Unaged sample: irreversible domain switching

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Explanation on domain switching behavior:

The left figure shows a series of domain micrographs and its correspondence to the measured P-E hysteresis loop for unaged Mn-BaTiO3 samples. Beside each micrograph, the corresponding defect symmetry state (small square) and Ps (thick arrow) state are also illustrated.

 For the unaged multi-domain sample, defect symmetry is cubic everywhere with zero defect polarization. We start from point A, which has a negative averaged polarization in P-E curve. With electric field increasing, Ps is switched to the direction of electric field and domain switching occurs. At maximum electric field, a single domain state is observed, corresponding to a maximum polarization (point B in P-E curve). During such a process, defect symmetry remains cubic. Consequently, when electric field decreases to zero, single domain state should remain unchanged, as cubic defect symmetry does not favor a particular domain state. Thus there should be no reason to switch back to the original domain state. Nevertheless, because of the existence of depolarization field, the single-domain state will change into multi-domain state, but such a multi-domain state has no relation with the initial multi-domain state (compare micrographs at point C with that at point A). Correspondingly, polarization does not come back to zero and stays at a positive value (point C). Moreover, in reverse field cycling, single-domain state changes into another new multi-domain state and polarization keeps the negative non-zero value (see C-D-E cycle). We consequently observed an irreversible domain switching and the well-known normal hysteresis loop in unaged sample.