多鐵奈米材料TbMn2O5之尺寸效應與磁相變交互作用研究

The interplay between size effect and magnetic ordering in TbMn2O5 nanorods

黃緯倫(Wei-Lun Huang)

指導教授 ： 楊仲準

中原大學/理學院/物理研究所/碩士(2011年)

https://doi.org/10.6840/cycu201100619

摘要

我們以水熱合成法(hydrothermal synthesis)製備TbMn2O5 nanorods 並經過高溫熱退火製備出不同尺寸與
不同長寬比之樣品。同時以固態反應法(conventional solid-state reaction)製備塊材樣品以作為比較。首先我們利用
X-ray 跟 FE-TEM 鑑定晶格結構與尺寸大小，接著以PPMS 量測其磁性質，除此之外，我們還量測了0.3 K ~ 60 K 的比熱，以觀察其豐富的相變。
我們藉由FE-TEM 照片統計出nanorods 的尺寸分別為：
33(8) nm×6(2) nm、48(9) nm×13(3) nm、
65(10) nm×17(4) nm 以及 209(45) nm×98 nm，並發現
長軸沿著c-axis 成長。其中僅有209(45) nm×98 nm 之nanorod 跟 bulk 在40 K 處觀察到反鐵磁特徵的磁化率峰，並且此兩個樣品的Néel temperature 以及沿c-axis的
Mn4+Oct -O(2)鍵長彼此接近而遠小於其他尺寸的nanorods。這指出了沿c-axis 的磁性質影響著nanorods 的成長方向，並存在一臨界尺寸介於65 ~ 209(nm)之間。而比熱量測結果說明在40 K 的磁化率峰其來自於43 K 與38 K 的兩個相變，並且在約7 K 以下觀察到Tb3+的磁矩緩慢凍結比熱貢獻，但是，只有bulk 在24 K 處有反鐵磁commensurate 轉incommensurate 的相轉變訊號。

關鍵字

比熱 ； 鋱 ； 多鐵奈米

並列摘要

Series of TbMn2O5 nanorods was synthesized by hydrothermal method.Different sizes and the aspect ratio of nanorods were growth with different annealing temperatures. The bulk sample was prepared by conventional solid-state reaction for comparison. X-ray and FE-TEM were used to characterize the crystal structure,
appearances, and sizes. The ACMS option of PPMS was used to study magnetic properties. The specific heat capacity of nanorods and bulk were also taken to determinate the phase transition between 0.3 K and 60 K.
Four sizes and the appearance of TbMn2O5 nanorods were identified by FE-TEM images and labeled in length × width as 33(8) nm×6(2) nm,
48(9) nm×13(3) nm, 65(10) nm×17(4) nm and
209(45) nm×98 nm. A magnetic susceptibility peak
appeared at around 40 K only in
209(45) nm×98(20) nm nanorods and bulk. SAED
experiments also revealed the long axis of the nanorod is corresponding to the c-axis of the crystal. We also found the Néel temperature is correlated with the length of Mn4+Oct -O(2) bound length along the c-axis. This implies the size effect is controlled by the magnetic behavior of TbMn2O5 nanorods system and the critical size of TbMn2O5 nanorods is between 65 nm and 209 nm along c-axis. The heat capacity
measurement revealed the magnetic susceptibility peak at ~40 K is composed with two phase transitions at ~43 K and ~38 K. A broad peak of specific heat measurement showing the slow frozen process on Tb3+ was observed below ~7 K in both 209(45) nm×98(20) nm nanorods and bulk. The commensurate to incommensurate AFM
transition was only appeared in the bulk sample at ~24 K.

並列關鍵字

TbMn2O5 nanorods ； specific heat ； magnetic susceptibility

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