First-principles study of the Fe

MgO bilayer systems emphasizing the influence of the iron layer thickness on the geometry, the electronic structure and the magnetic properties. Our calculations ensure the unconstrained structural relaxation at scalar relativistic level for various numbers of iron layers placed on the magnesium oxide substrate. Our results show that due to the formation of the interface the electronic structure of the interface iron atoms is significantly modified involving charge transfer within the iron subsystem. In addition, we find that the magnetic anisotropy energy increases from 1.9 mJ m-2 for 3 Fe layers up to 3.0 mJ m-2 for 11 Fe layers

Calculation of solubility in titanium alloys from first-principles

We present an approach to calculate the atomic bulk solubility in binary alloys based on the statistical-thermodynamic theory of dilute lattice gas. The model considers all the appropriate ground states of the alloy and results in a simple Arrhenius-type temperature dependence determined by a {\it "low-solubility formation enthalpy"}. This quantity, directly obtainable from first-principle calculations, is defined as the composition derivative of the compound formation enthalpy with respect to nearby ground states. We apply the framework and calculate the solubility of the A specie in A-Ti alloys (A=Ag,Au,Cd,Co,Cr,Ir,W,Zn). In addition to determining unknown low-temperature ground states for the eight alloys, we find qualitative agreements with solubility experimental results. The presented formalism, correct in the low-solubility limit, should be considered as an appropriate starting point for determining if more computationally expensive formalisms are otherwise needed.Comment: 10 pages, 12 figure

Site-resolved contributions to the magnetic anisotropy energy and complex spin structure of Fe/MgO sandwiches

Fe/MgO-based Magnetic Tunnel Junctions (MTJs) are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite size effects and temperature dependent magnetic properties which must be carefully controlled for practical applications. In this letter, we investigate the electronic, structural and magnetic properties of MgO/Fe/MgO sandwiches using first principles calculations and atomistic spin modeling based on a fully parameterized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin spiral ground state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layer-wise temperature dependence of the magnetization. The different temperature dependences of the interface and bulk-like layers results in an unexpected non-monotonic temperature variation of the effective magnetic anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure Fe/MgO interface and the role of elevated temperatures providing new insight when interpreting experimental data of nanoscale MTJs

First-principle solubilities of alkali and alkaline earth metals in Mg-B alloys

By devising a novel framework, we present a comprehensive theoretical study of solubilities of alkali (Li, Na, K, Rb, Cs) and alkaline earth (Be, Ca, Sr, Ba) metals in the he boron-rich Mg-B system. The study is based on first-principle calculations of solutes formation energies in MgB$_2$, MgB$_4$, MgB$_7$ alloys and subsequent statistical-thermodynamical evaluation of solubilities. The advantage of the approach consists in considering all the known phase boundaries in the ternary phase diagram. Substitutional Na, Ca, and Li demonstrate the largest solubilities, and Na has the highest (0.5-1 % in MgB$_7$ at $T=650-1000$ K). All the considered interstitials have negligible solubilities. The solubility of Be in MgB$_7$ can not be determined because the corresponding low-solubility formation energy is negative indicating the existence of an unknown ternary ground state. We have performed a high-throughput search of ground states in binary Mg-B, Mg-$A$, and B-$A$ systems, and we construct the ternary phase diagrams of Mg-B-$A$ alloys based on the stable binary phases. Despite its high temperature observations, we find that Sr$_{9}$Mg$_{38}$ is not a low-temperature equilibrium structure. We also determine two new possible ground states CaB$_{4}$ and RbB$_{4}$, not yet observed experimentally.Comment: 5 figure

Surface segregation in nanoparticles from first principles: The case of FePt

FePt nanoparticles are known to exhibit reduced L1_0 order with decreasing particle size. The phenomenon is addressed by investigating the thermodynamic driving forces for surface segregation using a local (inhomogeneous) cluster expansion fit to ab initio data. Subsequent Monte Carlo simulations reveal that first surface layer Pt segregation is compensated by Pt depletion in the second subsurface layer. This indicates that the core’s ordered state is not affected by surface thermodynamics as much as previously thought