Characterization of anti-phase boundaries in epitaxial magnetite films

The occurrence of anti-phase domain boundaries (APBs) in epitaxial Fe₃O₄ films has a strong influence on the resistivity, magnetic and magneto-resistance properties of these films. It is therefore important to understand the configuration and magnetic coupling across the boundary. We have studied the distribution of shift vectors and the relationship between the shift vector and the boundary plane and the resulting magnetic coupling at the boundary. The vast majority of APBs have shifts while those with 1/2[100] shift are very uncommon. Approximately 45% of APBs have shift vectors in the plane of the film. Their boundary plane is perpendicular to the shift vectors and in this case the magnetic coupling can be either ferromagnetic or anti-ferromagnetic. The remaining 55% of APBs have shift vectors out of the film plane, with the boundary planes not perpendicular to the shift vector but close to {100} or {310}, resulting in a ferromagnetic coupling when the boundary plane is {100} and in an anti-ferromagnetic coupling when the boundary plane is {310}.Received: 22 May 2003, Published online: 8 December 2003PACS: 68.37.Lp Transmission electron microscopy (TEM) (including STEM, HRTEM, etc.) - 75.30.Et Exchange and superexchange interactions - 75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)W. Eerenstein: Present address: Dept. of Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK     

An experimental study of anti-phase boundaries contrast of Fe-Si alloys in superlattice reflexions

Thin foils of ordered alloys of different composition are studied by transmission electron microscopy. The contrast on antiphase boundaries 1/4a ₀ ^′ 〈111〉, 1/2a ₀ ^′ 〈100〉 is compared with contrast theories. It is shown that the dependence of the contrast profile on the deviation parameter from the Bragg reflexion position and the thickness of the foil for oblique boundaries is qualitatively in agreement with the theory. On nearly perpendicular boundaries the observed dependence of contrast is qualitatively the same for higher silicon contents. For lower silicon contents, however, only dark contrast (in the dark field image) is observed for all values of the deviation parameter and foil thickness. This behaviour of nearly perpendicular boundaries at lower silicon contents is explained by the existence of a disordered layer of finite thickness in the boundary. The disordered layer also causes the anomalous contrast of 1/2a ₀ ^′ 〈100〉 boundaries in S₂ reflexions for which zero contrast is predicted by the theory. In contrast to the idealized model of a boundary of this type with disordered second nearest neighbours only, the experiments show also some disorder of the first nearest neighbours. Finally, examples of complicated fringe contrast are shown which can possibly be interpreted as many-beam cases with superlattice and fundamental reflexions excited simultaneously.     

Antiphase boundary (APB) orientation and energy in the superstructure L12

This paper is devoted to an electron microscope study of the antiphase boundaries (APB) in the ordered alloy Pd₃Fe, a theoretical explanation of the reasons for their preferred orientation, and a computation of the antiphase boundary energy with =1/2 110 {001} in alloys with the superstructure L1₂.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 59–61, March, 1979.     

Dislocation retardation due to the destruction of short-range order in the L12 superstructure

The magnitude and temperature dependence of the dislocation retardation due to the destruction of short-range order are calculated in the quasichemical approximation. The atomic interaction in two coordination spheres and dislocation slip in cubic and octahedral planes are taken into account. The retardation stress for the first superdislocation is essentially the same in these planes for T < T _k (where T _k is the phase-transition temperature), while the retardation stress for subsequent superdislocations is much lower, so large planar accumulations of superdislocations may arise. The retardation stress is maximal at T = T _k , and for T > T _k the first dislocations should move in pairs. Comparison of the experimental cleavage stresses with calculated values shows that, by itself, superdislocation retardation due to correlation destruction cannot explain the behavior of the yield point.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 78–85, April, 1971.     

Accumulation of dislocations and thermal strengthening of alloys having an L12 superstructure

This paper discusses the dislocation-accumulation mechanism in alloys having an L1₂ superstructure, which is associated with the formation of Kira-Wilsdorf barriers and the retardation of superdislocations during plastic deformation. A model of the dislocation-accumulation kinetics during plastic deformation is constructed, on the basis of which a mathematical model is formulated for the thermal and deformation strengthening of single crystals of alloys having the L1₂ superstructure. The results of numerical calculations based on the model are compared with the experimentally observed regularities of the deformation and thermal strengthening of single crystals of Ni₃Ge. Fiz. Tverd. Tela (St. Petersburg) 41, 454–460 (March 1999)     

Segregation at antiphase boundaries in a superstructure of the β-brass type of stoichiometric composition

Using the Gorskii-Bragg-Williams approximation, the authors examine the equilibrium configuration of a regular solid solution of the -brass type in the region of the antiphase boundary 1/2 110 {100}. They show that at these boundaries, with temperatures above 0 ° K, it is possible for segregation of the atoms of one of the components to occur with a stoichiometric composition of the solid solution. The deviation of the concentration of one of the elements at the antiphase boundary (APB) from the concentration of this element in the uniform solid solution remains large (up to 28%) right up to the temperature of disorganization.The authors thank N. S. Golosov for evaluating the paper and for help with the calculations.     

Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni₃Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E _sp, and adatom creation, E _ad, are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E _sp of 6.5?eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0?eV for a perfect M surface. High values of E _sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E _sp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E _ad imply that an electron beam could create more Al adatoms on surfaces intersected by APBs than on those without. The equilibrium, minimum energy structures for a (001) surface intersected by either Σ5[001](210) or Σ25[001](340) symmetric tilt grain boundaries are computed. E _sp for surface Al atoms near these GBs increases monotonically with increasing recoil angle to the surface normal, with a minimum value, which is only about 1?eV different from that obtained for a perfect surface. Temperature up to 300?K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for E _sp studied here. Possible reasons for this are discussed.     

Simulation of the process of deformation-induced destruction of long-range order in alloys with an L12 superstructure

Plastic-deformation-induced destruction of long-range order in alloys with an L1₂ superstructure is considered. A mathematical model is suggested which takes into account the following mechanisms that lead to the destruction of long-range order: generation of superdislocations, generation of single dislocations, multiplication of antiphase boundaries (APBs) upon the conservative motion of dislocations, multiplication of APBs upon dislocation climb, formation of APB tubes on superdislocations, generation of point defects, and thermal ordering. A mathematical model of deformation strengthening and long-range order destruction with allowance for the change in the type of shear-forming dislocations from superdislocations to single dislocations is formulated.     

Mathematical modeling of the plastic deformation of a material with an FCC matrix and strengthening particles with L12 superstructure

A mathematical model of the plastic deformation of metallic materials containing a strengthening phase with L1₂-type ordering that exhibits both coherent and incoherent coupling with a matrix in various ratios is used to study the effect the scale characteristics of the strengthening phase has on the features of deformation strengthening at different temperatures and strain rates.     

Thermal hardening of Ni3Ge alloy single crystals with an L12 superstructure at low temperatures

The mechanical properties and the dislocation structure of Ni₃Ge alloy single crystals have been experimentally studied at low temperatures. It is found that the flow stresses increase beginning with 4.2 K, and the observed rise in the stresses depends on the orientation of the strain axes of the crystals. The dislocation structure is investigated thoroughly. It is revealed that the mean density of dislocations and the interdislocation interaction parameter α anomalously increase as the temperature increases in the range 4.2–293 K. The mechanisms providing an explanation for the temperature anomaly of flow stresses and the α parameter are considered. The activation energy of thermal hardening is evaluated. It is assumed that the low activation energies of thermal hardening are due to the motion of dislocations at velocities close to the velocity of sound at these temperatures.     

Influence of urea on polyvinyl alcohol molecular superstructure formation

Whiskers up to 1 cm in length were grown in polyvinyl alcohol (PVA) and urea solution. Raman and IR spectra discover an interaction between PVA and urea molecules. Optical and electronic microscopy data show that urea influences on PVA molecular superstructure formation. PVA whiskers prepared in urea solution can be used for organic semiconductors production which properties are determined by arrangement of polymer macromolecules.     

Slipping superdislocation a/2〈112〉 in the L10 superstructure

A new type of slipping superdislocation with a Burgers vector a/2 (112) is proposed for alloys with the Ll₀ superstructure. A superdislocation of this kind has a configuration similar to that of the well-knowna 110 superdislocation, but differs from the latter (in the case of imperfect long-range order) by having a high value of the retarding force due to the disruption of correlation.Translated from Izvestiya Uchebnykh Zavedenii, Fizika, Vol. 16, No. 7, pp. 123–126, July, 1973.     

Model of periodic superstructure formation induced by mobile defects on a semiconductor surface

A model for forming a periodic superstructure on a semiconductor surface with mobile defects is proposed. It is shown that a self-organized system of such defects could lead to considerable modulation of incommensurate phase parameters in near-surface layers under the conditions of defect interaction with an incommensurate displacement wave. This type of defect system could induce a substantial energy decrease for the incommensurate phase and its additional stabilization on the one hand, and an increase in modulation amplitude that might be accompanied by a soliton-like nonsinusoidal incommensurate displacement wave on the other. The model allows us to explain the experimental results regarding the periodic superstructure discovered by G.V. Benemanskaya et al. on the surface of a GaN semiconductor. In particular, the model enables us to explain the nature of such superstructures based on the self-organization of mobile neutral triad-type defects, composed of negatively charged (3−) vacancies of the host lattice centers of Ga³⁺ and surface impurity ions of Cs⁺ and Ba²⁺, under conditions of triad interaction with an incommensurate displacement wave.     

Superdislocation blocking in B2 and L12 superlattices by interaction with dislocations at antiphase boundaries

The paper discusses the formation of unconforming dislocations at antiphase boundaries APB. The stress due to the interaction with superdislocations is estimated, and this stress is evaluated as a function of the size of the antiphase domain. It is shown that a gliding superdislocation cutting through APB of type 1/2 <111> {110} in CuZn and through cubic APB of (+) type in AuCu₃ facilitates passage of the superdislocation, while the converse applies for cubic APB of type in AuCu₃. There is only slight resistance to the motion of the superdislocation on account of octahedral APB in AuCu₃.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 21–27, April 1972.We are indebted to A. N. Orlov and L. E. Popov for valuable discussion.