Electric and Magnetic Orientations Shift Together in Nanoparticles

One promising approach involves materials in which electrical and magnetic properties are closely linked. In these materials, positive and negative charges shift slightly relative to each other, creating an electrical imbalance with a specific direction. This is known as ferroelectric polarization. Previous studies have shown that when an applied voltage switches the direction of polarization, the direction of magnetization can change along with it.

At very small scales, however, the picture becomes more complicated. Instead of pointing in a single direction, polarization can divide into several regions, forming patterns that converge toward the center or even swirl like a vortex. Until now, it was not well understood how these complex polarization patterns were linked to magnetism-or how both would change together when a voltage was applied.

A research team led by Specially Appointed Assistant Professor Kei Shigematsu at Institute of Science Tokyo (Science Tokyo) studied tiny particles of cobalt-substituted bismuth ferrite, a material with a crystal structure known as the perovskite structure. The particles were about 190 nanometers across-roughly one five-thousandth of a millimeter.

The word "perovskite," which many people may have heard in connection with solar cells, does not actually refer to a single material. Rather, it describes a particular type of crystal structure.

Inside each particle, the polarization was divided into four regions, all pointing toward the center. When a voltage was applied, the polarization reversed, reorganizing into a pattern that pointed outward from the center.

The magnetic state changed along with it. Using a specialized microscope capable of imaging magnetic directions at high resolution, the researchers directly observed for the first time that not only the magnetization along the surface of the particle, but also the magnetic component pointing from the particle interior toward the exterior, reversed as well.

Even more strikingly, the four magnetization did not change in the same way across all four regions. In different regions, it rotated by different angles, including 120 and 180 degrees. Within a particle only 190 nanometers wide, the directions of electric polarization and magnetization were changing in a complex but coordinated manner.

If magnetization can be switched using voltage without sending a large electric current, it could reduce heat generation and lead to lower-power memory technologies.

In addition, if researchers can eventually control the complex magnetic-state changes observed in this study, it may become possible to store information using more than just two states-0 and 1. That could open the door to higher-density memory devices capable of storing more information in the same space.

For now, however, the magnetic state cannot yet be switched freely into any desired direction. The next challenge is to better understand these complex changes and learn how to control them reliably.

This experiment was carried out in collaboration with research teams in the United States. We used microscopes at UC Berkeley on the West Coast and Northeastern University on the East Coast to observe the electrical and magnetic properties, respectively. During one month of experiments, I traveled between the two universities-about a six-hour flight apart-to examine regions only a few hundred nanometers across. That experience remains especially memorable to me.

Seeing directly how the electric and magnetic directions changed together, despite their complex relationship, gave me a real sense of the fascination of materials science. Looking ahead, I hope to develop simpler ways to control magnetization for future memory operation.

(Lee Koomok, International Research Fellow (Postdoctoral Researcher), Laboratory for Materials and Structures, Institute of Integrated Research, Institute of Science Tokyo)

Our goal in this study was to clarify the electrical and magnetic properties of materials at the nanometer scale. Experiments like these were once extremely difficult, but recent advances in science and technology have greatly improved the precision of measurement techniques, making this work possible.

By building on these results, we hope to unlock the remarkable potential hidden within materials and advance the development of new materials for memory devices.

(Kei Shigematsu, Specially Appointed Assistant Professor, Laboratory for Materials and Structures, Institute of Integrated Research, Institute of Science Tokyo)

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