Precise magnetic measurement for efficient power electronics

Soft magnetic materials can be easily magnetised and demagnetised, making 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-less soft magnetic materials. The efficiency of these materials is 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, eddy current loss becomes 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 arises due to localised 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 magnetised 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, researchers from Tokyo University of Science, led by Assistant Professor Takahiro Yamazaki, have 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. Yamazaki said the researchers used the MBN measurement system to obtain a high-fidelity, single-shot capture of individual MBN pulses, providing direct experimental evidence of magnetic DW relaxation in metallic ribbons.

The MBN measurement system integrates a dual-layer coil jig with full electromagnetic shielding, wiring and a custom low-noise amplifier. Designed to minimise noise while maintaining a wide bandwidth, the system enables the capture of individual MBN pulses with high fidelity. This system enabled the researchers to visualise the relaxation behaviour and precise evaluation of DWs, focusing on the microstructural features associated with energy dissipation.

Using this setup, the researchers observed clear isolated MBN pulses, indicative of DW relaxation, in amorphous NANOMET ribbons. These materials have low coercivity and are 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, the researchers constructed a new physical model of DW relaxation; this 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 theoretically clarifies the physical origin of excess eddy current losses, offering insights for future material design.

“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,” Yamazaki said.

The research findings, published in the journal IEEE Access, can also help with the design of devices with improved driving performance and low power consumption.

Image credit: Tokyo University of Science