M. Wiertel

Influence of annealing temperature on structural and magnetic properties of MnFe2O4 nanoparticles

Abstract Nanoparticles of manganese ferrite were obtained by the impregnation of highly ordered mesoporous MCM-41 silica support. The investigated sample contained 20% wt. Fe. The obtained nanocrystallites were strongly dispersed in silica matrix and their size was about 2 nm. The sample annealing at 500°C led to increase of particle size to about 5 nm. The Mössbauer spectroscopy investigations performed at room temperature show on occurrence of MnFe2O4 nanoparticle in superparamagnetic state for the sample annealed in all temperatures. The coexistence of superparamagnetic and ferromagnetic phase was observed at liquid nitrogen temperature. The sample annealed at 400°C and 500°C has bigger manganese ferrite particle and better crystallized structure. One can assign them the discrete hyperfine magnetic field components.

Influence of the local environment on the hyperfine interactions in Zr(Fe1-xCox)2 compounds

Ferromagnetic Laves phase compounds Zr(Fe1-xCox)2 have been investigated by means of the Mössbauer effect (57Fe) and by the time-dependent perturbed angular correlation of \gamma -rays (181Ta) technique. It has been concluded from ME experiments that by exchange of Fe by Co in the nearest neighbour shell of the nuclear probe the hyperfine magnetic field acting on 57Fe decreases by 10--12 kG.The analysis of the TDPAC experiments revealed that two different hyperfine magnetic fields: B1hf(Ta)~ 61 kG and B2hf(Ta)~ 88 kG act on the 181Ta nuclei. Both have a negative sign.

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

Mössbauer spectroscopy and the TDPAC method have been used to study Zr(Fe1−xNix)2 compounds forx⩽0.30. 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.

Surface morphology and phase stability of titanium irradiated with 168 MeV 136Xe ions

Abstract A titanium sample was irradiated with Xe 168 MeV energy at fluences of 1 × 1014, 2.2 × 1014, and 5 × 1014 ions · m−2. Changes induced by the ions with irradiation were analysed with an atomic force microscope, scanning electron microscope, and X-ray diffractometer. The irradiation at a fluence of 1 × 1014 ions · cm−2 leads to formation of hillocks with a density of 9 × 106 hillocks · cm−2. An increase in the fluence value results in disappearance of the hillocks and formation of void–dislocation lines. The changes in the surface topography are accompanied by a change in the relative content of the hcp (α) and fcc phases. The substantial changes in the crystalline structure (increased content of the fcc phase, reduction of lattice constants, changes in stresses) resulted from irradiation at a fluence of 1 × 1014 ions · cm−2. Irradiation of massive titanium samples with xenon ions at a fluence of 168 MeV 1 × 1014 ions · cm−2 at room temperature leads to a transition of ∼10% of the hexagonally closed-packed phase – hcp (α) into a face centred cubic structure (the fcc phase). Higher Xe ion fluences contribute to further reduction of lattice constants and crystallite sizes as well as reduction of the phase fcc content and lower values of stresses. It has been found for the first time that irradiation at higher fluences leads to partial recovery of the hcp (α) phase. Irradiation changes the texture of∼50% of the fcc phase crystallites.

Magnetic and structural properties of Sc(Fe1−xSix)2 Laves phases studied by Mössbauer spectroscopy and neutron diffraction

Abstract The aim of the presented paper is to study an influence of replacement of Fe atoms by Si atoms in quasibinary Sc(Fe1−xSix)2 Laves phases on their structural and magnetic properties. Powder X-ray diffraction (XRD) and neutron diffraction (ND) measurements carried out at different temperatures from 4.3 K up to about 700 K revealed that samples were single phase with cubic C15 structure for Si concentration x from 0.05 to 0.20 and hexagonal C14 structure for higher concentration. The results of 57Fe Mössbauer measurements showed that the Sc(Fe1−xSix)2 compounds with x ≤ 0.30 are ferrimagnetic at 4.3 K. At temperature 80 K in the samples with x = 0.20 and 0.30, a magnetic cluster spin-glass state has been observed, as ferrimagnetic long-range order disappears. Such picture was supported by the results of ND measurements carried out at 8 K, which confirmed the lack of long-range order for x above 0.10 and an occurrence of hyperfine field distributions in the corresponding Mössbauer spectra. At room temperature, samples with x ≥ 0.20 became paramagnetic. A substitution of Si atoms for Fe ones leads to a decreasing of mean values of hyperfine magnetic fields in samples under investigation. From the neutron diffraction pattern analysis of Sc(Fe0.90Si0.10)2Fe magnetic moment was determined as to be equal to 1.5 μB at 8 K. Combining this result with a value of hyperfine magnetic field on 57Fe probes, the hyperfine coupling constant A in Sc(Fe0.90Cu0.10)2 phases is estimated at about 11.6 T/μB at 8 K.

Study on the effect of atmospheric gases adsorbed in MnFe2O4/MCM-41 nanocomposite on ortho-positronium annihilation

Abstract In this paper, results of positron annihilation lifetime spectroscopy (PALS) studies of MnFe2O4/MCM-41 nanocomposites in N2 and O2 atmosphere have been presented. In particular, the influence of manganese ferrite loading and gas filling on pick-off ortho-positronium (o-Ps) annihilation processes in the investigated samples was a point of interest. Disappearance of the longest-lived o-Ps component with τ5 present in the PAL spectrum of initial MCM-41 mesoporous material in the PAL spectra of MnFe2O4-impregnated MCM-41 measured in vacuum is a result of either a strong chemical o-Ps quenching or the Ps inhibition effects. The intensity I4 of the medium-lived component initially increases, reaching a maximum value for the sample with minimum manganese ferrite content, and then decreases monotonically. Analogous dependence for the intensity I3 of the shortest-lived component shows a maximum at higher MnFe2O4 content. Filling of open pores present in the studied nanocomposites by N2 or O2 at ambient pressure causes partial reappearance of the τ4 and τ5 components, except a sample with maximum ferrite content. The lifetimes of these components measured in O2 are shortened in comparison to that observed in N2 because of paramagnetic quenching. Anti-inhibition and anti-quenching effects of atmospheric gases observed in the MnFe2O4/MCM-41 samples are a result of neutralization of some surface active centers acting as inhibitors and weakening of pick-off annihilation mechanism, respectively.