For the first time ever, a research team from Tokyo University and JEOL, a company specializing in scientific instruments, have succeeded in directly observing the magnetic field of an atom with an independently-developed electron microscope. Their paper was published in the scientific journal Nature.
Their achievement is the result of solving the long-standing issue of being unable to observe magnetic objects with an electron microscope, and it adds a new page to the history of microscope research. In practical application, it is hoped that this will contribute to improving the performance of magnetic materials used in electric vehicle (EV) motors and semiconductor devices.
Powerful Magnetic Fields
Magnetic materials are necessary in the motors of EVs, as well as next-generation semiconductor technologies like magnetic memory. The development of high-performance magnetic materials therefore has become a critical issue for achieving progress in energy-saving automobiles and information technology.
The magnetic force of a substance comes from the magnetic fields of the atoms, the basic building blocks of each material. For example, every iron atom is like a small magnet and has a magnetic field. The magnets that we are familiar with, like bar magnets, can be thought of as a collection of atomic magnets.
In order to develop high-performance magnetic materials, it is necessary to know the properties of magnets and how they differ, depending on the type and arrangement of atoms, as well as to control the unique magnetic field produced. For this, technology that can directly observe the structure of the atomic magnet and the magnetic field is indispensable.
Harnessing the Magnetic Field as a Lens
Electron microscopes on principle require strong magnetic fields. Therefore, samples that are themselves magnetic cannot be observed.
An optical microscope uses a combination of glass lenses to refract light and magnify the image of a small object, so it is difficult to observe objects smaller than the wavelength of the light being used. For instance, a typical virus is too small to be observed by a normal optical microscope.
An electron microscope uses a beam of electrons instead of light, and strong magnetic fields instead of glass lenses. When an electron is incident on the magnetic field, it bends due to the force received from the magnetic field (the Lorentz force). This phenomenon acts as a lens and magnifies the image.
In other words, if one tries to observe magnets, magnetic devices, or the magnetic materials used for them with an electron microscope, the structure of the material can change or break due to the magnetic field of the microscope itself.
Solving an 88-Year-Old Problem
In 2019, a research team led by Professor Naoya Shibata, PhD, of the University of Tokyo (Electron Microscope Materials Science) independently developed the Magnetic Field Free Atomic Resolution STEM known as MARS.
MARS has two upper and lower magnetic fields that play the role of a magnifying lens, with each magnetic field generated in the opposite direction. The sample is placed in between, so the magnetic fields cancel each other out. In this way, the team achieves a “magnetic-field-free” electron microscope that doesn’t affect the observation of magnetic materials.
This discovery solves problems that have been present since the invention of the electron microscope in 1931, and thus has attracted attention.
However, the absence of a magnetic field in the microscope doesn’t guarantee being able to see the magnetic fields of atoms. There is a strong electric field inside an atom along with the magnetic field. When it comes to bending the electron beam incident from the microscope, the magnetic field is only about 1% of the electric field and is hidden by a strong electric field. Thus, it becomes necessary to visualize the actions of the magnetic fields that are extremely weak compared to the electric fields.
Therefore, the team developed an ultra-sensitive detector that captures signals from extremely weak magnetic fields and image processing technology that removes the effects of the electric field. By combining these technologies, they succeeded in observing the magnetic field of iron atoms in the crystals of hematite, a type of iron ore.
Hematite crystals have a structure in which layers of iron atoms and layers of oxygen atoms are alternately stacked. The magnetic fields of iron atoms are aligned horizontally at room temperature and the direction is staggered between layers. Furthermore, theory states that when cooled to a low temperature, the direction of the magnetic field rotates 90 degrees and becomes perpendicular to the layers.
As a result of directly observing the crystals, it was possible to see that the directions of the magnetic fields were staggered between layers. The sample was cooled to about 113 Kelvin (minus 160 degrees Celsius), and it was also confirmed that the direction of the magnetic field rotated by 90 degrees.
A Step Forward in the History of Microscopes
In the future, the goal is to further observe samples that are at low temperatures. At low temperatures, there is a curious phenomenon in which the magnetic structure of matter changes rapidly. If this can be observed, it may lead to significant discoveries in condensed matter physics, the field that explores the properties of matter.
Professor Shibata is enthusiastic that “we may see the moment when superconductivity (when the electrical resistance of matter becomes zero at extremely low temperatures) occurs.”
Continuing the pursuit of making invisible things visible, this achievement marks a new step within the history of microscopic research. Ernst Ruska, the German physicist who invented the electron microscope, received the Nobel Prize in Physics in 1986. In 2017, the Nobel Prize in Chemistry was awarded to three scientists from the United States and Europe who contributed to the development of “cryo-electron microscopy,” which allows for the observation of biomolecules by freezing samples. There are numerous other awards that have gone to microscopy research.
The sophistication of technology for analyzing the microscopic world has accelerated the progress of science and technology. If the electron microscope developed is used to develop new magnetic materials and create results such as the discovery of physical science, it can be expected that it will be the subject of the Nobel Prize.
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(Read the report in Japanese at this link.)
Author: Maki Matsuda