鑭系錳氧化物之光譜及磁力顯微術研究

Abstract

我們研究不同厚度之La0.7Sr0.3MnO3薄膜與Nd0.5Ca0.5-ySryMnO3 (y = 0.08及0.09)塊材的光譜響應，分別探討薄膜結構上之奈米顆粒尺度，及薄膜與基板間晶格不匹配的效應、改變二價離子的平均半徑、溫度與外加磁場變化，對錳氧化物的晶格與電子結構造成的影響。 首先，我們觀察到當La0.7Sr0.3MnO3薄膜厚度超過30 nm時，由於奈米顆粒尺寸增大，遠紅外光譜呈現出與塊材相近的聲子振動響應，並在中紅外光區出現 的電子躍遷吸收；而低溫光譜顯示隨著溫度降低，300 nm厚度之La0.7Sr0.3MnO3薄膜的居德權重增加，中紅外光區極化子的束縛能逐漸下降，顯示樣品在低溫下的導電性漸增，與極化子響應有緊密的關連性，同時也印證了磁力影像上所觀察到鐵磁態磁區隨溫度降低而成長的情形。 另外，Nd0.5Ca0.5-ySryMnO3在些微Sr摻雜量的改變後，造成了光譜上極大的差異性。y = 0.08樣品在降溫過程中，由於電荷有序性現象，在190 K出現了金屬－絕緣的相轉變現象，並在490 cm-1左右形成了一個能隙，而y = 0.09樣品則一直保持為絕緣態，能隙隨著溫度愈低而增大，但在3 T以上的外加磁場時，卻破壞了其原來的電荷及軌道有序性所造成的絕緣態，展現了金屬態的行為。

We report a systematic study on the thickness, temperature, and magnetic field dependences of the optical properties of La0.7Sr0.3MnO3 thin films and polycrystalline Nd0.5Ca0.5-ySryMnO3 (y = 0.08 and 0.09). The magnetic force microscope (MFM) images show the stripe magnetic domains, characteristic of 300-nm-thick of La0.7Sr0.3MnO3 films with the perpendicular magnetic anisotropy. Optical reflectance and transmittance measurements provide evidence that there are significance changes of the peak position of the infrared phonon modes and the charge transfer absorption band as a function of thickness. With decreasing temperature, the Drude response of the 300-nm-thick La0.7Sr0.3MnO3 film increases, along with the small polaron binding energy decreases, suggesting that the polaron correlation effect plays an importance role on the transport properties of La0.7Sr0.3MnO3. On the other hand, with decreasing temperature from 300 to 20 K in zero field, the spectral weight of the optical conductivity for both Nd0.5Ca0.5-ySryMnO3 (y = 0.08 and 0.09) samples is suppressed below 0.1 eV, indicating an opening of the charge gap in the charge ordered state. The application of magnetic fields up to 17 T produces no discernible field dependence. Only after ten cyclic runs between 300 and 77 K, the large spectral weights are transferred from high- to low-frequency regions with increasing fields. Additionally, the internal bending and stretching phonon modes of the MnO6 octahedra show a noticeable hardening and broadening above the particular fields. The observed changes in the electronic and phononic excitations of the optical spectra with the external magnetic fields are attributed to a melting of the cooperative charge and orbital orderings in these materials.