JAEA directly observes nanomagnet spin excitations

A research group led by Visiting Researcher Shin-ichi Shamoto of the Advanced Science Research Center at the Japan Atomic Energy Agency, Associate Researcher Kazuki Iida of the Neutron R&D Division at CROSS, Group Leader Katsuaki Kodama at the Materials Sciences Research Center of the JAEA, and Yasuhiro Inamura of the J-PARC Center has developed a new analysis program that has allowed them to directly observe spin excitations in small magnetic materials, which could not be observed before. "Since neutron beam intensities have improved, we thought it might be possible to observe spin excitation in nanomagnets with inelastic neutron scattering, which reduces the energy to 1/100, so we tried it and were surprised that it was possible," explained Shamoto. "The analysis program is available to all J-PARC users, so we hope they will use it as much as possible." The group's research was published in Scientific Reports.

The performance of a magnet is related to the spin excitation of its electrons. To date, this excitation was measured as a dispersion by neutron scattering. This dispersion has wave-like nature, showing the relationship between the wavenumber and the energy. In other words, the wavenumber can be converted to distance through Fourier transformation.

However, in the past, neutron scattering measurements have required centimeter-size single-crystal magnets, and measurements of smaller sizes have been difficult because the lack of data intensity makes Fourier transformation impossible.

Recently, the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC) is nearing its final target of a neutron beam intensity of 1 MW. As a result, the strength of the data is increasing, so the group developed a new analysis method using Fourier transformation that can manage weak data strengths. This analytical method enables them to directly observe spin excitation on a sub-nanometer scale, even in powder magnets.

To investigate the validity of their analytical method and the software using it, they studied the magnetic material FeTiO₃ in ilmenite form, which has been well-studied for spin waves. The two spin excitations, which are sub-nanometers apart in this magnetic material, are said to have two different modes, one in the same direction and the other in the opposite direction. The researchers succeeded in observing those two modes by the analytical method they developed.

It is expected that the results of this study will enable direct observation of spin excitations even in nanomagnetic materials, which will be miniaturized in the future, and make a significant contribution to the understanding of the field.