Spin glass ordering of Zn0.75Co0.25Fe0.5Cr1.5O4 cubic spinel

The linear and nonlinear low field AC susceptibilities of Zn_0.75Co_0.25Fe_0.5Cr_1.5O₄ show peaks due to non-critical contributions, which mask the peak due to spin glass ordering. They extend into the region of temperatures in which Mössbauer spectra do not show any magnetic component. When a DC field of 200 Oe suppresses the non-critical contributions, peak due to spin glass ordering is clearly visible. The spin glass ordering is thus shown to be a thermodynamic transition. The critical exponent is found to fall within the range found using other spin glasses. Mössbauer spectra in zero fields provide T_SG, which agrees with the peak temperature of AC susceptibilities in the absence of non-critical contributions. 〈S_Z〉 determined using Mössbauer spectra does not show any anomaly. In the presence of a field of 5 T, the spectra show SG ordering at 4.2 K, which converts into ferrimagnetic ordering at higher temperatures.     

Mössbauer spectroscopy as a probe of magnetic states in La0.5Ca0.5FeO3−δ perovskite

Charge disproportionation in La_0.5Ca_0.5FeO_3−δ perovskite has been detected by zero-field Mössbauer spectra from 20 K to room temperature. On the basis of the parameters of center shifts and hyperfine fields, Mössbauer spectra identified that the iron ionic states are Fe³⁺ and Fe⁵⁺ below 150 K, Fe³⁺, Fe⁴⁺ and Fe⁵⁺ in the intermediate temperature region, as well as Fe³⁺ and Fe⁴⁺ above 220 K. At low temperatures, the system exhibits a cluster-glass-like state resulting from competition between antiferromagnetic interaction of Fe³⁺–Fe³⁺ and ferromagnetic interaction of Fe³⁺–Fe⁵⁺.     

Hyperfine interactions in some Aurivillius Bim+1Ti3Fem−3O3m+3 compounds

X-ray diffraction and Mössbauer spectroscopy were applied as complementary methods to investigate the structure and hyperfine interactions in the series of Bi_m+1Ti₃Fe_m−3O_3m+3 Aurivillius compounds with m=4, 6, 7 and 8. Samples were synthesized by the solid-state sintering method at various temperatures. As X-ray diffraction analysis proved, the compounds formed single phases at temperature above 993 K. Mössbauer studies have confirmed diffraction measurements. Compounds synthesized at 993 K contained residual hematite, however these sintered at elevated temperatures were single-phased materials. Room-temperature Mössbauer spectra of Bi_m+1Ti₃Fe_m−3O_3m+3 compounds revealed their paramagnetic properties, what is consistent with the literature data concerning the Néel temperature of these ceramics (T_N is smaller than room temperature). Detailed analysis of MS spectra allowed to state that iron ions may occupy both tetrahedral and octahedral sites in the crystallographic lattice of Aurivillius compounds.     

Mössbauer spectroscopy studies of spin reorientations in amorphous and crystalline (Co0.2Fe0.8)72.5Si12.5B15 glass coated micro-wires

Thermo-gravimetric, differential scanning calorimetry and comprehensive ⁵⁷Fe Mössbauer spectroscopy studies of amorphous and crystalline ferromagnetic glass coated (Co_0.2Fe_0.8)_72.5Si_12.5B₁₅ micro-wires have been recorded. The Curie temperature of the amorphous phase is T_C(amorp) ∼730 K. The analysis of the Mössbauer spectra reveals that below 623 K the easy axis of the magnetization is axial-along the wires, and that a tangential or/and radial orientation occurs at higher temperatures. At 770 K, in the first 4 hours the Mössbauer spectrum exhibits a pure paramagnetic doublet. Crystallization and decomposition to predominantly α-Fe(Si) and Fe₂B occurs either by raising the temperature above 835 K or isothermally in time at lower temperatures. Annealing for a day at 770 K, leads to crystallization. In the crystalline material the magnetic moments have a complete random orientation. After cooling back to ambient temperature, both α-Fe(Si) and Fe₂B in the glass coated wire show pure axial magnetic orientation like in the original amorphous state. The observed spin reorientations are associated with changes in the stress induced by the glass coating.     

The role of iron in the formation of the magnetic structure and related properties of La0.8Sr0.2Co1−xFexO3 (x=0.15, 0.2, 0.3)

La_0.8Sr_0.2Co_1−xFe_xO₃ (x=0.15, 0.2, 0.3) samples were studied by means of AC magnetic susceptibility, magnetization, magnetoresistance and ⁵⁷Fe Mössbauer spectrometry. Iron was found to take on a high spin 3d^5−α electronic state in each of the samples, where α refers to a partly delocalized 3d electron. The compounds were found to exhibit a spin-cluster glass transition with a common transition temperature of ∼53 K. The spin-cluster glass transition is visualized in the ⁵⁷Fe Mössbauer spectra as the slowing down of magnetic relaxation below ∼70 K, thereby showing that iron takes part in the formation of the glassy magnetic phase. The paramagnetic-like phase found at higher temperatures is identified below T_c≈195 K as being composed of weakly interacting, magnetically ordered nanosized clusters of magnetic ions in part with a magnetic moment oriented opposite to the net magnetic moment of the cluster. For each of the samples a considerable low-temperature negative magnetoresistance was found, whose magnitude in the studied range decreases with increasing iron concentration. The observed results obtained on the present compounds are qualitatively explained assuming that the absolute strengths of magnetic exchange interactions are subject to the relation ∣J_Co–Co∣<∣J_Fe–Co∣<∣J_Fe–Fe∣.     

Magnetic properties of RE0.7Ca0.3Mn0.95Fe0.05O3 (RE=Sm and Gd) manganites at low temperature

The magnetic properties of RE_0.7Ca_0.3Mn_0.95Fe_0.05O₃ perovskite with rare-earth cations (RE=Sm and Gd) were investigated by means of X-ray diffraction, Mössbauer spectroscopy, and low temperature (4.2-266 K) magnetization measurements. Structural characterization of these compounds shows that they both have orthorhombic (Pbnm) structure. The Mössbauer spectra show clear evidence of local structural distortion of the Mn(Fe)O₆ octahedron, which is based on the non-zero nuclear quadrupole interactions for high-spin Fe³⁺ ions. It was found that the local structural distortion increases significantly when Sm³⁺ is replaced by Gd³⁺. This distortion is attributed to the Jahn-Teller coupling strength as estimated from the Mössbauer effect results. The magnetic results indicate that the Curie temperature decreases as a result of replacing Sm by Gd. This is due to the decrease of the average A-site cationic radius 〈r_A〉. The rapid increase of magnetization at low temperature indicates the magnetic ordering of rare earth ions at the A-site.     

Mössbauer and magnetic studies of orthorhombic FeSiO3

Magnetic properties of orthoferrosilite FeSiO₃ have been examined using susceptibility, magnetization measurements and Mössbauer spectroscopy. From magnetic and Mössbauer measurements, one obtains close values of the magnetic ordering temperature, T_N=39±1 K and T_N=41±1 K, respectively. The magnetic order is characterized by strong ferromagnetic coupling of Fe²⁺ moments within the ribbons and a weak antiferromagnetic coupling of the moments between adjacent ribbons. The 4.2 K Mössbauer spectra can be fitted with two different hyperfine magnetic fields H_hf=68 kOe and H_hf=314 kOe which can be assigned to Fe²⁺ in the octahedrally coordinated M1 and M2 sites, respectively, of the FeSiO₃ structure.     

Temperature-dependent Fe Mössbauer spectroscopy and local structure of the mullite-type Bi2(FexGa1−x)4O9 (0.1≤x≤1) solid solution

The Bi₂(Fe_xGa_1−x)₄O₉ oxide solid solution possessing a mullite-type structure has been investigated by ⁵⁷Fe Mössbauer spectroscopy in dependence of composition (0.1≤x≤1) and temperature (293≤T/K≤1073). The spectra have been fitted with two doublets for tetrahedrally and octahedrally coordinated high-spin Fe³⁺ ions, respectively. The experimental areas of the subspectra were used to determine the distribution of iron on the two inequivalent structural sites. The fraction of iron cations occupying the octahedral site is found to increase with decreasing Fe content and the cation distribution is almost independent of temperature. The unusual temperature dependence of the quadrupolar splitting, QS, observed for the octahedral site with dQS/dT>0 is discussed in connexion with structural data for Bi₂Fe₄O₉. The temperature dependence of Mössbauer isomer shifts and signal intensities is examined in the context of local vibrational properties of iron on the two inequivalent sites of the mullite-type lattice structure.     

Antiferromagnetism and spin-glass transition in the FeInxCr2−xSe4 series of selenides

The magnetic behavior of the FeIn_xCr_2−xSe₄ system (with x=0.0, 0.2 and 0.4) has been investigated by magnetic and Mössbauer spectroscopy. Hyperfine parameters indicate that iron is in the Fe²⁺ oxidation state, with a minor (∼9%) Fe³⁺ fraction, located at different layers in the structure. Low-field magnetization curves as a function of temperature showed that the antiferromagnetic (AFM) order temperature is T_N=208(2) K for FeCr₂Se₄ and decreases to 174(3) K for FeIn_0.4Cr_1.6Se₄. The effective magnetic moment μ_eff decreases with increasing In contents, and shows agreement with the expected values from the contribution of Fe²⁺ (⁵D) and Cr³⁺ (⁴F) electronic states. A second, low-temperature transition is observed at T_G∼13 K, which has been assigned to the onset of a glassy state.     

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

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.     

Effects of annealing temperature on structure and magnetic properties of amorphous Fe61Co27P12 nanowire arrays

Arrays of Fe₆₁Co₂₇P₁₂ nanowire with an aspect ratio about 70 were prepared in anodic aluminum oxide templates by electrodeposition. The influences of annealing temperature on structure and magnetic properties of Fe₆₁Co₂₇P₁₂ nanowires were studied. When the specimens were annealed below 400 °C, there are no obvious changes in structure except relaxation. With the annealing temperature increasing from 400 to 600 °C, the Fe-Co phase is detected by X-ray diffraction and Mössbauer spectra. The crystalline fraction and hyperfine field can be derived from Mössbauer spectra. The room temperature magnetic hysteresis loops show that the coercivity and squareness of the nanowire arrays in parallel to the wire axis increase with the increasing of annealing temperature, which mainly attributes to the strengthening of anisotropy.     

Magnetic properties of Co-ferrite-doped hydroxyapatite nanoparticles having a core/shell structure

The magnetic properties of Co-ferrite-doped hydroxyapatite (HAP) nanoparticles of composition Ca_10−3xFe_2xCo_x(PO₄)₆(OH)₂ (where x=0, 0.1, 0.2, 0.3, 0.4 and 0.5% mole) are studied. Transmission electron microscope micrograms show that the 90 nm size nanoparticles annealed at 1250 °C have a core/shell structure. Their electron diffraction patterns show that the shell is composed of the hydroxyapatite and the core is composed of the Co-ferrite, CoFe₂O₄. Electron spin resonance measurements indicate that the Co²⁺ ions are being substituted into the Ca(1) sites in HAP lattice. X-ray diffraction studies show the formation of impurity phases as higher amounts of the Fe³⁺/Co²⁺ ions which are substituted into the HAP host matrix. The presence of two sextets (one for the A-site Fe³⁺ and the other for the B-site Fe³⁺) in the Mössbauer spectrum for all the doped samples clearly indicates that the CoFe₂O₄.cores are in the ferromagnetic state. Evidence of the impurity phases is seen in the appearance of doublet patterns in the Mössbauer spectrums for the heavier-doped (x=0.4 and 0.5) specimens. The decrease in the saturation magnetizations and other magnetic properties of the nanoparticles at the higher doping levels is consistent with some of the Fe³⁺ and Co²⁺ which being used to form the CoO and Fe₂O₃ impurity phase seen in the XRD patterns.     

Magnetic ordering in the frustrated system FeSc2S4

The sample of FeSc₂S₄ was prepared by solid reaction method. The crystallographic structure and the magnetic properties of the fabricated compound were investigated by X-ray, and superconducting quantum interference device (SQUID) magnetometer and Mössbauer spectroscopy. The polycrystalline FeSc₂S₄ confirmed the normal cubic spinel structure (space group Fd3m). The lattice constants a₀ and anion parameter u are 10.519 Å and 0.255, respectively. The Mössbauer spectroscopy has been studied for the FeSc₂S₄ at various temperatures, ranging from 4.2 K to room temperature. The spectra consist of two doublets at 4.2 K while a single line at room temperature. It is noticeable that the Mössbauer spectra of two doublet patterns with large electric quadrupole splitting (ΔE_Q) remain over the Néel temperature. Those are interpreted as a result of large electric quadrupole interaction compared to magnetic dipole interaction. The magnetic susceptibility measurements were performed with a SQUID magnetometer for temperatures 2<T<320 K, in external fields up to 5 kOe. Magnetic behavior shows antiferromagnetic behavior and the magnetic superexchange interactions between the Fe ions are weakly antiferromagnetic. The paramagnetic susceptibilities follow Curie–Weiss (CW) law with CW temperature Θ_CW=−100 K, and frustration parameter f=−Θ_CW/T_N is of the order of 1000. We conclude that two sublattices are coupled antiferromagnetically, leading to strong frustration effects.     

Spectroscopic and magnetic properties of Fe3Fe4(AsO4)6

The infrared (IR) and ⁵⁷Fe-Mössbauer spectra of Fe₃^IIFe₄^III(AsO₄)₆ were recorded and analyzed on the basis of its structural characteristics. The IR spectrum presents a high complexity, showing an important number of bands and splittings, as a consequence of the presence of three structurally independent AsO₄³⁻ groups. The analysis of the four quadrupole signals shown by the Mössbauer spectrum allowed to attain a detailed insight into the cation distribution over the available crystallographic sites. The alternating current susceptibility measurements indicate a paramagnetic to ferrimagnetic transition in the material at about 59 K.     

Synthesis and structural determination of the new oxide fluoride BaFeO2F

Low temperature fluorination of BaFeO_3−x using poly(vinylidene fluoride) leads to the formation of the oxide fluoride BaFeO₂F. Mössbauer spectroscopy shows that this phase exhibits magnetic ordering at room temperature due to interactions between the Fe³⁺ ions, with an ordering temperature of 645 (±5) K. Neutron diffraction studies show that the phase has cubic symmetry and confirm the presence of magnetic ordering (G-type antiferromagnetic) at room temperature.     

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.     

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 antiferromagnetic CuFeO2

We have studied the magnetic spin structure of antiferromagnetic CuFeO₂ by X-ray diffraction (XRD) and Mössbauer spectroscopy. Its crystal structure determined by XRD analysis was a rhombohedral structure (space group R-3m) and lattice constants a₀ and c₀ were 3.0333 and 17.1595 Å, respectively. In spite of 4-Fe sublattices in a delafossite CuFeO₂, its Mössbauer spectra were analyzed with 1-set (6-Lorentzian lines) below 10 K due to the collinear-commensurate spin structure, but the spectra were fitted with 4-sextet above 10 K due to the incommensurate spin structure. This phenomenon was attributed to the spin–lattice relaxation effect. Magnetic Néel temperature was also determined at 18 K, which corresponded to the high-spin Fe³⁺ valance state. On the other hand, CuFe_0.98Al_0.02O₂ powder with a noncollinear spin structure was fitted with 4-sextet at 4.2 K.     

Structural study by X-ray diffraction and Mössbauer spectroscopy of a new synthetic iron phosphate: K4MgFe3(PO4)5

A new iron phosphate K₄MgFe₃(PO₄)₅ has been synthesized by the flux method and characterized by single-crystal X-ray diffraction and Mössbauer spectroscopy. It crystallizes in the tetragonal system with the space group and the unit cell parameters a=9.714(3) Å and c=9.494(5) Å. The crystal structure is of a new type. It exhibits a three-dimensional framework built up from corner-sharing MO₅ (M=0.75Fe+0.25Mg) trigonal bipyramids and PO₄ tetrahedra. The K⁺ ions are occupying large eight-sided tunnels running along c. A room temperature Mössbauer study confirmed the +3 valence state of iron and its five-coordination.