W. T. Geng

Coordination and chemical effects on the structural, electronic, and magnetic properties in Mn pnictides

Simple structures of MnX binary compounds, namely hexagonal NiAs and zincblende, are studied as a function of the anion (X = Sb, As, P) by means of the all-electron FLAPW method within local spin density and generalized gradient approximations. An accurate analysis of the structural, electronic and magnetic properties reveals that the cubic structure greatly favours the magnetic alignment in these compounds leading to high magnetic moments and nearly half-metallic behaviour for MnSb and MnAs. The effect of the anion chemical species is related to both its size and the possible hybridization with the Mn

The Effect of Li, He and Ca on Grain Boundary Cohesive Strength in Ni

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Magnetism and Magnetic Anisotropy in Exchange BIAS Systems

states; both contributions are seen to hinder the magnitude of the magnetic moment for small and light anions. Our results are in very good agreement with experiment - where available - and show that the generalized gradient approximation is essential to correctly recover both the equilibrium volume and magnetic moment.

EMBRITTLING AND STRENGTHENING EFFECTS OF HYDROGEN, BORON, AND PHOSPHORUS ON A SIGMA 5 NICKEL GRAIN BOUNDARY

Boron is added to nickel-base superalloys such as Alloy X-750 in order to enhance high temperature strength and ductility so that the alloy may be more easily hot worked[1]. Boron additions also have been shown to ameliorate intergranular hydrogen embrittlement in nickel[2], and to improve the high temperature resistance of Alloy X-750 to aqueous stress corrosion cracking (SCC) in the absence of irradiation[3]. Recent quantum mechanical calculations demonstrate that boron strengthens grain boundaries in pure nickel[4], and may contribute to the observed benefits of boron on workability and fracture resistance of nickel alloys. Alloy X-750 exhibits greater susceptibility to intergranular stress corrosion cracking (IGSCC) when irradiated[5], and it has been proposed that the presence of grain boundary helium and/or lithium is responsible. Arguments have been advanced that helium embrittlement of the grain boundaries is primarily responsible for the greater observed susceptibility to IGSCC in irradiated X-750[1]. Alternatively, it has been proposed that lithium promotes IGSCC either by entering the water at the crack tip and lowering the local pH, or by inducing a restructuring of the grain boundary itself[1]. Direct embrittlement of grain boundaries by lithium also has been investigated by ion bombardment in Nimonic PE16, illustrating that under certain conditions lithium can produce degrees of embrittlement in nickel comparable to that produced by helium[6]. It is important to understand the relative roles of these species in grain boundary embrittlement in nickel alloys so that better predictive abilities and mitigation strategies can be developed. Toward that end, quantum mechanical calculations have been performed to investigate the influence of isolated lithium and helium atoms on the cohesive strength of an ideal grain boundary in pure nickel.

Influence of alloying additions on grain boundary cohesion of transition metals: First-principles determination and its phenomenological extension

Magnetism in man made ultra-thin materials has become an area of intense activity and excitement, mainly driven by applications in magnetic recording. For the purpose of high-density magnetic recording, the understanding and control of the magnetic properties, such as complex magnetic ordering, enhanced magnetic moments, magneto-crystalline anisotropy and magnetostriction are desired. In this paper, we present results of magnetism and magnetic anisotropy in ferromagnetic NiFe thin film, antiferromagnetic NiMn and their interface determined from first-principles FLAPW (full-potential linearized augmented plane wave) calculations, and demonstrate how their properties behave sensitively on the environment (such as in bulk, at surfaces or interfaces). These results may be important for understanding exchange bias materials.