New Stable Thin Film Could Improve Field of Spintronics

The ever-expanding field of spintronics exploits the spin of electrons, and not their charge, to improve solid-state devices such as cell phone components and hard drives by increasing the battery life. However, advancements in spintronics are being hindered by an obstacle called the Slater-Pauling limit, or the extent to which a material can tightly pack its magnetization. At present, an innovative thin-film looks propitious enough to surpass this decades-old standard.

Scientists from Montana State University and Lawrence Berkeley National Laboratory have developed a stable thin film made of cobalt, iron, and manganese, with an average atomic moment prospectively 50% higher than the Slater-Pauling limit. The thin film has been described in Applied Physics Letters, published by AIP Publishing, this week. Developed by adopting a method called molecular beam epitaxy (MBE), the ternary body-centered cubic (bcc) alloy has a magnetization density of 3.25 Bohr magnetons per atom, surpassing the earlier known maximum of 2.45.

“What we have is a potential breakthrough in one of the most important parameters of magnetic materials,” stated Yves Idzerda, one of the authors of the paper, from Montana State University. “Large magnetic moments are like the strength of steel—the bigger the better.”

Magnetization density of alloys is outlined by the Slater-Pauling curve. For many years now, iron-cobalt (FeCo) binary alloys have been largely used, with a maximum average atomic moment of 2.45 Bohr magnetons per atom and interpreting the current limit in the case of stable alloy magnetization density. Earlier, scientists combined FeCo alloys with transition metals that had a high magnetic moment, such as manganese. However, upon synthesizing these ternary alloys, much of their bcc structure is lost, which is an important component of their high magnetism.

Rather, the researchers adopted MBE, a painstaking method similar to covering a substrate with beads of individual metal atoms, in individual layers, to develop a Fe₉Co₆₂Mn₂₉ film with a thickness of 10-20 nm. Approximately 60% of the available compositions maintained the bcc structure to be a thin film, in the place of only 25% in bulk.

In order to gain an in-depth knowledge of the structure and composition of the alloy, the team adopted reflection high-energy-electron diffraction as well as X-ray absorption spectroscopy. The outcomes of X-ray magnetic circular dichroism demonstrated that the new material produced an average atomic moment of 3.25 Bohr magnetons per atom. Upon investigating the material with a more standard vibrating sample magnetometry, although the magnetization density decreased, it was still considerably greater than the Slater-Pauling limit, that is, 2.72.

According to Idzerda, this inconsistency will open the door for further research in this area. He also stated that the interface between the substrate and manganese inside the crystal might explain the gap.

“I have guarded optimism for this because the technique we used is a little bit non-standard and we have to convince the community of this material’s performance,” stated Idzerda.

Next, Idzerda and his colleagues will test the vigor of iron-cobalt-manganese alloys and more efficacious fabrication methods. They also aim to investigate the way in which molecular beam epitaxy might result in highly magnetic thin films, prospectively combining together four or more transition metals.