V. Rusanov

Magnetic properties of nanophase CoFe2O4 particles

Abstract Fine CoFe 2 O 4 particles with average size of 50 A have been produced by coprecipitation from Fe(II) and Co(II) solutions followed by a low-temperature calcination. The ac and dc magnetic susceptibilities, as well as magnetization, were studied between 5 and 300 K. The disordered system of such particles behaves superparamagnetically above T B = 250 K and ferrimagnetically below this temperature. T B can be considered as mean blocking temperature at zero field in the timescale of ac (111.1 Hz) magnetic susceptibility measurements. The anisotropy field exceeds 50 kOe below 150 K. The maximal coercivity measured at 5 K is 14.5 kOe and the maximal measured magnetization is 14.6 emu/g, which is much lower than the saturation magnetization of bulk CoFe 2 O 4 .

The effect of inert additives on the explosive mechanochemical synthesis of some metal chalcogenides

The effect of different inert additives on the mechanochemical synthesis of SnS was studied. A transition from explosive to nonexplosive mechanochemical reaction was observed. It was shown that this change takes place only when a critical volume xc is occupied by inert material. The critical volume for a transition from an explosive to nonexplosive mechanochemical synthesis were determined for the chalcogenides of Zn, Cd, and Sn. The xc values obtained are discussed. On the basis of the new data concerning the critical dimensions rc of the exploding mechanical aggregates and the percolation phenomena, a mechanism for the transition from an explosive to nonexplosive mechanochemical synthesis is suggested.

Mössbauer parameters of the two long-lived metastable states in Na2[Fe(CN)5NO]·2H2O single crystals

We have measured the quadrupole splitting, isomer shift and line broadening of the ground and two metastable states in Na2[Fe(CN)5NO]·2H2O single crystals. Pure state II could be observed by Mössbauer spectroscopy after irradiation of state I with light in the near infrared spectral region. The difference between the two nearly infinitely long-lived electronic states is shown. A possible change of the Fe(3d, 4s, 4p) electron densities is discussed on the basis of the Mössbauer parameters and thev(FeN) andv(NO) stretching vibrations. The resulting new orbital population allows to explain the observed properties of state I. A rearrangement of the electron density due to state II would lead to a contradiction with the vibrational behaviour of the molecule. The larger line width of both metastable states could not yet be interpreted.

On the Development Mechanism and Kinetics of the Explosive Mechanochemical Synthesis

Abstract Data are summarized for mechanochemical processes leading to aggregate formation in the mechanical activation and the classical theory on thermal explosions. It has been shown that explosion occurs as a result of mechanochemical synthesis through a thermal mechanism. A kinetic equation of explosive synthesis mechanochemical processes has been proposed which accounts for the mechanical activation features and for the suggested thermal mechanism of explosion. On the basis of the specificity of explosive mechanochemical processes it is suggested that explosive mechanochemical processes of synthesis should be regarded as a separate class of chemical reactions.

Mössbauer, magnetic, X‐ray fluorescence and transmission electron microscopy study of natural magnetic materials from speleothems: haematite and the Morin transition

Detrital magnetic materials within cave stalagmitic formations, e.g., haematite or magnetite, carry remanence whose vector is of value in dating. Magnetometry measurements on a particular haematite‐bearing sample reveal that remanence was substantially restored and/or conserved on rewarming after cooling below the Morin transition temperature. Mössbauer measurements indicate the presence of two types of haematite, distinguished primarily by particle size. The majority is small in size, partially exhibiting superparamagnetism, and does not undergo a Morin transition above liquid nitrogen temperature. Superparamagnetic goethite is the second major component. Mine haematite samples of surface location with different color and mineralogical composition have also been studied. Possible relations between the mineralogical composition of the mine samples and detrital stalagmitic magnetic material, the modifications and the origin of this mineralization are discussed. Special attention is paid to the “irreversible” Morin transition in large enough (>20 nm) haematite particles and the possible loss of natural remanent magnetization and hence of palaeomagnetic records.

Percolation phenomena in explosive mechanochemical synthesis of some metal chalcogenides

Abstract Percolation phenomena are observed during the transition from an explosive to nonexplosive mechanochemical synthesis in Met-Met-Halc systems. The transition occurs under the effect of inert additives. In the systems investigated the inert admixtures are unreacted metals and metal chalcogenides synthesized nonexplosively. Two explosive synthesis mechanochemical reactions and two critical volumes x c 1 and x c 2 for the transition from an explosive to a nonexplosive mechanochemical reaction, respectively, correspond to the Met-Met-Halc system. Depending on the x c 1 and x c 2 values, three possible cases, considered in the percolation theory, are observed. It is shown that the final product synthesized in a nonexplosive mechanochemical way plays the role of an inert impurity. On this basis, a mechanism is proposed which explains the existence of a series of mechanochemical, in principle, nonexplosive reactions.

Mössbauer spectroscopy in the time domain applied to the study of single‐crystalline guanidinium nitroprusside

Guanidinium nitroprusside GNP, (CN3H6)2[Fe(CN)5NO] has been investigated in single‐crystalline form by nuclear resonant forward scattering (NFS) using synchrotron radiation (Mössbauer spectroscopy in the time domain). This method provides a direct measure of effective absorber thickness and therefore also of the Lamb–Mössbauer factor fLM. GNP has the advantage that all [Fe(CN5)NO]2- anions are practically aligned within the crystal. For the two different crystal orientations, with the crystallographic a- and c-direction parallel to the synchrotron beam, respectively, we have obtained fLM(a)= 0.122(10) and fLM(c)= 0.206(10), i.e., GNP exhibits significant anisotropic vibrational behavior. The quantum beat pattern of the NFS spectra obtained for the two different crystal orientations is discussed on the basis of radiation characteristics of the polarized synchrotron beam and the multipole transitions of oriented 57Fe nuclei.

Observed difference between the probabilities of recoilless resonance absorptionf′ and reemissionf″ of gamma-quanta in single crystals of Na2[Fe(CN)5NO]·2 H2O

The recoilless absorption probability factor,f′, and recoilless reemission,f″, both measured on Na2[Fe(CN)5NO]·2 H2O single crystals using the “black” filter technique, were found to be different. Unexpectedly, the results found weref″>f′. In the calculation off″, selfabsorption in the scatterer, non-ideality of the “black” filter and the influence of non-resonant scattering processes have all been taken into account. By varying the scattering geometry for the incoming and outgoing γ-beam relative to the crystallographic axes only a change in the reemitted valuesf″a, f″b, f″c could be detected because of the long lifetime of the excited nucleus (∼10−7 s) relative to the lattice vibration frequencies (∼1012 Hz).

Some Mo¨ssbauer and X-ray investigations of theα2-Snformation during the reduction of SnCl2 water solutions

Abstract Tin structures with very high values of the isomer shifts (δ = +4.3 ± 0.1 mm/sec and δ = +7 ± 0.4 mm/sec) of the Mo¨ssbauer lines, have been observed not only in thin tin films but also in bulk samples. The samples have been obtained by the reduction of SnCl 2 water solutions with Mg, carried out either in the presence of ferromagnetic impurities (Fe, Co, Ni) or in an external magnetic field. The parallel X-ray studies are in accordance with the RHEED observations of thin tin films. The temperature dependence of the Mo¨ssbauer effect on samples rich in α 2 -Snhas been studied and microcalorimetric measurements on samples containing β 1 -Snand α 2 -Snhave also been performed.

Investigation of the Mossbauer line fine structure of a new α2-Sn allotropic form

A fine structure of the Mossbauer line of the new alpha 2-Sn allotropic form has been observed. The isomeric shifts of five alpha 2-Sn substructures have been determined. The Mossbauer data have been considered together with X-ray diffraction ones. It is shown that the isomeric shift increases when the lattice parameter of alpha 2-Sn substructure decreases. Magnetic measurements of the susceptibility of samples containing a significant quantity of alpha 2-Sn have been carried out. The problem with the alpha 2-Sn isomeric shift interpretation has been discussed. Considerations have been given to the causes of the formation of the new crystalline structures.