多鐵材料YMnO3薄膜之光譜性質研究

Abstract

我們研究不同結構的多鐵性材料YMnO3薄膜之光譜性質，分別以YSZ基板製成的六角晶系結構與SrTiO3基板製成的正交晶系結構，並探討在光譜上是否可以觀察得到磁電耦合的存在。 首先，比較基板與薄膜樣品的室溫光譜，我們觀察到六角晶系與正交晶系YMnO3薄膜在遠紅外光區皆有數個聲子吸收峰，可分別對應於[Phys. Rev. B 56, 2488 (1997)]及[Physica B 262, 1 (1999)]的第一原理計算結果，在近紅外至紫外光區的吸收峰，六角晶系YMnO3薄膜在1.6、2.4及5 eV，正交晶系YMnO3薄膜在5 eV，顯示著有電子能階躍遷的貢獻，由此印證了YMnO3薄膜在不同的基板下製備會形成不同的結構，同時也會展現各自的光譜性質及物理意義。 其次，我們研究這些樣品的變溫光譜響應，隨著溫度降低，六角晶系YMnO3薄膜在156、202及595 cm-1與正交晶系YMnO3薄膜在584 cm-1的振動模因為熱效應的緣故，頻率位置出現藍移現象且半高寬變窄；正交晶系YMnO3薄膜在450 cm-1處，隨溫度降低有下陷的趨勢，這是由於SrTiO3基板的結構相轉變所致；六角晶系YMnO3薄膜在高頻1.7 eV處，接近相變溫度時，頻率位置之斜率變化較陡峭，此現象可能與錳離子間磁矩之超交換相互作用效應有關。We report the optical properties of multiferroic YMnO3 thin films. Single-crystalline YMnO3 thin films were grown on YSZ in a hexagonal structure and on SrTiO3 substrates in an orthorhombic structure. Our goal is to investigate the couplings of lattice, electronic, and magnetic structures of these materials. The room-temperature infrared spectra of hexagonal and orthorhombic YMnO3 thin films are consistent with the results of first- principles calculations [Phys. Rev. B 56, 2488 (1997)] and [Physica B 262, 1 (1999)]. At higher frequencies, three optical excitations are observed at about 1.6, 2.4, and 5 eV in hexagonal YMnO3 thin film, while an interband transition appears at about 5 eV in orthorhombic YMnO3. This is mainly due to the fact that different substrates induce different crystal structures of YMnO3, and moreover influence their electronic structures. With decreasing temperature, phonon modes at about 156、202 and 595 cm-1 in hexagonal YMnO3 thin film and 584 cm-1 in orthorhombic YMnO3 exhibit blue shift in positions and narrower linewidths. We also notice that a dip appears in the reflectivity at around 450 cm-1 to ascribe the cubic-to-tetragonal phase transition in the SrTiO3 substrate. Near the antiferromagnetic phase transition temperature, the slope of temperature-dependent frequency of 1.7 eV peak becomes sharper in hexagonal YMnO3 thin film, which may be associated with Mn-Mn superexchange interactions.