Magnetic Barkhausen noise measurement unlocks possibilities for soft magnetic materials

Researchers in Japan have developed a highly-sensitive magnetic Barkhausen noise (MBN) measurement system to understand the energy loss mechanisms in soft magnetic materials, which can be easily magnetized and demagnetized, and are widely used in power electronics devices such as generators, transformers, and amplifiers.

It’s an important development because power electronics is moving toward high-frequency operations, which in turn demand low-loss soft magnetic materials. However, the efficiency of soft magnetic materials is fundamentally limited by iron loss, where energy is lost as heat when a varying magnetic field passes through them.

Iron loss mainly comprises three entities: hysteresis loss, classical eddy current loss, and excess eddy current loss. Eddy currents are generated when a varying magnetic field passes through a conductor; the currents that waste energy as heat are known as classical eddy current loss.

On the other hand, excess eddy current loss arises due to localized eddy currents induced by irregular movement of magnetic domain walls (DWs) under a varying magnetic field. Magnetic DWs are boundaries that separate tiny magnetic domains.

Enter magnetic Barkhausen noise or MBN, a key probe for DW dynamics. Here, it’s important to note that the current MBN measurement systems don’t facilitate the wide frequency coverage and high sensitivity needed to capture the individual MBN events. That makes it hard to understand the relationship between DW dynamics and eddy current losses.

MBN measurement system

The Japanese research team, aiming to address this gap, has developed a wideband and high-sensitivity MBN measurement system. The team is led by assistant professor Takahiro Yamazaki from the Department of Materials Science and Technology at the Tokyo University of Science (TUS). It also includes professor Masato Kotsugu from TUS and senior researcher Shingo Tamaru from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan.

The MBN measurement system investigated the magnetic DW dynamics in 25-μm-thick Fe–Si–B–P–Cu NANOMET ribbons, a class of soft magnetic alloys. It comprises a dual-layer coil jig with full electromagnetic shielding, wiring, and a custom low-noise amplifier. And it’s designed to minimize noise while maintaining a wide bandwidth.

Magnetic Barkhausen noise (MBN) serves as a key probe for DW dynamics.

The system allows the capture of individual MBN pulses with the highest possible fidelity. That, in turn, enabled the team to effectively visualize the relaxation behavior and precise evaluation of DWs. As a result, they were able to observe clear and isolated MBN pulses indicative of DW relaxation in amorphous NANOMET ribbons. These materials, well known for their soft magnetic properties, have exceptionally low coercivity.

Cause of excess eddy current loss

Statistical analysis of the captured pulses also revealed a mean relaxation time constant of approximately 3.8 μs with a standard deviation of around 1.8 μs. It’s much smaller than the values predicted by conventional models.

So, the research team constructed a new physical model of DW relaxation to explain this difference. Subsequently, this model showed that the damping caused by eddy currents generated during DW motion is the main cause of excess eddy current loss. That negates the common perception that the intrinsic magnetic viscosity of DWs is the cause this phenomenon.

It provided experimental and theoretical clarification on the physical origin of excess eddy current losses. Next, the team used this system to analyze heat-treated nanocrystalline NANOMET ribbons and found a significant decline in the amplitude of MBN pulses. This led to a substantial reduction in the irregularity of the DW motion.

Moreover, it demonstrated that it’s possible to smooth DW motion and thus reduce energy loss through microstructural control. “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,” said team leader Yamazaki. “It paves the way for smaller, lighter, and more efficient devices.”

He added that this wideband, high-sensitivity MBN measurement system has successfully captured high-fidelity, single-shot pulses. That provides direct experimental evidence of magnetic DW relaxation in metallic ribbons, Yamazaki concluded.

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