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ICQM faculty members publishes an article in Physical Review Letters reporting ‘Watching Spin Currents’
“Watching Spin Currents”——《物理评论快报》报道量子材料科学中心最新成果

Future spintronic circuits will use the spin orientation of an electron current to store and manipulate information. Such spin-polarized currents, however, cannot be directly observed and are typically characterized using indirect techniques. J. Li and Z.Q. Qiu et al. have now demonstrated that x-ray pulses from a synchrotron source can be used to measure the precession of spins in a material, which directly reveals the flow of a spin current. The technique allowed the researchers to observe a spin current as it propagated through different layers of materials. 

Spin currents are generally probed using measurements of the effects they produce in the metals through which they flow. They can, for instance, create a voltage drop perpendicular to the direction of the spin current or a torque that turns the magnetization direction of a magnetic film. However, these indirect measurements are often ambiguous because they are influenced by factors other than the spin current, such as magnetic proximity effects. 

J. Li and Z.Q. Qiu et al. used a “pump-probe” technique to detect the spin current flowing between two thin ferromagnetic layers separated by a nonmagnetic spacer layer. A microwave “pump” was delivered to the sample, resonantly exciting the precession of the spins in the first layer. This precession injected an alternating current (ac) of spins into the spacer layer, which traveled towards the second magnetic layer. The spin precession of the ac spin current was monitored through the changes it caused in the absorption of circularly polarized “probe” x-ray pulses. By tuning the energy of the x rays to characteristic resonances of the atomic elements within the different layers, the researchers could selectively detect the spin current flowing in each layer.


北京大学物理学院的栗佳研究员和加州大学伯克利分校的邱子强教授等近期在实验研究中,成功实现了针对交流自旋流的直接测量。这也是首次在实验中成功探测到交流自旋流,对自旋电子学领域的进一步发展将具有重要的意义。该工作已经在《物理评论快报》上发表(Phys. Rev. Lett. 117, 076602 (2016)),并且作为亮点工作进行重点报道。

未来的自旋电子学器件将利用自旋流来实现信息的存储与处理。然而时至今日,在实验研究中依然缺乏直接测量自旋流的有效手段,对自旋流的实验研究不得不依赖于自旋流在各类材料中所产生的间接物理效应。这些间接物理效应通常都具有伴随效应,例如磁近邻效应(magnetic proximity effects)等等,对自旋电子学的深入研究造成了不可避免的困扰。

栗佳研究员和邱子强教授等人利用“pump-probe”技术,研究了磁性自旋阀中的磁动力学性质。在实验中,微波信号作为泵浦(pump),驱动磁性自旋阀中的铁磁薄膜产生自旋泵浦效应(spin pumping),在此过程中向邻近的非磁性层和磁性层输出自旋流。利用同步辐射脉冲X射线和磁圆二色谱技术,针对邻近的非磁性层和磁性层进行了元素分辨和时间分辨的测量。实验结果显示自旋泵浦效应所产生的是随时间变化的交流自旋流,这个交流自旋流与铁磁薄膜保持严格的相位一致,并会对邻近的磁性层施加自旋力矩。


这个工作不仅首次从实验上揭示了交流自旋流的特征,而且首次在实验上完整地表征了自旋阀中自旋转移力矩(spin transfer torque)的物理过程。