以氫氣及厚度調控 CoPd, FePd Alloy/Co/[Pt/Co]4/Pt 多層薄膜之磁異向性

Abstract

多年來，對過渡金屬-鈀合金 (TM-Pd) 系統的研究未曾中斷，特別是鐵磁金屬(鐵、鈷、鎳) 與 Pd 的結合能在氫氣環境中對磁性造成改變，這吸引許多人的關注，在理論和實驗上有許多研究被報導出來，但關於確切的機制仍未有定論。我們的研究中，將可吸收氫氣的鐵鈀合金和鈷鈀合金分別鍍在垂直異向性的鈷/白金多層底層上，透過調整鐵鈀合金、鈷鈀合金及白金緩衝層的厚度，可以改變薄膜的磁異向性，進而成長出分別具有水平異向性、傾斜異向性、垂直異向性的合金薄膜。當樣品放置在氫氣環境中，具有傾斜異向性的鐵鈀合金/鈷/白金多層膜，觀察到薄膜吸收氫氣後，薄膜磁易軸由垂直膜面方向往平行膜面方向旋轉，而自旋取向轉變沒有在水平異向性和垂直異向性的鐵鈀合金/鈷/白金多層膜上觀察到，這代表氫氣誘發的自旋取向轉變具有選擇性，只有在具有傾斜異向性的樣品可觀察到樣品磁化方向會由垂直膜面方向往平行膜面方向旋轉，這在先前的文獻中是不曾報導過的。這個發現對於探討氫氣引發的磁性變化機制來說提供了新的方向。另一方面，在鈷鈀合金/鈷/白金多層膜的系統中，多數樣品的磁性為垂直異向性，少數樣品的磁性為傾斜異向性。我們所使用的薄膜厚度，沒有發現具有水平異向性的樣品。在氫氣環境中，此系統樣品沒有觀察到自旋取向轉變。

此外，對垂直異向性的樣品來說，樣品的矯頑力會隨著氫氣吸附和脫附發生可逆性改變，而在水平異向性的樣品中，氫氣吸附沒有對樣品的磁滯曲線的方正度和矯頑力造成改變。但在傾斜異向性的樣品中，樣品矯頑力和方正度會隨著氫氣吸附和脫附發生可逆的變化，透過與文獻的比較，我們試圖解釋造成這些變化的來源，目前文獻中普遍認為氫氣造成的磁性變化可能來源有二，一個是鐵磁金 屬與鈀分別與氫原子鍵結造成了自旋相依的電荷轉移使原子軌域的電子結構發生改變，影響了磁性。另一個是鈀在氫氣吸附後，造成晶格的膨脹，透過磁彈性效應引發垂直異向性的減弱。一般而言，氫氣吸附對晶格的影響不大，但這些文獻討論的系統為單一的合金或多層膜，對比我們的系統來說，結構上相對簡單。氫氣吸附可能對鐵鈀合金或鈷鈀合金與鈷/白金多層膜介面造成額外影響，改變了樣品的應力而引發樣品磁性的改變。 在應用上，透過適當的厚度搭配，鐵鈀合金和鈷鈀合金都能在氫氣環境迅速產生磁性上的變化，對氫氣感測來說是適合候選材料，極具開發價值。

Research into transition metal-palladium (TM-Pd) alloy systems has been ongoing for many years, especially regarding the impact of the combination of ferromagnetic metals (Fe, Co, Ni) with Pd in hydrogen environments. This combination has attracted considerable attention, and both theoretical and experimental studies have been reported, yet the underlying mechanisms remain inconclusive. In our research, we deposited hydrogen-absorbent FePd and CoPd alloys on a perpendicular magnetic anisotropic Co/Pt multilayer. By adjusting the thickness of the FePd, CoPd, and buffer Pt layers, we could alter the film’s magnetic anisotropy, resulting in alloy films with in-plane magnetic anisotropy (IMA), tilted magnetic anisotropy (TMA), and perpendicular magnetic anisotropy (PMA). When these samples were exposed to a hydrogen environment, the FePd/PMA multilayer films with TMA exhibited a clear change in the film’s easy-axis, rotating from the perpendicular direction to the in-plane direction. This rotation was not observed in the FePd/PMA multi-layer films with IMA and PMA, indicating a selective spin reorientation transition (SRT) induced by the hydrogen. This had not been previously reported in the literature, thus providing a new direction for investigating the mechanism of hydrogen-induced magnetic changes.

Additionally, for the PMA samples, the coercivity of the samples exhibited reversible changes upon hydrogen adsorption and desorption. In IMA samples, hydrogen adsorption did not affect the squareness of the magnetic hysteresis loop and coercivity. However, in TMA samples, the coercivity and squareness of the sample exhibit reversible changes with hydrogen adsorption and desorption. By comparing these findings with existing literature, we attempted to explain the sources of these changes. Current literature generally suggests that the magnetic changes induced by hydrogen can originate from two sources. One is the spin-dependent charge transfer caused by the bonding of hydrogen atoms with ferro- magnetic metals and Pd, which alters the electronic structure of atomic orbitals, affecting magnetism. The other is the expansion of the lattice due to palladium after hydrogen adsorption, leading to a weakening of the perpendicular magnetic anisotropy through magnetoelastic effects. In general, hydrogen adsorption has little impact on the lattice, but these systems discussed in the literature are single alloys or multilayer films, which are relatively simple in structure. Hydrogen adsorption may have an additional influence on the interfaces between FePd or CoPd alloys and Co/Pt multilayer films, therefore altering the sample’s stress and thereby causing changes in its magnetism.

In practical applications, with the appropriate thickness combinations, both iron-palladium and cobalt-palladium alloys exhibit rapid magnetic changes in a hydrogen environment, making them promising candidates for hydrogen sensing, with significant development potential.