Exploring the surface properties of NiO with low-energy electron diffraction

Spintronics is a field that deals with electronics that exploit the intrinsic spin of electrons and their associated magnetic moment for applications such as quantum computing and memory storage devices. Owing to its spin and magnetism exhibited in its insulator-metal phase transition, the strongly correlated electron systems of nickel oxide (NiO) have been thoroughly explored for over eight decades. Interest in its unique antiferromagnetic (AF) and spin properties has seen a revival lately, since NiO is a potential material for ultrafast spintronics devices.

Despite this rise in popularity, exploration of its surface magnetic properties using low-energy electron diffraction (LEED) technique has not received much attention since the 1970s. To review the understanding of the surface properties, Professor Masamitsu Hoshino and Emeritus Professor Hiroshi Tanaka, both from the Department of Materials and Life Sciences at Sophia University, Japan, revisited the surface LEED crystallography of NiO. The results of their quantitative experimental study investigating the coherent exchange scattering in Ni²⁺ ions in AF single crystal NiO were reported in The European Physical Journal D on 11^th December 2023.

For the study, the researchers had two major objectives: to improve upon old experimental techniques used to decipher coherent spin exchange scattering of low-energy electrons by Ni²⁺ ions of NiO and to provide a reliable theoretical analysis using recent techniques.

They first carried out the quantitative characterization of surface atoms of NiO crystal using the LEED method. This allowed them to explore the energy dependence of LEED for “half-order beam” intensity via I-V spectra. Upon inspection of the I-V curve, the researchers observed a resonance enhancement which was attributed to the surface wave resonance (SWR) effect. This led the team to analyze the temperature dependence of LEED at the maximum intensity and surface spin properties under SWR conditions—a state where propagating diffracted beams emerge nearly parallel to the crystal surface.

For building a robust theoretical basis (to clarify the theoretical background), the researchers used (the more sophisticated) LEED dynamical theory to interpret the experimental results and revealed clearly the SWR as observed in the I-V curve. The temperature dependence measured over a wide temperature range enabled a more quantitative comparison with conventional molecular field theory.

This study not only succeeds in reaffirming previous experimental data on surface-spin structure and magnetic properties but also provides for the first time an I-V spectra of a half-order beam, the SWR conditions, and the temperature dependence over a wide temperature range.

 “Unlike ferromagnetic materials that exhibit magnetism, AF materials, which do not exhibit magnetic properties as indicated by their spin arrangement, have been regarded as ‘unusable materials’. However, they are now being reborn. This phrase is often used now and the term, unusable materials was derived from Néel's Nobel Prize lecture (1970),” say the researchers when asked about the motivation behind revisiting NiO LEED experiments. “Furthermore, this research is at the cusp of a classic and new research theme that began in the 1970s through a personal communication from Nobel laureate Prof. N.F. Mott, known for breakthroughs in the research of Mott insulators such as NiO, as noted in the reference.” They further commented, “This research is specialized, focusing on academic and fundamental aspects, and is not intended for the public, but rather may help to elucidate the physical and chemical properties of promising antiferromagnetic materials.” 

Reference

About Sophia University

Established as a private Jesuit affiliated university in 1913, Sophia University is one of the most prestigious universities located in the heart of Tokyo, Japan.  Imparting education through 29 departments in 9 faculties and 25 majors in 10 graduate schools, Sophia hosts more than 13,000 students from around the world.

Conceived with the spirit of “For Others, With Others,” Sophia University truly values internationality and neighborliness, and believes in education and research that go beyond national, linguistic, and academic boundaries. Sophia emphasizes on the need for multidisciplinary and fusion research to find solutions for the most pressing global issues like climate change, poverty, conflict, and violence. Over the course of the last century, Sophia has made dedicated efforts to hone future-ready graduates who can contribute their talents and learnings for the benefit of others, and pave the way for a sustainable future while “Bringing the World Together.”

Website: https://www.sophia.ac.jp/eng/

About Professor Masamitsu Hoshino from Sophia University

Dr. Masamitsu Hoshino is a Professor at the Faculty of Science and Technology, Department of Materials and Life Sciences at Sophia University, Japan. Before joining the faculty at his alma mater in 2005, he got his Ph.D. in physics from the university’s Graduate School of Physics. He later went on to pursue his special postdoctoral research at RIKEN, Japan. Over the years he has published over 165 papers. His current areas of interest include semiconductors, atomic physics, high-resolution spectroscopy, and excitation processes in atoms, molecules and materials.

Funding information

N/A