase separation of α-Fe₂O₃ and SnO₂ was observed for x≥0.4: the α-Fe₂O₃ structure containing tin decreases simultaneously with the increase of the SnO₂ phase containing substitutional iron ions. The mean particle dimension decreases from 70 to 6 nm, as the molar fraction x increases up to x=1.0. The estimated solubility limits in the nanoparticle system (1−x)α-Fe₂O₃-xSnO₂ synthesized under hydrothermal conditions are: x≤0.2 for Sn⁴⁺ in α-Fe₂O₃ and x≥0.7 for Fe³⁺ in SnO₂.     

Magnetic contribution to heat capacity and entropy of nickel ferrite (NiFe2O4)

The heat capacity of nickel ferrite was measured as a function of temperature from 50 to 1200 °C using a differential scanning calorimeter. A thermal anomaly was observed at 584.9 °C, the expected Curie temperature, T_C. The observed behavior was interpreted by recognizing the sum of three contributions: (1) lattice (vibrational), (2) a spin wave (magnetic) component and (3) a λ-transition (antiferromagnetic-paramagnetic transition) at the Curie temperature. The first was modeled using vibrational frequencies derived from an experimentally-based IR absorption spectrum, while the second was modeled using a spin wave analysis that provided a T^3/2 dependency in the low-temperature limit, but incorporated an exchange interaction between cation spins in the octahedral and tetrahedral sites at elevated temperatures, as first suggested by Grimes [15]. The λ-transition was fitted to an Inden-type model which consisted of two truncated power law series in dimensionless temperature (T/T_C). Exponential equality (m=n=7) was observed below and above T_C, indicating symmetry about the Curie temperature. Application of the methodology to existing heat capacity data for other transition metal ferrites (AFe₂O₄, A=Fe, Co) revealed nearly the same exponential equality, i.e., m=n=5.     

Oxygen non-stoichiometry and defect thermodynamics in La2Ni0.9Fe0.1O4+δ

The oxygen hyperstoichiometry of K₂NiF₄-type La₂Ni_0.9Fe_0.1O_4+δ, studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10⁻⁵-0.7 atm at 923-1223 K, is considerably higher than that of undoped lanthanum nickelate. The p(O₂)-T-δ diagram of iron-doped lanthanum nickelate can be adequately described by introducing point-defect interaction energy in the concentration-dependent part of defect chemical potentials and accounting for the site-exclusion effects. The critical factors affecting the equilibrium oxygen incorporation process include coulombic repulsion of interstitial anions, trapping of the p-type electronic charge carriers by iron, and interaction between Fe³⁺ and holes localized on nickel cations. Due to low chemical expansion of La₂Ni_0.9Fe_0.1O_4+δ lattice, the thermodynamic functions governing oxygen intercalation, site-blocking factors and hole mobility are all independent of the defect concentrations. The predominant 3+ state of iron cations under oxidizing conditions was confirmed by the Mössbauer spectroscopy. The stability of La₂NiO₄-based phase in reducing atmospheres is essentially unaffected by doping.     

Mössbauer study of Zn0.4Cu0.6Fe1.2Cr0.8O4

Zn_0.4Cu_0.6Fe_1.2Cr_0.8O₄ has been studied by Mössbauer spectroscopy, SQUID magnetometry, and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant The iron ions are in ferric states and occupy both the tetrahedral (A) and octahedral (B) sites; the fractions of the iron ions at the A-sites and B-sites are 0.52 and 0.34, respectively. While spin orderings are collinear at higher temperatures, spin canting begins to appear around 25 K and increases with decreasing temperature; the canting angle at 4.7 K reaches up to 27°. Debye temperatures of the tetrahedral and octahedral sites are determined to be 339 and 335 K, respectively.     

Orbital fluctuations and orbital order in FeCr2S4

Heat-capacity investigations on the ferrimagnetic spinel FeCr₂S₄ poly- and single crystals provide experimental evidence of orbital liquid and orbital glass states. The low-temperature transition in the polycrystals at 10 K arises from orbital order and is very sensitive to the sample stoichiometry. In the single crystals the orbital order is fully suppressed resulting in an orbital glass state with the heat capacity following a strict T² dependence towards zero temperature. At elevated temperatures, FeCr₂S₄ exhibits an unexpected large linear term of about 100 mJ mol⁻¹ K⁻² as the fingerprint of the orbital liquid.     

Critical behaviour and magnetic properties of A-B spinel ZnxCu1−xFe2O4

Polycrystalline Zn_0.6Cu_0.4Fe₂O₄ ferrites have been prepared using a solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and Mössbauer and magnetic measurements. These results have been compared to a more general theoretical study, on Zn_xCu_1−xFe₂O₄, based on mean field theory and high-temperature series expansions (HTSE), and extrapolated with the Padé approximant method. The nearest neighbour super-exchange interactions for the intra-site and the inter-site of Zn_xCu_1−xFe₂O₄ spinel ferrites, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The Curie temperature T_C is calculated as a function of Zn concentration. The theoretical results obtained are in good agreement with the experimental results obtained by magnetic measurements.     

Vibrational and Fe-Mössbauer spectra of LaFeGe2O7 and NdFeGe2O7

The infrared, Raman and ⁵⁷Fe-Mössbauer spectra of LaFeGe₂O₇ and NdFeGe₂O₇ were recorded and analysed on the basis of their structural characteristics. Some comparisons with the stoichiometrically related materials containing the heavier lanthanides are made, showing that it is possible to differentiate spectroscopically both groups of materials. The Mössbauer parameters clearly reflect the small structural differences in the FeO₅-polyhedra present in these compounds.     

Magnetic properties and magnetic structures of Sr3−xCaxRu2O7

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden-Popper type ruthenate Sr_3−xCa_xRu₂O₇ (x=1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b-axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T_N, an additional reflection was observed at (0 0 1) for each x, and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca₃Ru₂O₇. We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca₃Ru₂O₇.     

α-Fe2O3-In2O3 mixed oxide nanoparticles synthesized under hydrothermal supercritical conditions

α-Fe₂O₃-In₂O₃ mixed oxide nanoparticles system has been synthesized by hydrothermal supercritical and postannealing route, starting with (1−x)Fe(NO₃)₃·9H₂O·xIn(NO₃)₃·5H₂O aqueous solution (x=0-1). X-ray diffraction and Mössbauer spectroscopy have been used to study the phase structure and substitutions in the nanosized samples. The concentration regions for the existence of the solid solutions in the α-Fe₂O₃-In₂O₃ nanoparticle system together with the solubility limits of In³⁺ ions in the hematite lattice and of Fe³⁺ ions in the cubic In₂O₃ structure have been evidenced. In general, the substitution level is considerably lower than the nominal concentration x. A justification of the processes leading to the formation of iron and indium phases in the investigated supercritical hydrothermal system has been given.     

Characterization of partially inverse spinel ZnFe2O4 with high saturation magnetization synthesized via soft mechanochemically assisted route

ZnFe₂O₄ was prepared by a soft mechanochemical route from two starting combinations of powders: (1) Zn(OH)₂/α-Fe₂O₃ and (2) Zn(OH)₂/Fe(OH)₃ mixed in a planetary ball mill. The mechanochemical treatment provoked reaction leading to the formation of the ZnFe₂O₄ spinel phase that was monitored by XRD, TEM, IR and Raman spectroscopy. The spinel phase was first observed after 4 h of milling and its formation was completed after 18 h in both the cases of starting precursors. The synthesized ZnFe₂O₄ has a nanocrystalline structure with a crystallite size of about 20.3 and 17.6 nm, for the cases (1) and (2), respectively. In the far-infrared reflectivity spectra are seen four active modes. Raman spectra suggest an existence of mixed spinel structure in the obtained nanosamples. In order to confirm phase formation and cation arrangement, Mössbauer measurements were done. Estimated degree of inversion is about 0.58 for both starting mixtures. The magnetic properties of the prepared ZnFe₂O₄ powders were also studied. The results show that the samples have a typical superparamagnetic-like behavior at room temperature. Higher values of magnetization in the case of samples obtained with starting mixture (2) suggest somewhat higher degree of cation inversion.     

Defect formation and transport in mixed-conducting La0.90Sr0.10Al0.85−xFexMg0.15O3−δ

The redox behavior of perovskite-type La_0.90Sr_0.10Al_0.85−xFe_xMg_0.15O_3−δ (x=0.20-0.40) mixed conductors was analyzed by the Mössbauer spectroscopy and measurements of the total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10⁻²⁰ to 0.5 atm at 1023-1223 K. The results combined with oxygen-ion transference numbers determined by the faradaic efficiency technique in air, were used to calculate defect concentrations, mobilities, and partial ionic and p- and n-type electronic conductivities as a function of oxygen pressure. The redox and transport processes can be adequately described in terms of oxygen intercalation and iron disproportionation reactions, with the thermodynamic functions independent of defect concentrations. No essential delocalization of the electronic charge carriers was found. The oxygen non-stoichiometry values estimated from the conductivity vs. p(O₂) dependencies, coincide with those evaluated from the Mössbauer spectra.     

Van Vleck paramagnetism of the thulium garnet Tm3Al5O12

The magnetic susceptibility of the garnet-type single crystal Tm₃Al₅O₁₂ exhibits the typical Van Vleck temperature independent paramagnetism below ≈8 K. The temperature dependence of the susceptibility over the range 2.0-300 K has been analyzed on the assumption that the cubic crystal-electric-field dominates the energy level on ³H₆ (J=6) ground multiplet for Tm³⁺ ion having 12-electrons in 4f shell. The ground state of the ³H₆ is nonmagnetic with Γ₂ singlet, avoiding the Kramers doublet. The energy separation between Γ₂ and the first excited state Γ⁽²⁾₅ triplet is evaluated to be 68.0 K. The whole energy interval Δ between Γ₂ and the highest state Γ₁ in ³H₆ is estimated to be 339.5 K.