鐵電效應被引入有機自旋閥中可控制磁輸運，界面的能量分佈和界面晶體結構對於界面處的自旋傳輸都是至關重要的。
美國內布拉斯加州立大學Xiaoshan Xu副教授對採用鐵電效應控制有機自旋閥的磁阻的最新研究進展進行了綜述，發表在Journal of Materiomics第4卷第1期，題目為A brief review of ferroelectric control of magnetoresistance in organic spin valve。免費下載地址：https://www.sciencedirect.com/science/article/pii/S2352847817300813#sec6！內容梳理
通過“三明治”結構可實現巨大的磁阻效應，即一個非磁性（NM）隔離層夾在兩個鐵磁（FM）電極之間。根據隔離層材料和厚度，自旋閥通常分為3種，1）隔離層為金屬；2）隔離層為一塊厚的半導體；3）隔離層為一塊薄的絕緣體。有機自旋閥通常屬於後2種分類。有機自旋閥具有壽命長、柔性佳、環境友好以及各式各樣化學組合的大量有機物可作為候選材料等優點。
Fig. 1. (a) Schematics of an FM/NM/FM trilayer spin valve. (b) and (c) are the magnetic-field dependence of resistance of the spin valve for positive (normal) and negative (inverse) MR respectively. (d) Schematics of the electronic structure of the FM and NM materials, where n↑and n↓ are the number of states of the up spin and down spin respectively.
為理解擴散自旋閥的磁阻效應，先討論了鐵磁和非鐵磁材料的界面電阻。單純增加NM電阻不會顯著改變總界面電阻。即存在阻抗不匹配問題。
Fig. 2. Calculated spatial dependence of the electrochemical potential and the normalized current in the two-current model. (a) and (b) are near a FM/NM interface. (c) and (d) are for a trilayer spin valve when the spin polarization of the two FM layers are aligned. (e) and (f) are for a trilayer spin valve when the spin polarization of the two FM layers are anti-aligned.
對於隧道自旋閥不存在阻抗不匹配問題。與擴散自旋閥相比，隧道自旋閥具有更大的磁阻效應。垂直傳輸的有機旋轉閥需要解決的一個重要問題就是有機層和頂部電極之間的邊界互擴散問題。
作者利用以上原理介紹並分析了有機自旋閥中利用鐵電性控制磁阻。鐵電材料極化的轉換既涉及原子的位移，也涉及電荷的相應轉換；兩者均可用於主動控制界面性質。
Fig. 3. (a) Spatial dependence of the electric potential caused by a ferroelectric film (indicated using the charge distribution). (b) and (c) are the charge distribution and the vacuum electric potential of a metal/FE/metal junction. (d) Energy level bending and shift at the metal/FE/metal junction. The lower curve represents the metal states that are at the Fermi energy far away from the FE.
Fig. 4. The schematics of the charge transport across a metal/OSC/metal junction. (a) When the Fermi energy of the metal is close to the LUMO of OSC, electron transport is favored. (b) When the Fermi energy of the metal is close to the HOMO of OSC, hole transport is favored.
鐵電材料對界面的影響可分為2類：能帶分佈的靜電變化和極化反轉引起的界面電子結構變化。
Fig. 5. MR of the LSMO/PZT/Alq3/Co junction. (a)–(c) are the MR measured at 0.1, −0.2, and −1.0 V respectively when a 1.2 or a −1.2 V initial voltage is applied. (d) Is the MR as a function of measurement voltage when a 1.2 or a −1.2 V initial voltage is applied. (e) Is the schematic diagram indicating that the shift of vacuum level and the change of effective voltage on the OSC caused by the FE dipole of PZT.
Fig. 7. MR of the LSMO/PVDF/Co junction. (a) MR measured when the PVDF polarization is pointing toward Co and toward LSMO respectively. (b) Resistance of the LSMO/PVDF/Co junction as a function of the poling voltage. (c) Tunneling MR as a function of the poling voltage. (d) Tunneling MR as a function of measurement voltage for 1.2 and −1.5 V poling voltages respectively. (e) Tunneling MR as a function of temperature for 1.5 and −1.5 V poling voltages respectively. Reproduced with permission from Ref. [34].
Fig. 8. Crystal structure at the Co/PVDF interface when the polarization is pointing away (a) and toward (b) Co respectively. Reproduced with permission from Ref. [34].
展望
有機自旋閥中自旋輸運的確切機制尚不明晰，因此需在更多類似器件上採用不同鐵電材料進行研究工作，以及對界面晶體結構和界面電子結構的進一步表徵，可以幫助闡明鐵電效應以及其他伴隨效應，例如電極的磁性變化以及輸運機制變化。同時，隨著有機自旋電子學領域的不斷髮展，鐵磁性/有機界面的自旋極化仍將是研究焦點。大量通過改變能級排列或界面晶體結構來調整界面自旋的研究有望得以開展。這些研究不僅對有機自旋電子學很重要，對於調整涉及有機/無機界面的其他電子設備（例如發光二極管和光伏電池）也將有重要價值。
摘要
Magnetoelectric coupling has been a trending research topic in both organic and inorganic materials and hybrids. The concept of controlling magnetism using an electric field is particularly appealing in energy efficient applications. In this spirit, ferroelectricity has been introduced to organic spin valves to manipulate the magneto transport, where the spin transport through the ferromagnet/organic spacer interfaces (spinterface) are under intensive study. The ferroelectric materials in the organic spin valves provide a knob to vary the interfacial energy alignment and the interfacial crystal structures, both are critical for the spin transport. In this review, we introduce the recent efforts of controlling magnetoresistance of organic spin valves using ferroelectricity, where the ferroelectric material is either inserted as an interfacial layer or used as a spacer material. The realization of the ferroelectric control of magneto transport in organic spin valve, advances our understanding in the spin transport through the ferromagnet/organic interface, and suggests more functionality of organic spintronic devices.
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