High Ferroelectric Transition Temperature Research Articles

Hexagonal RFeO3 (R = Dy, Er, and Lu) Films Grown on Glass Substrates with Both Magnetic and Ferroelectric Orders

Hexagonal rare-earth ferrites (h-RFeO3) are promising multiferroic materials owing to the spontaneous magnetization, high ferroelectric transition temperature, and fascinating features provided by strong magnetoelectric coupling. However, h-RFeO3 is metastable, and its synthetic methods are limited, particularly when ionic size of R is large. One of useful synthesis methods is epitaxial thin-film growth on single-crystal substrates, because the metastable phase is stabilized by the lattice match between h-RFeO3 and the substrate. Compared to single-crystal substrates, glass substrates are more cost-effective. However, films on glass tend to consist of a thermodynamically stable phase owing to the amorphous nature of glass substrates. Herein, we report that a h-RFeO3 pure phase can be obtained as thin film even on glass substrates. The metastable phase was formed owing to the nonequilibrium synthesis process of pulsed laser deposition. The obtained h-RFeO3 thin films on glass with R = Dy, Er, and Lu exhibited weak ferromagnetism with Néel temperatures of 120–140 K. Furthermore, a clear magnetic phase transition from weak ferromagnetic to antiferromagnetic phase was observed in the h-ErFeO3 films. Additionally, piezoelectric force microscopy measurements exhibited the ferroelectricity of the film at room temperature. These results provide a convenient synthesis method of a pure phase multiferroic h-RFeO3 system.

Effects of Sm-substitution on dielectric, ferroelectric, and piezoelectric properties of 0.36(Bi1-xSmx)ScO3-0.64PbTiO3 ceramics

Dielectric, ferroelectric, and piezoelectric properties of 0.36(Bi1-xSmx)ScO3-0.64PbTiO3 (BSPT-xSm) ceramics were investigated to assess effects of Sm-substitution on 0.36BiScO3-0.64PbTiO3 for high temperature piezoelectric device application. Optimal sintering was achieved at 1200°C when the BSPT-xSm ceramics were fully densified and crystallized with a perovskite structure without any secondary phase. The substitution of Bi3+ with Sm resulted in degradation of rhombohedral side in BSPT-xSm ceramics having morphotropic phase boundary. In addition, variations of grain size and ferroelectric behavior after Sm-substitution were insignificant. However, dielectric constant (εT33/ε0) was significantly enhanced with an increasing of amount of Sm to 5%. Although a slight decrease of relative density in case of x exceeding 3% led to deterioration of piezoelectric values of d33, kp, and d33*, the BSPT-3%Sm ceramic exhibited excellent values of d33 of 628 pC/N, kp of 62.4%, and d33* of 718 pm/V at 4.5 kV/mm, along with a high ferroelectric transition temperature of 421°C. The highly increased diffusion coefficient of 1.909 also implies that the Sm-substitution contributed to relaxor-like ferroelectric behavior of BSPT ceramics.

High-TC Two-Dimensional Ferroelectric CuCrS2 Grown via Chemical Vapor Deposition.

Two-dimensional (2D) ferroelectrics have attracted intensive attention. However, the 2D ferroelectrics remain rare, and especially few of them represent high ferroelectric transition temperature (TC), which is important for the usability of ferroelectrics. Herein, CuCrS2 nanoflakes are synthesized by salt-assisted chemical vapor deposition and exhibit switchable ferroelectric polarization even when the thickness is downscaled to 6 nm. On the contrary, a CuCrS2 nanoflake shows a TC as high as ∼700 K, which can be attributed to the robust tetrahedral bonding configurations of Cu cations. Such robustness can be further clarified by a theoretically predicted high order-disorder transition barrier and structure evolution from 600 to 800 K. Additionally, the interlocked out-of-plane (OOP) and in-plane (IP) ferroelectric domains are observed and two kinds of devices based on OOP and IP polarizations are demonstrated.

Tunable spin textures in polar antiferromagnetic hybrid organic\u2013inorganic perovskites by electric and magnetic fields

The hybrid organic–inorganic perovskites (HOIPs) have attracted much attention for their potential applications as novel optoelectronic devices. Remarkably, the Rashba band splitting, together with specific spin orientations in k-space (i.e., spin texture), has been found to be relevant for the optoelectronic performances. In this work, by using first-principles calculations and symmetry analysis, we study the electric polarization, magnetism, and spin texture properties of the antiferromagnetic (AFM) ferroelectric HOIP TMCM-MnCl3 (TMCM = (CH3)3NCH2Cl+, trimethylchloromethyl ammonium). This recently synthesized compound is a prototype of order–disorder and displacement-type ferroelectric with a large piezoelectric response, high ferroelectric transition temperature, and excellent photoluminescence properties as reported by You (Science 357:306, 2017). The most interesting result is that the inversion symmetry breaking coupled to the spin–orbit coupling gives rise to a Rashba-like band splitting and a related robust persistent spin texture (PST) and/or typical spiral spin texture, which can be manipulated by tuning the ferroelectric or, surprisingly, also by the AFM order parameter. The tunability of spin texture upon switching of AFM order parameter is largely unexplored and our findings not only provide a platform to understand the physics of AFM spin texture but also support the AFM HOIP ferroelectrics as a promising class of optoelectronic materials.

Pressure-induced large enhancement of Néel temperature and electric polarization in the hexagonal multiferroic Lu0.5Sc0.5FeO3

Hexagonal ferrites ($h\text{\ensuremath{-}}R\mathrm{Fe}{\mathrm{O}}_{3}$) have attracted great attention for their high ferroelectric transition temperature, strong magnetoelectric couplings, and tunable N\'eel temperature (${T}_{N}$) and electric polarization. While introducing structural distortion has been previously found to be effective to raise ${T}_{N}$ and polarization in $h\text{\ensuremath{-}}R\mathrm{Fe}{\mathrm{O}}_{3}$, it is generally difficult to create sizable structural distortion by common approaches including substrate-induced epitaxial strain and chemical doping. Here, we use high-pressure x-ray-diffraction measurements to show that pressure can generate large structural distortion and R-layer displacement of $h\text{\ensuremath{-}}R\mathrm{Fe}{\mathrm{O}}_{3}$, resulting with dramatically enhancement of polarization and ${T}_{N}$. Density-functional theory calculations reveal that the enlarged $c/a$ ratio results in an \ensuremath{\sim}70 K increase of ${T}_{N}$ along with a significant enhancement of ferroelectric polarization. Our results suggest that pressure is effective to tune structural distortions and related multiferroicity of the $h\text{\ensuremath{-}}R\mathrm{Fe}{\mathrm{O}}_{3}$ system, making $h\text{\ensuremath{-}}R\mathrm{Fe}{\mathrm{O}}_{3}$ a promising material for spintronic applications.

Doping effects on the ferroelectric transition of multiferroic Y(Mn,Al/Ga)O3

Multiferroic hexagonal manganites ${\mathrm{RMnO}}_{3}$ have a very high ferroelectric transition temperature around and above 1200 K, depending on the rare-earth elements, and a reasonably large electric polarization of about $5.5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$ at room temperature. It is generally believed that the ferroelectric transition is driven by the combination of $\text{R-}{\mathrm{O}}_{\mathrm{P}}$ displacement and ${\mathrm{MnO}}_{5}$ tilting, and hence called improper ferroelectric. In order to better understand the improper ferroelectric transition, we studied doping effects, using two elements with the same valence but a different ionic size: Al and Ga on the Mn site of ${\mathrm{YMnO}}_{3}$. Through detailed structural studies and nanoscale measurements of piezoresponse force microscopy (PFM) we conclude that there is a drastic doping effect for Al, whose ionic size is much smaller than Mn. It is in stark contrast with our observation that Ga, having a slightly smaller ionic size with Mn, does not change the ferroelectric transition up to 50% doping. This drastic difference in the doping effect is due to local strain induced by the difference in the ionic size of Al and Mn as compared with that of Mn, and sheds light on the intriguing nature of the improper ferroelectric transition.

Multiferroicity in B-site ordered double perovskite Y2MnCrO6

Double perovskite manganite Y2MnCrO6 ceramic is synthesized and its multiferroic properties are investigated. Novel multiferroic properties are displayed with respect to other multiferroics, such as high ferroelectric phase transition temperature, and the coexistence of ferrimagnetism and ferroelectricity. Moreover, the ferroelectric polarization of Y2MnCrO6 below the magnetic phase temperature can be effectively tuned by an external magnetic field, showing a remarkable magnetoelectric effect. These results open an effective avenue to explore magnetic multiferroics with spontaneous magnetization and ferroelectricity, as well as a high ferroelectric transition temperature.

Spin-Reorientation, Ferroelectricity, and Magnetodielectric Effect inYFe1−xMnxO3(0.1≤x≤0.40)

We report the observation of magnetoelectric and magnetodielectric effects at different temperatures in Mn-substituted yttrium orthoferrite, YFe(1-x)Mn(x)O(3)(0.1 ≤ x ≤ 0.40). Substitution of Mn in antiferromagnetic YFeO(3)(T(N) = 640 K) induces a first-order spin-reorientation transition at a temperature, T(SR), which increases with x whereas the Néel temperature (T(N)) decreases. While the magnetodielectric effect occurs at T(SR) and T(N), the ferroelectricity appears rather at low temperatures. The origin of magnetodielectric effect is attributed to spin-phonon coupling as evidenced from the temperature dependence of Raman phonon modes. The large magnetocapacitance (18% at 50 kOe) near T(SR) = 320 K and high ferroelectric transition temperature (∼115 K) observed for x = 0.4 suggest routes to enhance magnetoelectric effect near room temperature for practical applications.

Formation of local electric dipoles with no unique polar axis inTb3Fe5O12

Using neutron scattering and the pair-density-function analysis, we investigated, the local atomic structure of the ferrimagnetic dielectric ${\text{Tb}}_{3}{\text{Fe}}_{5}{\text{O}}_{12}$ garnet. Pronounced magnetic diffuse scattering is observed at high temperatures that gradually decreases with cooling as the Tb spins take on an ordered structure at about 50 K. In the temperature range where the dielectric $(\ensuremath{\epsilon})$ constant is enhanced, a volume striction is observed from the diffraction measurements under a magnetic field that couples the magnetic response to the dielectric properties. At the same time, large oxygen displacements, on the order of $0.1--0.2\text{ }\text{\AA{}}$, are observed starting first at 90 K that persist up to 550 K, resulting in the formation of electric dipoles. However, they are not ordered and the lack of a unique polar axis in the hyperkagome structure may be linked to the absence of a polarized phase in this system. If it were present, the system could have a high ferroelectric transition temperature.