Hyperfine magnetic interactions of 57Fe nuclei in NaFeAs arsenide

The results of the Mössbauer effect studies of layered NaFeAs arsenide in a wide temperature range are presented. The measurements at T > T _N demonstrate that the main part (～90%) of iron atoms are in the low-spin state Fe²⁺. The other atoms can be attributed to the impurity NaFe₂As₂ phase or to the extended defects in NaFeAs. The structural phase transition (at T _S ≈ 55 K) does not produce any effect on hyperfine parameters (δ, Δ) of iron atoms. At T < T _N, the spectra exhibit the existence of a certain distribution of the hyperfine magnetic field (H _Fe) at ⁵⁷Fe nuclei, indicating the inhomogeneity of the magnetic environment around iron cations. The analysis of the temperature behavior of the distribution function p(H _Fe) allows us to determine the temperature of the magnetic phase transition (T _N = 46 ± 2 K). It has been found that the magnetic ordering in the iron sublattice has a two-dimensional type. The analysis of the H _Fe(T) dependence in the framework of the Bean-Rodbell model reveals a first-order magnetic phase transition accompanied by a drastic change in the electron contributions to the main component (V _ZZ ) and the asymmetry parameter (η) of the tensor describing the electric field gradient at ⁵⁷Fe nuclei.     

57Fe Mössbauer study of new multiferroic AgFeO2

The present work reports results of the ⁵⁷Fe Mössbauer measurements on AgFeO₂ powder sample recorded at various temperatures including the points of both magnetic phase transitions. The ⁵⁷Fe Mössbauer spectra of AgFeO₂ measured in the paramagnetic range (T > T _N1) consist of one quadrupole doublet with rather high quadrupole splitting of Δ_300K = 0.66 ± 0.01 mm/s for Fe³⁺ ions. In order to predict the sign of electric field gradient (EFG) at ⁵⁷Fe nuclei, we calculated the lattice contribution to the electric field gradient (EFG) at ⁵⁷Fe nuclei, which emphasized the importance of the dipolar contributions, with resultant oxygen polarizabilities in the range of α _O = 0.83 ų, in agreement with the results obtained previously for other delafossite-like oxides. In the temperature range of T _N2 < T < T _N1, Mössbauer spectra gave clear evidence for the existence of a distribution of the hyperfine magnetic fields H _hf at ⁵⁷Fe nuclei. We present the results of a model fitting of the spectra based on an assumption of the cycloid magnetic structure of AgFeO₂ at T < T _N2. The obtained data were analysed in comparison with published data on Mössbauer studies of oxide multiferroics.     

Mössbauer study of the modulated magnetic structure of FeVO<Subscript>4</Subscript>

Mössbauer spectroscopy is used to study the FeVO₄ multiferroic, which undergoes two magnetic phase transitions at T _N1 ≈ 22 K and T _N2 ≈ 15 K. The first transition (T _N1) is related to transformation from a paramagnetic state into a magnetically ordered state of a spin density wave, and the second transition (T _N2) is associated with a change in the type of the spatial magnetic structure of the vanadate. The electric field gradient tensor at ⁵⁷Fe nuclei is calculated to perform a crystal-chemical identification of the partial Mössbauer spectra corresponding to various crystallographic positions of Fe³⁺ cations. The spectra measured in the range T _N2 < T < T _N1 are analyzed on the assumption about amplitude modulation of the magnetic moments of iron atoms μ_Fe. The results of model intersection of the spectra recorded at T < T _N2 point to a high degree of anharmonicity of the helicoidal magnetic structure of the vanadate and to elliptic polarization of μ_Fe. These features are characteristic of type-II multiferroics. The temperature dependences of the hyperfine interaction parameters of ⁵⁷Fe nuclei that were obtained in this work are analyzed in terms of the Weiss molecular field model on the assumption of orbital contribution to the magnetic moments of iron cations.     

Density correlations of hadron gas and the origin of early multiplicity scaling

We will define a sequence of functions called density correlations which measure statistical correlations between momentum space densities of particles in the ensemble of inelastic final states produced by scattering a pair of hadrons. Then we will show that early multiplicity scaling of the distribution of charged particles produced by pp collisions can primarly be explained by the assumption that fluctuations of the density of charged particles are quite random and therefore higher correlation functions are small in an appropriate sense, combined with the assumption that the first three density correlations fulfill an approximate form of Feyman scaling at small values of x. These assumptions will aslo be used to derive an expression for the scaling function Ψ(η) which is completely determined by <N²>_S/<N>_S² and <N³>_S/<N>_S³, and recursion relation to calculate <N^m>_S/<N>_S^m, m ? 4, from <N²>_S/<N>_S² and <N³>_S/<N>_S³. The prediction for Ψ(η) accurately reproduces data in the interval 0.2 ? η ? 3.1, 50 GeV/c ? p_L ? 303 GeV/c, and the predictions for <N_S>^m/<N>_S^m, 4 ? m ? 10, agree with observed values in the interval 50 GeV/c ? p_L ? 303 GeV/c.     

Fe-induced magnetic transition in YBa2(Cu1−x 57Fe x )3O6 by NQR and Mössbauer spectroscopy

Nuclear quadrupole resonance spectroscopy (around 30 MHz) on the chain site Cu(1) nuclei in oxygen deficient YBa₂(Cu_1?x Fe _x )₃O₆ doped with different amounts of⁵⁷Fe (x≤0.01) reveal an onset of magnetic order at low temperatures represented by a symmetrical doublet contribution to the nuclear quadrupole resonance (NQR) spectrum. The onset temperatureT _N2 depends on the concentration of Fe reaching 130 K forx=0.01. The splitting forx=0.01 at 100 K corresponds to a net internal field of 0.09 T with a distribution of ≈0.08 T representing about 70 percent of the Cu(1) nuclei.⁵⁷Fe and⁵⁷Co Mössbauer spectroscopy at 4.2 K with and without an external magnetic field of 5 T revealed that belowx=0.00015 Fe spins are decoupled from the Cu(2) moments in the antiferromagnetic state. Results are interpreted in terms of the magnetic model structure suggested by Kadowaki et al. [1].     

Pion photoproduction and T> versus < analog states in N>Z nuclei

The T_> and T_< components of giant resonance levels in N>Z nuclei cannot be unambiguously identified in photonuclear reactions. In charged pion photoproduction, their analogs may be so identified (at least for the spinflip levels), as illustrated here with a ¹³C target.     

Hysteresis effects at a cooperative high-spin (5T2) ⇌ low-spin (1A1) transition in dithiocyanatobis (4, 7-dimethyl-1, 10-phenanthroline) iron (II)

Hysteresis has been observed at the cooperative high-spin (⁵T₂) ? low-spin (¹A₁) transition in Fe (4, 7-(CH₃)₂-phen)₂(NCS)₂ where phen = 1, 10-phenanthroline. The transition is centered on T_c> = 121.7 K for rising and on T_c< = 118.6 K for lowering of temperature. The observations are in only qualitative agreement with the thermodynamic theory of Slichter and Drickamer.     

Electronic structure and hyperfine fields in non-stoichiometric magnetite above the Verwey transition

The NMR spectra of ⁵⁷Fe nuclei were measured below and above the Verwey transition temperature T_V in single crystal samples of Fe_3(1?δ)O₄ (0?δ?0.009). The measured crystals were grown from the melt using the cold crucible technique. In a cubic phase above T_V the satellite structure induced by vacancies in the octahedral sublattice is clearly resolved. The satellite lines originate from those iron nuclei in the tetrahedral sublattice that are nearest to the vacancy. Temperature dependence of satellite lines indicates that the presence of vacancy leads to a redistribution of the electrons in octahedral sublattice and the appearance of iron ions possessing appreciable orbital momentum.     

Possible oxygen hopping in the YBa2Cu3O7 system observed by Mössbauer spectroscopy

In situ high temperature⁵⁷Fe Mössbauer measurements have been performed on samples of YBa₂ (Cu_1?xFe_x)₃O_7?δ withx=0.005 andx=0.010, in pure oxygen or in air, in the range 295 K≤T≤640 K. TheT-dependence of Fe species, of the Debye-Waller factorf(T) as well as of the hyperfine parameters is analysed.     

Perovskites Bi0.8La0.2FeO3 and Bi0.8La0.2Fe0.95Cr0.05O3: Crystal structure and magnetic and charge states of iron ions

Perovskites of the Bi_0.8La_0.2Fe_{1 ? x} Cr _x O₃ system (x = 0, 0.05) were investigated by Mössbauer spectroscopy in the temperature range of 298–800 K. The samples were fabricated by solid-state synthesis and had a rhombic structure. Iron ions in Bi_0.8La_0.2FeO₃ and Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ are situated in trivalent states. The magnetic transition temperatures (the Néel temperatures T _N ) T _N = 677.5 ± 2.5 K for Bi_0.8La_0.2FeO₃ and T _N = 647.6 ± 2.5 K for Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ are measured. The substitution of trivalent iron ions from trivalent chromium ions in the amount x = 0.05 in Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ perovskite decreases the hyperfine magnetic field at nuclei ⁵⁷Fe in Fe⁺³-O-Cr⁺³ chains by 30 kOe.     

Mössbauer studies of magnetic behavior of 3d-doped REBa2Cu3−x Fe x O7+δ (RE=Y,Er, Dy,Gd)

A Mössbauer study has been made on⁵⁷Fe ions substituted into the Cu(1) site of REBa₂Cu_3?x Fe _x O_7+δ (RE=Y, Er, Dy, Gd;x=0.15, 0.30). At low temperature, the iron atoms antiferromagnetically order with a transition temperature which is dependent on the Fe concentration. The temperature dependence of the magnetic subspectra representing Fe ions with various local oxygen environments in YBa₂Cu_3?x Fe _x O_7+δ and ErBa₂Cu_3?x Fe _x O_7+δ fit a 2D-Ising model with a ratio of the anisotropic exchange between the two directions on the order of 0.5–1.0(10^?3) for the Y-compounds and on the order of 1 for the Er-compounds. The magnitude of the local dopant magnetization is related to a short-range chemical order which determines the magnetic chain size and defines the correlation lengths. For the Y-compound, the order is quasi-1D with strong intrachain but very weak interchain coupling. For the Er-compounds, the magnetic coupling is Ising 2D. The strong fluctuation behavior expected in low dimensional systems above and belowT _N is observed via characteristic relaxation in the Mössbauer linewidth nearT _N. For both the Dy- and Gd-compounds, the magnetic order is 3D. The magnitude of the rare-earth magnetic moments appears to affect the character of the magnetic interaction in the Cu(1)-site. However, a Mössbauer effect measurement at¹⁵⁵Gd nuclei in GdBa₂Cu_2.85Fe_0.15O_7+δ (T _N(Fe)～14 K) shows paramagnetic behavior at 4.9 K.     

Morin transition suppression in Polycrystalline 57Hematite (α-Fe2O3) exposed to 56Fe(II)

We used the isotope selectivity of ⁵⁷Fe Mössbauer spectroscopy to investigate changes in the magnetic properties of polycrystalline hematite exposed to ferrous iron (Fe(II)). We found that sorption of ⁵⁶Fe(II), followed by interfacial electron exchange, alters the bulk magnetic properties of ⁵⁷hematite. After reaction with ⁵⁶Fe(II), we observed partial suppression of the Morin transition of ⁵⁷hematite to below 13 K. This is significantly lower than the Morin temperature (T _M) of ～230 K measured for isotopically enriched polycrystalline ⁵⁷hematite, as well as the T _M of 264?±?2 K reported for normal polycrystalline hematite.     

A Brillouin study of the temperature-dependence of the acoustic modes across the insulator-metal transitions in V2O3 and Cr-doped V2O3

Brillouin scattering studies have been carried out on high-quality single crystals of undoped and 0.9% Cr-doped V₂O₃. The observed modes in both the samples at ∼12 and ∼60 GHz are associated with the surface Rayleigh wave (SRW) and bulk acoustic wave (BAW), respectively. In the undoped sample, the mode frequencies of the SRW and BAW modes decrease as the temperature is lowered from room temperature to the insulator-metal transition temperature (T_IM=T_N=∼130 K). Below the transition, the modes show hardening. In the doped sample, the SRW mode shows a similar temperature-dependence as the undoped one, but the BAW mode shows hardening from room temperature down to the lowest temperature (50 K). This is the first measurement of the sound velocity below T_IM in the V₂O₃ system. The softening of the SRW frequency from 330 K to T_IM can be qualitatively understood on the basis of the temperature-dependence of C₄₄, which, in turn, is related to the orbital fluctuations in the paramagnetic metallic phase. The hardening of the mode frequencies below T_IM suggests that C₄₄ must increase in the antiferromagnetic insulating phase, possibly due to the orbital ordering.     

EPR of Cr2O3 in the phase transition region

The temperature dependence of the Cr₂O₃ EPR spectra is obtained. The linewidth of the EPR is defined by the phonon's modulation of the exchange interactions at T >T_N + 4–5 K though at T_N < T < T_N + 4 K the critical broadening of the EPR in Cr₂O₃ is determined by fluctuation effects.     

Structure of the local environment and hyperfine interactions of <Superscript>57</Superscript>Fe probe atoms in DyNiO<Subscript>3</Subscript> nickelate

The local structure of DyNiO₃ nickelate at both sides of the insulator (T < T _im) ? metal (T > T _im) phase transition was studied by probe ⁵⁷Fe Mössbauer spectroscopy. The character of change in the hyperfine parameters of probe iron atoms specifically near the phase-transition temperature (T ≈ T _im) was analyzed.     

The thermoelectric power in AgxTiS2 and AgxNiPS3. Part II. The ionic component of the thermoelectric power

The EMF of the isothermal cells: Ag/AgI/Ag_xTiS₂: 0<x<1, T=150–200°C/Ag_xNiPS₃: 0<x<3, T=150–350°C has been measured. From the EMF-x curves the existence ranges of the 2-phase (stage I and II) regions ?0.16<x<0.32 for the Ag/Ag_xTiS₂ system at 190°C; 0.20 < x < 0.50 and 1 < x < 2 for the Ag/Ag_xNiPS₃ system at 400°C - have been determined. The results are sustained by X-ray diffraction and electrical conductivity measurements. From the EMF-T curves the partial enthalpy (ΔH?_Ag) and entropy (ΔS?_Ag) of dissolution of silver in the Ag_xSSE (solid solution electrode) materials were obtained. In the case of Ag_xTiS₂, ΔH?_Ag has a low absolute value, while ΔS?_Ag is distinctly positive. The EMF of the Ag/Ag_xNiPS₃ system also has a positive temperature coefficient. Furthermore, the ionic component of the thermoelectric power, ΔE/ΔT, of the thermogalvanic cells: Ag/AgI/Ag_xSSE/AgI/Ag Ag_xTiS₂: 0 < x < 1, T = 150–200°C( T ) (T+ΔT) Ag_xNiPS₃: 0 < x < 1, T= 150–350°C has been measured. The kinetically important heat of transport of silver ions in the Ag_xSSE materials has been determined in two ways: first from the dependence of the ionic Seebeck coefficient (?_Ag+) on reciprocal temperature; and second from direct calculation, using the data for ?_Ag+ and ΔS?_Ag. The heat of transport is much smaller than the activation enthalpy for Ag⁺-conduction, indicating a high ionic polaron binding energy in these materials.     

Crystal field analysis of the one-dimensional antiferromagnet CsCoCl3

In the past quadrupole splitting data for⁵⁷Fe doped CsCoCl₃ has been fitted to a model with crystal fields and spin-orbit coupling for T>T _N1. However there is poor agreement with data below T _N1. In this paper the results of attempting to refit the data over all temperatures, and the effects of the orbit lattice interaction are reported.     

Magnetic collapse and the change of electronic structure of FeBO3 antiferromagnet under high pressure

The effect of high pressures up to 60 GPa on single-crystal and polycrystalline samples of iron borate ⁵⁷FeBO₃ was studied by Mössbauer absorption spectroscopy (⁵⁷Fe nuclei) in a diamond anvil cell. Magnetic field H _hf at the ⁵⁷Fe nuclei increases with pressure but abruptly drops to zero at 46±2 GPa, indicating the crystal transition from the antiferromagnetic to nonmagnetic state. This is accompanied by an abrupt change in the isomer shift and quadrupole splitting. Their values in the high-pressure phase are evidence for the transition of Fe⁺³ ions from a high-spin (S=5/2, ⁶ A _1g ) to low-spin (S=1/2, ⁶ T2g) state (spin crossover). This correlates with an abrupt decrease in the unit-cell volume (by ～9%) and optical gap. The change of the magnetic and electronic structures is explained by Mott’s transition with rupturing of strong d-d-electron correlations.     

The influence of the metalloid elements on the magnetic properties of amorphous Fe40Ni40PyB20−y

The Curie temperatures (T_C) and the hyperfine interactions of amorphous Fe₄₀Ni₄₀P_yB_20?y(0≤y>≤20) have been determined by ⁵⁷Fe Mössbauer spectroscopy. In this series of amorphous solid with a fixed transition-metal content, the metalloid atoms are shown to have a noticeable effect on T_C, isomer shift and hyperfine field distribution.     

Spatially modulated magnetic structure of AgFeO2: Mössbauer study on 57Fe nuclei

The results of the Mössbauer study of ferrite AgFeO₂ manifesting multiferroic properties (at T ≤ T _N2) have been presented. The hyperfine interaction parameters of ⁵⁷Fe nuclei have been analyzed in a wide temperature range including the points of two magnetic phase transitions (T _N2 ≈ 7–9 K and T _N1 ≈ 15–16 K). It has been shown that the Mössbauer spectra of the ⁵⁷Fe nuclei are sensitive to the variations of the character of the magnetic ordering of Fe³⁺ ions in the studied ferrite. The results of the model identification of a series of spectra (4.7 K ≤ T ≤ T _N2) under the assumption of the cycloid magnetic structure of ferrite AgFeO₂ have been presented. The analysis of the results has been performed in comparison with the literature data for other oxide multiferroics.