I am measuring dielectric constant of a magneto-dielectric which has a low temperature incommensurate (LT-ICM) magnetic structure and a high temperature commensurate (HT-CM) one. At the same T regions I observe dielectric anomalies. When I apply external magnetic/electric (H, E) field the dielectric anomaly of the LT-ICM one is greatly affected ( enhanced, shifted to lower T) whereas the effect is less strong when it comes to the HT-CM dielectric anomaly.
If I think in terms of energy, then an external field (E, H) corresponds basically to a perturbation on the overall system's energy. What is the difference of the Hamiltonian of a ICM structure and a CM one?
If I try to imagine the physical picture in the lattice, then the commensurate structure should be more "rigid" and stable than the ICM one(?).
Is that why the effect of an external field is more strong to the ICM phase?
Any references/comments to get a more clear picture of the above phenomenon are highly welcomed!
Dear Joanna,
your results seem to be consistent with manifestation of the so-called ‘‘colossal" magnetodielectric (MD) effect in DyMn2O5 multiferroics due to an unusual commensurate-incommensurate magnetic transition with high sensitivity of the incommensurate state to external perturbation. Recall that the typical incommensurate-commensurate transition results in the commensurate state being the low-temperature ground state, which is a consequence of the ‘‘locking in’’ for both magnetic and lattice modulations that is energetically favored at LT. The unusual ICM phase in DyMn2O5 coincides with the appearance of multiple FE components, large lattice polarizability, and an enormously large MD effect (at relatively low magnetic fields). All the above is due to an anomalous "softness" (both magnetical and structural) of this ICM phase in DyMn2O5. See the attached paper for more details.
Sergei
Are you by chance investigating RMn2O5 system which also has also similar behavior? It has been speculated that the ferroelectricity in this system is not due to inverse D-M interaction, but due to exchange striction. For exchange striction mechanism of ferroelectricity the magnetic structure need not be incommensurate and non-collinear. In fact in this system the commensurate and and almost collinear intermediate phase is ferroectric. This commensurate structure is however not strictly commensurate because its propagation vectpr k = (1/2,0,1/4) does not correspond to a symmetry point of the lattice. This structure should be called commensurately modulated. The picture of commensurate structure being rigid and incommensurate structure not-so-rigid is completely naive. Field effect in magnetic structure is governed by magnetic anisotropy. But in multiferroic system the situation is more complex and we cannot make any simple mental picture of the energetics.
Dear Sergei, Many thanks for the paper, It is helpful to know that the phenomenon has also been observed in other systems. I was trying understand if I should correlate the phenomenon to the strength of magnetic interactions (in both CM and ICM phase) and the effect an external field.
Dear Tapan,
Many thanks for the comment, I am measuring a dellafossite, AMnO2, (bulk). I understand the situation is quite complex in magnetodielectrics, nevertheless, I am searching a good starting direction in order to understand better the phenomenon.
Even though the magnetoelectric coupling in IC structure is two orders of magnitude larger than that of commensurate structure in relative terms(I have Orthorhombic and Hexagonal RMnO3 in mind), it’s not true in terms of the actual value. The polarization values in IC structure is two orders of magnitude weaker in comparison to commensurate structure (hexagonal RMnO3). Thus the magnitude of polarization variation is same in both commensurate and incommensurate magnetic structures. Such a weak polarization (0.05micro Coulomb /cm2) generally arise from improper ferroelectrics where the polarization is not the primary order parameter. As far as orthorhombic RMnO3 are concerned the high temperature structure is incommensurate. upon decreasing the temperature, the propagation vectors locks at a particular value, ( some call it as a commensurate structure LT CM) with the onset of polarization. In HTICM the magnetoelectric response is purely elastic effect due to magnetic ordering without the onset of polarization ( a non ferroelectric phase) thus the dielectric constant cannot be tuned by applying magnetic field. However the change of magnetic structure to LTCM induces a net polarization but of low values which can be readily varied by applying magnetic field thus showing large variation in the dielectric constant.
I hope the reference on RMnO3 will be helpful
http://prb.aps.org/abstract/PRB/v71/i22/e224425
Thank you for the references, it is important to know how the phenomenon has been approached in similar cases.
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