Magnetic structures of Fe32+δGe33As2 and Fe32+δ'Ge35-xPx intermetallic compounds: a neutron diffraction and 57Fe Mössbauer spectroscopy study.

Fe32+δGe33As2 and Fe32+δ'Ge35-xPx are quasi-binary intermetallic compounds that possess a rare variant of intergrowth-type crystal structure, which is a combination of the column shaped Co2Al5 and MgFe6Ge6 structure type blocks. The compounds are antiferromagnets with the Néel temperatures around 125 K. Neutron powder diffraction experiments on the samples with δ≈ 0.1, δ'≈ 0.5 and x≈ 3 reveal commensurate magnetic ordering of low symmetry in both compounds and a non-monotonic change in the intensities of magnetic reflections. On the other hand, temperature dependence of the hyperfine fields obtained from 57Fe Mössbauer spectroscopy indicates a gradual, monotonic increase in local magnetic fields upon cooling. We interpret these results as a spin reorientation within the Co2Al5-type block of the crystal structure, with the possible formation of a non-collinear magnetic order at low temperatures. Between the compounds, the reorientation occurs at significantly different temperatures, however the resulting magnetic structures themselves are similar as well as the average values of the magnetic moments and the hyperfine fields.

57Fe Mössbauer study of unusual magnetic structure of multiferroic 3R-AgFeO2.

We report new results of a 57Fe Mössbauer study of hyperfine magnetic interactions in the layered multiferroic 3R-AgFeO2 demonstrating two magnetic phase transitions at T N1 and T N2. The asymptotic value ß * ≈ 0.34 for the critical exponent obtained from the temperature dependence of the hyperfine field H hf(T) at 57Fe the nuclei below T N1 ≈ 14 K indicates that 3R-AgFeO2 shows quasi-3D critical behavior. The spectra just above T N1 (T N1 < T < T * ≈ 41 K) demonstrate a relaxation behavior due to critical spin fluctuations which indicates the occurrence of short-range correlations. At the intermediate temperature range, T N2 < T < T N1, the 57Fe Mössbauer spectra are described in terms of collinear spin-density-waves (SDW) with the inclusion of many high-order harmonics, indicating that the real magnetic structure of the ferrite appears to be more complicated than a pure sinusoidally modulated SDW. Below T < T N2 ≈ 9 K, the hyperfine field H hf reveals a large spatial anisotropy (ΔH anis ≈ 30 kOe) which is related with a local intra-cluster (FeO6) spin-dipole term that implies a conventional contribution of the polarized oxygen ions. We proposed a simple two-parametric formula to describe the dependence of H anis on the distortions of the (FeO6) clusters. Analysis of different mechanisms of spin and hyperfine interactions in 3R-AgFeO2 and its structural analogue CuFeO2 points to a specific role played by the topology of the exchange coupling and the oxygen polarization in the delafossite-like structures.

Local structure and hyperfine interactions of 57Fe in NaFeAs studied by Mössbauer spectroscopy.

Detailed 57Fe Mössbauer spectroscopy measurements on superconducting NaFeAs powder samples have been performed in the temperature range 13 K ≤ T < 300 K. The 57Fe spectra recorded in the paramagnetic range (T > TN ≈ 46 K) are discussed supposing that most of the Fe2+ ions are located in distorted (FeAs4) tetrahedra of NaFeAs phase, while an additional minor (<10%) component of the spectra corresponds to impurity or intergrowth NaFe2As2 phase with a nominal composition near NaFe2As2. Our results reveal that the structural transition (TS ≈ 55 K) has a weak effect on the electronic structure of iron ions, while at T ≤ TN the spectra show a continuous distribution of hyperfine fields HFe. The shape of these spectra is analyzed in terms of two models: (i) an incommensurate spin density wave modulation of iron magnetic structure, (ii) formation of a microdomain structure or phase separation. It is shown that the hyperfine parameters obtained using these two methods have very similar values over the whole temperature range. Analysis of the temperature dependence HFe(T) with the Bean­Rodbell model leads to ζ = 1.16 ± 0.05, suggesting that the magnetic phase transition is first order in nature. A sharp evolution of the VZZ(T) and η(T) parameters of the full Hamiltonian of hyperfine interactions near T ≈ (TN,TS) is interpreted as a manifestation of the anisotropic electron redistribution between the dxz-, dyz- and dxy-orbitals of the iron ions.

A new layered triangular antiferromagnet Li4FeSbO6: spin order, field-induced transitions and anomalous critical behavior.

Structure, electrochemical, magnetic and resonance properties of new layered antimonate Li(4)FeSbO(6) were comprehensively studied using powder X-ray diffraction, cyclic voltammetry, magnetic susceptibility, heat capacity, electron spin resonance and Mössbauer spectroscopy. In the crystal structure the iron ions form the triangular network within (LiFeSbO(6))(3-) layers alternating with nonmagnetic lithium layers. The electrochemical activity studied implies an Fe(3+)/Fe(4+) redox couple at 4.3 V (ox.) and 3.9 V (red.) thereby revealing that Li can be reversibly extracted. The long-range antiferromagnetic order was found to occur at the Néel temperature, T(N) ≈ 3.6 K, confirmed both by the magnetic susceptibility data and specific heat ones. The effective magnetic moment is estimated to be 5.93 µ(B)/f.u. and satisfactorily agrees with theoretical estimations assuming high-spin configuration of Fe(3+) (S = 5/2). In the magnetically ordered state, though, the magnetization demonstrates rather peculiar behavior. An additional anomaly on the M(T) curves appears at T(2) < T(N) in moderate magnetic field. The positions of transitions at T(N) and T(2) separate increasingly with increasing external field. Multiple measurements consistently demonstrated field-sensitive moving of magnetic phase boundaries constituting a unique phase diagram for the compound under study. The complex low-dimensional (2D) nature of magnetic coupling was confirmed by the dynamic magnetic properties study. Electron spin resonance from Fe(3+) ions in paramagnetic phase is characterized by a temperature independent effective g-factor of 1.99 ± 0.01. However, the distortion and broadening of the ESR line were found to take place upon approaching the magnetically ordered state from above. The divergence of the temperature-dependent linewidth is analyzed in terms of both critical behavior close to long-range magnetic order and the Berezinskii-Kosterlitz-Thouless (BKT)-type transition. Heat capacity measurements even at zero field manifested an appearance of the additional anomaly at temperatures below the Néel temperature. The temperature dependence of ESR intensity, linewidth and shift of the resonant field imply an extended region of short-range order correlations in the compound studied. The rich variety of the anomalies in magnetic and resonance properties makes this new antimonate a very interesting system to investigate the multiple phase transitions and competing exchange interaction due to the critical role of the layered structure organization accompanied by the frustration effects in triangular antiferromagnets.

Thermodynamic properties and neutron diffraction studies of silver ferrite AgFeO2.

We present thermodynamic and neutron scattering data on silver ferrite AgFeO(2). The data imply that strong magnetic frustration Θ/T(N)∼10 and magnetic ordering arise via two successive phase transitions at T(2) = 7 K and T(1) = 16 K. At T

On the evolution of the DyNiO3 perovskite across the metal-insulator transition though neutron diffraction and Mössbauer spectroscopy studies.

The structural changes of polycrystalline DyNiO3 perovskite across the metal-insulator transition (TMI = 564 K) have been studied by high resolution neutron diffraction techniques together with Mössbauer spectroscopy, in a sample doped with 1.5 at.% 57Fe. In the insulating (semi-conducting) regime, below T(MI), the perovskite is monoclinic, space group (SG) P21/n, and the crystal structure contains two chemically different Ni1 and Ni2 cations, as a result of the charge disproportionation of Ni3+ cations. The beta parameter, characterizing the low-temperature monoclinic distortion, is smaller than 90.04 degrees for T < TMI, indicating a strongly pseudo-orthorhombic symmetry, although the internal monoclinic symmetry, implying the splitting and shifts of oxygen positions around the two Ni sites is perfectly detected by neutrons. Above TMI, DyNiO3 becomes orthorhombic, SG Pbnm. Upon heating across TMI, there is an abrupt convergence of the two sets (Ni1 and Ni2) of three Ni-O bond lengths, in the monoclinic-insulating phase, to three unique Ni-O distances in the orthorhombic-metallic phase upon entering the metallic region. The 57Fe Mössbauer spectra of an iron-doped (1.5 at.%) DyNiO3 sample recorded in the insulating, paramagnetic temperature range (TN < T < TMI) are discussed by supposing that the Fe3+ probe cations replace nickel in the two octahedral Ni1 and Ni2 sites. Electric field gradient calculations have shown that the 57Fe hyperfine parameters of Fe1 and Fe2 subspectra reflect a specificity of local structure corresponding to large (Ni1O6) and small (Ni2O6) octahedra. At T > TMI, the 57Fe spectrum gives clear evidence for the formation of an unique state for iron probe atoms and could, therefore, imply that the charge disproportionation in the (NiO6) subarray completely vanishes at the insulator-->metal transition.