鈷鈀合金在氫化效應下導致可逆性長程有序磁排列

Abstract

不同比例及厚度下的鈷鈀合金長在藍寶石基板(Al_2 O_3 (0001))上，觀察樣品氫化效應對磁性的影響。 樣品皆在超高真空系統下(〖10〗^(-8) torr)利用熱蒸鍍原理將鈷及鈀兩金屬對鍍形成合金，鍍完成後用歐傑能譜儀(Auger Spectrum)測量其成分比例、用磁光柯爾效應(MOKE)測量氫氣吸附後的磁光特性改變、原子力顯微鏡(AFM)觀測表面結構對於氫氣吸附所改變的磁性行為，接著在低溫下用超導量子干涉震動磁量儀測量磁的特性。 鈀吸附氫氣後會變成氫化鈀，隨著曝的氫氣量愈來愈多，氫化鈀晶格常數從原本的 3.89 Å (α-phase)上升至 4.02 Å(β-phase)。樣品固定Pd的鍍量且Pd比例為33 %時，曝完氫氣後其磁光特性並沒有明顯變化，隨著鈀金屬比例升高至61 % 時，曝完氫氣後，樣品磁滯曲線的矯頑力、殘磁比及光訊號皆會改變。將鈀的比例提升至 76& 及 86 %時，矯頑力上升10倍；磁滯曲線的飽和磁化量及殘磁的比值(squareness = M_r/M_s )也在曝完40 mbar氫氣2-3秒內從10% 上升至100%。這是因為當鈀的比例變高而磁性金屬鈷的比例下降時，合金的磁矩排列從原本的長程無序狀態下，氫化後變成長程有序排列的稀磁性金屬特性。固定Pd比例(61 %)下改變樣品厚度，發現20奈米厚度的樣品，其吸附氫氣後的磁光效應改變比10奈米來的明顯。除此之外，當氫氣被機械幫浦抽出時，樣品會在2~3秒內從β-phase回到α-phase，且該反應是可以被重複的。 用原子力顯微鏡觀察 Co_14 Pd_86 樣品的表面形貌，其表面上有許多直徑約100奈米的團簇，團簇中間充滿著直徑約20奈米的顆粒，這些顆粒除了可以增加樣品接收氫氣的表面積外，還可以形成不同的截面以增加氫氣吸收的效率。

The hydrogenation effect on the various thickness Co_(1-x) Pd_x/Al_2 O_3 (0001) films investigated the magnetic and reversible properties. The concentration of Co and Pd was determined by Auger electron spectrum. The magnetic properties were measured using Magneto-Optical Kerr Effect (MOKE) and Superconducting Quantum Interference Device Vibrating Sample Magnetometer (SQUID). The morphology was measured using AFM ex-situ. Palladium were transferred into Palladium hydride (PdH_x) and the lattice constant was expanded during hydrogen adsorption and absorption process. The hydrogenation effect were unobvious in Co-rich (Co_67 Pd_33) sample. When the concentration of Pd was increased to 86 %, the magnetic coercivity was enhanced 10 times. Moreover, the squareness of hysteresis loop was also enhanced from 10 % to 100 % after exposed 40 mbar of hydrogen gas within 2-3 sec. This behavior might cause from dilute magnetic material with long-range magnetic ordering. Besides, the efficiency of hydrogenation effect was influenced by different thickness of Co_39 Pd_61 alloy. After removed the hydrogen atoms by mechanical pump, the shape of hysteresis loops returned from β-phase to α-phas during 2-3 sec. This behavior means that the hydrogenation process in palladium is reversible.

Atomic force microscopy (AFM) figure shows the Co_0.14 Pd_0.86 alloy’s morphology, which has many 100 nm diameter nano-clusters and mounts of 20 nm nano-dots at nearby clusters can not only enhance the interaction surface but also create the different surface cross section. Both of properties can improve the hydrogenation effect in CoPd alloy.