Although Ta is a necessary component of permalloy (Py)-based hetero-structures used in magnetic random access memory devices, it has deleterious effects on the magnetic properties, resulting in magnetic dead layers and reduced performance. First principles studies of Permalloy show that the addition of Ta not only reduces the average magnetic moment associated with the Ni and Fe atoms but also results in large fluctuations in the Ni site moment magnitude. Left: Length and color of arrows show the magnitude of the magnetic moments associated with the Ni sites (mid-gray spheres) in Py. The much larger moments of the Fe site (dark-gray spheres) have been truncated to aid the visualization. Right: For an alloy of 90% Py and 10% Ta, color and length coding of the magnitude has been preserved to illustrate the reductions and variations in the size of Fe and Ni moments. Interestingly, the magnetic moments associated with the Ta atoms (purple spheres) are antiferromagnetically aligned with respect to the Fe and Ni moments due to hybridization effects.

Magnetic Materials: Bridging Basic and Applied Science
G. Malcolm Stocks, Oak Ridge National Laboratory
Bruce N. Harmon, Ames Laboratory, Iowa State University
Michael Weinert, Brookhaven National Laboratory

Research Objectives
The central goal of this project is to develop rigorous approaches to both refining and bridging the models that describe magnetic phenomena on different length scales. An important aspect of this is the development of modeling tools capable of integrating atomic-level understanding of magnetic properties and interactions with structure and microstructure.

Computational Approach
First principles density functional (DFT) methods are applied to calculating fundamental magnetic properties. Constrained density functional theory is used to calculate magnetic moments and constraining (forcing) fields. Spin dynamics (SD) is used to treat the spin degrees of freedom in simulations of finite temperature and non-equilibrium properties. At the macroscopic length scale, micromagnetics calculations based on the Landau-Lifshitz-Gilbert (LLG) equations are used to study microstructural effects on domain wall motion and switching in devices.  

Accomplishments
The effects of tantalum on the magnetic structure of permalloy were studied using the coherent potential approximation to Korringa-Kohn-Rostoker (KKR-CPA) method. The effects of Ta additions on local moments were also investigated using the locally-self-consistent multiple-scattering (LSMS) method and large supercell calculations.

Using first principles electronic structure methods, the energy of a magnetic domain wall in CoPt was evaluated for the ideal crystal and for one with an anti-phase boundary (APB) defect. This is the first time first principles calculations have been used to study domain walls and to evaluate quantitatively the consequences of defects on domain wall pinning.

We determined the structural properties and relative phase stability of NiAl₃ and NiAl₂. The compositional defects for NiAl₃ were found to be Ni antisites for the Ni-rich alloys and Ni vacancies on the Al-rich side.

Significance
Modeling tools capable of integrating atomic-level understanding of magnetic properties and interactions with structure and microstructure would allow the prediction of technologically relevant magnetic properties and the design of improved electromagnetic devices as well as electric motors with reduced weight and improved performance.

Publications
B. Újfalussy, X.-D. Wang, D. M. C. Nicholson, W. A. Shelton, G. M. Stocks, Y. Wang, and B. L. Gyorffy, “Constrained density functional theory for first principles spin dynamics,” J. Appl. Phys. 85, 4824 (1999).

V. P. Antropov, B. N. Harmon, and A. V. Smirnov, “Aspects of spin dynamics and magnetic interactions,” J. Magnetism and Magnetic Mater. 200, 148 (1999).

Q. Niu, Xindong Wang, L. Kleinman, Wu-Ming Liu, D. M. C. Nicholson, and G. M. Stocks, “Adiabatic dynamics of local spin moments in itinerant magnets,” Phys. Rev. Letters 83, 207 (1999).