The magneto-acoustic emission (invited)

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Abstract
When cycling a hysteresis loop, bursts of high frequency ultrasonic waves are emitted. This general phenomenon, which must not be confused with the classical magnetostrictive effect at twice the cycling frequency, is called the magnetoacoustic emission (MAE). By using literature and authors’ results, qualitative and quantitative, the authors review the past and present knowledge of the MAE phenomenon to stress its fundamental origin: the domain wall (DW) creation/annihilation processes. On a qualitative point of view, the shape demagnetizing effect is shown to drastically change the MAE profiles, which allow us to clearly distinguish the MAE from the classical magnetic Barkhausen noise. Quantitatively and in agreement with their interpretation, the authors have established that the total MAE activity along an hysteresis loop is proportional to the hysteresis losses during the same loop; this law has been confirmed on several series of polycrystalline ferrimagnets and is valid whatever the composition, the grain size, or the measuring temperature is. The noncorrelation between the MAE and the magnetostriction is also demonstrated, contrary to some previously reported results and interpretations. Related to the DW creation/annihilation processes, the MAE might be the missing link in the process of the conversion of the magnetic energy into heat during a hysteresis loop.

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