“自旋过滤”现象是石墨烯的量子力学属性与晶体镍薄膜的量子力学属性之间相互作用的结果。当晶体镍与石墨烯结构对齐时，只有向同一个方向自旋的电子容易从一种材料穿越至另一种材料（即自旋过滤效应），从而产生自旋极化电流。

在一组跨学科科学家团队的协助下，美国海军研究实验室（NRL）成功利用镍-石墨烯-铁薄膜连接设备，在室温环境下演示了金属自旋过滤效应。电子除了带电之外，还有另一种基本属性——自旋。未来，这种属性很有可能在数据的传输、处理与存储领域发挥重要作用。另外，NRL实验室首次在室温环境下取得突破，这也为该技术的广泛应用奠定了一定基础。

“过去，理论分析与实验结果均说明，自旋过滤仅存在于低温下的高阻抗结构之中。”NRL实验室材料科学与技术部首席研究员Enrique Cobas博士表示，“但最新实验结果证实，自旋过滤效应也存在于一系列室温下的超低阻抗设备中。”

在低温和室温环境下，薄膜连接点均表现出低电阻及自旋过滤界面的磁电阻特性。考虑到理论中的自旋过滤效应，研究小组还开发了一个设备模型，利用自旋电流转换处理金属自旋向下通道，并最终发现石墨烯层的自旋极化至少达到了80%。

Cobas说，“石墨烯一直以其非凡的平面特性著称，但我们想去研究层叠石墨烯层的导电性及其与其他材料的反应特性。”

为此，NRL实验室的研究人员想出一种方法，可直接在光滑的晶体镍合金薄膜上“种植”大片的多层石墨烯薄膜，并在此过程中同时保留镍薄膜的磁性，使其形成连接点阵列。

Cobas说，“我们也想证明，我们还可以利用行业通用的工具制造这些设备，而并不需要打造专门的工具。”

NRL实验室材料科学与技术分部研究科学家Olaf van't Erve博士表示，理论证明，改变石墨烯薄膜的层数还可以将自旋效应提升一个量级，因此该设计还有一定优化空间。

“但是，目前的模型并未考虑铁磁触点内的自旋转换现象，”他说，“一旦我们将这些效应也考虑在内，那几乎就可以达到理想状态，将石墨烯层的自旋极化提升至100%。这样一来，我们就可以改变设备的几何形状与材料选择，尽最大可能增强这种效应。”

石墨烯技术的进展可能将同时推动下一代非易失性磁性随机存储器（MRAM）的研发（这种存储器可利用自旋极化脉冲，切换存储区域内的“0”或“1”位）。此外，未来，石墨烯还有可能在自旋逻辑技术或磁传感器领域找到用武之地。

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