Exchange Effects in the Invar Hardening: Fe0.65Ni0.35Fe_{0.65}Ni_{0.35} as a test case

An increase of the critical resolved shear stress of Invar alloys (Invar hardening) with a lowering temperature is explained. The effect is caused by a growth of the exchange interaction between dangling $d$-electron states of dislocation cores and paramagnetic obstacles (e.g., Ni atoms in FeNi alloys) which occurs below the Curie temperature. The spins of the two electrons align along the magnetization due to the exchange interaction with the surrounding atoms of the ferromagnetic. The exchange interaction between the dislocations and obstacles is enhanced in Invars due to a strong growth of the magnetic moments of atoms under the action of elastic strains near the dislocation cores. Parameters characterizing the exchange interaction are determined for the case of the Fe$_{0.65}$Ni$_{0.35}$ Invar. The influence of the internal magnetic field on the dislocation detachment from the obstacles is taken into account. The obtained temperature dependence of the critical resolved shear stress in the Fe$_{0.65}$Ni$_{0.35}$ Invar agrees well with the available experimental data. Experiments facilitating a further check of the theoretical model are suggested.Comment: 8 pages, 2 figure

"Cold Melting" of Invar Alloys

An anomalously strong volume magnetostriction in Invars may lead to a situation when at low temperatures the dislocation free energy becomes negative and a multiple generation of dislocations becomes possible. This generation induces a first order phase transition from the FCC crystalline to an amorphous state, and may be called "cold melting". The possibility of the cold melting in Invars is connected with the fact that the exchange energy contribution into the dislocation self energy in Invars is strongly enhanced, as compared to conventional ferromagnetics, due to anomalously strong volume magnetostriction. The possible candidate, where this effect can be observed, is a FePt disordered Invar alloy in which the volume magnetostriction is especially large

Magnetic structural effect in nonequilibrium defective solids

Scientific study of the effect of structural memory of nonequilibrium defective solids about the processing in magnetic field (the magnetic structural effect (MSE) was continued in this paper. The study was aimed to reveal the universal nature of the MSE, which was investigated in several new nonequilibrium defective solids. The results of investigation of the processing in the vortical magnetic field (PVMF) and its effect on the structure of the natural magnetite Fe3O4 and the SnO2 films were presented. The methods of Mössbauer and X-ray spectroscopy were used. The PVMF reduction of a defectiveness of Fe3O4 structure in the magnetite was detected. The MSE was also observed in the Mössbauer spectra of diamagnetic tin oxide SnO2 films after the PVMF. One of the possible explanations of the MSE was given in the paper.Comment: 6 pages, 6 figures, 3 table

Dynamics of ferroelectric domain growth in the field of atomic force microscope

Application of very high voltage to atomic force microscope tip leads to the growth of narrow, string-like domains in some ferroelectrics, a phenomenon that was named “ferroelectric domain breakdown.” In this work the dynamics of domain breakdown have been studied experimentally and theoretically in stoichiometric lithium niobate (LN). The theory has been found to be in a good agreement with the measured domain radius temporal dependence. Dynamics of domain growth has also been studied in ultrathin LN crystals, where the domain breakdown phenomenon does not take place. It is also shown that domain formation processes occurring in bulk and ultrathin crystals are very different, and this is ascribed to the observed difference in depolarization energy dependence on the domain length