151Eu and57Fe Mössbauer effect studies of magnetic interactions in europium transition element oxides solution

The solid state solutions of europium transition element oxides Eu (Fe0.8M0.2)O₃ (M=Sc,Cr,Mn,Co) are synthesized. The X-ray diffraction of the compound shows that all the compounds possess the perovskite structures. Both the¹⁵¹Eu Mössbauer spectra and the⁵⁷Fe Mössbauer spectra are measured. The hyperfine magnetic field and non-axisymmetric electric field gradient are observed in the¹⁵¹Eu Mössbauer spectrum. The⁵⁷Fe Mössbauer spectrum shows that there are four components of hyperfine fields corresponding to four kinds of different neighbours of the Fe ion.     

Mössbauer studies of hyperfine fields in disordered Fe2CrAl

Heusler-like alloy Fe₂CrAl was prepared and studied. Structure determination was done by X-ray. The structure was found to conform to the B₂ type. Magnetic hyperfine fields in this sample were studied by the Mössbauer effect. The Mössbauer spectra were recorded over a range of temperature from 40 to 296 K. The Mössbauer spectra showed the co-existence of a paramagnetic part with a magnetic hyperfine portion at all recorded temperatures. Even with the distribution in the magnetic hyperfine field, the average hyperfine field follows the (T/T _c)^3/2 law. The paramagnetic part of the hyperfine field is explained in terms of the clustering of Cr atoms.     

Temperature dependence of the hyperfine fields in NiFe2O4 and CuFe2O4 oxides

We report the temperature dependence of the magnetic properties of (Ni, Cu)Fe₂O₄ spinel oxides. Mössbauer spectra for NiFe₂O₄ at various temperatures (79 ≤?T?≤ 900 K) are well fitted by two sextets associated with ⁵⁷Fe nuclei at tetrahedral (A) and octahedral (B) sites. The Curie point T _C was deduced by zero velocity Mössbauer technique to be 873 ± 3 K. The hyperfine fields are observed to vary with temperature according to the equation $B_{\rm hf} (T)=B_{\rm hf} (0)[{1-(T/T_{\rm C})^n}]^{\beta_n}$ where n?=?1 (based on the Landau–Ginzburg theory) and n?=?2 (based on the Stoner theory). A systematic decrease of the Mössbauer spectrum shift with increasing temperature is observed.     

237Np and 57Fe Mössbauer study of NpFeGa5

⁵⁷Fe and ²³⁷Np Mössbauer ōmeasurements have been performed for NpFeGa₅, which is one of the so-called neptunium 1-1-5 compounds. The ⁵⁷Fe Mössbauer spectra below T _N = 118 K show the magnetically ordered state. The magnitude of the hyperfine magnetic field at the ⁵⁷Fe nucleus is determined to be 1.98 ± 0.05 T at 10 K. From the ²³⁷Np Mössbauer spectrum at 10 K, the hyperfine magnetic field at the ²³⁷Np nucleus is 203 T and the hyperfine coupling constant is determined to be 237 T/μ_B using the Np atomic magnetic moment of 0.86 μ_B determined by the neutron diffraction study.     

Mössbauer spectroscopy of Fe-based nanomaterials

X-ray diffraction, magnetic measurements and Mössbauer spectroscopy were used to study magnetic properties and hyperfine interaction parameters of nanocrystalline (< 10 nm) and bulk bcc Fe, Fe₉₀Ge₁₀, and Fe₇₇Al₂₃ alloys. It has been established that nanocrystalline state does not influence the formation of specific saturation magnetization, Curie temperature, isomer shift and hyperfine magnetic field. No additional sextets in Mö ssbauer spectra as well as special features in temperature dependences of a.c. magnetic susceptibility have been found. A slight increase (～ 20%) of the width of the nanocrystalline Fe Mössbauer spectral lines has been observed.     

Temperature investigations of the spatial spin-modulated structure of multiferroic BiFeO<Subscript>3</Subscript> by means of Mössbauer spectroscopy

The results from ⁵⁷Fe Mössbauer spectroscopic studies of multiferroic BiFeO₃ in a range of tem-peratures including that of the magnetic phase transition are presented. The Mössbauer spectra are processed and analyzed by reconstructing the hyperfine magnetic field distributions and interpreting the spectra with a cycloid-type spatial spin-modulated structure model. The temperature dependences of the hyperfine spectrum parameters (the Mössbauer line shift, the quadrupole shift, and the isotropic and anisotropic contributions to the hyperfine magnetic field) are obtained, along with the anharmonicity parameter of an incommensurate spin wave.     

The magnetic properties of the MgZn ferrite system by Mössbauer spectroscopy

Samples of the magnetism-zinc ferrite series Zn_xMg_1?xFe₂O₄ (x = 0.0 to 1.0) have been studied by the Mössbauer effect technique at 77 K. Mössbauer spectra for x = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octachedral ions (B-sites), while for x=0.7 it shows relaxation behaviour and for x?0.8 it exhibits a paramagnetic quadrupole doublet. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of the AB and BB supertransferred hyperfine interactions. Analysis of the average Mössbauer line width as a function of zinc concentration suggests that the relaxation spectrum observed at x=0.7 (77 K) is possibly due to domain wall oscillations.     

Tin-doped spinel-related oxides of composition M3O4 (M = Mn, Fe, Co)

Tin-doped compounds of spinel-related M₃O₄ (M = Fe, Mn, Co) have been studied by ¹¹⁹Sn and ⁵⁷Fe Mössbauer spectroscopy in the temperature range of 20–600 K. The ¹¹⁹Sn Mössbauer spectra recorded down to 20 K from the non-iron-containing compounds of Co₃O₄ and Mn₃O₄ contained only doublets showing no transfer of magnetic properties from cobalt or manganese to the dopant tin ions. In contrast, the tin-doped-(FeCo)₃O₄ and (FeMn)₃O₄ gave ¹¹⁹Sn and ⁵⁷Fe Mössbauer spectra, which showed magnetic hyperfine interactions. The Curie temperature has been estimated for the former sample.     

Magnetic hyperfine fields at Fe,Cd and Sn sites in an FeCo alloy

The magnetic hyperfine field at an Fe site in the ferromagnetic alloy Fe_0.475Co_0.525 was measured using the Mössbauer effect. The value obtained at room temperature was 343 kOe. The hyperfine field at a substituted Cd impurity was measured by the method of time differential perturbed angular correlations. A single frequency was observed at room temperature, corresponding to a field of -177 kOe. Using the Mössbauer effect, the Sn site hyperfine field was measured in a sample in which 0.3 atomic percent of ¹¹⁹Sn had been substituted. The room temperature spectrum consisted of the superposition of a single line, together with a six-line hyperfine spectrum, corresponding to a field of 231 kOe. A phenomenological interpretation is proposed for Fe, Cd and Sn fields in the binary alloys of iron.     

Hyperfine fields and Mössbauer isomer shifts of 57Fe in PdAgFe alloys

The hyperfine fields B_hf and Mössbauer isomer shifts S of ⁵⁷Fe in two PdAgFe alloy series containing 3 and 7 at% have been measured at 4.2 K and 295 K. With increasing Ag content, a change in the magnetic ordering from ferromagnetism towards a spin-glass behaviour is indicated in the 3 at% Fe series.     

Temperature evolution of frustrated spin states in the system with competing exchange interactions. Fe0.65Ni0.35)1−x Mnx (x=0, 0.024, 0.034)

The influence of temperature on the distribution function P(B _hf) of the magnetic hyperfine fields for ⁵⁷Fe in (Fe_0.65Ni_0.35)_1?x Mn_x alloys (x=0, 0.024, 0.034) are investigated by Mössbauer spectroscopy. The Mössbauer absorption spectra are measured in the temperature interval 5–300 K; in the interval 5–80 K the measurements are performed in a magnetic field of 0.2 T. Anomalies are found in the temperature curves of the intensity of the principal maximum of the functions P(B _hf)[B _hf=30–38 T] and the total (integrated) intensities of the low-field components [B _hf=(4–13) T]. The detected anomalies in the behavior of the total intensities are interpreted as resulting from a change in the balance of competing exchange interactions due to the thermal annihilation of antiferromagnetic Fe-Fe exchange interaction. The emergence of strong satellite lines in the interval B _hf=20–29 T in Mn-doped alloys is attributed to reorientation of the spins of Fe atoms under the influence of strong antiferromagnetic Mn-Fe exchange interaction.     

Spatial spin-modulated structure and hyperfine interactions of 57Fe nuclei in multiferroics BiFe1–x T x O3 (T = Sc,Mn; x = 0, 0.05)

The results of the Mössbauer studies on ⁵⁷Fe nuclei in multiferroics BiFe_1–x T _x O₃ (T = Sc, Mn; x = 0, 0.05) in the temperature range of 5.2–300 K have been presented. The Mössbauer spectra have been analyzed in terms of the model of an incommensurate spatial spin-modulated structure of cycloid type. Information has been obtained about the effect of the substitution of Sc and Mn atoms for Fe atoms on the hyperfine parameters of the spectrum: the shift and the quadrupole shift of the Mössbauer line, the isotropic and anisotropic contributions to the hyperfine magnetic field, and also the parameter of anharmonicity of the spatial spin-modulated structure.     

Surface and bulk Mössbauer effect studies of annealed NANOPERM-type alloys

Structure, hyperfine interactions, and magnetic behaviour of Fe₈₀M₇Cu₁B₁₂ (M=Mo, Nb, Ti) nanocrystalline alloys are studied by Mössbauer spectrometry. As-quenched and heat-treated specimens are investigated. Transmission and Conversion Electron Mössbauer effect techniques are used to compare surface and bulk crystallization as a function of annealing temperature with the aim to unveil the crystallization onset. In addition, magnetic structure comprising distributions of hyperfine fields is discussed as a function of composition and annealing temperature. Hyperfine field distributions are obtained separately for the amorphous residual phase and for interface regions. Crystalline phases are represented by discrete components.     

Magnetic properties of the R1+eFe4B4 compounds (R = rare earth) from magnetization and Mössbauer measurements

The magnetic properties of rare earth-iron-boron alloys with composition R_1+eFe₄B₄ have been determined using Mössbauer and magnetization measurements. Magnetic ordering occurs at temperatures between 4.2 and 25 K for the compounds with R = Pr, Nd, Sm, Gd, Tb, Dy, Ho. The Curie temperature scales very well with the de Gennes factor for the heavy rare earth members of the series, while significant deviations are observed for the light rare earths indicating the presence of strong CEF effects. The absence of magnetic hyperfine splitting even at 4.2 K indicates that the Fe ion has a zero magnetic moment. This is confirmed by Mössbauer spectra in an applied magnetic field.     

Effective magnetic fields on Fe in R1+εFe4B4 alloys

⁵⁷Fe Mössbauer spectra of magnetically ordered R_1+εFe₄B₄ alloys have been measured at low temperatures. Small (⋍ 0.8 T) hyperfine fields have been found for R = Sm and Dy. Analysis of the spectra in terms of simultaneous magnetic and quadrupolar interactions has revealed the magnetization to be parallel to the c axis for R = Sm and perpendicular to the c axis for R = Dy. These results are consistent with a 2^nd-order CEF mechanism for anisotropy. The ordering temperature of Sm_1+εFe₄B₄ was determined from Mössbauer and magnetization measurements to be T_c = 37 ± 2 K, the highest in the R_1+εFe₄B₄ series.     

57Fe Mössbauer studies of nickel hydride

Nickel loaded with hydrogen electrolytically or under high pressure was studied by⁵⁷Fe Mössbauer spectroscopy on dilute substitutional iron solutes. Experiments in external magnetic fields at 4.2 K show that Fe species in nickel hydride have magnetic moments between about 4.7 and 2.5 μ_B and saturation hyperfine fields between 24 and 17 T, depending on the number of nearest hydrogen neighbours. By quenching hydride samples from ambient to liquid nitrogen temperature, non-equilibrium hydrogen distributions in the vicinity of the iron can be frozen in. They relax towards equilibrium between 130 and 150 K within hours, permitting information on hydrogen jump rates to be obtained.     

57Fe and 166Er Mössbauer and magnetic studies of RFe4B (R = Er,Tm, Lu) compounds

Magnetic and Mössbauer studies have been carried out on a series of ternary borides RFe₄B (R = Er, Tm, Lu) which have the hexagonal CeCo₄B type structure. These compounds are found to be magnetically ordered at room temperature. Magnetization studies in the temperature range from 5 to 300 K reveal the presence of compensation temperatures in Er and Tm compounds and indicate antiferromagnetic coupling between the rare earth and Fe moments. Room temperature ⁵⁷Fe Mössbauer studies yield values of hyperfine fields at the two Fe sites as 246 and 185 kOe in ErFe₄B and TmFe₄B, and 204 and 145 kOe in LuFe₄B. The ¹⁶⁶Er Mössbauer studies give nearly free-ion hyperfine field at the Er sites which indicates that the exchange interaction in ErFe₄B is much stronger than crystal field interaction.     

Mössbauer studies of spatial spin-modulated structure and hyperfine interactions in multiferroic Bi<Superscript>57</Superscript>Fe<Subscript>0.10</Subscript>Fe<Subscript>0.85</Subscript>Cr<Subscript>0.05</Subscript>O<Subscript>3</Subscript>

Results of Mössbauer investigations on ⁵⁷Fe nuclei in multiferroic material Bi⁵⁷Fe_0.10Fe_0.85Cr_0.05O₃ in the temperature range from 5.2 to 300 K are presented. Bulk rhombohedral samples were obtained by solidstate synthesis at high pressure. Mössbauer spectra were analyzed using the model of spatial incommensurate spin-modulated structure of the cycloidal type. Information on the influence of substituting Cr cations for Fe cations on hyperfine spectral parameters was obtained: the shift and quadrupolar shift of a Mössbauer line, and isotropic and anisotropic contributions into the hyperfine magnetic field. The anharmonicity parameter m of the spatial spin-modulated structure increases almost 1.7 times at 5.2 K when BiFeO₃ is doped with chromium. The data on m were used for calculation of the uniaxial magnetic anisotropy constants and their temperature dependences for pure and chromium-doped BiFeO₃.     

Mössbauer effect in natural strunzite, ferristrunzite and ferrostrunzite

The temperature dependence of the ferric and ferrous hyperfine fields in natural samples of strunzite, ferristrunzite and ferrostrunzite is determined by Mössbauer spectroscopy between 4.2 K and their magnetic transition temperatures (T _N), i.e. 50.5±0.5 K, 43.0±0.5 K and 44.0±0.5 K respectively, which are determined by Mössbauer thermoscanning. Two dominating magnetically split ferric subspectra were consistently present in all of the samples and are related to the Fe(1) and Fe(2) sites in the crystallographic structure, but an unambiguously assignment to a specific site is not possible. The difference between the corresponding hyperfine fields is very small. In the strunzite sample these fields are well defined and rather weakly dependent of temperature. In the other samples the corresponding hyperfine fields are more distributed especially at higher temperatures (below T _N). The relative contribution in the spectra of the third magnetic ferric component differs strongly between the samples and is assigned to ferric ions at the Mn site. At the lowest temperatures applied, its hyperfine field exceeds all other field values, but it decreases rather rapidly with increasing temperature, in so far that the corresponding spectral lines make a crossover with the lines of the other ferric subspectra. The magnetically split spectra of ferrostrunzite consist additionally of a ferrous magnetic component, which could be successfully analysed by introducing two magnetically split ferrous subspectra, which strongly overlap with each other but also with the ferric components. At higher temperatures in the magnetic region all subspectra overlap more and in the case of ferri- and ferrostrunzite the ferric hyperfine fields were distributed over a wider range.     

151Eu Mössbauer spectroscopic, electric, and magnetic measurements of EuAgGe and EuAuGe

The physical properties of EuAgGe and EuAuGe, the structures of which are derived from the CeCu₂ type, have been investigated in detail by means of magnetic susceptibility, electrical conductivity and ¹⁵¹Eu Mössbauer measurements. Above 50 K both germanides show Curie--Weiss behavior with experimental magnetic moments of \mu _exp=7.70(5) \mu _B (EuAgGe) and \mu _exp=7.40(5) \mu _B (EuAuGe) and Weiss constants of -2(1) K (EuAgGe) and 33(1) K (EuAuGe). For EuAgGe, a magnetic phase transition is observed below 18(1) K. Zero-field cooling and field cooling measurements indicate cluster glass behavior (weak ferromagnetism, mictomagnetism). Magnetization measurements at 5 K show a saturation magnetic moment of 3.3(2) \mu _B/Eu at 5.5 T. ¹⁵¹Eu Mössbauer measurements show a Eu(II) valence state (\delta =-10.4 mm/s). While magnetic hyperfine splitting appears in the spectra at temperatures as high as 15 K, complete magnetic ordering is not reached at temperatures down to 4.2 K. EuAuGe orders ferromagnetically at 32.9(2) K. Magnetization measurements at 2 K show a saturation magnetic moment of 6.2(1) \mu _B/Eu at 5.5 T, respectively, indicating that all spins are ordered ferromagnetically at low temperatures. ¹⁵¹Eu Mössbauer measurements show a Eu(II) valence state (\delta =-10.6 mm/s) and two spectral components in an approximate 1:1 ratio, subjected to magnetic hyperfine splitting effects at T₁=32(2) and T₂=18(4) K, respectively. Thus, the transition temperature of 32.9 K observed in the susceptibility measurements appears to be associated with ordering of only one of the two crystallographically distinct europium sites in this compound. Electrical conductivity measurements indicate metallic behavior for both germanides.