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تسجيل مستخدم جديدThe Persistence and Memory of Polar Nano-Regions in a Ferroelectric Relaxor Under an Electric Field
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The response of polar nanoregions (PNR) in the relaxor compound Pb[(Zn$_{1/3}$Nb$_{2/3}$)$_{0.92}$Ti$_{0.08}$]O$_3$ subject to a [111]-oriented electric field has been studied by neutron diffuse scattering. Contrary to classical expectations, the diffuse scattering associated with the PNR persists, and is even partially enhanced by field cooling. The effect of the external electric field is retained by the PNR after the field is removed. The ``memory of the applied field reappears even after heating the system above $T_C$, and cooling in zero field.
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A new class of superparamagnetism was found in relaxor ferroelectric 2/3BiFeO$_{3}$-1/3BaTiO$_{3}$. The size of the magnetic particle, estimated from the superparamagnetic magnetization curve, coincides with the size of the polar nanoregion (PNR), wh
ich governs the relaxor ferroelectric property. This suggests that the magnetic domain is identical to the PNR. The temperature variations in the sizes of the magnetic domains and PNRs estimated by our neutron diffraction measurements support this picture. Since the same domain provides both electric and magnetic properties, strong coupling between the two properties through the domain size is expected.
We report measurements of the neutron diffuse scattering in a single crystal of the relaxor ferroelectric material 95.5%Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 (PZN-4.5%PT). We show that the diffuse scattering at high temperatures has a quasielastic component wi
th energy width $agt$ 0.1 meV. On cooling the total diffuse scattering intensity increases, but the intensity and the energy width of the quasielastic component gradually diminish. At 50 K the diffuse scattering is completely static (i.e.the energy width lies within the limits of our instrumental resolution). This suggests that the dynamics of the short-range correlated atomic displacements associated with the diffuse scattering freeze at low temperature. We find that this depends on the wave vector q as the quasielastic diffuse scattering intensities associated with <001> (T1-type) and <110> (T2-type) atomic displacements vary differently with temperature and electric field.
indent It is a generally accepted fact that the unique dielectric properties of relaxor ferroelectrics are related to the formation of polar nanoregion (PNRs). Less well recognized is the corollary that, because they are polar and therefore lack inve
rsion symmetry, PNRs are also piezoelectric at the nanoscale and can therefore behave as nanoresonators. Using the particular relaxor ferroelectric K$_{tt1-x}$Li$_{tt x}$TaO$_{tt 3}$ (KLT), we show that, when electrically excited into oscillation, these piezoelectric nanoresonators can drive macroscopic electro-mechanical resonances. Unexpectedly however, pairs of coupled resonances are observed, with one of the two exhibiting a characteristic Fano-like lineshape. The complex resonance spectra can be described equally well by two alternative but complementary models both involving two resonances coupled through a relaxation: a purely classical one based on two coupled damped harmonic oscillators and a semi-classical based on two discrete excitations coupled to each other through a continuum. Together, they provide complementary perspectives on the underlying physics of the system. Both reproduce the rapid evolution of the resonance spectrum across three wide temperature ranges, including a phase transition range. In the high temperature range, the coupling between modes is due to the collective $pi$ relaxation of the lithium ions within PNRs and in the phase transition range to heterophase relaxation of the surrounding lattice between its high temperature cubic and low temperature tetragonal phases. The coupling is suppressed in the intermediate range of the collective $pi/2$ relaxation of the lithium ions. Incidentally, the measured dielectric spectra are shown to bear a surprising but justifiable resemblance to the optical spectra of certain atomic vapors that exhibit electromagnetically induced transparency.
Local inhomogeneities known as polar nanoregions (PNR) play a key role in governing the dielectric properties of relaxor ferroelectrics - a special class of material that exhibits an enormous electromechanical response and is easily polarized with an
external field. Using neutron inelastic scattering methods, we show that the PNR can also significantly affect the structural properties of the relaxor ferroelectric Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 (PZN-4.5%PT). A strong interaction is found between the PNR and the propagation of sound waves, i.e. acoustic phonons, the visibility of which can be enhanced with an external electric field. A comparison between acoustic phonons propagating along different directions reveals a large asymmetry in the lattice dynamics that is induced by the PNR. We suggest that a phase instability induced by this PNR-phonon interaction may contribute to the ultrahigh piezoelectric response of this and related relaxor ferroelectric materials. Our results also naturally explain the emergence of the various observed monoclinic phases in these systems.
We have performed a series of neutron diffuse scattering measurements on a single crystal of the solid solution Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$ (PZN) doped with 8% PbTiO$_3$ (PT), a relaxor compound with a Curie temperature T$_C sim 450$ K, in an effor
t to study the change in local polar orders from the polar nanoregions (PNR) when the material enters the ferroelectric phase. The diffuse scattering intensity increases monotonically upon cooling in zero field, while the rate of increase varies dramatically around different Bragg peaks. These results can be explained by assuming that corresponding changes occur in the ratio of the optic and acoustic components of the atomic displacements within the PNR. Cooling in the presence of a modest electric field $vec{E}$ oriented along the [111] direction alters the shape of diffuse scattering in reciprocal space, but does not eliminate the scattering as would be expected in the case of a classic ferroelectric material. This suggests that a field-induced redistribution of the PNR has taken place.
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