Yu. P. Chernenkov

Atomic displacements and short-range order in the FeSi soft magnetic alloy: Experiment and ab initio calculations

In order to determine the mechanism responsible for the formation of short-range order in dilute FeSi solid solutions, the chemical bonding, atomic displacements near the metalloid, and the enthalpy of silicon dissolution in iron have been studied within density functional theory. It is found that the directed character of the Si-Fe chemical bond formed upon the p-d hybridization brings about an anisotropy in atomic displacements near silicon atoms. Calculations of the Si-Si effective pairwise interaction energy offer an explanation for the observed features in short-range order in FeSi and suggest that ferromagnetic bcc Fe does not have a tendency toward Si atom clusterization. The mechanism of formation of the anisotropy induced by application of an external load or a magnetic field is discussed.

Relaxation of the state with induced transverse magnetic anisotropy in the soft magnetic nanocrystalline alloy Fe73.5Si13.5Nb3B9Cu1

The residual lattice strains of nanocrystals, which are responsible for the formation of states with transverse magnetic anisotropy in samples of the Fe-Si-Nb-B-Cu alloys (Finemets) subjected to annealing under tensile loading with the subsequent relaxation annealing at temperatures in the range from 500 to 600°C, have been measured using X-ray diffraction. The relative extension and compression of interplanar spacings have been compared with the induced magnetic anisotropy constants determined from the magnetic hysteresis loops. It has been shown that, during the relaxation annealing at the nanocrystallization temperature (500–540°C), the observed decrease in the residual strains is accompanied by a decrease in the transverse magnetic anisotropy constant. A linear correlation between the relative extension and compression of the interplanar spacings for different crystallographic planes and magnetic anisotropy constant has been revealed. The deviation from linearity is observed after annealing at a temperature of 600°C, which is explained by a possible increase in sizes of nanocrystals, changes in their structure, and partial crystallization of the amorphous matrix.

Effect of thermomagnetic and thermomechanical treatments on the magnetic properties and structure of the nanocrystalline soft magnetic alloy Fe81Si6Nb3B9Cu1

The structural and magnetic states of ribbon samples of the soft magnetic alloy Fe-Si-Nb-B-Cu (6 at % Si) have been investigated after the nanocrystallization at a temperature of 550°C in a constant magnetic field (thermomagnetic treatment), in a field of mechanical tensile stresses (thermomechanical treatment), and without external effects. It has been shown that exposure to a constant magnetic field or a field of mechanical tensile stresses gives rise to a longitudinal anisotropy of magnetic properties. The mag- netic hysteresis loop transforms and becomes close to rectangular. This is accompanied by a significant increase in the residual magnetic induction, which approaches the saturation magnetic induction. While the time required to complete the processes of nanocrystallization is as short as 20 min and, under thermome- chanical treatment, the magnetic anisotropy is induced for 20 min, the time it takes to decrease significantly the coercive force of the alloys under thermomagnetic treatment is substantially longer (up to 60 min). After the thermomagnetic treatment, no lattice strains of α-FeSi nanocrystals have been found. Either they do not exist at all, or their values are within the error of the X-ray diffraction experiment. In the samples subjected to annealing under tensile loading, anisotropic lattice strains of nanocrystals with the values increasing pro- portionally to the applied stress have been revealed. The highest strains reaching 1% have been observed after the annealing under a stress of 860 MPa.

Evolution of the atomic order and valence state of rare-earth atoms and uranium in a new carbon-metal composite—diphthalocyanine pyrolysate C64H32N16Me (Me = Y, La, Ce, Eu, and U)

The structure of a metal-carbon composite formed by the pyrolysis of diphthalocyanine of some rare-earth elements (Y, La, Ce, Eu) and uranium in the temperature range Tann = 800–1700°C has been investigated for the first time by the methods of X-ray diffraction analysis and X-ray line shift. It has been shown that, in the general case, the studied pyrolysates consist of three phases. One phase corresponds to the structure of graphite. The second phase corresponds to nitrides, carbides, and oxides of basic metal elements with a crystallite size ranging from 5 to 100 nm. The third phase is amorphous or consisting of crystallites with a size of ∼1 nm. It has been found that all the basic elements (Y, La, Ce, Eu, U) and incorporated iodine atoms in the third phase are in a chemically bound state. The previously unobserved electronic configurations have been revealed for europium. The possibility of including not only atoms of elements forming diphthalocyanine but also other elements (for example, iodine) in the composite structure is of interest, in particular, for the creation of a thermally, chemically, and radiation resistant metal-carbon matrix for the radioactive waste storage.

Lattice distortions near impurity atoms in α-Fe1−x Six alloys

Lattice distortions near impurity atoms in α-Fe1−xSix alloys (x≈0.05–0.06) are studied both experimentally using x-ray diffraction and theoretically by means of ab initio calculations. It is found that the distortions are more complex than experimental data suggest. The displacements of atoms near impurities are not determined by the concentration dependence of the average lattice constant nor by a difference in the ion radii.

Anisotropy of the local atomic structure in Fe-(5–6 at. %) Si single crystals as the cause of formation and stability of induced magnetic anisotropy

Fe1 − xSix (x = 0.05–0.06) single crystals were prepared and subjected to various heat treatments for structural studies. X-ray diffuse scattering measurements detected an anisotropy of regions with local B2-type atomic ordering in samples with induced magnetic anisotropy. It was shown that the average size of ordered clusters as measured along an applied dc magnetic field during heat treatment is slightly larger than that in a transverse direction and reaches 10 Å. Such anisotropy of B2-type regions is not observed in magnetically isotropic samples obtained by rapid quenching or by annealing and cooling in a rotating field (or in the absence of an external field). A comparative analysis of the atomic structure, domain structure, and hysteresis loop shape in samples subjected to various treatments demonstrated a correlation between the short-range order and magnetic properties.

Effect of annealing under tensile loading on the structure of nanocrystals in the Finemet alloy

The effect of nanocrystallization annealing under tensile loading on the structure of nanocrystals in the soft magnetic alloy Fe-Si-Nb-B-Cu (Finemet) has been investigated. It has been shown that the body-centered cubic (bcc) lattice of α-FeSi nanocrystals is extended along the direction of the application of the load upon annealing and is compressed in the transverse direction. Nanocrystals in the Finemet alloy have a higher degree of anisotropy of mechanical properties as compared to bulk crystals of α-FeSi, so that agreement between the measured and calculated values of the elongation is achieved only with a significant increase in the elastic moduli. Substantial changes in mechanical properties of the crystals with a decrease in their size to the nanometer scale are caused by the influence of the rigid amorphous matrix of the Fe(Nb)-B phase surrounding the nanocrystals.

Structure of the amorphous phase of pyrolisates of lanthanum diphthalocyanine according to X-ray scattering data

Data on X-ray diffraction in lanthanum diphthalocyanine pyrolysates synthesized at temperatures of 800–1800°С demonstrate the formation of an amorphous carbon phase with embedded lanthanum atoms. Low-temperature pyrolysis (800–900°С) creates layered carbon structures. Due to annealing at 1000°С, carbon integrates into globules whose number of atoms is m ~ 100. Such structures with gyration radii of Rg ~ 0.4–0.5 nm on the order of the precursor molecule size are synthesized in the temperature range of 1000–1800°С, and are stable in terms of size and mass. In this case, their density approaches that of graphite.

Thermal stability of strains induced in Fe81Si6Nb3B9Cu1 alloy nanocrystals during annealing under tensile loading

The effect of nanocrystallization annealing under tensile loading on residual strains of nanocrystals in the soft magnetic alloy FeSiNbCuB (Finemet) has been investigated using X-ray diffraction. It has been shown that the body-centered cubic lattice of α-FeSi nanocrystals is extended along the direction of the application of the load upon annealing and is compressed in the transverse direction. It has been established that the extension of the lattice along the crystallographic axes coinciding with the stretching direction as well as the compression in transverse directions are well described within the framework of the elastic strain theory taking into account the symmetry of the cubic lattice. The isotropic and anisotropic Poisson’s ratios of nanocrystals have been determined. The thermal stability of the strains of nanocrystals has been analyzed. The annealing without external influences leads to a decrease in the strain and the Poisson’s ratio.

Effect of crystallization annealing under loading on the magnetic properties and the structure of a soft magnetic FeSiNbCuB alloy doped with chromium

The changes of quasi-static magnetic hysteresis loops and X-ray diffraction patterns of the Fe73.5Si13.5B9Nb3Cu1 doped to 10 at % chromium instead of iron have been studied to elucidate the influence of the thermomechanical treatment consisting of annealing and cooling of the alloy under the tensile stress (tensile-stress annealing (TSA)) on the magnetic properties and the structure of these alloys. It is shown that the treatment results in the induction of the magnetic anisotropy of the hard axis type at which the magnetization reversal along the direction of applying the external stress during annealing is hampered. The energy of the induced magnetic anisotropy decreases as the chromium content increases. During TSA, the nanocrystal lattices are deformed, and the deformation is retained after cooling. The interplanar spacings increase along the extension direction and decrease in the transverse direction. The deformation anisotropy is observed for crystallographic directions. The anisotropic deformation of the bcc lattice of nanocrystals with high content of the ordered Fe3Si phase characterized by a negative magnetoelastic interaction is the cause of formation of the state with the transverse magnetic anisotropy of the hard axis type.