Manipulating the electron spins or, more specifically, the magnetism of a system by means of electrostatic gating has been immensely challenging but of great importance in nanoelectronic applications. Recently, atomically thin magnetic semiconductors, for example 2D vDW materials with much attenuated electrostatic screening due to reduced dimension, have attracted significant attention due to their emerging new physical phenomena. However, many issues are yet to be resolved to convincingly demonstrate gate-controllable magnetism in these two-dimensional materials.

Here, researchers from Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS), in collaboration with colleagues from Shanxi University, Changsha University of Science & Technology, Chongqing University, Liaoning Shihua University, Central Research Institute of Electric Power Industry Japan and Beijing University, showed that a strong field effect can be observed in devices based on few-layered ferromagnetic semiconducting Cr₂Ge₂Te₆ via electrostatic gating, and demonstrated bipolar gate-tunable magnetization loops below the Curie temperature in Cr₂Ge₂Te₆. The main findings have been published on July 2^nd, 2018 in Nature Nanotechnology (doi: 10.1038/s41565-018-0186-z).

Several breakthroughs have been technically achieved in this study. They encapsulated atomically thick Cr₂Ge₂Te₆ few layers between h-BN layers to solve the stability problem of samples in ambient condition. Field-effect devices of few-layered Cr₂Ge₂Te₆ were obtained through micro-nano manufacturing processes and investigated systematically in cryogenic electrical and magnetic measurements. Few-layered Cr₂Ge₂Te₆ remains electrically conductive below Curie temperature (~ 65K) and exhibits bipolar effect. Open-circuit current is obtainable up to dozens of micro-Amperes under a bias voltage of 1V at room temperature, and the threshold on-off current ratio can reach above 10⁴. Cryogenic micro-area Kerr measurements show that magnetism in such a device can be effectively and bipolarly tuned by gating, validating the possibility of using vdW ferromagnetic semiconductors as spin field-effect transistors. Multi-scale simulations from microscopic ab intio DFT calculations to mesoscopic/macroscopic micromagnetics were performed. A decent agreement between computer simulations and the experiments shows that the moment rebalance of the spin-polarized band structure in such a special system gives rise to bipolar field effect on both spin and charge as experimentally observed.

“Few-layered Cr₂Ge₂Te₆, as the first-known 2D system having gate tunability on both charge and spin, can potentially help integrate both memory storage and logic operation units into one, thereby circumventing the long-lived Von-Neumann bottleneck in the conventional computers.” the paper’s last author, Prof. ZHANG Zhidong said. “Such findings of electric-field-controlled magnetism in van der Waals magnets show bright future for potential applications in new-generation magnetic memory storage, sensors and spintronics.”

This work was supported by Project of the National Key R&D Program of China, the National Natural Science Foundation of China and Shenyang National Laboratory for Materials Science (SYNL).

Figure 1.Few-layered Cr₂Ge₂Te₆ vdW heterostructure device and field effect of charge. (Image by IMR)

Figure 2. Bipolar effect of charge carrier and spin in few-layered Cr₂Ge₂Te₆ (Image by IMR)