An ab initio spin dynamics calculation has been performed on the FeMn/Co(111) system to understand properties of this interface, a candidate to be used in magnetic read heads. Top: Section of the FeMn/Co interface showing the initial configuration of the magnetic moments. The visualization shows the Co layer and four FeMn layers closest to the interface. The Co layer is initialized in a ferromagnetic state, and the FeMn layers are initialized in the 3Q structure. Gold spheres represent the Fe and gray the Mn atoms. Bottom: Section of the FeMn/Co interface showing the final configuration of the magnetic moments for the same five layers as shown for the initial configuration. This calculation involved 2,016 nodes on the IBM SP to achieve 2.3 teraflop/s performance.

G. Malcolm Stocks, Oak Ridge National Laboratory
Bruce N. Harmon, Ames Laboratory, Iowa State University
Michael Weinert, Brookhaven National Laboratory
David P. Landau, University of Georgia

Research Objectives
Our goal is understanding the magnetic properties of real materials by developing a comprehensive multi-length-scale modeling capability. Using this capability, we will study the magnetic structure of systems including disordered alloys, nanoparticles, nanowires, interfaces in multilayers, and quantum corrals, and to study domain walls and their interaction with structural defects.

Computational Approach
First-principles density functional theory (DFT) methods are applied to calculating fundamental magnetic properties including magnetic moments, exchange interactions, and magneto-crystalline anisotropy. First-principles spin dynamics is used to treat the spin degrees of freedom at finite temperature and in the presence of external fields, and to find complex ground states using relaxation techniques. Tight binding methods based on fits to first principles are being developed to implement spin dynamics at an intermediate level (systems containing several thousands of atoms) between first-principles techniques and extended Heisenberg models.  

Accomplishments
We have performed first-principles calculations of the magnetic structure of gamma-FeMn based on large cell models (up to 2,048 atoms) of the disordered alloy and have discovered a new ground state magnetic structure. The calculations are based on the constrained local moment model and use of first-principles spin dynamics to obtain the ground state orientational configuration.

We have made the first calculations of the magnetic ground states of iron inclusions embedded in fcc copper using first-principles calculations. Our calculations show that, depending on whether the chains of magnetic atoms are embedded in copper along the 100 or 110 directions, the ground state orientation of the magnetic moments on the iron sites can be either parallel or perpendicular to the chain.

In ab initio calculations of the reorientation transition for overlayers of Fe on Cu(111), we find that the direction of the magnetization is perpendicular to the surface for films up to two monolayers thick, and it is oriented in-plane for larger thicknesses, in complete agreement with recent experiments.

The order-N locally self-consistent multiple scattering (LSMS) method has been extended to treat relativistic systems. The fully relativistic method, in which solution of the Schrödinger equation is replaced by the solution of the Dirac equation, is capable of treating systems comprising ~2,000 atoms on currently available machines.

Significance
The goal of this project is to develop modeling tools capable of integrating atomic-level understanding of magnetic properties and interactions with structure and microstructure. Such a capability would enable the science-based understanding and prediction of technologically relevant magnetic properties and the design of improved permanent magnets and magneto-electronic devices, such as magnetic recording media, read heads, and MRAM.

Publications
Markus Eisenbach, G. Malcolm Stocks, Balazs Ujfalussy, and Balazs L. Gyorffy, "Magnetic anisotropy of monoatomic iron chains embedded in copper," Phys. Rev. B. (submitted).

H. A. De Raedt, A. H. Hams, V. V. Dobrovitski, M. Al-Saqer, M. I. Katsnelson, and B. N. Harmon, "Many spin effects in tunneling splittings in Mn12 magnetic molecules," Phys. Rev. B. (submitted).

Nassrin Y. Moghadam, G. M. Stocks, M. Kowalewski, and W. H. Butler, "Effects of Ta on the magnetic structure of permalloy," J. Appl. Phys. 89, 6886 (2001).