Magnetic materials: Forging ahead with a back-to-basics approach

Their results could lead to novel magnetic recording devices.

One promising route to such spintronic devices is to design structures consisting of alternating layers of different magnetic atoms. The strength of the magnetic influence is stronger in the direction of the multilayer stack than it is parallel to the planes of the atoms. This so-called perpendicular magnetic anisotropy is useful for spintronic memory devices because it allows a greater storage density than a conventional electronic device.

The properties of these structures, however, are highly sensitive to the precise arrangement of the crystal. Just one misplaced layer of atoms -- a stacking fault -- can noticeably alter device performance (see image). Previous studies usually ignored these special defects, "but nature sometimes makes 'mistakes'," explains Gan. "It is important to understand these defects and subsequently use them to control the material's physical properties."

Gan and his team went back to basics to better understand how atom-level imperfections affect the properties of these multilayers. They used a powerful mathematical approach known as density functional theory. This approach uses only fundamental equations from quantum mechanics to model the behavior of electrons in these structures, without requiring any prior assumptions.

The researchers modeled a material consisting of alternating layers of cobalt and palladium atoms. Multilayers of these atoms have previously exhibited a large perpendicular magnetic anisotropy when the cobalt layers are less than 0.8 nanometers thick. Gan and co-workers then assessed how stacking faults and the ratio of cobalt to palladium atoms affected this anisotropy. Their results showed that a stacking fault could enhance the magnetic anisotropy in structures with a relatively thick cobalt layer. They also found that the anisotropy increased almost linearly with increasing cobalt content.

High magnetic anisotropy materials have potential for use in the next generation of ultrafast and high-capacity magnetic random-access memory, Gan explains. The improved understanding of these materials from this research will guide the way to realizing such devices.