Mössbauer and magnetic studies of orthorhombic FeSiO3

Magnetic properties of orthoferrosilite FeSiO₃ have been examined using susceptibility, magnetization measurements and Mössbauer spectroscopy. From magnetic and Mössbauer measurements, one obtains close values of the magnetic ordering temperature, T_N=39±1 K and T_N=41±1 K, respectively. The magnetic order is characterized by strong ferromagnetic coupling of Fe²⁺ moments within the ribbons and a weak antiferromagnetic coupling of the moments between adjacent ribbons. The 4.2 K Mössbauer spectra can be fitted with two different hyperfine magnetic fields H_hf=68 kOe and H_hf=314 kOe which can be assigned to Fe²⁺ in the octahedrally coordinated M1 and M2 sites, respectively, of the FeSiO₃ structure.     

Mössbauer effect study of fine atomic structure of Fe50Al40Ni10 powders

In the present paper, we discuss the local atomic environment of Fe atoms in the mechanically alloyed Fe₅₀Al₄₀Ni₁₀ powders on the basis of hyperfine data estimated from ⁵⁷Fe Mössbauer spectra. B_hf decreases with increasing milling time due to the diffusion of Al and/or Ni into Fe grains. Nickel atoms did not diffuse inside the first coordination sphere of Fe and if the diffusion takes place the number is not more than one atom. Analyses of P(B_hf), indicate that the high hyperfine field values ranging from 30 to 33 T have to be partially attributed to Fe crystalline nanograins and the presence of the defects in them, the hyperfine field values ranging from 15 to 30 T can be associated to the nanocrystalline bcc Fe(Al, Ni) solid solution while the low hyperfine field values (<15 T) result from Fe atoms located in the disordered grain boundaries.     

Antiferromagnetic spin correlations in palladium-based Pd-Fe, Pd-Fe-Ag, and Pd-Fe-Rh magnetic alloys

The magnetic structure of Pd_1?x Fe_x (x=0.03, 0.06, 0.10, 0.15, and 0.20) alloys is investigated using the method of ⁵⁷Fe-Mössbauer spectroscopy. The distribution functions P(B _hf) of hyperfine magnetic fields have a discrete structure defined by variations of the contribution to B _hf from the magnetic moment of the neighboring Fe atoms. The anomalies of intensities of components of the functions P(B _hf), which increase with the concentration of iron, are indicative of the instability of configurations with a large total spin and of the formation of local spin configurations with the antiferromagnetic orientation of magnetic moments. The probability of formation of such configurations is defined by the competition of the ferromagnetic Fe-Pd exchange interaction with the direct antiferromagnetic exchange between the nearest neighboring atoms of Fe. An Ag or Rh impurity effectively induces the process of spin flipping, which explains the anomalously strong effect of impurities on the magnetic ordering temperature. The results confirm the presence in Pd-Fe alloys of perturbations of long-range ferromagnetic order revealed by neutron diffraction.     

Anti-Invar properties and magnetic order in fcc Fe-Ni-C alloy

Anti-Invar effect was revealed in the fcc Fe-25.3%Ni-0.73%C (wt%) alloy, which demonstrates high values of thermal expansion coefficient (TEC) (15-21)×10⁻⁶ K⁻¹ accompanied by almost temperature-insensitive behavior in temperature range of 122-525 K. Alloying with carbon considerably expanded the low temperature range of anti-Invar behavior in fcc Fe-Ni-based alloy. The Curie temperature of the alloy T_C=195 K was determined on measurements of temperature dependences of magnetic susceptibility and saturation magnetization. The Mössbauer and small-angle neutron scattering (SANS) experiments on the fcc Fe-25.3%Ni-(0.73-0.78)%C alloys with the varying temperatures below and above the Curie point and in external magnetic field of 1.5-5 T were conducted. Low value of the Debye temperature Θ_D=180 K was estimated using the temperature dependence of the integral intensity of Mössbauer spectra for specified temperature range. The inequality B_eff=(0.7-0.9)B_ext was obtained in external field Mössbauer measurement that points to antiferromagnetically coupled Fe atoms, which have a tendency to align their spins perpendicular to B_ext. Nano length scale magnetic inhomogeneities nearby and far above T_C were revealed, which assumed that it is caused by mixed antiferromagnetically and ferromagnetically coupled Fe atom spins. The anti-Invar behavior of Fe-Ni-C alloy is explained in terms of evolution of magnetic order with changing temperature resulting from thermally varied interspin interaction and decreasing stiffness of interatomic bond.     

A study of the magnetic moments and Fe site hyperfine fields in iron-chromium alloys

The magnetization of Fe-Cr alloys ranging from 1 to 15 atomic % of Cr has been measured at room temperature in order to study the relationship between the Fe site hyperfine fields and the magnetic moment. The average moment decreases linearly, at a rate of -2.36 μ_B per Cr atom, up to 10% Cr concentration. The Fe site hyperfine fields were measured in a previous study¹ using the same samples. It is found that the hyperfine fields measured are not proportional to the corresponding magnetic moments. The results are interpreted using a model previously developed for other binary alloys of iron².     

Influence of segregation on magnetic order in Cr75(Fe x Mn1−x )25, 0.64 ≤x ≤ 0.85

Mössbauer and magnetic measurements on Cr₇₅(Fe,Mn)₂₅ alloys, which were segregated and re-disordered by appropriate heat treatments, are reported. A primary factor for the kind of magnetic order which appears is the Cr concentration around the Mössbauer probe atom. At around 15 K, a transition to a re-entrant spin-glass state occurs. For this state, the hyperfine field distributions of all investigated disordered and re-disordered samples are independent of the Fe concentration.     

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.     

Hyperfine interactions of tin atoms in samarium

The hyperfine interactions of ¹¹⁹Sn impurity atoms in samarium at temperatures from 5 to 70 K are investigated by Mössbauer spectroscopy. The distributions P of magnetic hyperfine fields B _hf for tin atoms at sites of the hexagonal [P _h(B _hf)] and cubic [P _c(B _hf)] samarium sublattices are determined from the experimental absorption spectra. Ion ordering in pairs of magnetic centers located in layers of the cubic sublattice is observed by Mössbauer spectroscopy for the first time. Each magnetic center involves ordered ions at the nearest neighbor sites of the tin atom replacing the samarium ion at the hexagonal lattice site. The quadrupole coupling constant e ² q _hQ=0.59±0.12 mm/s is determined for tin atoms at the hexagonal sublattice sites of samarium. The quadrupole interaction of tin atoms in heavy rare-earth metals (from Tb to Er) with a hexagonal close-packed structure is discussed.     

Magnetic Structure of FexCu100−x Magnetoresistive Alloys Produced by Mechanical Alloying

Fe _x Cu_100–x magnetoresistive alloys were produced by mechanical alloying. X-ray diffraction shows fcc structure. The room-temperature Mössbauer spectra evolves from an asymmetrical doublet below x=25%, to a broad magnetic hyperfine field distribution above this concentration. Quadrupole splitting of the doublet varies between 0.48 and 0.57 mm/s, and its isomer shift from 0.16 to 0.29 mm/s. Low-temperature Mössbauer spectroscopy displays a B _hf distribution. Magnetization measurements display different features depending on concentration, from mictomagnetism to ferromagnetism. Low-temperature magnetoresistance is measured. Samples with x20% exhibit larger magnetoresistivity ratios. Bulk and hyperfine magnetic properties are correlated in order to explain magnetoresistivity features of these samples.     

Structural and magnetic correlation of Finemet alloys with Ge addition

The correlation between saturation magnetization and the magnetic moment per Fe atom in the nanocrystalline state is studied for Finemet-type alloys. These studies were performed on nanocrystalline ribbons whose compositions were Fe_73.5Si_13.5−xGe_xNb₃B₉Cu₁ (x=8, 10 and 13.5 at%). We used a simple lineal model, X-ray diffraction and Mössbauer spectroscopy data to calculate the magnetic contribution of the nanocrystals and the results were contrasted with the measured saturation magnetization of the different alloys. The technique presented here provides a very simple and powerful tool to compute the magnetic contribution of the nanocrystalline phase to the alloy. This calculus could be used to determine the volume fraction of nanocrystalline and amorphous phases in the nanocrystallized alloy, without using a very sophisticated microscopy method.     

Magnetic hyperfine fields on119Sn nuclei in uranium compounds

119Sn Mössbauer spectroscopy studies were performed on 12 uranium intermetallic compounds in order to investigate correlations between the formation of the magnetic moment on the U atom and the magnetic hyperfine field transferred to 119Sn nuclei in magnetically ordered materials. The measured hyperfine fields (H_hf) are related to the values of the ordered U magnetic moments (UB) by U_oH_hf/n=A UU. The parameter A varies between 0.73 (UGa2) and 1.55 (UGe2). It seems to correlate with the extent of the hybridization of the 5f states with the conduction electron states.     

The radial dependence of the magnetic hyperfine field for Sn atoms in the ferromagnetic Heusler alloys

The magnetic hyperfine fields,B _hf, for impurity¹¹⁹Sn atoms in Z sites of ferromagnetic Heusler alloys Co₂MnZ (Z=Si, Ge) are measured by the Mössbauer effect. At 77 KB _hf=–1.43±0.04 T in Co₂MnSi andB _hf=+1.05±0.05 T in Co₂MnGe. From the comparison between the values ofB _hf for Sn atoms in Co₂MnZ (Z=Si, Ge, Sn), it follows that the negative contribution toB _hf drops as the interatomic distance begins to increase. This radial dependence also manifests itself in the anomalies of the temperature dependences of the hyperfine fields. The temperature anomaly is positive for Sn in Co₂MnGe and negative for Sn in Co₂MnSi.     

Magnetic properties of CoFeRu alloys in the f.c.c. and h.c.p. phases

Measurements have been made of the magnetization, the Fe⁵⁷ Mössbauer effect, and the crystal structures of Co-rich CoFeRu alloys. The alloys with f.c.c. structure behave in a typical ferromagnetic way, while those with h.c.p. structure show non-saturation of the magnetization in magnetic fields as high as 80 kG, and they show relatively small magnetic moments per atom and a broad single Mössbauer absorption.     

Determination of the long-range order parameters of Fe3Al alloys by Mössbauer spectroscopy and X-ray diffraction technique

Binary Fe-Al alloys in the range (21–25.2) at.% Al were investigated with use of Mössbauer spectroscopy and X-ray diffraction technique. A linear correlation between the hyperfine field H_hf and the isomer shift IS was found for ordered and disordered alloys. The results of Mössbauer investigations are compared with those obtained by X-ray diffraction technique.The Institute for Metal Research Polish Academy of Sciences of Cracow is thanked for financial support from MR.I.18 problem.     

Magnetic and119Sn Mössbauer studies on a new Heusler alloy Co2ScSn

A new alloy, having the formula, Co₂ScSn, has been prepared. This is found to have the cubic Heusler L2₁ type structure (a = 6.19A). Magnetization measurements reveal a large drop in magnetization around 270K. Low field ac susceptibility also shows a transition at 268K which we take to be the Curie temperature of the alloy. The magnetic moment at 5K is found to be 0.55 _B per cobalt atom.¹¹⁹Sn Mössbauer measurements at 8K show a hyperfine split spectrum with a hyperfine field of about 40 kOe. Attempts to prepare other Co₂RSn alloys particularly with R=Lu, were not successful.     

Influence of hydrogen absorption on the electronic structure of V-Fe alloys

The results of the Mössbauer spectroscopy investigation of the hydrogenated V-Fe alloys show a distinct changes in both the isomer shift and the line-width of the Mössbauer spectra when the mean electron concentration of the alloy exceeds 5.6 el./at. No magnetic hyperfine splitting is observed in the spectra. It seems that the tetragonal deformation of the crystalline structure of the alloys, related to the hydrogen absorption, is responsible for the generation of the localized magnetic moments on the Fe atoms in the alloy with n_e> 5.6 el./at.     

Magnetism of ultrathin Fe(110) films

Application of conversion electron Mössbauer spectroscopy (CEMS) to structural and magnetic analysis of ultrathin films and their interfaces is reviewed. Fe(110) films were prepared on W(110) under UHV conditions and analyzed in situ. CEMS provides detailed information on the mode of growth and film structure and on magnetic hyperfine fields, B _hf. Local structure of B _hf across the film is discussed in relation to modifications of magnetic order caused by the finite (including monolayer) film thickness and by the electronic structure of the interface.     

Magnetic hyperfine fields for Sn atoms in rare-earth intermetallides: huge deviation from proportionality between Bhf and Sz

The magnetic hyperfine fields for ¹¹⁹Sn impurity atoms, localized in Ga sites of ferromagnetic intermetallic compounds RGa (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), were measured by the Mössbauer spectroscopy technique. At T=5 K, the hyperfine field value (B_hf) varies from 3.3 T in TmGa to 28.0 T in GdGa. Huge deviation from the proportionality between B_hf and the projection of the R³⁺ ion spin (S_z=(g−1)J) was found. As the atomic number of the R element increases, the B_hf/S_z ratio drastically decreases from 12.6 T for PrGa to 3.3 T for TmGa. This unexpected result can be explained by the strong dependency of B_hf value on the relationship between the Sn-R atomic separation (R_nn) and the radius of the magnetic 4f shell (R_4f). In the framework of this concept, the available experimental data for Sn atom in the rare-earth compounds with non-magnetic sp elements were considered. The data may be described by the universal dependency on the single parameter, λ=R_nn/R_4f.     

Consideration of the spin density of conduction electrons in the formation of a hyperfine interaction field on 57Fe cores in disordered FexCr1 ? x alloys

M?ssbauer spectra of quenching-disordered Fe _x Cr_1−x alloys were measured over a wide range of chromium concentrations (0.9–45 at %), and a detailed analysis of their hyperfine structure was performed. Considering the known data that chromium atoms have an all but negligible perturbation effect on the magnetic moments of matrix atoms, it was concluded that the RKKY polarization of conduction electrons plays a leading part in the formation of the hyperfine magnetic structure of FeCr alloys. It is necessary to take into account the Zener effect.     

Mössbauer spectroscopy study of the disordering process of Fe-rich FeAlSi alloys

In this work we study systematically the influence of different Al/Si ratios on the magnetic and structural properties of mechanically disordered powder Fe₇₅Al_25?x Si _x , Fe₇₀Al_30?x Si _x and Fe₆₀Al_40?x Si _x alloys by means of Mössbauer spectroscopy and X-ray diffraction measurements. In order to obtain different stages of disorder the alloys were deformed by ball milling annealed (ordered) alloys during different number of hours. X-ray and Mössbauer data show that mechanical deformation induces the disordered A2 structure in these alloys. The results indicate that addition of Si to binary Fe–Al alloys makes the disordering more difficult. The study of the hyperfine fields indicates that depending on the Fe content the magnetic behaviour of these ternary alloys varies. For Fe₇₅Al_25?x Si _x series, the alloys have different magnetic behaviours with deformation depending on the Si content. The magnetization of the alloys with high Si content decreases with deformation, as it happens to binary Fe₇₅Si₂₅ and the magnetization of the alloys with low Si content increases with deformation, as it happens to binary Fe₇₅Al₂₅. For Fe₇₀Al_30?x Si _x series the mean hyperfine fields show that there are two different stages with the disordering, in a first stage the mean hyperfine fields decrease and in the second stage they increase. Finally, for Fe₆₀Al_40?x Si _x alloys there is a magnetic transition, from a paramagnetic ordered state to a magneticdisordered state.