Signature of negative domain wall mass in soft magnetic materials

Magnetic properties of ferromagnetic materials often originate from domain wall motion, involving different damping mechanisms, an effective mass and various interactions with the surrounding media. In metallic materials, eddy current damping overwhelms inertia and thus the effect of the mass is usually neglected. We have recently reported experimental evidence that in soft metallic ferromagnets eddy currents yield an observable negative contribution to the effective domain wall mass. The weight of this mass is of the order of 10-5 kg/m2, much larger than the positive Doring mass (∼10-9 kg/m2). This negative effective mass is responsible for the leftward asymmetry of Barkhausen noise pulse shapes. In particular, this asymmetry depends on the pulse duration and it is found to encode important information on the characteristic time of the underlying domain wall dynamics. Only on long timescales the pulse shapes are symmetric and show the universal features typical of the Barkhausen effect. This result clarifies the general significance of pulse shape asymmetry commonly observed in systems showing a similar crackling noise, and contributes to better understand the microscopic phenomena responsible of magnetic hysteresis.