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Register a new userOrigin of ferroelectricity in high-$T_c$ magnetic ferroelectric CuO
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Cupric oxide is a unique magnetic ferroelectric material with a transition temperature significantly higher than the boiling point of liquid nitrogen. However, the mechanism of high-T$_c$ multiferroicity in CuO remains puzzling. In this paper, we clarify the mechanism of high-T$_c$ multiferroicity in CuO, using combined first-principles calculations and an effective Hamiltonian model. We find that CuO contains two magnetic sublattices, with strong intrasublattice interactions and weakly frustrated intersublattice interactions, which may represent one of the main reasons for the high ordering temperature of the compound. The weak spin frustration leads to incommensurate spin excitations that dramatically enhance the entropy of the mutliferroic phase and eventually stabilize that phase in CuO.
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Magnetic ferroelectric has been found in a wide range of spiral magnets. However, these materials all suffer from low critical temperatures, which are usually below 40 K, due to strong spin frustration. Recently, CuO has been found to be multiferroic at much higher ordering temperature ($sim$ 230K). To clarify the origin of the high ordering temperature in CuO, we investigate the structural, electronic and magnetic properties of CuO via first-principles methods. We find that CuO has very special nearly commensurate spiral magnetic structure, which is stabilized via the Dzyaloshinskii-Moriya interaction. The spin frustration in CuO is relatively weak, which is one of the main reasons that the compound have high ordering temperature. We propose that high $T_c$ magnetic ferroelectric materials can be found in double sublattices of magnetic structures similar to that of CuO.
The successful theoretical prediction and experimental demonstration of hybrid improper ferroelectricity (HIF) provides a new pathway to couple octahedral rotations, ferroelectricity, and magnetism in complex materials. To enable technological applications, a HIF with a small coercive field is desirable. We successfully grow Sr3Sn2O7 single crystals, and discover that they exhibit the smallest electric coercive field at room temperature among all known HIFs. Furthermore, we demonstate that a small external stress can repeatedly erase and re-generate ferroelastic domains. In addition, using in-plane piezo-response force microscopy, we characterize abundant charged and neutral domain walls. The observed small electrical and mechanical coercive field values are in accordance with the results of our first-principles calculations on Sr3Sn2O7, which show low energy barriers for both 90{deg} and 180{deg} polarization switching compared to those in other experimentally demonstrated HIFs. Our findings represent an advance towards the possible technological implemetation of functional HIFs.
Ferroelectric hafnia is being explored for next generation electronics due to its robust ferroelectricity in nanoscale samples and its compatibility with silicon. However, its ferroelectricity is not understood. Other ferroelectrics usually lose their ferroelectricity for nanoscopic samples and thin films, and the hafnia ground state is non-polar baddeleyite. Here we study hafnia with density functional theory (DFT) under epitaxial strain, and find that strain not only stabilizes the ferroelectric phases, but also leads to unstable modes and a downhill path in energy from the high temperature tetragonal structure. We find that under tensile epitaxial strain $eta$ the tetragonal phase will distort to one of the two ferroelectric phases: for $eta > 1.5$%, the $Gamma^{-}_{5}$ mode is unstable and leads to oII , and at $eta > 3.75$% coupling between this mode and the zone boundary M1 mode leads to oI. Furthermore, under compressive epitaxial strain $eta < 0.55$% the ferroelectric oI is most stable, even more stable than baddeleyite.
Remarkably high values of polarization as well as a significant magnetic susceptibility have been observed in multiferroic Bismuth Ferrite (BFO) in the form of nanorods protruding out. These were developed on porous Anodised Alumina (AAO) templates using wet chemical technique. Diameters of nanorods are in the range of 20-100 nm. The high values of polarization and magnetic susceptibility are attributed to the BFO nanorod structures giving rise to the directionality. There is no leakage current in P-E loop examined at various frequencies. Magnetocapacitance measurements reflect a significant enhancement in magnetoelectric coupling also.
High resolution ultrasonic velocity measurements have been used to determine the temperature -- magnetic-field phase diagram of the monoclinic multiferroic CuO. A new transition at TN3 = 230 K, corresponding to an intermediate state between the antiferromagnetic non-collinear spiral phase observed below TN2 = 229.3 K and the paramagnetic phase, is revealed. Anomalies associated with a first order transition to the commensurate collinear phase are also observed at TN1 = 213 K. For fields with B along the b axis, a spin-flop transition is detected between 11 T - 13 T at lower temperatures. Moreover, our analysis using a Landau-type free energy clearly reveals the necessity for an incommensurate collinear phase between the spiral and the paramagnetic phase. This model is also relevant to the phase diagrams of other monoclinic multiferroic systems.
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