Researchers discover a surprising quantum effect on the material of a hard drive

Scientists are finding a surprising way to affect the information storage properties in metal alloys.

Sometimes scientific discoveries can be found on the beaten track. This proved the case of an alloy of iron and cobalt that is commonly found in hard disk drives.

As stated in a recent issue of Letters of physical examinationResearchers at the Argonne National Laboratory of the US Department of Energy (DOE), as well as universities in Oakland, Michigan and Fudan, China, have discovered a surprising quantum effect with this alloy.

The effect involves the ability to control the direction of electron spin and could allow scientists to develop more powerful and energy-efficient materials for storing information. By changing the spin direction of electrons in a material, researchers have been able to modify its magnetic state. This better magnetization control allows you to store and retrieve more information in a smaller space. Better control could also lead to additional applications, such as more energy-efficient electric motors, generators, and magnetic bearings.

The effect discovered by the researchers is related to "damping," in which the direction of the electron spin controls how the material dissipates energy. "When you drive your car on a flat road with no wind, the energy dissipated by the drag is the same, regardless of the direction in which you are heading," said Olle Heinonen, materials scientist for the company. Argonne, author of the study. "With the effect we have discovered, it's as if your car was experiencing more drag if you were traveling north-south than if you were traveling east-west."

"On the technical side, we have discovered a considerable effect of magnetic damping on nanometric layers of iron and cobalt alloy coated on one side with a magnesium oxide substrate", a added Axel Hoffmann, materials scientist at Argonne, also author of the study. "By controlling the spin of the electrons, the magnetic damping determines the rate of energy dissipation, thereby controlling the aspects of magnetization."

The team's discovery was particularly surprising as the cobalt iron alloy was widely used in applications such as magnetic hard drives for several decades and its properties have been thoroughly studied. It was common knowledge that this material had no preferred direction for the electronic spin and thus the magnetization.

In the past, however, scientists have prepared the alloy to be used by "baking" it at high temperature, which orders the arrangement of cobalt and iron atoms in a regular network, thus eliminating the directional effect. The team observed the effect by examining uncooked cobalt iron alloys in which cobalt and iron atoms can randomly occupy their respective sites.

The team was also able to explain the underlying physics. In a crystalline structure, the atoms are normally at perfectly regular intervals in a symmetrical arrangement. In the crystalline structure of some alloys, there are slight differences in the separation of atoms that can be removed by the cooking process; these differences remain in an "uncooked" material.

By pressing such a material at the atomic level, the separation of the atoms is modified, resulting in different interactions between the atomic spins in the crystalline environment. This difference explains how the damping effect on magnetization is important in some directions and low in others.

The result is that very small distortions in the atomic arrangement within the crystal structure of the cobalt-iron alloy have giant implications for the damping effect. The team performed calculations at the Argonne Leadership Computing Facility, a user facility of the DOE Office of Science, which confirmed their experimental observations.

Researchers' work appears in the online edition of March 21 Letters of physical examination and entitled "Giant anisotropy of Gilbert's damping in epitaxial films in CoFe".