Magnetic coupling in Gd(Fe6−x Cr x )Al6: A57Fe Mössbauer study

⁵⁷Fe Mössbauer measurements have been made on the ternary ThMn₁₂-type intermetallic compounds Gd(Fe_6–x Cr _x )Al₆ withx=0, 0.5, 1.0, 1.5 and 2.0, at temperatures of 4.2 and 77 K. The principal effect of the Cr substitution is to reduce the⁵⁷Fe magnetic hyperfine field at 4.2 K in this series. The analysis of the⁵⁷Fe Mössbauer spectra is consistent with a ferromagnetic coupling between the Gd and Cr magnetic moments. These results are in agreement with previous studies by Felner et al. on GdCr₆Al₆, in which a ferromagnetic ordering withT _C=170 K was observed.On leave from Applied Acoustics Institute, Shaanxi Teachers University, Xian, PR China.     

Mössbauer study of the hyperfine fields at Fe sites in orthorhombic Fe3C1−xBx

The orthorhombic Fe₃C_1–xB_x system (0.6 < x < 0.8) was studied by Mössbauer Effect Spectroscopy at both 100K and room temperature. The data were analyzed by both assuming the existence of two and four hyperfine field distributions in the Mössbauer spectra. The extrapolation of our results to x=1 give values of 268 kOe and 238 kOe for the hyperfine field values at the crystallographic inequivalent Fe sites I and II in orthorhombic Fe₃B. These results are in good agreement with the values previously reported by Choo and Kaplow/1/.On a fellowship from CONICET, Republica Argentina.     

The influence of the local surrounding on the hyperfine interactions in Zr(Fe1−x Ni x )2 compounds

Mössbauer spectroscopy and the TDPAC method have been used to study Zr(Fe_1–x Ni _x )₂ compounds forx0.30. The hyperfine magnetic field at the Fe sites and the quadrupole splitting as functions of nickel concentration were analysed by use of⁵⁷Fe Mössbauer spectroscopy. Values of the internal magnetic field on¹⁸¹Ta nuclei have been found by means of the TDPAC method.     

Mössbauer effect study of the MnxZnyNizFe2O4

A⁵⁷Fe Mössbauer effect study of the triangular Mn_xZn_yNi_zFe₂O₄ system at 78 K has been undertaken. The mean values of the hyperfine magnetic field, the values of standard deviations and the asymmetry parameters have been derived from the distribution diagrams of the hyperfine magnetic field. The dilution process affects only slightly the A-site hyperfine field but it is responsible for a wide distribution of the B-site field. For the highly diluted samples, relaxation processes play an important role. The results of Mössbauer effect investigations can be understood in terms of the lack of the Zn₂₊-O-Fe₃₊ magnetic coupling and the weaker Mn₂₊-O-Fe₃₊ and Ni₂₊-O-Fe₃₊ superexchange interactions as compared to the Fe₃₊-O-Fe₃₊ interactions.     

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.     

Hyperfine interactions and site population in the spinel compounds Ca x Co1−x Fe2O4

Mössbauer spectra of the spinel series Ca _x Co_1–x Fe₂O₄, withx=0.0, 0.05 and 0.10, have been collected between 80 and 300 K and in an applied field of 6 T at 4.2 K. From the shape-independent hyperfine field distribution fits of the external field spectra, and from the binomial distribution fits of the temperature-dependent spectra, the hyperfine field and the center shift could be unambiguously assigned to octahedrally and tetrahedrally coordinated ferric ions. The importance of the application of an external field in conjunction with a reasonable physical model to obtain a correct assignment of hyperfine parameters to a specific site is demonstrated.     

57Fe Mössbauer studies of U(Fe1−x Co x )2

In the pseudo-binary alloy system U(Fe_1–x Co_x)₂, the ordering transition temperature and the magnetic moment decrease rapidly with x.⁵⁷Fe Mössbauer measurements on the alloys (x=0.08 and 0.20) have been performed as a function of temperature. The observed hyperfine field at the⁵⁷Fe nucleus is much reduced with respect to that in UFe₂.     

Mössbauer study of Zn x Ni5/3−x Fe1Sb1/3O4 spinel ferrite

The antimony substituted nickel ferrite Zn_xNi_5/3–xFe₁Sb_1/3O₄ with different values ofx are prepared, checked by x-ray and studied with⁵⁷Fe Mössbauer spectroscopy over a wide temperature range. Characteristic spectra of paramagnetic, magnetic and electronic relaxation types for the different compositions have been observed. The interpretation of the spectra allows the cation distribution of the compounds to be deduced. The Mössbauer effect parameters at different temperatures are calculated and their dependence on the substitution of non-magnetic Zn⁺² for the magnetic nickel ions are discussed. The temperature dependence of the hyperfine magnetic fields and the respective Néel temperature points are obtained.     

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.     

Effects of nitrogen and vanadium on the57Fe hyperfine field of RFe12−x V x N y (R=Y,Nd)

Magnetization and⁵⁷Fe Mössbauer measurements were carried out on RFe_12–x V _x N _y compounds (R=Y and Nd,x=1.7 and 2.2) and the effects of nitrogen and vanadium atoms on the⁵⁷Fe hyperfine fields at the different iron crystallographic sites were investigated. The hyperfine field decreases with increasing number of vanadium neighbour atoms at all the iron sites. The hyperfine field is strongly enhanced in the nitrogen composition withy>1 where the compound tends to transform into an amorphous-like solid. The iron moment deduced from the hyperfine field increases more upon nitrogenation for the 8i-site than for the other sites, and exceeds the moment of bcc iron.     

Mössbauer studies of the irradiation effects of Y1.6Ca1.4V0.45Sn0.5Fe4.05O12

Y_1.6Ca_1.4V_0.45Sn_0.5Fe_4.05O₁₂ has been bombarded with a 0.56 GeV C⁶⁺ ion beam. The vacancies and vacancy clusters induced by the irradiation change the directions of hyperfine fields around the defects. The reduction of the magnetic hyperfine field is caused by the lower superexchnage field due to the formation of the O^2– vacancies. The change of direction of the hyperfine field has also been observed in Mössbauer spectra. X-ray diffraction showed that the defects give rise to an increase of the lattice constant of the samples.     

Co/Cu multilayers studied by using the119Sn probe

To observe spin polarization in nonmagnetic layers sandwiched by magnetic layers,¹¹⁹Sn Mössbauer spectra of [Co(20 Å)/Cu(20-x Å)/¹¹⁹Sn(1.5 Å)/Cu(x Å)] (x=0, 5 and 10) multilayers were measured. A magnetic fraction is observed in every spectrum, and the average hyperfine field ¯H _f at Sn nuclei in a Cu layer changes from 14 kOe (x=0) to 8 kOe (x=10). It was also observed that the polarization is greatly reduced by adding a Cr layer of only 2 Å to the Co/Cu interfaces. The spectrum of thex=10 film, measured under an external field of 30 kOe, cannot be interpreted without assuming magnetic fractions both in parallel and antiparallel to the external field, which indicates an oscillation of spin polarization in a Cu layer.     

Spectral studies of Co substituted Ni-Zn ferrites

The spinel ferrites Zn_0.35Ni_0.65−xCo_xFe₂O₄, 0≤x≤1, have been prepared using the standard ceramic technique. Room temperature Mössbauer, X-ray and infrared IR spectra were used for carrying out this study. X-ray patterns reveal that all the samples have single-phase cubic spinel structure. The Mössbauer spectra of the samples show a paramagnetic phase for x=0 and a six-line magnetic pattern and a central paramagnetic phase for x≥0.1. They are analyzed and attributed to two magnetic subpatterns and two quadrupole doublets due to Fe³⁺ ions at the tetrahedral A-sites and octahedral B-sites. Four absorption bands are observed in IR spectra. They confirm the spinel structure of the samples and existence of Fe³⁺ ions in the sample sublattices. The deduced hyperfine interactions, lattice parameters, absorption band positions and intensities and force constant are found to be dependent on the substitution factor x, where the cation distribution is estimated. The hyperfine magnetic fields, magnetization and lattice resonant frequency are found to be dependent on the interionic distance.     

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.     

Influence of radiation damage on the hyperfine field of triton irradiated nickel foils

The recently observed Mössbauer effect in⁶³Ni enables the study of the hyperfine field in nickel with high resolution. In the present paper, the line broadening of the Mössbauer resonance is analyzed in order to study the influence of radiation damage on the magnetic hyperfine field in⁶⁴Ni foils. Each of these foils was irradiated by tritons at 15 K with a total current integral of 2.4 A h in order to produce the short-lived⁶³Co sources (T _1/2=27 s). An upper limit of 4.6 kG was estimated for the standard deviation of the magnetic hyperfine field distribution which resulted from the accumulated radiation damage. For the annealed absorber material the standard deviation of the magnetic field is less than 53 G.     

Mössbauer study of magnetic critical behavior of F0.875V0.125

The disordered alloy Fe_1–xV_x has a well defined maximum in T_c at x=0.125. Working at this value to suppress T_c broadening, we have made high precision Mössbauer measurements in the ferromagnetic region. When analyzed in terms of four hyperfine field components our spectra suggest that the sample isannealed. Fitting the hyperfine field over reduced temperatures 1.7×10^–3 < t < 5×10^–1 with corrections to scaling yields p =0.362(8), in agreement with earlier work on pure Fe, and with Fisher's predictions for annealed disordered systems.     

Mössbauer study of cobalt ferrite nanocrystals substituted with rare-earth Y ions

Mössbauer spectroscopy is used to characterize the crystallite size and structure of CoFe_2−xY_xO₄ (x=0, 0.1, 0.3, 0.5) ferrite nanocrystallites synthesized by the sol-gel auto-combustion method. The effect of the substitution of Fe³⁺ ions by Y³⁺ ions on the structure of cobalt ferrite nanocrystallites is investigated. The Mössbauer spectra showed two sets of six-line hyperfine patterns for all the samples, indicating the presence of Fe in both A and B-sites. On increasing the concentration of doped Y, the hyperfine field strength and the isomer shift first increase and then decrease, whereas the quadrupole splitting continuously increases. The superparamagnetism was observed for all the samples and the change of ratio of the superparamagnetism component reflects the size of crystal grain.     

Mössbauer study of spin states in amorphous Fe100−x Zr x alloys

Spin states in amorphous Fe_100–x Zr_x (7x12) are investigated using hyperfine field distributions (HFDs) as derived from⁵⁷Fe transmission Mössbauer experiments. The existence of low-, medium- and high-spin states is demonstrated by three-dimensional projections of HFDs. Temperature dependences of the corresponding average values revealed anomalous behaviour in the vicinity ofT _fc. An increase in the hyperfine magnetic fields observed towards low temperatures is discussed in terms of a ferromagnet-to-spin-glass transition. Utilizing also theT _c values, which were obtained from a thermal scanning Mössbauer technique, we propose a diagram of spin states in amorphous Fe-Zr.     

Properties of Fe–Ga based powders prepared by mechanical alloying

Alloys of Fe–Ga with starting compositions of 17, 19, 21, 23, and 25 at% Ga and Fe₈₁Ga₁₇Z₂ (Z=Si, Sn) have been prepared by mechanical alloying. Samples were milled in a SPEX Model 8000 mill with a ball to sample weight ratio of about 4:1. Phase formation as a function of milling time has been investigated for the 19 at% Ga sample and suggests that milling times of 12 h produce fully alloyed samples. Alloys have been studied by electron microprobe, X-ray diffraction, vibrating sample magnetometery and ⁵⁷Fe Mössbauer effect spectroscopy. Fully milled powders have measured compositions of Fe_100−xGa_x with x=15.7, 17.0, 19.0, 22.4, and 24.0 and Fe_83.1Ga_15.2Z_1.7 (for both Z=Si and Sn). X-ray diffraction showed the presence of a disordered bcc phase with no indication of an ordered D0₃ phase. However, the latter is difficult to observe with X-ray diffraction because of the low intensity of the fcc superlattice peaks. A bimodal Fe hyperfine field distribution as obtained from Mössbauer effect spectra indicated the presence of two discrete Fe environments. The results suggested a lower degree of Ga clustering than has been previously observed in Fe–Ga alloys, of similar composition, prepared by melt spinning. The microstructure is similar to that of Fe–Ga thin films prepared by combinatorial sputtering. Some samples have also been studied after annealing at 800 °C for 8 h. No changes were observed in X-ray diffraction patterns after annealing. However, Mössbauer effect studies show the formation of D0₃ and L1₂ order in annealed samples analogous to the phases observed in melt spun ribbons of similar composition.     

Mössbauer study of substituted barium ferrite BaFe11−x−y Co0.5Ti0.5Ni x ZnyO19−r

Substituted barium ferrite BaFe_11–x–y Co_0.5Ti_0.5Ni _x Zn_yO_19–r powders were prepared using a coprecipitation method and investigated by X-ray diffraction (XRD) and⁵⁷Fe Mössbauer spectroscopy. The results show that the as-prepared magnetic powders possess the typical hexagonal structure and demonstrate both a good dispersibility and a narrow particle size distribution. The hyperfine fields for all sites decrease slightly asx (ory) increases. The Ni²⁺ ions prefer to occupy the 2a and 12k sites, and Zn²⁺ ions occupy the 4f_IV site.