Research Highlights

A neglected aspect of the physics of magnetic thin films plays a crucial role in their magnetic properties, AIMR researchers have discovered¹. This finding has important implications for practical technologies employing spin-based nanodevices.

Many emerging technologies rely on how the magnetism of a material varies with time. Magnetization dynamics, as it is known, is especially important for spintronic devices, which exploit the angular momentum of an electron — its spin — rather than its charge, which is the basis of conventional electronics.

The spectroscopic technique known as ferromagnetic resonance (FMR) is a standard tool that material scientists employ to explore magnetization dynamics. It is used to probe the magnetization of ferromagnetic materials — those which, like iron, have a permanent magnetization because the spins of their electrons are aligned.

Now, Fumihiro Matsukura of the AIMR at Tohoku University and his co-workers have found that a missing piece of physics is needed to correctly interpret FMR spectra of ferromagnetic systems made from different materials.

They explored how the widths of the lines in FMR spectra of thin films of CoFeB–MgO — a promising building block for high-performance nanoscale spintronic devices — varied with temperature and film thickness. To their surprise, the team found that the FMR linewidths became narrower with increasing temperature. After eliminating other possible causes, they concluded that this was due to motional narrowing — an effect that originates from thermal fluctuations at the interface between two different materials. Motional narrowing had been overlooked in previous studies as it was masked by the bulk properties of ferromagnetic materials. It has come to light now because of advances in technology associated with ferromagnetic systems made from different materials.

“This finding came as a surprise to us,” says Matsukura. “About two years ago, research groups at Tohoku University and Nanyang Technological University in Singapore independently observed that the widths of some lines in FMR spectra varied strangely with temperature. We started collaborating with these groups, but we initially couldn’t explain the experimental results. It was through discussions with a theoretical group at the Japan Atomic Energy Agency that we came to understand what was happening.”

“Since systems containing interfaces between two different materials have been used to develop many spintronic applications, it’s vital to examine motional narrowing in other material systems besides CoFeB–MgO,” notes Matsukura. “Our result is expected to bring a new concept to spintronic devices, namely the control of interfacial anisotropy by external means,” he adds.