Ferroelectric Single-Molecule Magnet with Toroidal Magnetic Moments.

Materials that coexist magnetic and electric properties on the molecular scale in single-molecule magnets (SMMs) with peculiar quantum behaviors have promise in molecular electronics and spintronics. Nevertheless, such molecular materials are limited in potentials because their magnetic signal cannot be transformed into an electrical signal through magnetoresistance or Hall effects for their high insulativity. The discovery of an entirely new material, ferroelectric SMMs (FE SMMs) is reported. This FE SMM also shows single-molecule magnetic behaviors, toroidal magnetic moments, and room-temperature ferroelectricity. The toroidal moment is formed by a vortex distribution of magnetic dipoles in triangular Dy3 clusters. The analysis of ac magnetic susceptibility reveals the coexistence of three distinct magnetic relaxation processes at low temperatures. The ferroelectricity is introduced by incorporating polar alcohol molecules in the structure, which is confirmed by the X-ray diffraction and optical second harmonic generation (SHG) measurements. Moreover, the dielectric measurements reveal a ferroelectric-to-ferroelectric phase transition around 150 K due to the symmetry change from Pc to Pna21 . The coexistence of toroidal moment and ferroelectricity along with quantum magnetism in the rare-earth single-molecule magnets yields a unique class of multiferroics.

Tunable electronic structure and magnetic anisotropy in bilayer ferromagnetic semiconductor Cr2Ge2Te6.

The emergence of ferromagnetism in two-dimensional van der Waals materials has aroused broad interest. However, the ferromagnetic instability has been a problem remained. In this work, by using the first-principles calculations, we identified the critical ranges of strain and doping for the bilayer Cr2Ge2Te6 within which the ferromagnetic stability can be enhanced. Beyond the critical range, the tensile strain can induce the phase transition from the ferromagnetic to the antiferromagnetic, and the direction of magnetic easy axis can be converted from out-of-plane to in-plane due to the increase of compressive strain, or electrostatic doping. We also predicted an electron doping range, within which the ferromagnetism can be enhanced, while the ferromagnetic stability was maintained. Moreover, we found that the compressive strain can reverse the spin polarization of electrons at the conduction band minimum, so that two categories of half-metal can be induced by controlling electrostatic doping in the bilayer Cr2Ge2Te6. These results should shed a light on achieving ferromagnetic stability for low-dimensional materials.

Direct evidence of correlation between the second harmonic generation anisotropy patterns and the polarization orientation of perovskite ferroelectric.

For ferroelectric materials, where the polar state breaks the inversion symmetry, second harmonic generation is a useful tool to prove their ferroelectric properties. However, the correlation between the anisotropy patterns and the polarization orientation of the ferroelectric domains has not been clarified yet. In this work, we systematically investigated this correlation in a typical perovskite oxide ferroelectric, Barium Titanate (BaTiO3) crystal, by second harmonic generation and the piezoresponse force microscopy technique. The evolution of polarization-dependent anisotropy patterns proves that there is a linear relationship between the rotation angle of second harmonic generation anisotropy patterns and the polarization angle of BaTiO3 single crystals. It is a direct evidence illustrating that the polarization of BaTiO3 crystal can be qualitatively identified in 0°-180° by second harmonic generation technology. This work gives a glance at improving a nonintrusive and convenient method to identify the polarization of perovskite ferroelectric materials.

Evolution of structural distortion in BiFeO3 thin films probed by second-harmonic generation.

BiFeO3 thin films have drawn much attention due to its potential applications for novel magnetoelectric devices and fundamental physics in magnetoelectric coupling. However, the structural evolution of BiFeO3 films with thickness remains controversial. Here we use an optical second-harmonic generation technique to explore the phase-related symmetry evolution of BiFeO3 thin films with the variation of thickness. The crystalline structures for 60 and 180-nm-thick BiFeO3 thin films were characterized by high-resolution X-ray diffractometry reciprocal space mapping and the local piezoelectric response for 60-nm-thick BiFeO3 thin films was characterized by piezoresponse force microscopy. The present results show that the symmetry of BiFeO3 thin films with a thickness below 60 nm belongs to the point group 4 mm. We conclude that the disappearance of fourfold rotational symmetry in SHG s-out pattern implies for the appearance of R-phase. The fact that the thinner the film is, the closer to 1 the tensor element ratio χ31/χ15 tends, indicates an increase of symmetry with the decrease of thickness for BiFeO3 thin films.