Research
My research area is experimental condensed matter physics. A major focus of my research is the spin physics in magnetic materials. There are two aspects throughout my research career: (1) Growing the ultra-high quality thin films and heterostructures by off-axis magnetron sputtering and (2) fabricating devices to explore the spin dynamics such as the generation, detection, and transportation of spin current.
Due to magnon scattering by conduction electrons, the damping of conducting ferromagnet is generally one or two orders of magnitude larger than the insulating ferromagnet for example Y3Fe5O12 (YIG). While metallic ferromagnets are commonly used for charge-based spintronic devices such as spin Hall nano oscillator and spin transfer torque MRAM (STT-MRAM), low damping ferromagnetic metal are highly desired.
Previously, it is been predicted that Co-Fe alloy at Co concentration around 25% has the lowest damping and experimentally achieved a relatively low damping in polycrystal Co25Fe75. In our work, we prove that through growing Co25Fe75 epitaxial thin film with excellent crystalline quality, the damping constant of Co25Fe75 can be as low as . This is the first direct measurement of a Gilbert damping constant in the 10−4 regime for metallic FM films, making it an ideal material for exploration of spintronic applications requiring metallic FMs.
Furthermore, we systematically explore the different contributions to the measured ferromagnetic resonance linewidth broadening. Unlike the YIG thin film with a dramatically increase of the damping, the damping of metallic ferromagnet is not sensitive to the temperature.
2. Antiferromagnetic spintronics
Antiferromagnetic spintronics is a fast-booming subfield due to the unique properties of antiferromagnet: no stray field, robust to the external field and THz response. Although it looks very promising, for a long time, AFM plays a auxiliary role in spintronics research. Our group is one of the few pioneers that notice the great potential in spintronics as we first reveal the efficient spin transport in AFM insulator NiO. The spin dynamics in AFM becomes one of my major research topic during my PhD career.
2.1 Spin transport in YIG/NiO/Pt trilayers
The efficient spin transport in AFM impressed the community but also left a lot of open questions. To better understand the spin transport in antiferromagnetic insulator, angular dependence spin pumping measurement for YIG/NiO/Pt trilayers was performed. It is found that spin current polarization in NiO is determined by the orientation of Neel order in NiO which can be tuned by varying the temperature and thickness of NiO.
2.2 Spin magnetoresistance and proximity effect in Pt/Fe2O3 bilayer
Spin magnetoresistance (SMR) is a power tool to detect the spin states in both FM and AFM. Fe2O3 is an easy-plane AFM at room temperature with very small spin-flop transition field. This provide a unique advantage in controlling the Neel order and make Fe2O3 an ideal platform to study the spin transport and manipulation compared with other AFMs. We performed a systematic magneto transport measurement in Pt/ Fe2O3 bilayers. A large SMR was measured, which indicates the large spin mixing conductance at the Pt/ Fe2O3 interface. Besides, angular dependence measurements shows how the spin-flop transition influence the SMR signal. Surprisingly, an anomalous Hall (AHE) and anisotropic MR (AMR) were also shown up at 10K, as a clear evidence of proximity effect in Pt by antiferromagnetic insulator for the first time.
2.3 Switching of antiferromagnetic spin
Spin orbit torque (SOT) induced switching in antiferromagnetic has get a lot of attentions since switching speed can reach ~THz regium. Not like FM where the switching feasibility can be cross checked using AHE with field induced switching, reported AFM switching in either metallic CuMnAs family and NiO are inert to the external field. Thus, to better understanding the switching behavior in AFM spins, we choose the Fe2O3, an AFM with low spin-flop transition field. This means when the applied field is large enough, the AFM spins are frozen and there would be no more SOT induced switching. In experiment, we find that there are two switching signal: A saw-tooth like switching which is not influenced by the external field and a step-like switching which disappears when the field is beyond the spin-flop transition. Furthermore, we shows that such saw-tooth like switching can exist even in bare Pt thin film and will disappear when anneal the sample with large pulse current while the step-like switching is stable, which means such saw-tooth switching is an artifact from the Pt layers and the step-like switching due to the SOT induced switch. The ambiguously demonstration of the SOT switching in AFM insulator paves the way for the following explore of the application and properties in AFM switching. It also suggests that earlier research results need a more rigorous check.
2.4 Topological spin texture in Pt/Antiferromagnetic insulator bilayers
Topological spin textures for example skyrmion is a promising candidate for the next-generation data storage unit. While the skyrmion hosted in ferromagnet is suffered by the skyrmion Hall effect which hindered the motion of skyrmion along the current direction, antiferromagnet skyrmion is treated as an ideal solution. We choose the Pt/Cr2O3 bilayers where Cr2O3 is an easy-axis antiferromagnetic insulator. Pt layer provides conducting channel for the electric measurement as well as the interfacial Dzyaloshinskii–Moriya interaction (DMI). It is shown that as temperature near and above the Neel temperature of Cr2O3 (TN = 307 K), a pronounced topological Hall (THE) signal is detected. The THE is the direct evidence of the topological spin texture in the Pt/Cr2O3 bilayers. By varying the thicknesses of the Cr2O3, the THE disappears at above 5-6 nm, which indicates its interfacial origin. The Monte-Carlo simulation proves that at a certain range of DMI strength, the non-zero topological charge can show up at the temperature near or even above the TN of the antiferromagnet. The emergence of spin textures in AF insulator films significantly expands our materials base to include the large family of AF insulators for the exploration of AF-based skyrmion technology.
Nonlocal spin transport using lateral structures is attractive for spintronic devices. Typically, a spin current is generated by a ferromagnetic (FM) or a heavy metal (HM) electrode in a nonlocal structure, which can be detected by another FM or HM electrode. Here, we report a new nonlocal spin injection scheme using uniform-mode ferromagnetic resonance (FMR) spin pumping in Pt/Y3Fe5O12 (YIG) lateral structures. This scheme is enabled by well-separated resonant fields of Pt/YIG and bare YIG due to substantial change of anisotropy in YIG films induced by a Pt overlayer, allowing for clearly distinguishable local and nonlocal spin pumping. Our first observation of the nonlocal spin pumping through high efficiency angular momentum transfer from the uniform FMR mode in a resonant region into a non-resonant region opens up a new path for spin current generation, propagation, and detection, which offers unique advantage of high microwave efficiency, nonlocal device structure, and spin transport for future spintronic application.