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Hexagonal Ferroelectricity and Multiferroism in Rare-Earth Ferrites Thin Films

Hexagonal Ferroelectricity and Multiferroism in Rare-Earth Ferrites Thin Films
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Multiferroics are an interesting group of materials that have ferroelectric, ferromagnetic, and ferroelastic order parameters simultaneously. Multiferroic materials are currently the subject of intensive scientific investigation because these materials potentially offer a whole range of new applications, such as multiple state nonvolatile memories, ferromagnetic spin devices and magnetic data storage. However, the current research activities of single-phase multiferroics are limited to a few oxides because transition-metal d electrons reduce the tendency of an off-centering ferroelectric distortion. The scarcity of room-temperature multiferoics has led many workers to investigate the materials for having both electric and magnetic order. Considering the rareness of multiferroic materials, we propose the possibility of artificially imposing new multiferroics by a structural modification. First, we have carried out research on the artificially imposed hexagonal ferroelectricity in canted antiferromagnetic YFeO3 epitaxial thin films. Antiferromagnetic YFeO3, a family of centrosymmetric orthoferrites, is known to be nonferroelectric (space group Pbnm). In the present study, contrary to this common knowledge, that an epitaxial YFeO3 thin film fabricated by adopting a hexagonal template is characterized by a six-fold hexagonal symmetry and exhibits ferroelectricity with the ferroelectric Curie temperature (Tc) of ~460 K. According to first-principles calculations, the P63/mmc-P63cm structural phase transition at Tc by the freezing-in of the zone-boundary K3 phonon is primarily responsible for the observed room-temperature ferroelectricity and the asymmetric Y 4dz2 -O 2pz hybridization along the c-axis of P63cm over the Γ-M first Brillouin zone is the electronic origin of this artificially imposed ferroelectricity. Seconds, in choosing a rare-earth cation for the purpose of artificially imposing hexagonal ferroelectricity using a ReFeO3 type orthoferrite, we proposed another multiferroic material, epitaxially grown TmFeO3 thin film. TmFeO3 is also known to be characterized by the FeO6 octahedral conner-linked to form a three-dimensional network in an orthorhombic unit cell (space group Pbnm). Thus, it is expected to be non-ferroelectric. Here we show that a heteroepitaxial TmFeO3 thin film fabricated by adopting a hexagonal template surprisingly exhibits ferroelectricity with the Curie temperature (Tc) of 430K. On the basis of our experimental results and first principle calculations demonstrate that the spontaneous electric polarization along the c-axis is induced by absence of mirror symmetry (P63cm) parallel to the ab-plane. The temperature-dependent magnetization of the h-TFO film show a rapid increase in the magnetization beginning at ~120 K which is considered the AFM ordering Neel temperature. The magnetization-field (M-H) hysteresis curves measured at various temperatures reveal that the conventional weak ferromagnetic character is shown below TN. Finally, we have investigated the multiferroic properties of hexagonal rare-earth ferrites on the basis of our previous work. Consequently, h-ReFeO3 (Re=Y, Sc, Tm, Yb, Lu) has possessed different ferroelectric transition temperature (Tc), remnant polarization values (Pr) and unique physical characteristics depending on types of Re ions.
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