A. E. Austin

Grain‐Boundary Diffusion

The diffusion of nickel into grain boundaries of various tilt angles in copper bicrystals was studied. The concentration contours from the grain boundary and the lateral lattice diffusion were measured by means of electron‐probe microanalysis. The product of grain‐boundary width and diffusion coefficient were calculated, and comparisons were made with theoretical solutions. The grain‐boundary coefficient is concentration dependent above 3% nickel in a 45° tilt boundary. With lower tilt angles, the coefficient is concentration dependent above 0.5% nickel.

Magnetic Structures of Mn2As and Mn2Sb0.7As0.3

Neutron diffraction has been performed on powder specimens of Mn2As and Mn2Sb0.7As0.3. Mn2As has the antiferromagnetic structure found in Mn1.9Cr0.1Sb below the exchange inversion temperature. The atomic moments are 3.7 and 3.5 Bohr magnetons for Mn(I) and Mn(II), respectively. The moments lie perpendicular to the tetragonal axis. The magnetic structure of Mn2Sb0.7As0.3, which undergoes exchange inversion over the 35° to 115°C temperature range, is the same as that of Mn2As at room temperature and is the same as that of ferrimagnetic Mn2Sb at 150°C. The atomic moments at room temperature (antiferromagnetic) are 2.3 and 2.8 Bohr magnetons, respectively, for Mn(I) and Mn(II). The moments are perpendicular to the tetragonal axis at room temperature and parallel to it at 150°C.

Magnetic properties of heavy-rare-earth orthomanganites

Abstract The orthorhombic phases of YMnO3, HoMnO3 and YbMnO3 were made by transformation of the normal hexagonal phases at 1000°C and 35–40 kbar. The magnetic susceptibilities were measured from 4·2 to 400 K. Antiferromagnetic ordering is evident at 42 K for YMnO3 and 9 K for HoMnO3. The paramagnetic Curie temperatures were decreased considerably in magnitude from those of the hexagonal phases, being −67 K for YMnO3, −23K for HoMnO3 and −83 K for YbMnO3. HoMnO3 is metamagnetic below 9 K. In YbMnO3 and HoMnO3, both the rare-earth and Mn ions carry approximately their full moment. The magnetic structures are discussed in comparison with the orthoferrites. It is shown that strong Jahn-Teller distortion around the Mn3+ ion leads to a slight reduction in the lattice parameter c(b′), and thence to the low magnetic ordering temperatures.

Theoretical Solutions of Grain‐Boundary Diffusion Problem. Approximations and Interpretation of Experiments

Whipples exact solution to the idealized grain‐boundary diffusion problem has been evaluated numerically for ranges of parameters appropriate to existing and contemplated experiments. From these evaluations the concentration at the grain boundary, the integrated concentration at fixed depth of penetration, and the angle at which isoconcentration contours intersect the grain boundary are obtained, these results being presented in either tabular or graphical form. The ranges of validity of approximate solutions obtained by various authors are discussed on the basis of the numerical results, as is the use of the graphs and tables for the interpretation of experimental data.

Stoichiometry dependence of lithium outdiffusion in LiNbO3

Measurements by Mach‐Zehnder interferometry of the refractive‐index changes near the surface of outdiffused LiNbO3 crystals indicate that, in agreement with recent results of Noda et al. [J. Appl. Phys. 51, 1379 (1980)], the rate of lithium outdiffusion is greater for more nearly stoichiometric crystals. The activation energy for diffusion of Li does not vary significantly with stoichiometry. The Li diffusion coefficient is also higher in the relatively Li‐depleted regions near the surface.

Grain‐Boundary Diffusion of Gold in Copper

The diffusion of gold into the grain boundaries of copper bicrystals was studied for couples of both a continuous and an instantaneous source. The concentration contours along the grain boundaries were measured by means of electron‐probe microanalysis. For the continuous source conditions at 750°C, the ratio of the coefficients of grain‐boundary diffusion and lattice diffusion decreased from 6×105 to about 103 with increasing gold concentration from 0.5 to 15 at. % gold. The rate of grain‐boundary diffusion decreases even though there is some increase in the rate of lattice diffusion. There was found some contribution of surface diffusion to the couples with an instantaneous or thin source, i.e., 0.5 μ thick. The concentration contours observed in these couples agreed generally with the theoretical solutions.

Surface and Grain‐Boundary Diffusion of Gold‐Copper

The diffusion of gold into copper grain boundaries from thin sources has been studied from 760° to 625°C. The activation energy for the grain‐boundary diffusion of gold into copper is 25 kcal/mole. At the lower temperature, there was observed surface depletion of gold at the grain boundary in accordance with theoretical solutions. The surface diffusion of gold on copper (100) surface was studied from 705° to 580°C under 10−9 Torr. The activation energy for surface diffusion was about 25 kcal/mole. The rates of surface diffusion were found to be less than those for grain‐boundary diffusion at the same temperature. Gold‐copper whiskers were generated adjacent to the grain boundaries for diffusion at or below 660°C. The growth of such whiskers is diffusion‐controlled and is attributed to the stress resulting from volume increase due to gold diffusion into the grain boundary.

A Positive‐Replica Technique for Electron Microscopy

A positive‐replica technique for electron microscopy has been developed, which reproduces the contour variations of the specimen surface, and permits direct visual interpretation of elevation. The method utilizes two resins, each mutually insoluble in the solvent for the other, specifically, polyvinyl alcohol plus Formvar. Shadow casting of the Formvar positive replica enhances detail and highlights the areas in relief in the original surface. Examples are given, in particular, applied to wear‐test specimens.

Weak Ferromagnetism in BiFeO3 ‐ NdFeO3 Solid Solutions

There are three phases in the pseudobinary solid solution Bi1−xNdxFeO3 — the rhombohedral ferroelectric BiFeO3 phase for x ≤ 0.1, a tetragonal phase near x = 0.15, and the orthorhombic NdFeO3 structure for x ≥ 0.2. Intrinsic weak ferromagnetism is present in all three phases. In rhombohedral Bi.9Nd.1FeO3, the room temperature moment is about 3 mμ B/formula unit after thermomagnetic annealing. The moment increases fairly smoothly with increasing Nd content through all three phases, at least up to x = 0.5. The presence of weak ferromagnetism in the BiFeO3 phase is explained on the basis of the effect of Nd in regularizing the Fe‐O distances, thus affecting superexchange at least locally. These results are compared with those found by others for Bi1−xLaxFeO3 and Bi1−xPrxFeO3.

Magnetic Structures of Mn 2 As and Mn 2 Sb 0.7 As 0.3

Neutron diffraction has been performed on powder specimens of Mn2As and Mn2Sb0.7As0.3. Mn2As has the antiferromagnetic structure found in Mn1.9Cr0.1Sb below the exchange inversion temperature. The atomic moments are 3.7 and 3.5 Bohr magnetons for Mn(I) and Mn(II), respectively. The moments lie perpendicular to the tetragonal axis. The magnetic structure of Mn2Sb0.7As0.3, which undergoes exchange inversion over the 35° to 115 °C temperature range, is the same as that of Mn2As at room temperature and is the same as that of ferrimagnetic Mn2Sb at 150°C. The atomic moments at room temperature (antiferromagnetic) are 2.3 and 2.8 Bohr magnetons, respectively, for Mn(I) and Mn(II). The moments are perpendicular to the tetragonal axis at room temperature and parallel to it at 150°C.