Ferroelectric Critical Temperature Research Articles

Persistence of Structural Distortion and Bulk Band Rashba Splitting in SnTe above Its Ferroelectric Critical Temperature.

The ferroelectric semiconductor α-SnTe has been regarded as a topological crystalline insulator, and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the past decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of α-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about Tc ≈ 110 K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution of local dipoles to the phase transition that requires the presence of fluctuating dipoles above Tc. We conclude that no topological surface state can occur under these conditions at the (111) surface of SnTe, at odds with recent literature.

Can the ferroelectric soft mode trigger an antiferromagnetic phase transition?

Type-II multiferroics, where spin interactions induce a ferroelectric polarization, are interesting for new device functionalities due to large magnetoelectric coupling. We report on a new type of multiferroicity in the quadruple-perovskite BiMn3Cr4O12, where an antiferromagnetic phase is induced by the structural change at the ferroelectric phase transition. The displacive nature of the ferroelectric phase transition at 125 K, with a crossover to an order-disorder mechanism, is evidenced by a polar soft phonon in the THz range and a central mode. Dielectric and pyroelectric studies show that the ferroelectric critical temperature corresponds to the previously reported Néel temperature of the Cr3+ spins. An increase in ferroelectric polarization is observed below 48 K, coinciding with the Néel temperature of the Mn3+ spins. This increase in polarization is attributed to an enhanced magnetoelectric coupling, as no change in the crystal symmetry below 48 K is detected from infrared and Raman spectra.

Effect of A-site covalency on ferroelectric phase transition in Ba doped morphotropic phase boundary PbTiO3–BiFeO3 solid solutions

This work investigates the role of A-site chemical bonding in morphotropic phase boundary (MPB) compositions of PbTiO3–BiFeO3 solid solutions exhibiting high tetragonality. Ba substitution significantly reduces the ferroelectric critical temperature, which correlates well with the structural parameters and manifests marked change in the free energy profile of structural phases within the MPB regime. A reduction in the grain size with Ba substitution has been observed which shows the reduction in the grain growth with Ba incorporation. The presence of pinned defect dipoles has been confirmed from the observed elliptical hysteresis loops at room temperature, making high-temperature permittivity studies unique to unraveling the finer details of this system. Inter and Intra grain contributions in the electrical response are quantified from the equivalent circuit analysis and compared with the complex permittivity profile. The frequency and temperature-dependent ac conductivity responses were analyzed, and observed conductivity was related to charge carriers hopping between vacant sites.

Multiferroic properties of RFe0.5Co0.5O3 with R=Tm , Er, Ho, Dy, and Tb

We present a detailed study of the thermal dependence of the neutron powder diffraction (NPD) of $R\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{o}}_{0.5}{\mathrm{O}}_{3}$ perovskites ($R=\mathrm{Tm}$, Er, Ho, Dy, and Tb) combined with a complete characterization of their magnetic, electric transport and ferroelectric properties. All samples are described with an orthorhombic (Pbnm) crystallographic structure. The inverse of magnetic susceptibility at high temperature shows an effective magnetic moment with contributions of high-spin $\mathrm{C}{\mathrm{o}}^{3+}$, $\mathrm{F}{\mathrm{e}}^{3+}$, and the corresponding rare-earth cations (${R}^{3+}$) moments. Negative Curie-Weiss temperatures indicate antiferromagnetic (AFM) correlations between the magnetic ions. At low temperature, the reduced magnetic saturation values are associated with crystal-field effects on ${R}^{3+}$ ions. Below 300 K, magnetic reflections in NPD data show that the spin configuration in the ordered state is AFM type-G with a weak ferromagnetic component along the $c$ axis (${\mathrm{\ensuremath{\Gamma}}}_{4}$) for transition metal ions. Spin reorientation (SR) transitions are observed, changing the irreducible representation from ${\mathrm{\ensuremath{\Gamma}}}_{4}$ (${G}_{x}$) to ${\mathrm{\ensuremath{\Gamma}}}_{2}({G}_{z})$ at low temperatures, except for the Ho compound where it changes from ${\mathrm{\ensuremath{\Gamma}}}_{4}$ (${G}_{x}$) to ${\mathrm{\ensuremath{\Gamma}}}_{1}({G}_{y})$. Magnetization data under field cooling (FC) and zero field cooling (ZFC) protocols show reversal magnetization for $R=\mathrm{Er}$ and Tb. A significant change of the slope of the M vs T curves is associated with the onset of magnetic order (${T}_{N1}\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) of Fe/Co sublattice. The electric conductivity at room temperature shows that Tm value is ten times less than the other rare earths. The temperature dependence of the electrical conductivity can be described with a variable range hopping model ($\mathrm{ln}\ensuremath{\sigma}\ensuremath{\sim}{T}^{\ensuremath{-}1/4}$). Also, in the Tm case, the complex electrical permittivity shows a different behavior in comparison with the other members of the series. Finally, the Tm sample can be polarized when the specimen is cooled under different values of electrical field (E). Pyroelectric current can be detected while the sample is warming and depolarized, showing the ferroelectric critical temperature (${T}_{C}$) at ${T}_{N1}$. A low electric polarization of $400\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ was measured for $\mathrm{TmF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{o}}_{0.5}{\mathrm{O}}_{3}$.

Double hysteresis loops in 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 near the ferroelectric critical endpoint

Polarization-electric field hysteresis loops in 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 have been investigated near the ferroelectric critical temperature in the bipolar field applied along the [001]c direction in the pseudo-cubic coordinate. The double hysteresis loop has been observed above the transition temperature indicating the field induced transition, and the ferroelectric critical endpoint has been found at 192 °C and 1.6 kV/cm. The temperature field phase diagram under the electric field applied along the [001]c direction has been clarified.

Carrier concentration dependence of ferroelectric transition in multiferroic Li doped and Li-Co co-doped ZnO nanoparticles

Dielectric measurements on both Li doped and Li-Co co-doped multiferroic nanoparticles are presented and correlated with the hole carrier concentrations measured by the Hall effect. The ferroelectric Curie temperatures lay in the range 443–512 K with the dielectric constant increasing monotonically with Li concentration. However, we find that both for these p-type systems, the Curie temperature varies non-monotonically with Li concentration, being larger for compositions with higher hole concentrations and vice versa. We find a similar trend in the variation of the magnetic moment with hole concentration. Hence, the multiferroic behavior in this system is strongly correlated with the hole concentration. The ferroelectric behavior is explained in terms of the model of electric dipoles, formed by Li off-centre impurities, interacting indirectly via the free hole carriers. The variation of the ferroelectric critical temperature with hole concentration is explained within this model in terms of the dependence of the indirect interaction strength on the Fermi wave vector.

Increasing the Phase-Transition Temperatures in Spin-Frustrated Multiferroic MnWO4 by Mo Doping

Ceramic samples of MnW1–xMoxO4 (x ≤ 0.3) solid solution were prepared by a solid-state route with the goal of increasing the magnitude of the spin-exchange couplings among the Mn2+ ions in the spin spiral multiferroic MnWO4. Samples were characterized by X-ray diffraction, optical spectroscopy, magnetization, and dielectric permittivity measurements. It was observed that the Neel temperature TN, the spin spiral ordering temperature TM2, and the ferroelectric phase-transition temperature TFE2 of MnWO4 increased upon the nonmagnetic substitution of Mo6+ for W6+. Like pure MnWO4, the ferroelectric critical temperature TFE2(x) coincides with the magnetic ordering temperature TM2(x). A density functional analysis of the spin-exchange interactions for a hypothetical MnMoO4 that is isostructural with MnWO4 suggests that Mo substitution increases the strength of the spin-exchange couplings among Mn2+ in the vicinity of a Mo6+ ion. Our study shows that the Mo-doped MnW1–xMoxO4 (x ≤ 0.3) compounds are spin-frustrat...

Multiferroic properties of CaMn7O12

We report that CaMn$_7$O$_{12}$ is a new magnetic multiferroic material. The appearance of a ferroelectric polarization coinciding with the magnetic phase transition ($\sim90$ K) suggests the presence of ferroelectricity induced by magnetism, further confirmed by its strong magnetoelectric response. With respect to other known magnetic multiferroics, CaMn$_7$O$_{12}$ displays attractive multiferroic properties, such as a high ferroelectric critical temperature and large polarization. More importantly, these results open a new avenue to search for magnetic multiferroics in the catalogue of doped oxides.

Effect of chemical substitution on the Néel temperature of multiferroicBi1−xCaxFeO3

Multiferroic BiFeO3 ceramics have been doped with Ca. The smaller ionic size of Ca compared with Bi means that doping acts as a proxy for hydrostatic pressure, at a rate of 1%Ca=0.3GPa. It is also found that the magnetic Neel temperature (TNeel) increases as Ca concentration increases, at a rate of 0.66K per 1%Ca (molar). Based on the effect of chemical pressure on TNeel, we argue that applying hydrostatic pressure to pure BiFeO3 can be expected to increase its magnetic transition temperature at a rate around ~2.2K/GPa. The results also suggest that pressure (chemical or hydrostatic) could be used to bring the ferroelectric critical temperature, Tc, and the magnetic TNeel closer together, thereby enhancing magnetoelectric coupling, provided that electrical conductivity can be kept sufficiently low.

Strain relaxation and critical temperature in epitaxial ferroelectric Pb(Zr0.20Ti0.80)O3 thin films

Strain relaxation and the ferroelectric critical temperature were investigated in a series of epitaxial Pb(Zr0.20Ti0.80)O3 thin films of different thicknesses grown on metallic 0.5% Nb-doped SrTiO3 substrates. Detailed x-ray diffraction studies reveal that strain relaxation progressively occurs via misfit dislocations as the film thickness is increased from fully coherent films (for films below 150Å) to essentially relaxed films (for thicknesses above typically 800Å). It is found that this change in the strain state does not modify the ferroelectric critical temperature which is found for all the samples to be around 680°C, a value much higher than the bulk.

Microscopic theory of superconductor-ferroelectric heterostructures: Interface charge redistribution

We present a theory of periodic ferroelectric-superconductor (FE-$S)$ heterostructures containing ferroelectric layers sandwiched between superconducting planes. We analyze the electronic charge-carrier redistribution at the FE-S interface in the presence of the spontaneous polarization in the ferroelectric layer. On the other hand, we study the influence of the superconductor on the structural dynamics in the ferroelectric layer. The effect of FE-$S$ contacts on the ferroelectrics is found to be crucial leading to a structural transformation from the state with the homogeneous-type polarization to the phase with a set of asymmetric stable polarization domains. FE-$S$ interface phenomena induce a decrease of the temperature of the transition to the symmetric phase with two symmetric (negative and positive) polarization domains. Nevertheless, even above the ferroelectric critical temperature, we find in the ferroelectric layer a stable contact-induced enhanced spontaneous polarization. The domain structure in the symmetric phase appears as the response to the charge-carrier redistribution at the contact with the superconducting subsystem. An increase of the FE-$S$ interface coupling results in a complex nonmonotonic behavior of the superconducting transition temperature and finally, for the strong-coupling regime, in a complete suppression of the superconductivity. The results are expected to be especially important for the analysis of high-temperature cuprate superconductor films grown on perovskite-type ferroelectrics.

Anharmonic effects in potassium-dihydrogen-phosphate-type ferroelectrics.

A Green's-function technique is used to study the influence of spin-phonon interactions in the Ising model with a transverse field (describing KDP-type ferroelectrics) including higher-order anharmonic terms. The renormalized spin-wave and phonon energy, the damping of the spin waves, and the phonon damping have been evaluated. The contribution of the anharmonicity is at low temperatures very small, but at temperatures near the ferroelectric critical temperature ${\mathit{T}}_{\mathit{c}}$, and above ${\mathit{T}}_{\mathit{c}}$, it increases strongly.

ESR Evidence for an incommensurate phase in doped KH2PO4 and KD2PO4

Experiments show that ESR lineshapes in doped crystals of KH2PO4 and KD2PO4 are altered in a way that suggests the existence of an incommensurate phase in a temperature range below the ferroelectric critical temperature. In both cases the normal shape reappears at approximately 100K. Data can be interpreted in terms of the one-dimensional phase soliton theory, and are possibly related to a four-domain structure proposed by early X-ray and neutron scattering investigators.