Mössbauer and magnetization studies of the NpM2Ge2(M=Cr,Fe, Co,Ni, Cu) intermetallic compounds

Magnetization and ²³⁷Np Mössbauer studies were performed on tetragonal NpM₂Ge₂ intermetallic compounds. All the compounds order antiferromagnetically. The Néel points and the magnetic hyperfine interactions of the NpM₂Ge₂ compounds are 62(3), 28(3), 36(5), 27(3), 34(3) K and 2860(40), 1800(40), 3040(40), 2600(40), 2330(90) Mc/sec for M=Cr, Fe, Co, Ni and Cu respectively.     

Combined Mössbauer and LIII-edge X-ray absorption study of mixed- valent EuPd2Si2 and EuNi2P2

The mean valence of Eu is determined by L_III-edge X-ray absorption (L_III-XA) and ¹⁵¹Eu Mössbauer isomer shift measurementd for the mixed-valent compounds EuPd₂Si₂ and EuNi₂P₂ in the temperature range from 1.7 to 300 K. For EuPd₂Si₂, the behaviour of the satellite line observed in the Mössbauer spectra was investigated in detail. From a comparison of the L_III-Xa and Mössbauer results obtained on identical samples the valence/isomer shift calibration problem as well as possible final-state effects in L_III-XA spectra are discussed.     

Magnetism and hyperfine interactions in EuM2Ge2 and GdM2Ge2(M = Mn,Fe, Co,Ni, Cu)

X-ray, magnetic susceptibility and ¹⁵¹Eu, ¹⁵⁵Gd Mössbauer effect studies of EuM₂Ge₂ and GdM₂Ge₂ were performed. All compounds crystallize in the ThCr₂Si₂ body centered tetragonal structure. In all compounds, except those with M = Mn and in EuM₂Ge₂, the M component carries no magnetic moment. All compounds except those with Mn are antiferromagnetic at low temperatures. In EuMn₂Ge₂ the Mn moments order ferromagnetically at 330 K and change to antiferromagnetic order when the Eu moments order ferromagnetically (9 K). This behaviour is different from that in GdMn₂Ge₂, where the Mn sublattice orders antiferromagnetically at 365 K and becomes ferromagnetic and antiparallel to the ferromagnetic Gd sublattice at 96 K. The Mössbauer studies of ¹⁵¹Eu and ¹⁵¹Gd provide values for the magnetic hyperfine fields, the quadrupole interactions and the orientation of the magnetic moments relative to the local fourfold axis (c-axis). It turns out that in the Eu compounds the easy axis of magnetization is close to the c-axis, while in the Gd compounds it is in the basal plane. In all systems, excluding those with Mn, the interatomic rare earth-rare earth distances have the dominant effect on the conduction electron charge density and polarization at the rare earth site and on the Curie point.     

Magnetic sublattices in Np2Co17 and Np2Ni17

Rare-earth-based compounds R₂T₁₇ (R=Rare earth; T=Transition metal) have been extensively studied and developed for applications as permanent magnets. The actinide-based analogues, however, are much less documented and we report here about the magnetic properties of Np₂Co₁₇ and Np₂Ni₁₇, as inferred from ²³⁷Np Mössbauer spectroscopy, the best resonance in actinides, and specific heat.     

Mössbauer and magnetization studies of the U1−xNpxO2 fluorites

²³⁷Np Mössbauer effect and magnetic susceptibility measurements of the U_1?Np_xO₂ fluorite solid solution have been performed in the composition range 0.15 ? x ? 0.75. For x = 0.15 and 0.25, the Np ions order magnetically at a lower temperature T₀ than the bulk material (T_N) (T₀ ～ 19 K, T_N ～ 27 K for x = 0.15). For x = 0.50, T₀ ～ 10 K (T_N ～ 12 K from recent neutron diffraction measurements). For x = 0.75, T₀ ～ T_N ～ 9 K. The Np (induced) ordered moment is ～ 0.5 μ_B. The ²³⁷Np Mössbauer isomer shift shows that the Np ions are in a IV charge state.     

151Eu-Mössbauer investigation in some new ternary Eu2M3Si5 systems (M=Ni,Pd, Cu,Rh)

The series Eu₂M₃Si₅ (M=Ni, Cu, Pd, Rh) have been synthesised for the first time. The Ni, Cu, Pd systems form in U₂Co₃Si₅ type structure.¹⁵¹Eu Mössbauer spectroscopy shows that Eu is in valence fluctuating (VF) state in Ni system, while in Cu and Pd systems it is in stable divalent state. These observations are further confirmed by magnetic susceptibility studies. The material Eu₂Rh₃Si₅ shows two lines in the Mössbauer resonance, one of which shows VF behaviour.     

Analysis of impact-induced Fe2+ disorder in the pyroxene of the Ibitira meteorite

Mössbauer spectroscopy and X-ray diffraction analysis indicate that the only iron compound present in the Ibitira meteorite is the pyroxene (Mg, Fe, Ca)₂Si₂O₆ known as pigeonite [1]. The Mössbauer spectrum shows Fe²⁺ in the two different crystallographic sites, M₁ and M₂, and the population ratio Fe²⁺ (M₁)/Fe²⁺ (M₂) indicates a reasonable degree of cation order. Comparison between our results and results obtained from pyroxene shocked under controlled conditions in the laboratory [2] suggests that, the meteorite was subjected to a impact of low intensity.     

Europium silicides and germanides of the EuM2X2 type: Crystal structure and the valence states of europium

X-ray diffraction studies of EuM₂X₂ compounds (M = Fe, Co, Ni, Cu; X = Si, Ge) revealed that these compounds crystallize in the ThCr₂Si₂ type body-centered tetragonal structure, with the space group $\text{I4}\text{mmm}$. Distribution of the atoms among the lattice sites, the free parameter of the Si and Ge atoms, and the interatomic distances of the compounds were determined by crystal structure calculations. Europium, as determined by Mössbauer spectroscopy, is present in the di- and tri-valent states in the EuM₂Si₂ compounds, the relative amounts of the two states being different in each of the compounds. In EuNiSi₃, EuNi₂Ge₂ and EuCu₂Ge₂ all the Eu are divalent. The relationship between the structure properties and the valence states of Eu in the compounds is discussed.     

Magnetic and hyperfine properties of NpFe2−xCoxSi2 tetragonal systems

Magnetization, ²³⁷Np Mössbauer effect and neutron diffraction studies of the tetragonal NpFe_2?xCo_xSi₂ (x = 0, 1, 1.5, 2) intermetallic compounds were performed. The Mössbauer studies of the²³⁷Np show a magnetic order below 87 (3)_, 15 (3), 37 (3), 42 (3) K, hyperfine fields of 2535 (50), 1600 (50), 2210 (50), 2600 (50) MHz and isometric shifts of -2.3 (3), +7.6 (3), 0 (3), -2.9 (3) mm/s (relative to NpAl₂), respectively. An extremely low magnetization of non-saturated character (at 4.2 K, 20 KOe) is observed.A polycrystalline ingot sample of NpCo₂Si₂ was studied by neutron diffraction at temperatures from 2 to 160 K. Five superlattice lines were observed at temperatures below 46 K and are consistent with an antiferromagnetic structure of the Np(2a) sublattice of type I with T_N = 46 (3) K. The Np ion magnetic moment consistent with the diffracted intensities is 1.5 (1) μ_B with no localized moment on the Co ion.A direct correlation between the Isomer shift, Hyperfine fields and ordering temperature is reported for the first time. This unusual correlation can be explained only if strong f-d, f-s hybridization are assumed.     

Transferred hyperfine fields at 119Sn in Fe1−xMxSn [M≡Mn, Co and Ni]

The transferred hyperfine fields at ¹¹⁹Sn, using Mössbauer spectroscopy are reported for the hexagonal B-35 compounds with a general formula Fe_1?xM_xSn, where MMn, Co and Ni. In these compounds, Sn atoms occupy two crystallographically inequivalent sites. For FeSn the observed spectrum consists of a quadrupole doublet and a magnetic pattern corresponding to 2(d) and 2(a) sites respectively. The data have been analysed to resolve the controversy regarding hyperfine parameters. On replacing Fe by Mn atoms, additional lines appear in the higher velocity region of the Mössbauer spectrum and the intensity of the nuclear Zeeman pattern increases at the expense of quadrupole doublet. The resulting Mössbauer spectra have been analysed by taking only the nearest neighbour interactions into account. This analysis shows that on replacing each Fe atom by a Mn atom, the hyperfine field at 1(a) Sn site increases by about 40 kOe and a field of about 35 kOe is produced at the 2(d) Sn sites. Further, from the nuclear Zeeman pattern for 2(d) sites, the sign of quadropole splitting for these sites could also be determined and was found to be positive. However, the substitution of Co and Ni in place of Fe atoms results in a broad unresolved pattern suggesting that the hyperfine field at the 1(a) sites decreases and a finite field develops at the 2(d) site. The origin of transferred hyperfine fields at the two inequivalent Sn sites is discussed, the magnetic transition temperatures of these compounds have been estimated and the magnetic moments of M-atoms have been inferred.     

Mössbauer study of fluorides: Ba2MFeF9 and NaBaFe2F9

The Mössbauer spectra of the compounds Ba₂NiFeF₉, Ba₂FeCrF₉ and NaBaFe₂F₉ have been studied as a function of temperature. Values of the Néel temperature are obtained and the effects of cationic inversion between the two sites of M^II and M^III in compounds Ni^IIFe^III and Fe^IICr^III are observed. In the latter compound, we observe broad lines at all temperatures and a smearing of the magnetic ordering temperature. However, the Fe^IIFe^III compound shows strict structural order. The two sites of Fe^III in NaBaFe₂F₉ have not been resolved.     

Observation by mössbauer spectroscopy of electron hoppings in the spinel series Zn1-xGexFe2O4

The Mössbauer spectra of the spinel series Zn_1-xGe_xFe₂O₄ with x = 0.25, 0.5 and 0.75 are observed between 77 and 623 K. In the detection limit of the Mössbauer spectrometry, the ferrous and ferric ions are all found on the B sites and the broadening of the spectra between the Curie point and 623 K is correlated with an electron hopping process via a conduction band. This process favors a “double exchange” which can explain the magnetic order in the compounds with x = 0.25 and x = 0.5 below 200 and 250 K.     

Magnetic properties and hyperfine interactions in RCo2B2 (R = Nd,Gd, Tb)

The magnetic properties of RCo₂B₂ compounds which crystallize in the ThCr₂Si₂ structure with R = Nd, Gd, Tb have been investigated. The magnetic structure is ferromagnetic for NdCo₂B₂ and GdCo₂B₂, T_c equals 32 and 26 K respectively and antiferromagnetic for TbCo₂B₂ (T_N = 19 K). Curie-Weiss behaviour is exhibited by all the compounds and the effective moments derived indicate that Co is diamagnetic. The difference in magnetic properties between RCo₂B₂ and other isomorphous RCo₂X₂ (X = Si, Ge) is discussed. Mössbauer studies of ¹⁵⁵Gd in GdCo₂B₂ yielded the hyperfine interaction parameters and determined the direction of the magnetization to be in the basal plane. The electric quadropole interaction at 4.1 K is 580 MHz sec^?1, this is the largest ever found in an intermetallic Gd containing compound.     

57Fe Mössbauer effect evidence for magnetostriction in UFe2

Temperature dependent ⁵⁷Fe Mössbauer experiments on UFe₂ have shown that above T_c (158 K) the logarithm of the reoil-free fraction is a linear function of temperature, with θ_M = 325 K. Below T_c there is a non-linear increase in z.hsl which is interpreted in terms of a reduction in the mean square amplitude of the iron atom motion associated with magnetostriction of the unit cell.     

Observation of paramagnetic relaxation in the NpCu4Al8 intermetallic compound

²³⁷Np Mössbauer spectroscopy between 77 and 4.2 K was carried out in connection with neutron diffraction studies between 300 and 2 K on the tetragonal intermetallic NpCu₄Al₈. The Mössbauer spectra give an isomer shift of + 14.3 mm/s vs. NpAl₂ indicating the non-Kramers Np³⁺(⁵I₄)-ion to be present. Below 45 K the onset of magnetic hyperfine splitting is observed. It develops into a fully resolved Zeeman pattern at 4.2 K with B_eff≈330 T. Neutron diffraction finds no evidence for magnetic order over the whole temperature range scanned. All these results can consistently be explained by paramagnetic relaxation phenomena involving a low lying Γ^t₅ doublet closely followed by a Γ^t₄ singlet as the tetragonal crystalline field states of the Np³⁺ ion.     

237Np Mössbauer study of a novel intermetallic NpRh2Si2

NpRh₂Si₂ (ThCr₂Si₂ structure) has been studied by ²³⁷Np Mössbauer spectroscopy between 4.2 and 100 K. The isomer shift value suggest a Np⁴⁺ electronic configuration. A single site combined magnetic plus quadrupole pattern is observed up to the magnetic ordering temperature of T_c=73(1) K. A Np magnetic moment of 1.4μ_B is deduced from the hyperfine field measured at 4.2 K. The magnetic moments are estimated to make an angle of either 90° or 34° with the tetragonal axis.     

Mössbauer insight to metallurgy, materials science and engineering

A brief overview of the contributions which Mössbauer effect spectroscopy has made to areas of materials science is presented. A survey of the literature reveals the decreasing trends of established areas, with emergence in the past decade or so of new areas such as nanostructured materials and materials produced by mechanochemical treatment and the continuing importance of rare-earth magnetic materials. Examples of applications of ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectroscopy, both transmission and backscattering, are discussed. The complementary nature of Mössbauer spectroscopy and neutron diffraction in delineation of the magnetic behaviour and structures of materials is demonstrated by the La_1?x Y _x Mn₂Si₂ series of rare-earth intermetallic compounds.     

Spin rotations in HoxEr1−xFe2 cubic laves compounds

The temperature dependence of the elastic moduli and the Mössbauer effect in Ho_x,Er_1?x, Fe₂ cubic Laves compounds (x, between 0.3 and 0.9) has been investigated in the temperature region where spin rotation occurs. The composition-dependence minima in the elastic moduli, and the Mössbauer effect measurements, were used to determine the boundaries between the various directions of easy magnetization in these compounds. The experimental spin orientation diagram was found to deviate from the predictions of a one-ion model based on the rare-earth ions alone. From the Mössbauer effect measurement it was deduced that in compounds for which a spin reorientation was observed, the spin rotates continuously with temperature between the major axes of the cubic symmetry. This was attributed to the contribution of higher-order magnetocrystalline anisotropy constants. The ΔE effect, measured in external magnetic fields up to 25 kOe, was found to be constant, and relatively small, in the holmium composition range of x = 0.45–1.00 in the Ho_xEr_1?xFe₂; compounds.     

Investigations of crystal and magnetic properties of the Mn5−xFexGe3 intermetallic compounds

The Mn_5?xFe_xGe₃ intermetallic compounds are investigated with X-ray, neutron diffraction, magnetometric and Mössbauer effect methods. It is found that crystal structure of x = 1 compound is of D8₈ type while the structure of x = 3, 4 and 5 compounds is of B8₂ type. All are ferromagnets with collinearly ordered atomic spins. The lattice constants are derived from X-ray diffraction patterns, while magnetometric measurements yield the Curie temperatures and Weiss constants as well as the values of magnetic moments per molecule in ferromagnetic and paramagnetics states. The distributions of Fe and Mn atoms among two non-equivalent crystal sites are determined with the neutron diffraction method and are confirmed by the Mössbauer effect measurements. The parameters of hyperfine interactions are derived from Mössbauer absorption spectra and are attributed to iron atoms in two non-equivalent crystal sites.     

The coordination of Co2+ions in Co substituted magnetites,Fe3−xCoxO4, with x⩽0.04

⁵⁷Fe Mössbauer spectra at room temperature, both with and without external magnetic field, indicate that Co²⁺ ions in Co_xFe_3?xO₄spinels (x?0.04) are situated on the octahedral B sites. The Mössbauer parameters are listed and the existence of unpaired Fe³⁺ ions is evidenced.