The joint development team of Professor Shibata (University of Tokyo), JEOL Ltd. and Monash University succeeded in directly observing an atomic magnetic field, the origin of magnets (magnetic force), for the first time in the world. The observation was made using the new Magnetic Fieldless Atomic Resolution STEM (MARS) (1). This team had already succeeded in observing the electric field inside atoms for the first time in 2012. However, since the magnetic fields in atoms are extremely weak compared to electric fields, the technology to observe magnetic fields was unexplored. since the development of the electron. microscopes. This is a landmark achievement that will rewrite the history of microscope development.
Electron microscopes have the highest spatial resolution among all microscopes currently in use. However, in order to achieve ultra-high resolution so that the atoms can be observed directly, we must observe the sample by placing it in an extremely strong lensing magnetic field. Therefore, atomic observation of magnetic materials strongly affected by the magnetic field of the lens, such as magnets and steels, was impossible for many years. For this difficult problem, the team succeeded in developing a lens which has a completely new structure in 2019. Using this new lens, the team achieved atomic observation of magnetic materials, which is not affected by the magnetic field of the lens. The team’s next goal was to observe the magnetic fields of atoms, which are the origin of magnets (magnetic force), and they continued technology development to achieve this goal.
This time, the joint development team took on the challenge of observing the magnetic fields of iron (Fe) atoms in a crystal of hematite (α-Fe2O3) by loading MARS with a new high-speed, high-sensitivity detector and additionally using computer image processing technology. To observe the magnetic fields, they used the differential phase contrast (DPC) method. (2) atomic resolution, which is an ultra-high resolution local electromagnetic field measurement method using a scanning transmission electron microscope (STEM) (3), developed by Professor Shibata et al. The results directly demonstrated that the iron atoms themselves are small magnets (atomic magnet). The results also clarified the origin of magnetism (antiferromagnetism (4)) exhibited by hematite at the atomic level.
From the current research results, the observation of the atomic magnetic field has been demonstrated, and a method for observing atomic magnetic fields has been established. This method is expected to become a new measurement method in the future which will lead to the research and development of various magnetic materials and devices such as magnets, steels, magnetic devices, magnetic memory, magnetic semiconductors, spintronics and topological materials.
This research was conducted by the joint development team of Prof. Naoya Shibata (Director of the Institute of Engineering Innovation, School of Engineering, University of Tokyo) and Dr. Yuji Kohno et al. (JEOL Ltd. specialists) in collaboration with Monash University, Australia, under the Advanced Measurement and Analysis Systems Development (SENTAN), Japan Science and Technology Agency (JST).
(1) Atomic Resolution STEM without magnetic field (MARS)
An electron microscope is an instrument for directly observing the microstructure of a sample, where a beam of electrons is injected into the sample, and the beams of electrons transmitted and scattered by the sample are amplified using of a magnetic field lens. Currently, it is possible to observe atoms directly using an electron microscope. In an optical microscope, the spatial resolution is in principle limited to about one micrometer due to the light source (visible light). On the other hand, the electron microscope is an instrument where this limit of spatial resolution is overcome by using the wave nature of electrons. Therefore, it can be said that an electron microscope is an observation technology that applies the advantages of quantum mechanics in the most direct way. The Magnetic Field-Free Atomic Resolution STEM (MARS) is an electron microscope developed by the current joint development team in 2019, capable of measuring a sample in an environment without a magnetic field. For more details, please see the following press release.
“An innovative electron microscope overturning common knowledge of 88 years of history” (May 24, 2019)
(2) Differential Phase Contrast (DPC) Method
A method for measuring the electromagnetic field at each point in a sample. More precisely, when an electron beam is injected into a sample, the strength of the electromagnetic field existing in the sample causes a slight change in the trajectory of the incident electron beam, and by measuring the difference in intensity of the beam d electrons detected at each position of a split detector, the electromagnetic field can be measured. Since the spatial resolution of this method is essentially determined by the size of the electron probe, observation of an electromagnetic field at atomic resolution is in principle possible using the DPC method.
(3) Scanning transmission electron microscope (STEM)
An instrument for directly observing the structure inside a sample. Specifically, a micro-focused electron beam is scanned over the sample, and observation is performed by measuring the intensity of electrons transmitted and scattered by the sample. Currently, we can directly observe atoms using a STEM.
A magnetism where the spins of neighboring atoms are aligned with each other face to face antiparallel, and the material has no spontaneous magnetization as a whole.
Reference: Kohno Y, Seki T, Findlay SD, Ikuhara Y, Shibata N. Real-space visualization of the intrinsic magnetic fields of an antiferromagnet. Nature. 2022;602(7896):234-239. doi: 10.1038/s41586-021-04254-z
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