“自旋過濾”現(xiàn)象是石墨烯的量子力學(xué)屬性與晶體鎳薄膜的量子力學(xué)屬性之間相互作用的結(jié)果。當(dāng)晶體鎳與石墨烯結(jié)構(gòu)對(duì)齊時(shí)，只有向同一個(gè)方向自旋的電子容易從一種材料穿越至另一種材料（即自旋過濾效應(yīng)），從而產(chǎn)生自旋極化電流。
在一組跨學(xué)科科學(xué)家團(tuán)隊(duì)的協(xié)助下，美國海軍研究實(shí)驗(yàn)室（NRL）成功利用鎳-石墨烯-鐵薄膜連接設(shè)備，在室溫環(huán)境下演示了金屬自旋過濾效應(yīng)。電子除了帶電之外，還有另一種基本屬性——自旋。未來，這種屬性很有可能在數(shù)據(jù)的傳輸、處理與存儲(chǔ)領(lǐng)域發(fā)揮重要作用。另外，NRL實(shí)驗(yàn)室首次在室溫環(huán)境下取得突破，這也為該技術(shù)的廣泛應(yīng)用奠定了一定基礎(chǔ)。
“過去，理論分析與實(shí)驗(yàn)結(jié)果均說明，自旋過濾僅存在于低溫下的高阻抗結(jié)構(gòu)之中。”NRL實(shí)驗(yàn)室材料科學(xué)與技術(shù)部首席研究員Enrique Cobas博士表示，“但最新實(shí)驗(yàn)結(jié)果證實(shí)，自旋過濾效應(yīng)也存在于一系列室溫下的超低阻抗設(shè)備中。”
在低溫和室溫環(huán)境下，薄膜連接點(diǎn)均表現(xiàn)出低電阻及自旋過濾界面的磁電阻特性?？紤]到理論中的自旋過濾效應(yīng)，研究小組還開發(fā)了一個(gè)設(shè)備模型，利用自旋電流轉(zhuǎn)換處理金屬自旋向下通道，并最終發(fā)現(xiàn)石墨烯層的自旋極化至少達(dá)到了80%。
Cobas說，“石墨烯一直以其非凡的平面特性著稱，但我們想去研究層疊石墨烯層的導(dǎo)電性及其與其他材料的反應(yīng)特性。”
為此，NRL實(shí)驗(yàn)室的研究人員想出一種方法，可直接在光滑的晶體鎳合金薄膜上“種植”大片的多層石墨烯薄膜，并在此過程中同時(shí)保留鎳薄膜的磁性，使其形成連接點(diǎn)陣列。
Cobas說，“我們也想證明，我們還可以利用行業(yè)通用的工具制造這些設(shè)備，而并不需要打造專門的工具。”
NRL實(shí)驗(yàn)室材料科學(xué)與技術(shù)分部研究科學(xué)家Olaf van't Erve博士表示，理論證明，改變石墨烯薄膜的層數(shù)還可以將自旋效應(yīng)提升一個(gè)量級(jí)，因此該設(shè)計(jì)還有一定優(yōu)化空間。
“但是，目前的模型并未考慮鐵磁觸點(diǎn)內(nèi)的自旋轉(zhuǎn)換現(xiàn)象，”他說，“一旦我們將這些效應(yīng)也考慮在內(nèi)，那幾乎就可以達(dá)到理想狀態(tài)，將石墨烯層的自旋極化提升至100%。這樣一來，我們就可以改變?cè)O(shè)備的幾何形狀與材料選擇，盡最大可能增強(qiáng)這種效應(yīng)。”
石墨烯技術(shù)的進(jìn)展可能將同時(shí)推動(dòng)下一代非易失性磁性隨機(jī)存儲(chǔ)器（MRAM）的研發(fā)（這種存儲(chǔ)器可利用自旋極化脈沖，切換存儲(chǔ)區(qū)域內(nèi)的“0”或“1”位）。此外，未來，石墨烯還有可能在自旋邏輯技術(shù)或磁傳感器領(lǐng)域找到用武之地。
The phenomenon known as "spin filtering" is due to an interaction of the quantum mechanical properties of graphene with those of a crystalline nickel film. When the nickel and graphene structures align, only electrons with one spin can pass easily from one material to the other, or spin filtering, that results in spin polarization of an electric current.
The U.S. Naval Research Laboratory (NRL) took advantage of an interdisciplinary team of scientists to successfully demonstrate metallic spin filtering at room temperature using ferromagnet-graphene-ferromagnet thin film junction devices. Spin is said to be a fundamental property of electrons, in addition to charge, that can be used to transmit, process, and store data; and room temperature is said to be the breakthrough that may lead to the technology's more commonplace use.
“Spin filtering had been theoretically predicted and previously seen only for high-resistance structures at cryogenic temperatures,” said Dr. Enrique Cobas, Principal Investigator, NRL Materials Science and Technology Division. “The new results confirm the effect works at room temperature with very low resistance in arrays of multiple devices.”
The thin film junctions demonstrated low resistance, and the magnetoresistance characteristic of a spin filter interface from cryogenic temperatures to room temperature. The research team also developed a device model to incorporate the predicted spin filtering by explicitly treating a metallic minority spin channel with spin current conversion, and determined that the spin polarization was at least 80% in the graphene layer.
“Graphene is famous for its extraordinary in-plane properties, but we wanted to look at conductivity between stacked graphene sheets and how they interact with other materials,” said Cobas.
To do so, NRL researchers developed a method to grow large multi-layer graphene films directly on a smooth, crystalline nickel alloy film while retaining that film’s magnetic properties, then patterned the film into arrays of cross-bar junctions.
“We also wanted to show we could produce these devices with standard industry tools, not just make one device,” Cobas added.
According to Dr. Olaf van't Erve, a research scientist at NRL Materials Science and Technology Division, there is room for improvement as theory suggests the effect can be increased by an order of magnitude by fine-tuning the number of graphene layers.
“However, current models do not include the spin-conversion that happens inside the ferromagnetic contacts," he said. " Once we account for those effects, we’re already close to the ideal case of 100% spin polarization in the graphene layer, enabling us to revise our device geometry and materials to maximize the effect.”
The result is relevant to next-generation non-volatile magnetic random access memory (MRAM), which uses spin-polarized pulses to flip a magnetic bit from 0 to 1 and vice-versa. It may also find use in future spin logic technologies or as magnetic sensors.
Author: Jean L. Broge
Source: SAE Aerospace Engineering Magazine
如圖所示，鎳（藍(lán)）和鐵（紅）薄膜中混合了大量自旋方向不同的電子。金屬層之間的多層石墨烯（灰）可為向一個(gè)方向自旋的電子提供導(dǎo)電通道，并同時(shí)阻斷另一個(gè)方向的自旋。此時(shí)，將產(chǎn)生一個(gè)從金屬結(jié)點(diǎn)中穿過的自旋極化電流（標(biāo)有“J”）。（來源：美國海軍研究實(shí)驗(yàn)室）