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Consider a ferromagnetic material is subjected to a gradually increasing external magnetic field and magnetized to saturation. Then a reverse field is applied to demagnetize the material and then the magnetization is allowed to saturate in the opposite direction. Then the field is again increased in the positive direction until it saturates and a hysteresis loop is completed.

Why is it that at the end of the process the sample heats up? What happens inside the material so that heat is generated?

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  • $\begingroup$ Magnetocrystalline anisotropy makes that phonons are generated when a domain wall moves. $\endgroup$
    – user137289
    Nov 22, 2017 at 20:52
  • $\begingroup$ @Pieter Can you elaborate it a little? $\endgroup$
    – SRS
    Mar 23, 2018 at 20:04
  • $\begingroup$ Magnetocrystalline anisotropy is due to spin-orbit coupling, which is also linked to magnetostriction: when the spin direction changes, bond lengths change, and this generates atomic movement and phonons: heat. In metals, there is also a contribution from eddy currents when domain walls move. $\endgroup$
    – user137289
    Mar 23, 2018 at 22:08

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Hysteresis is a kind of non-time-reversible transition, so it is associated with entropy (which must, in theory, generate heat).

Because magnetic materials have crystal structure, the internal magnetic polarization is not uniform, but aligns in small patches to the crystal, and these patches (magnetic domains) shift in size and shape during a magnetic change. That changes the internal forces between the domains, which causes flexing of the material, in nearly random distribution, i.e. generates acoustic white noise.

The losses in moving a magnetic domain wall are increased by flaws and inclusions which 'pin' the boundary of the domain, and which determine whether the material is a 'hard' (highly hysteretic) or 'soft' magnet.

Even soft magnetic materials have losses with magnetism change, and part of that is induced electrical current (eddy current) which depends on the electrical resistivity, and on dB/dt, so is greatest in conductive materials, and at high frequencies. Iron is an effective transformer-core material at low frequency (50 to 400 Hz) but nonconductive ferrites are preferred for low loss at higher frequencies.

There is also a small dimensional signature to magnetic changes in ferromagnets (called magnetostriction), and this makes microscopic acoustic events every time there is a magnetic change.

Any internal acoustic energy will decay to thermal equilibrium by Umklapp scattering, becoming heat.

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  • $\begingroup$ Is there a reference that you can suggest? @Whit3rd $\endgroup$
    – SRS
    May 4, 2018 at 10:21
  • $\begingroup$ Indeed, the acoustic emissions in the Barkhausen noise are the clearest evidence for phonons being created when domain walls slip. $\endgroup$
    – user137289
    May 5, 2018 at 8:02

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