Department of Physics, University of Washington, Seattle, Washington 98195, United States.
Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States.
Nano Lett. 2021 Nov 10;21(21):9180-9186. doi: 10.1021/acs.nanolett.1c03027. Epub 2021 Nov 1.
van der Waals (vdW) magnets have emerged as a tunable platform for exploring a variety of layer-dependent magnetic phenomena. Here we probe the thickness-dependent magnetism of vanadium triiodide (VI), a material known as a layered ferromagnetic Mott insulator in its bulk form, using magnetic circular dichroism microscopy. Robust ferromagnetism is observed in all thin layers, down to the monolayer limit with large coercive fields. In contrast to known vdW magnets, the Curie temperature shows an anomalous increase as the layer number decreases, reaching a maximum of 60 K in monolayers. Second harmonic generation measurements reveal broken inversion symmetry in exfoliated flakes, down to trilayers. This observation demonstrates that the exfoliated flakes take a layer stacking arrangement that differed from the inversion-symmetric parent bulk counterpart. Our results suggest a coupling effect between magnetic and structural degrees of freedom in VI and its potential for engineering layer and twist angle-dependent magnetic phenomena.
范德华（vdW）磁体已成为探索各种层依赖磁现象的可调谐平台。在这里，我们使用磁圆二色显微镜探测三碘化钒（VI）的厚度依赖性磁体，三碘化钒是一种在其块状形式下被称为层状铁磁莫特绝缘体的材料。在所有的薄层中都观察到了强铁磁性，甚至在单层极限下也有较大的矫顽场。与已知的 vdW 磁体不同，居里温度随着层数的减少而异常增加，在单层中达到 60 K 的最大值。二次谐波产生测量表明，在剥落的薄片中存在破缺的反转对称性，甚至在三层薄片中也是如此。这一观察结果表明，剥落的薄片采用了与具有反转对称性的母体块状对应物不同的层堆叠排列。我们的结果表明 VI 中磁和结构自由度之间存在耦合效应，以及在工程层和扭转角依赖性磁现象方面的潜在应用。
