補償亞鐵磁中非共線磁相激發之磁振子自旋電流

Abstract

磁性絕緣體在自旋電子學扮演著重要的角色，正因它不導電的特性，使得在自旋相關的效應發生的時候不受電荷電流的影響，能夠體現自旋電流的完整特性。稀土鐵石榴石作為磁性絕緣體的一員，在與不同的稀土元素組合下提供了十分多樣的特性變化，舉例來說，釔鐵石榴石具有小的磁阻尼與高熱穩定性，這使得它在自旋擴散和自旋傳輸方面具有一定的潛力，銩鐵石榴石則是被發現有拓樸相關的特性，在磁場下能夠產生拓樸霍爾效應和磁泡。釓鐵石榴石，作為我們主要的研究材料，不僅具有亞鐵磁中補償點的特性，其發生的溫度也相對來的高 (塊材補償溫度~287 K) ，由於磁矩不同的溫度依賴性，讓亞鐵磁體可以在補償溫度時形成類似反鐵磁體的表現，因此補償溫度附近所發生的現象也是熱門的研究對象。 除此之外，透過不同基板的選擇，稀土鐵石榴石可以透過應力誘發異向性在單層膜的結構中形成垂直異向性。在結構上，釓鐵石榴石擁有三個次晶格，分別是c-site Gd, a-site Fe, d-site Fe,其中的兩個鐵次晶格因為強反鐵磁交換耦合作用成反平行排列，釓和鐵則是擁有較弱的交換耦合作用。比起鐵磁體，亞鐵磁體中的磁矩呈現更為複雜的行為，我們透過變溫的熱激發自旋波測量，也就是自旋賽貝克效應作為探測工具，研究了釓鐵石榴石在磁矩共線與非共線態的磁振子激發，在非常規自旋賽貝克效應中展示了它的貢獻，並加以定性分析其與異向性之關聯。此外，我們在變溫自旋賽貝克效應重現了兩個訊號反轉點, 分別是由自旋波譜中磁振子模式的主導權轉換和跨過補償溫度Tcomp時造成的次晶格反轉導致，此現象也存在於磁矩非共線態的情況,顯示了磁振子模式在低溫轉變的一致性。 結論來說，我們觀察到了補償亞鐵磁體中多變的磁矩行為，並發現在低溫時施加磁場於難軸出現的非共線態。對於亞鐵磁體內部的複雜行為有了進一步的了解，也能進一步期望將特性運用在更多磁性應用上。

Magnetic insulators play a crucial role in spintronics due to their non-conductive nature, allowing spin-related effects to occur independently of charge currents. This property enables the full manifestation of spin currents without being affected by electric charges. The rare earth iron garnets (REIGs), as one type of magnetic insulators, offer a wide range of characteristic variations when combined with different rare earth elements. For example, YIG (Yttrium Iron Garnet, Y3Fe5O12) exhibits low magnetic damping and high thermal stability, making it potentially promising in spin diffusion and spin transport. On the other hand, TmIG (Thulium Iron Garnet, Tm3Fe5O12) has been found to possess topologically related properties, with topological Hall effect and magnetic bubble domain features under field. GdIG (gadolinium Iron Garnet, Gd3Fe5O12), as our main research material, not only has the characteristics of a ferrimagnetic compensation point, but also exhibits a relatively high compensation temperature (bulk GdIG:Tcomp ~287 K). Due to the different temperature dependence of magnetic moments, ferrimagnets can exhibit characteristics similar to antiferromagnetic behavior at compensation temperature. Therefore, phenomena occurring near the compensation temperature are also a popular subject of study. In addition, through the selection of different substrates, REIGs can form perpendicular anisotropy (PMA) in the structure of a single-layer film through stress induce anisotropy. In the structure of GdIG (Gd3Fe5O12), it possesses three sublattices: c-site Gd, a-site Fe, and d-site Fe. Two of the Fe sublattices exhibit an antiparallel arrangement due to strong antiferromagnetic exchange coupling, while Gd and Fe have weaker exchange coupling between them. Because of this, compared with ferromagnets, the magnetic moment in ferrimagnets exhibits more complex behavior. We utilized thermally excited spin waves, which are Spin Seebeck Effect (SSE) measurements to study the magnon excitation in the collinear and non-collinear configuration of GdIG. Its contribution is demonstrated through unconventional SSE, and a qualitative analysis of its relationship with anisotropy is conducted. Additionally, in the temperature-dependent SSE, we reproduced a double sign change. These changes is attributed to the domination shifts of magnon modes within the spin wave spectrum and the sublattice reversal occurring across the compensation temperature Tcomp. This phenomenon is also present in the non-collinear state, demonstrating the consistency of the magnon mode transition at low temperatures. In conclusion, we observed diverse magnetic moment behaviors in compensating ferrimagnets and studied the magnon excitations from both the collinear and non-collinear magnetic configurations of GdIG. This deeper understanding of the complex behaviors within ferrimagnets may potentially expand their utility in various magnetic applications.