釔鐵石榴石之表面及塊材磁化強度之非共線行為

Noncollinear Magnetization between Surface and Bulk YIG

吳柏勳(Po-Hsun Wu)

指導教授 ： 黃斯衍

國立臺灣大學/理學院/物理學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201600409

摘要

Y3Fe5O12(YIG)為一種磁性絕緣體材料。近年來，實驗發現藉由外加縱向溫度梯度以及橫向磁場，可產生縱向自旋塞貝克效應(longitudinal spin Seebeck effect, LSSE)並於YIG中驅動純粹自旋電流(pure spin current)。然而純粹自旋電流為一不伴隨傳統電流(charge current)之自旋角動量流(flow of spin angular momentum)，並且YIG為一絕緣體，因此無法直接於YIG上量測純粹自旋電流。此時，利用反自旋霍爾效應(inverse spin Hall effect, ISHE)，則可藉由電性量測之方式探測到純粹自旋電流。為了產生反自旋霍爾效應，一般常見的方法是在YIG表面上鍍一層非磁性重金屬（如：鉑(Pt)、金(Au)、鎢(W)…等材料），由於自旋軌道交互作用(spin-orbit coupling)，由YIG中產生的自旋電流可注入重金屬層並藉由反自旋霍爾效應轉換成傳統電流，然後於此金屬層被探測到。由於鉑(Pt)具有較大之自旋電流與傳統電流之轉換率(Hall angle)，因此Pt/YIG雙層膜結構(bi-layer structure)被廣泛應用於利用縱向自旋塞貝克效應驅動自旋電流之研究。在Pt/YIG結構中，自旋電流在YIG中產生並在Pt經由反自旋霍爾效應被量測到。根據反自旋霍爾效應之描述式，自旋電流沿溫度梯動之方向流動而自旋方向則受控於YIG中之磁矩，因此在外加磁場量測下，於Pt中被量測到的電性訊號應隨YIG之磁矩隨外加磁場之變化而改變。然而，在Pt/YIG系統中由熱傳輸量測到的反自旋霍爾電壓(ISHE)以及由電傳輸量測到的磁阻(Magneto Resistance, MR)對磁場的結果，可於低磁場範圍觀察到一明顯的平台(plateau)行為，和YIG隨磁場變化之磁矩行為(磁滯曲線)並不一致，違反了原先預期之結果。
於本實驗中，利用具高解析度之磁光柯爾效應(magneto-optic Kerr effect, MOKE)量測平台，我們證實此存在於熱、電傳輸測量和磁滯曲線間之不一致性是源自於YIG中表面磁矩和塊材磁矩之非共線性(noncolinear)行為。即使將非磁性金屬Pt替換成磁性金屬Py，非預期之平台行為仍然可於熱、電傳輸量測中被觀察到。此結果顯示了覆蓋於YIG上之金屬層是否具有磁性與平台之存在性無關。此外，藉由研究各種不同製作方法之YIG，可以得知此異常平台行為存在於YIG塊材而不存在於YIG薄膜，並且此現象和YIG為單晶或多晶、具不同晶格方向單晶YIG、及YIG的製作方法皆無關。磁矩之非共線性行為是YIG之本質特性。
另外，在維持相同之量測表面下，減少YIG塊材厚度進行測量，可觀察到表面磁矩行為隨被改變的YIG厚度呈現出有系統的變化，表示在同一塊YIG中雖然具有明顯不同的表面磁矩與塊材磁矩結構，然而此兩者間具交互作用而非完全獨立。更進一步地，在本文中我們將展示YIG之表面磁矩和覆蓋其表面之鐵磁材料間具有遠距離之磁耦合(magnetic coupling)現象，我們推論此耦合機制是由磁偶極矩之交互作用(magnetic dipole-dipole moment)所造成。
近年來，在Pt/YIG雙層膜結構中之自旋電流傳輸訊號被發現可藉由在Pt和YIG間插入一層反鐵磁性(antiferromagnetic, AF)絕緣體NiO而在室溫量測下有明顯的提升(enhancement)。無論純粹自旋流(pure spin current)是以自旋幫浦(spin pumping)或是自旋塞貝克(spin Seebeck effect)之方式於YIG中驅動，皆可觀察到此現象。然而，自旋電流傳輸訊號皆是在NiO為極薄膜(ultra-thin film, ~1nm)時才可觀察得到訊號明顯提升。由於當NiO被製作成極薄膜時，其相對應之尼爾溫度(Neel temperature)遠低於室溫，因此此訊號提升現象可能不是由反鐵磁性之NiO磁矩結構所造成。目前普遍認為可能的物理機制為NiO中之反鐵磁性磁子(AF magnons)與自旋變動(spin fluctuation)造成此一訊號提升現象。
於本實驗中，我們展示即使最上層的自旋探測層替換成鐵磁性的Py，仍然可觀察到自旋電流傳輸訊號提升，表示此提升現象和自旋探測層是否具有磁性無關。另外，我們確認自旋電流傳輸訊號提升現象並非是因為NiO可以產生自旋電流或是NiO可作為自旋電流之探測層。此結果指出自旋電流傳輸訊號提升是透過NiO作為自旋電流傳輸層所造成，而背後的物理機制尚須進一步的研究與調查。

關鍵字

自旋塞貝克效應 ； 非共線性 ； 磁光柯爾效應 ； 自旋電流

並列摘要

Yttrium iron garnet (YIG, Y3Fe5O12) is a magnetic insulator that has been widely used to generate spin-wave spin current via the longitudinal spin Seebeck effect (LSSE). Spin current can be converted to charge current by the inverse spin Hall effect (ISHE) in an attached metal layer with strong spin-orbit coupling, such as Pt. The electric field of ISHE can be described by E(ISHE)∝Js×σ, where Js is the spin current and σ is the spin direction. The direction of spin current is along the applied temperature gradient and the spin direction is given by the magnetization of the YIG. However, both the ISHE voltage from the thermal transport measurement and the magnetoresistance (MR) from the electrical transport measurement of the metal/YIG structure show a clear plateau behavior in the low-field range, which is inconsistent with the magnetization reversal behavior of the YIG slab.
In this work, we provide direct evidences by using the highly sensitive micro-magneto-optic Kerr effect (MOKE) measurement to demonstrate that the plateau behavior in the thermal and electrical transport measurement of metal/YIG is due to the noncollinear magnetization configuration between the bulk and surface of YIG. Even when the normal metal Pt is replaced by ferromagnetic materials, the inconsistent behavior still exists, indicating that it is irrelevant to the magnetism. By conducting measurements in different types of YIG with various growth methods, we found that the unexpected plateau behavior in the electrical, the thermal, and the MOKE signal is due to the noncollinear magnetization in the intrinsic property of the bulk YIG, and is independent of the YIG growth method, chosen substrate, and crystal orientation.
In addition, keeping the measured surface of YIG without being altered, we show that its surface magnetization can be systematically controlled by varying the thickness. We further demonstrate that the magnetic coupling between the surface magnetization of YIG and an attached ferromagnetic layer exhibits the long-range interaction due to the magnetic dipole-dipole interaction.
Recently, spin current driven by spin pumping and spin Seebeck effect in Pt/YIG shows an enhancement when an antiferromagnetic (AF) NiO layer is inserted between YIG and Pt. However, in these reports, the enhancement is clearly observed at room temperature when the NiO is ultra-thin film (~1nm) even with Neel temperature only about 170K. Therefore, it is considered that the AF magnetization ordering is not the dominant role. Possible mechanisms include spin fluctuation and AF magnons are considered to be important.
In this work, we further demonstrate that the spin transport enhancement is independent of the top capping layer, normal metal or ferromagnetic metal. In addition, we show the inserted NiO layer cannot be either a generator or a detector of spin current; therefore, the spin transport enhancement is due to the transmission of spin current through NiO layer. Even we can exclude some possibilities for the role of NiO, the clear physical mechanism for this enhancement needs further investigation.

並列關鍵字

spin current ； spin Seebeck effect ； magneo-optic Kerr effect ； noncollinear ； magnetic dipole-dipole interaction

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