Magnetic Properties of Np2Ir2In Investigated by 237Np Mössbauer Spectroscopy

The present study of Np₂Ir₂In completes the ²³⁷Np Mössbauer spectroscopy investigations of Np₂T₂X (T = Co, Rh, Ir, Ni, Pd, Pt; X = In, Sn) compounds. Np₂Ir₂In is found to order at T _ord30 K and the complex Mössbauer spectra suggest the occurrence of a noncollinear modulated magnetic structure with an average magnetic moment on the neptunium _Np0.76 _B. The magnetic properties of Np₂Ir₂In are consistent with the general trends observed in the An₂T₂X (An =U, Np) isostructural family.     

Mössbauer effect studies of Sm(Rh1−xFex)3B2, x=0.005

Sm(Rh_1–xFe_x)₃B₂ compound (x=0.005) was studied by⁵⁷Fe Mossbauer spectroscopy as a function of temperature. The Fe doped compound has high temperature magnetic transition T_c ^*580 K in addition to the transition at T_c90 K exhibited by the pure SmRh₃B₂ compound. For Tc ^* the spectrum comprises of a quadrupole doublet along with magnetic splitting which merges into a single quadrupole doublet above T_c ^*. The results indicate that Fe induces a large polarization in the Rh-4d band due to the enhanced density of states at the Fermi level in the presence of the hybridized Sm 4f-electrons.     

Mössbauer spectroscopic study of FeAl1−x Co x

We report the results of a Mössbauer study of the alloy sytem FeAl_1?x Co _x forx ≥ 0.3 at temperatures down to 83 K. Magnetic splitting is observed forx ≥ 0.35 at all temperatures. However, forx=0.3, no splitting is observed at room temperature, and superparamagnetic behavior occurs at LN₂ temperature. The magnetically split spectra are fitted each with a distribution of hyperfine fields and the average hyperfine field \(\bar B_{hf} \) as a function of temperature is obtained. The variation of \(\bar B_{hf} \) withT is explained using the model of magnetic clusters with collective magnetic excitations from which the saturation hyperfine field and the magnetic anisotropy energy for these clusters are obtained. Also, the results are discussed using the model of random atomic distributions, and the agreement between the calculated and the experimentally obtained distributions of hyperfine fields is found improve asx increases.     

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 (237Np) and structural studies of some oxouranates and oxoneptunates

Two oxoneptunates have been investigated using²³⁷Np Mössbauer spectroscopy. The crystal structures of Na₂U₂O₇ and -Na₂UO₄ were recently determined by one of us. The two isotypic compounds Na₂Np₂O₇ and -Na₂NpO₄ were prepared. For -Na₂NpO₄ the Mössbauer spectrum confirms the structural study. For Na₂Np₂O₇ the structural study enabled the fit of the complex Mössbauer spectrum recorded at 77K.     

Mössbauer study of mixed valent silicides Eu(Ir1−x Pd x )2Si2

The solid solutions Eu(Ir_1–x Pd _x )₂Si₂, which exist for 0x0.125 and 0.75x1. cristallize with the tetragonal ThCr₂Si₂-type structure. The variation of the europium valence with composition has been thoroughly studied at temperatures 4.2T293 K by¹⁵¹Eu Mössbauer resonance. For 0x0.125 the europium valence at room temperature decreases asx increases. For 0.75x1 the valence transition temperature Eu³⁺Eu²⁺ increases asx increases.     

Mössbauer effect study of the pseudo-binary system (Ti1−x Nb x )Fe2

Mössbauer spectroscopy and X-ray diffraction measurements have been done on (Ti_1–x Nb _x )Fe₂ compounds in order to investigate the effect of Nb on the magnetic properties of TiFe₂. The experimental results show that Nb enters the lattice by filling Ti sites, thereby forming a continuous phase over the whole range of Nb concentrations. The Mössbauer spectra at 80 K fitted with a magnetic hyperfine field distribution show a continuous decrease of the average magnetic hyperfine field with increasing Nb concentration, as well as several different magnetic configurations forx0.3.     

237 Np Mössbauer isomer shifts in neptunium chalcogenides

We report²³⁷Np Mössbauer spectrometry measurements on some Np polychalcogenides: NpS₃,-NpS₂, Np₃Se₅, NpTe₃,-Np₂S₃ and-Np₂Se₃. The experiments have been performed at 4.2 K and 77 K. From the isomeric shift values, the oxidation state of the Np ion can easily be assigned in these kinds of compounds: Np(IV) in NpS₃,-NpS₂ and Np(III) in NpTe₃,-Np₂S₃ and-Np₂ Se₃. A mixture of Np(III) and Np(IV) ions has been observed in Np₃Se₅. Comparisons of theIS values have pointed out the influence of the electronegativity of the chalcogen atom and of the bond length Np-X(X S, Se, Te).     

Mössbauer study of amorphous (Fe1−x Ga x )84B16

Using the⁵⁷Fe Mössbauer effect the influence of the Ga content in amorphous (Fe_1?x Ga _x )₈₄B₁₆ on the average hyperfine fields \(\bar H\) and isomer shift has been studied. For the sample (Fe_0.98Ga_0.02)₈₄B₁₆ the \(\bar H\) , as well as the recoilless fraction,f _a were measured as functions of temperature ranging from 12 K to 300 K. The experimental results show a linear correlation between Inf _a and δ, and well as between δ andx. In the temperature range \(\bar H(T)\) can be described by the Brillouin function and the second-order Doppler shift is appreciable. The characteristic temperature for such an amorphous alloy is 372 K. the effective vibrating massM _eff=79 a.u.     

A Mössbauer study of YBa2(Cu1−x)Fex3O7−σ

⁵⁷Fe Mössbauer spectra of a sample of YBa₂(Cu_0.985Fe_0.015)₃O_7−σ for which T_c≈59 K show that long-range magnetic order is established below ∼ K. A model in which the Fe spins are aligned with the crystallographic c-axis provides satisfactory agreement with the observed relaxation spectra. This result is discussed with reference to current theoretical models of high-T_c superconductivity which involve magnetic coupling mechanisms.     

Mössbauer studies of Fe x Mn1 − x S single crystals

Single crystals of iron manganese sulfides Fe _x Mn_{1 ? x} S (0.25 ≤ x ≤ 0.29) are experimentally investigated using Mössbauer spectroscopy and x-ray diffraction. The Mössbauer spectra measured at 300 K exhibit a single broadened line characteristic of paramagnets. The isomer shift of this line is equal to 0.92–0.94 mm/s, which is typical of Fe²⁺ ions in the octahedral position. The quadrupole splitting (0.18–0.21 mm/s) suggests a distortion of the coordination polyhedron of iron ions in the Fe _x Mn_{1 ? x} S compounds.     

Tellurium-125 Mössbauer spectroscopy study of molybdenum metal tellurides of composition Mo6−x M x Te8 (M=Ru,Rh)

Tellurium-125 Mössbauer spectra have been recorded from some recently reported superconducting compounds Mo_6?x Ru _x Te₈ (x=0, 0.5, 1.0, 1.5) and Mo_5.5Rh_0.5Te₈. The spectra are characterised by single lines which probably contain contributions from the crystallographically dissimilar tellurium sites in these compounds. The trends in the linewidth and chemical isomer shift data are consistent with the enhanced polarisation of tellurium 5p electrons with increasing ruthenium content in the compounds. The results can be related to the nature of chemical bonding in the cluster of metal atoms.     

Electronic structure of Rh, Pt, In, and Sn in the mixed-valence systems Eu(Rh1−x Ptx)2 and U(In1−x Snx)3

The electronic structure of Rh, Pt, In, and Sn in the mixed-valence systems Eu(Rh_1−x Pt_x)₂ and U(In_1−x Sn_x)₃ has been studied by the x-ray K line-shift method. It has been found that the occupation of the Rh 4d-shell in Eu(Rh_1−x Pt_x)₂ is higher than that in the metal, and that it grows with decreasing Eu valence (i.e., with increasing 4f-shell occupation). The electronic structure of Pt, In, and Sn in Eu(Rh_1−x Pt_x)₂ and U(In_1−x Sn_x)₃ does not depend on the Eu and U valence and is practically the same as in the metals. These features in the electronic structure of Rh, Pt, In, and Sn in Eu(Rh_1−x Pt_x)₂ and U(In_1−x Sn_x)₃ suggest that the electron released in the f ⁿ → f ⁿ −1+e transitions, rather than transferring to the common conduction band, remains localized at the Eu and U atoms. Fiz. Tverd. Tela (St. Petersburg) 41, 1529–1531 (September 1999)     

237Np Mössbauer studies on actinide superconductors and related materials

Actinide materials play a special role in condensed matter physics, spanning behaviours of itinerant d-electron and localized 4f-electron materials. This duality of the 5f electrons confer to actinide-based intermetallic compounds a broad variety of physical properties such as magnetic or multipolar ordering, heavy fermion behaviour, quantum criticality, unconventional superconductivity... ²³⁷Np Mössbauer spectroscopy is a unique microscopic tool for gaining information on the electronic and magnetic properties of Np systems.     

Magnetic properties and Mössbauer studies in Y1−x Gd x Fe2

Alloys of Y_1???x Gd _x Fe₂B _y (x = 0, 0.25, 0.5, 0.75 and 1; y = 0, 0.1, 0.15 and 0.2) have been prepared and investigated for structural and magnetic properties. The compounds with x = 0 and 1 are found to form in single phase with C15-type cubic Laves phase structure, while those with x = 0.25, 0.5 and 0.75 are observed to form with small quantities of secondary (Y,Gd)Fe₃ phase. The lattice parameters, Curie temperature and the average Fe hyperfine field are found to increase with increasing x. The Gd–Gd and Gd–Fe interactions are attributed to be the main reason for the enhancement of magnetic properties. Boron was found to stabilize the (Y,Gd)Fe₂ phase without affecting the magnetic properties.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

Magnetic behaviour of U(MnxAl1−x)2 compounds

The results of magnetic measurements performed on U(Mn_xAl_1−x)₂ compounds in the temperature range 4.2K < T < 800K are reported. In the low temperature range (T < 200K), UMn₂ shows a Pauli-type paramagnetism. Above 420K a Curie-Weiss behaviour is evidenced. The magnetic properties of U(Mn_xAl_1−x)₂ compounds were analysed assuming a superposition of a temperature dependent term on a Pauli-type contribution, χ_O. The effective moments as well as the χ_O values were determined both in the low (T < 200K) and high (T > 420K) temperature range. The experimental data were discussed considering changes in the band structure and/or quenching of spin fluctuations.     

Mössbauer studies of GdFe2 − x Hf x alloys

GdFe_2???x Hf _x alloys, where x?=?0, 0.10, 0.15, 0.20, and 0.30, are produced by arc-melting of pure elements. The samples are investigated by x-ray diffraction and Fe⁵⁷ Mössbauer spectroscopy at 78 K and 300 K. We find that the alloy system GdFe_2???x Hf _x have the single phase cubic Cu₂Mg type structure in the whole concentration range. Mössbauer spectroscopic results show that all the samples studied are magnetically ordered at 78 K, and at room temperature. The room temperature spectra are fitted with two magnetic components where the direction of magnetization is along the [111] while the spectra at 78 K are fitted with four magnetic subspectra indicating a complex direction of magnetization for all samples under investigation. The average magnetic hyperfine field and the average isomer shift are found to decrease almost linearly with increasing the Hf concentration at 78 K and 300 K due to the replacement of Fe by nonmagnetic Hf.     

Superconductivity of (Sn1−z Pbz)1−x InxTe alloys

The temperature dependences of the electrical resistivity of (Sn_1?z Pb_z)_1?x In_xTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T _c and H _c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T _c and of the second critical magnetic field H _c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn_1?z Pb_z)_1?x In_xTe increases, T _c and H _c2 decrease as the Fermi level E _F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E _F leaves the LΣ saddle point in the valence-band energy spectrum.     

Mössbauer effect study of Ni1−xCuxMnyFe2−yO4 system

The Mössbauer effect technique has been employed for the study of magnetic properties of spinel series Ni_1?xCu_xMn_yFe_2?yO₄ with 0.0≤x≤1.0, and y=0.6. The substitution of Mn³⁺ and Cu²⁺ ions results in a slight decrease of the hyperfine field at B‐ as well as A‐sites. The area ratio of Fe³⁺ ions at the A‐ and B‐site at 77 K indicates that Cu²⁺, Ni²⁺ and Mn³⁺ ions occupy the octahedral sites in an evidence for complete inverse spinel in this system. The temperature dependence of the hyperfine parameters has been studied for composition with x=0.5 where Nèel point T_N and Debye temperature θ_D are found to be 650 and 679 K, respectively. The temperature dependence of the sublattice magnetization σ(T) obeys a one‐third‐power law in the range 0.5N<0.99.