Th. Sinnemann

A Mössbauer and NMR spectroscopy study of RFe12−xMx intermetallics

Several members of the ThMn12 type compounds RFe12−xMx with R=Y Gd, Er, Sm; M=V, Mo, Ti, W, Si were studied with57Fe Mössbauer spectroscopy. For GdFe10Mo2 and YFe10V2 NMR spectra at 4.2 K were also taken. The spectra were analyzed by taking into account the preferential distribution of the M atoms over the 3 crystallographic sites available for both Fe and M atoms in the ThMn12 lattice. A binomial distribution was used for calculating the probability of the possible nearest neighbor configurations of the Fe atoms, leading to fits with up to 4 separate subspectra for each of the 3 crystallographic sites 8i, 8j, and 8f which provided a consistent description of the experimental data. A spin reorientation transition was found for ErFe10V2 in the temperature range 4.2 K-77 K.

Mössbauer spectroscopy study of RFe12 − xVx intermetallics

Abstract The 57 Fe Mossbauer spectra of the ThMn 12 -type compounds GdFe 12 − x V x ( x = 1.5, 2, 3) and YFe 10 V 2 were measured. The spectra were analysed by taking account of the preferential occupation of the vanadium atoms and using a binomial distribution for calculating the probability of the possible nearest-neighbour configurations. This method of analysing the spectra was found to result in a consistent description of the 57 Fe Mossbauer data for compounds in which the concentration of the vanadium atoms residing at the 8i position was varied by a factor of two.

Sputtered NdFeB-films of high coercivity

Abstract Thin Nd(Dy)FeB-films prepared by rf-magnetron sputtering with different substrate temperatures and deposition rates were investigated for their magnetic and structural properties.

Fe magnetic moments in RFe10Si2 studied with mössbauer effect and spin echo NMR

Abstract 57Fe Mossbauer spectra of RFE10Si2 compounds with R = Gd, Er, Tm and Lu were taken at 77 and 293 K. The magnetic moments were determined for each of the three crystallographic Fe sites. Additional information was gained by nuclear magnetic resonance (NMR) spin echo spectra for LuFe10Si2 and ErFe10Si2. The average Fe magnetic moments lie between 1.80μB/Fe and 1.88μB/Fe at 77 K. The substitution of Si for Fe leads to a smaller reduction of the Fe magnetic moments as compared with the substitutions with M = Ti, V and Mo in the series RFe12−xMx.

A Mössbauer spectroscopy study of RE2Fe17Cx (with RE ≡ Y, Lu, Gd, Ce, Er)

Abstract The rare earth (RE) iron carbides of RE 2 Fe 17 C x with RE ≡ Y, Ce, Cd, Lu and Er were studied with 57 Fe Mossbauer spectroscopy. In the spectra taken at 4.2 K we observed that the iron hyperfine fields hardly change with increasing x , indicating that volume effects (which increase the iron moment) are compensated by bonding effects (which decrease the iron moment). In the spectra taken at higher temperatures than 4.2 K (77 K and room temperature) the strong enhancement of the Curie temperature with carbon content and the concomitant increase in the reduced magnetization are responsible for the overall increase in the hyperfine field with x . For Er 2 Fe 17 C 1.5 uniaxial magnetic anisotropy was found at 4.2 K, with a spin reorientation taking place in the narrow temperature range 122–127 K, and giving rise to planar anisotropy at higher temperatures.

Mössbauer spectroscopy of R2Fe14B and related compounds

The Mössbauer Spectroscopy (MBS) has been widely used in the last 4 years for the study of the recently discovered ternary compounds R2Fe14B where R means Y, Th or a rare earth element. The strong interest for this class of intermetallics arose drastically after the discovery of the exceptional properties of Nd2Fe14 as an ideal material for permanent magnet applications.The newest results about hyperfine fields BHF, quadrupole splitting EQ and isomer shifts I.S. at the 6 crystallographically different Fe sites and at the 2 R sites in the R2Fe14B and their impact on the understanding of the local magnetic moments and magnetocrystalline anisotropy will be reviewed.In the case of RFe12−xMx compounds where M=V, Ti, Si, Mo, W, Cr, complex Mössbauer spectra were obtained because of the presence of 3 crystallographically inequivalent Fe sites and the presence of differents amounts of the M component on one or more of these sites.

Hyperfine fields at iron and carbon nuclei in Gd2Fe14C

Abstract Nuclear magnetic resonance and 57 Fe Mossbauer spectroscopy were used to investigate hyperfine fields at the six crystallographically different iron sites and the single carbon site in the compound Gd 2 Fe 14 C. Shifts in the hyperfine fields at iron sites to values smaller than those determined in Gd 2 Fe 14 B provide evidence of a stronger electron transfer from the carbon atoms to the iron atoms. The hyperfine field at the carbon nuclei was found to be 2.38 T at maximum spin-echo amplitude which is the same value as that determined for the hyperfine field at the boron nuclei in Gd 2 Fe 14 B.

Antiferromagnetism of the new non-superconducting 2201 stacking polytype BiPbSr2Fe1-xMxO6+z with M = Co, Ni and Morin-like spin-flop-transition in BiPbSr2FeO6+z

Abstract The magnetic properties and the spin structure of the BiPbSr2Fe1-xMxO6+z compounds with M = Co, Ni, isotypical with the members of the Bi2(T12)Sr2(Ba2)Can-1CunO2n+4 compositional series with n=1, were studied using magnetic susceptibility measurements and 57Fe Mossbauer spectroscopy. All the compounds investigated (with x=0, xCo=0.5 and xNi=0.25) are basically antiferromagnetic. The highest Neel temperature of 212 K is reached for BiPbSr2FeO6+z, where the hyperfine field at the Fe site has with 48.9 T a value close to the expectation for Fe3+ in a high spin state. A comparison between the values of the quadrupole splitting in the paramagnetic and in the magnetically ordered state shows that a spin-flop transition from the basal plane to the c-axis occurs in the temperature range 120-77 K in BiPbSr2FeO6.1. Such a behaviour is reminiscent of the Morin transition in α-Fe2O3, where the spin direction of the antiferromagnetic sublattices rotates from the plane to the trigonal axis. No spin-flop transition takes place in BiPbSr2Fe0.5Co0.5O6.01.

Iron spin fluctuations in 57Fe-doped high-Tc superconductors (Bi,Pb)2Sr2Can-1CunO2n+4 with n=2,3

Bi-based superconductors of 2212 and 2223 type were doped with 1-3% 57Fe. 57Fe Mossbauer spectra were taken between 293 K and 2 K. In all samples indications for a slowing down of Fe spin fluctuations were found. In the 2223 superconductor doped with 3% Fe, a magnetically split spectrum with a hyperfine field of 40.5 T is measured at T=2 K. The Fe spins are aligned nearly parallel to the electric field gradient which is positive and lies close to the c axis. The Fe valence is +3 in all cases. The RT spectra are strongly dependent on the substitution of Pb for Bi. The spectra of both the Pb-containing 2212 and 2223 samples show one well-defined doublet with a quadrupole splitting value of 1.12 mm s-1. Without Pb substitution, the spectra of the 2212 system consist of two doublets with quadrupole splittings of about 0.6 mm s-1 and 1.7 mm s-1, with their relative intensities depending on the Fe concentration.

Mössbauer spectroscopy study of (Bi, Pb)2Sr2Bin−1FenO3n+3+z stacking polytypes with n = 2, 3, 4

Abstract The (Bi, Pb)2Sr2Bin-1FenO3n+3+z compounds are strongly related to the members of the high-Tc superconducto r series (Bi, Pb)2Sr2Can-1CunO2n+4+z. In this study we show that the Fe compounds with n = 2, 3, 4 are antiferromagnets with Neel temperatures of 512, 597 and 620 K. The saturation hyperfine field and isomer shift values correspond to Fe ions in the 3+ oxidation state. For the n = 3 and 4 compounds, the different subspectra belonging to the inequivalent Fe lattice sites were measured, and the temperature dependence of their hyperfine fields was interpreted in the molecular field approximation.