Magnetic Anisotropy of Iron Nano-Chains Embedded in Copper

As the size of magneto-electronic devices (such as recording media) continues to shrink, magnetic nanosystems are the focus of intensive experimental research. Nanosystems also provide a unique opportunity for theory and simulation to contribute to the understanding and prediction of magnetic properties. Understanding the relationship between microstructures and magnetism will be one of the keys to designing new magnetic materials with precisely engineered properties. Recent calculations have made important contributions to this understanding.

	Figure 5   The orientations of the iron chains in the copper matrix. (a) Chain along the copper 110 direction. Here two of the nearest neighbors of each iron atom are also iron atoms. (b) Chain along the 100 direction. All nearest neighbors to iron atoms are copper, and the closest iron atom is in the next nearest neighbor position.

Using first-principles relativistic density functional calculations, Eisenbach et al. have studied monoatomic iron chains embedded in copper, focusing on chains of atoms aligned along different directions in the copper matrix (Figure 5). They found that the magnetocrystalline anisotropy energy favored orientations of the magnetic moments perpendicular to the chain, while the magnetostatic energy was lowest when the moments were aligned parallel to the chain. Due to the delicate balance of these competing effects, the ground state orientation of the magnetic moments on the iron sites can be either parallel or perpendicular to the chain, depending on whether the chains are embedded in the copper along the 100 or 110 directions.

Due to the lower symmetry of these embedded chain systems (as compared to a bulk iron system with its cubic symmetry), the contribution of the spin-orbit coupling was significantly increased. The reduced symmetry led to a noticeably larger crystalline anisotropy energy, which favored the perpendicular orientation of the iron moments. The competing magnetic dipolar interaction preferred the parallel orientation. As it turned out, the nearest neighbor chains exhibited such a close balance between the magnetocrystalline and shape contributions to the total anisotropy that the preferred orientation in the system depended on the direction in which the iron chain was embedded in the copper.

INVESTIGATORS
G. M. Stocks and B. Újfalussy, Oak Ridge National Laboratory; B. N. Harmon, Ames Laboratory; M. Weinert, University of Wisconsin, Milwaukee; D. P. Landau, University of Georgia; M. Eisenbach and B. L. Györffy, University of Bristol, U.K.

PUBLICATIONS
M. Eisenbach, B. L. Györffy, G. M. Stocks, and B. Újfalussy, “Magnetic anisotropy of monoatomic iron chains embedded in copper,” Phys. Rev. B 65, 144424 (2002).
M. Eisenbach, G. M. Stocks, B. Újfalussy, and B. L. Györffy, “Magnetic anisotropy of iron chains embedded in copper,” J. Appl. Phys. 91, 6878 (2002).