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I'm a bit confused about when magnetic hysteresis occurs in ferromagnetic materials. Does it only occur with a rapidly changing external magnetic field or with a slowly changing field? Does the speed at which the external field changes matter?

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Magnetic hysteresis is inherently irreversible in the sense that no matter how slowly one executes a time periodic current drive and hence an $H$ field around a ferromagnetic sample the resulting $B(H)$ curve encloses a non-zero area representing the dissipated magnetic work. This is an interesting example of a thermodynamic system where a quasi-static process is not reversible.

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It happens no matter the frequency (low or high). However, the area of the cycle does depend slightly on the frequency, but usually, it is assumed constant. In this case, the hysteresis losses are $P=k(B)*f$ (the cycle is done every period).

When the magnetic field includes a large fundamental wave and smaller high-frequency waves (ex : PWM), one can observe minor cycles along the main hysteresis cycle.

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  • $\begingroup$ Addendum: If you have such a high frequency that the magnetic domains can't orient that fast, you will have a very different magnetisation behaviour. $\endgroup$
    – WalyKu
    Commented Dec 1, 2014 at 22:46
  • $\begingroup$ @Kurtovic Aye. Working in electrotechnics, I often forget really high frequencies. $\endgroup$
    – TZDZ
    Commented Dec 2, 2014 at 15:34
  • $\begingroup$ If the frequency is very low, couldn't it happen that the magnetic momenta have time enough to leave the metastable states and reach Boltzmann equilibrium, so going back to the no-hysteresis curve? $\endgroup$ Commented Sep 4, 2016 at 23:52

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