Soft magnetic materials can be easily magnetized and demagnetized, which makes them a key component in electrical power devices, such as generators, transformers, and amplifiers. As power electronics advance toward high-frequency operation, demand is growing for low-loss soft magnetic materials. The efficiency of these materials is fundamentally limited by iron loss, where energy is lost as heat when a varying magnetic field passes through them, as is typical in transformers and generators. Iron loss mainly consists of hysteresis loss, classical eddy current loss, and excess eddy current loss. Among these, excess eddy current loss becomes increasingly dominant at high frequencies, but its mechanisms are not clearly understood.
When a varying magnetic field passes through a conductor, it generates eddy currents, resembling swirling eddies in water. These currents waste energy as heat, known as classical eddy current loss. Excess eddy current loss, however, arises due to localized eddy currents induced by irregular movement of magnetic domain walls (DWs) under a varying magnetic field. Magnetic DWs are boundaries separating tiny magnetic domains, separating uniformly magnetized regions.
Magnetic Barkhausen noise (MBN) serves as a key probe for DW dynamics. Yet, current MBN measurement systems do not possess the wide frequency coverage and high-sensitivity needed to capture the individual MBN events, making it difficult to understand the relationship between DW dynamics and eddy current losses.
To address this gap, a research team led by Assistant Professor Takahiro Yamazaki from the Department of Materials Science and Technology at Tokyo University of Science (TUS), Japan, developed a wide-band and high-sensitivity MBN measurement system. They used this system to investigate the magnetic DW dynamics in 25 μm-thick Fe–Si–B–P–Cu (NANOMET) ® ribbons, a class of soft magnetic alloys. Dr. Yamazaki explains, "The most fundamental understanding based on our previous studies is that 'measure what was previously unmeasurable.' With our wide-band, high-sensitivity MBN measurement system, we successfully obtained high-fidelity, single-shot capture of individual MBN pulses, providing direct experimental evidence of magnetic DW relaxation in metallic ribbons."
The team also included Senior Researcher Shingo Tamaru from the National Institute of Advanced Industrial Science and Technology (AIST), Japan, and Professor Masato Kotsugu from TUS. The study, a collaboration between TUS and AIST, was published in Volume 13 of the journal IEEE Access on August 07, 2025.
The developed MBN measurement system integrates a dual-layer coil jig with full electromagnetic shielding, wiring, and a custom low-noise amplifier. Designed to minimize noise while maintaining a wide bandwidth, the system enables the capture of individual MBN pulses with the highest possible fidelity. This system enabled the team to effectively visualize the relaxation behavior and precise evaluation of DWs, focusing on the microstructural features associated with energy dissipation.
Using this setup, the team observed clear isolated MBN pulses, indicative of DW relaxation, in amorphous NANOMET® ribbons. These materials have exceptionally low coercivity and are well known for their soft magnetic properties. Statistical analysis of the captured pulses revealed a mean relaxation time constant of approximately 3.8 μs with a standard deviation of around 1.8 μs, much smaller than the values predicted by conventional models.
To explain this difference, they constructed a new physical model of DW relaxation. The model showed that the damping caused by eddy currents generated during DW motion is the main cause of excess eddy current loss, rather than the intrinsic magnetic viscosity of DWs themselves. This experimentally and theoretically clarifies the physical origin of excess eddy current losses, offering crucial insights for future material design.
The team further used their system to analyze heat-treated nanocrystalline NANOMET® ribbons, finding a significant decline in the amplitude of MBN pulses, indicating a substantial reduction in irregularity of the DW motion. This shows that it is possible to smoothen DW motion and therefore reduce energy loss through microstructural control. By collaborating with industries, NANOMET® could be translated into ultra-efficient components for renewable energy systems.
"Our method has the potential for wide application in the design of next-generation low-loss soft magnetic materials, especially in high-frequency transformers, electric vehicle motors, paving the way for smaller, lighter, and more efficient devices," concludes Dr. Yamazaki. The insights gained from this study can aid in the designing of devices with improved driving performance and low power consumption.
