Aurelian Rusanu

First principles calculation of finite temperature magnetism in Fe and Fe3C

Density functional calculations have proven to be a useful tool in the study of ground state properties of many materials. The investigation of finite temperature magnetism, on the other hand, has to rely usually on the usage of empirical models that allow the large number of evaluations of the systems Hamiltonian that are required to obtain the phase space sampling needed to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature. We have demonstrated a solution to this problem that harnesses the computational power of today’s large massively parallel computers by combining a classical Wang–Landau Monte-Carlo calculation [F. Wang and D. P. Landau, Phys. Rev. Lett. 86, 2050 (2001)] with our first principles multiple scattering electronic structure code [Y. Wang et al., Phys. Rev. Lett. 75, 2867 (1995)] that allows the energy calculation of constrained magnetic states [M. Eisenbach et al., Proceedings of the Conference on High Performance Comput...

First principles approach to the magneto caloric effect: Application toNi2MnGa

The magneto-caloric effect (MCE) is a possible route to more efficient heating and cooling of residential and commercial buildings. The search for improved materials is important to the development of a viable MCE based heat pump technology. We have calculated the magnetic structure of a candidate MCE material: Ni2MnGa. The density of magnetic states was calculated with the Wang Landau statistical method utilizing energies fit to those of the locally self-consistent multiple scattering method. The relationships between the density of magnetic states and the field induced adiabatic temperature change and the isothermal entropy change are discussed.

Calculated electronic and magnetic structure of screw dislocations in alpha iron

Local atomic magnetic moments in crystalline Fe are perturbed by the presence of dislocations. The effects are most pronounced near the dislocation core and decay slowly as the strain field of the dislocation decreases with distance. We have calculated local moments using the locally self-consistent multiple scattering (LSMS) method for a supercell containing a screw-dislocation quadrupole. Finite size effects are found to be significant indicating that dislocation cores affect the electronic structure and magnetic moments of neighboring dislocations. The influence of neighboring dislocations points to a need to study individual dislocations from first principles just as they appear amid surrounding atoms in large-scale classical force field simulations. An approach for the use of the LSMS to calculate local moments in subvolumes of large atomic configurations generated in the course of classical molecular dynamics simulation of dislocationdynamics is discussed.

Electronic and Magnetic Structure of

Nanocomposite permanent magnets made of hard and soft magnetic nanoparticles are important nanostructured materials. They, however, present substantial theoretical challenges due to the need to treat the electronic interactions quantum-mechanically whilst dealing with a large number of atoms. In this presentation, we show a direct quantum mechanical simulation of magnetic nano-structures made of spherical L1 0-FePt nanoparticles, with diameter within 2.5-5 nm, embedded in an fct-FePt random alloy. The calculation is performed using the locally self-consistent multiple scattering (LSMS) method, a linear scaling ab initio method capable of treating tens of thousands of atoms. We found that there exists a screening region below the surface of each nanoparticle. This screening region essentially screens out the effect of the external random alloy to keep the physical properties of the interior region unchanged from the bulk of L10-FePt. Interestingly, the depth of this screening region is around 4 Aring and is independent of the size of the nanoparticles we have investigated. We will discuss the formation of this screening region and the effect of the external random alloy on the electronic and magnetic structure of the nanoparticles

{\rm L}1_{0}

Classical Molecular Dynamics (MD) simulations characterizing dislocations and radiation damage typically treat 105-107 atoms. First principles techniques employed to understand systems at an atomistic level are not practical for such large systems consisting of millions of atoms. We present an efficient coarse grained (CG) approach to calculate local electronic and magnetic properties of large MD-generated structures from the first principles. Local atomic magnetic moments in crystalline Fe are perturbed by the presence of radiation generated vacancies and interstitials. The effects are most pronounced near the defect cores and decay slowly as the strain field of the defects decrease with distance. We develop the CG technique based on the Locally Self-consistent Multiple Scattering (LSMS) method that exploits the near-sightedness of the electron Green function. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with first-principles based on full local structure information. Atoms in the rest of the system are modeled by representative atoms with approximated properties. The calculations result in local moments near the defect centers with first-principles accuracy, while capturing coarse-grained details of local moments at greater length scales. This CG approach makes these large scale structures amenable to first principles study.

-FePt Nanoparticle Embedded in FePt Random Alloy

The Wang-Landau method [F. Wang and D. P. Landau, Phys. Rev. E 64, 056101 (2001)] is an efficient way to calculate the density of states (DOS) for magnetic systems, and the DOS can then be used to rapidly calculate the thermodynamic properties of the system. A technique is presented that uses the DOS for a simple Hamiltonian to create a stratified sample of configurations which are then used calculate a “warped’’ DOS for more realistic Hamiltonians. This technique is validated for classical models of bcc Fe with exchange interactions of increasing range, but its real value is using the DOS for a model Hamiltonian calculated on a workstation to select the stratified set of configurations whose energies can then be calculated for a density-functional Hamiltonian. The result is an efficient first-principles calculation of thermodynamic properties such as the specific heat and magnetic susceptibility. Another technique uses the sample configurations to calculate the parameters of a model exchange interaction ...

Coarse Grained Approach to First Principles Modeling of Radiation Cascade in Large Fe Supercells

Defects, defect interactions, and defect dynamics in solids created by fast neutrons are known to have significant impact on the performance and lifetime of structural materials. A fundamental understanding of the radiation damage effects in solids is therefore of great importance in assisting the development of improved materials - materials with ultrahigh strength, toughness, and radiation resistance. In this presentation, we show our recent theoretical investigation on the magnetic structure evolution of bulk iron in the region of the radiation defects. We applied a linear scaling ab-initio method based on density functional theory with local spin density approximation, namely the locally self-consistent multiple scattering method (LSMS), to the study of magnetic moment distributions in a cascade at the damage peak and for a series of time steps as the interstitials and vacancies recombined. Atomic positions correspond to those in a low energy cascade in a 10|000 atom sample, in which the primary damage state and the evolution of all defects produced were simulated using molecular dynamics with empirical, embedded-atom inter-atomic potentials. We will discuss how a region of affected moments expands and then recedes in response to a cascade evolution.

Improved methods for calculating thermodynamic properties of magnetic systems using Wang-Landau density of states

Mean-field approximations are used to find approximate solutions to the one-electron equations for the electronic states in disordered alloys because ordinary band-theory approaches are not applicable. The first mean-field approximation, the coherent potential approximation, does not treat Coulomb effects correctly. This has been improved by changing the way the mean-field approximation is implemented. It may be that this experience with mean-field approximations will be useful to the combination of many-body theory and mean-field theory that has produced the dynamical coherent potential approximation and dynamical mean field theory for treating strongly correlated electron systems.

A study of radiation damage effects on the magnetic structure of bulk Iron

Exact evaluations of partition functions are generally prohibitively expensive due to exponential growth of phase space with the number of degrees of freedom. For an sing model with sites the number of possible states is requiring the use of better scaling methods such as importance sampling Monte-Carlo calculations for all but the smallest systems. Yet the ability to obtain exact solutions for as large as possible systems can provide important benchmark results and opportunities for unobscured insight into the underlying physics of the system. Here we present an sing model for the magnetic sublattices of the important magneto-caloric material Ni2MnGa and use an exact enumeration algorithm to calculate the number of states for each energy and sublattice magnetizations MNi and MMn. This allows the efficient calculation of the partition function and derived thermodynamic quantities such as specific heat and susceptibility. Utilizing the jaguarpf system at Oak Ridge we are able to calculate for systems of up to 48 sites, which provides important insight into the mechanism for the large magnet-caloric effect in Ni2MnGa as well as an important benchmark for Monte-Carlo (esp. Wang-Landau method).