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Suppose a virgin unmagnetized ferromagnetic sample is magnetized to complete saturation. Usually, when we reverse the magnetic field, the system does not retrace its path in the M-H plane and there is also a remnant magnetization when the field the made to vanish from the opposite direction. So the process is not reversible. But if the reversed magnetic field is changed very very slowly, as slowly as we can think of, do we expect the process to become reversible and the system to retrace its path? Thanks!

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Ferromagnetic hysteresis is the prime example of having a thermodynamic process such that it is dissipative and thus is irreversible at all (macroscopic) time scales showing that despite what you were frequently and erroneously told in freshman physics class a quasi-static process is not necessarily reversible. You may find a beautiful analysis of this problem in Bridgman: "The Thermodynamics of Plastic Deformation and Generalized Entropy",doi:10.1103/RevModPhys.22.56

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No. If the material shows a remnant magnetization, going slow will not avoid it. To demagnetize a material, gradually remove it from an alternating field. (In a tape recorder, the tape is demagnetized before recording by moving past an AC electromagnet.) This takes the material around a hysteresis loop which shrinks a bit on each loop, ending at zero.

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  • $\begingroup$ I wasn't looking for a way of demagnetizing the sample. But by virtue of this example, I wanted to ask if a quasistatic (slow) process is always reversible. I admit that in the question I didn't mention that clearly. $\endgroup$ Mar 14, 2020 at 23:10
  • $\begingroup$ Apparently not. If you bring atomic dipoles into alignment in a ferromagnetic material, some of them may stay that way in spite of thermal agitation. $\endgroup$
    – R.W. Bird
    Mar 15, 2020 at 16:35

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