I have done electric polarization measurement on one of my substituted BiFeO3 compound. I observed that with the increase of applied field, coercive field (Hc) and polarization at maximum applied field (Pm) increased up to a certain field. But, beyond a certain field, both Hc and Pm did not change whereas saturation polarization increased a little bit. What is the reason for Hc and Pm maintaining constant value even as the applied field is increased. Also, the compound did not show dielectric breakdown throughout the measurement (up to 10 kV).
I agree with the above answer. Orientations of the spins compete at grain boundaries but I do not understand how the saturation polarization increases.
For a given hysteresis expt., beyond Hc if the field is increased the polarization will still increase upto saturation value, but Hc will remain constant when the field is withdrawn. So this evidence is somewhat incomprehensible. Pl. read my article for the critical field value for LN & LT ferroelectric in Euro Phys J. B65 (2008) 525. The breakdown field is defined as when soliton motion is zero or rather solitons are non-existent.
All the domain are switching at the same field value.
Coercive field is a material property depending on the ferroelectric composition (soft and hard materials), and in which form the materials exists (single crystal, ceramics)
Perfect single have a square loop, with a single value of Ec coercive field.
Polycrystalline samples with lot of defects or pining centers will show banana shaped loops where the switching of the domains is occurring at different fields, and the polarisation is increasing.
Coercive field does not depend on polarisation or the max field you apply.
Coercive field is the field required to switch a dipole in the opposite direction.
Since all the dipoles/domains have switched, and you have reached the saturation polarisation, why should the coercive field change.
K. Sreenivas
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