Wednesday, December 13, 2017
Ideal material platforms provide testing grounds for fundamental physics. In the past few decades, magnetic metal thin films have played the indispensable role for tremendous amount of condensed matter experiments on (quasi-)two-dimensional (2D) magnetism. However, at least two drawbacks exist in these epitaxially grown thin films. Firstly, the uniform thickness and single-crystallinity typically occur in crystallites of tens of nanometers size. Thus, any measurement with a lateral spatial resolution larger than this length scale acquires smeared signals averagely of differing crystallites, including edges and grain boundaries as well. Secondly, structural and electronic properties of thin films are drastically altered by substrates, on which epitaxy of thin films occurs. Therefore, intrinsic magnetic properties of 2D materials purely arising from quantum confinement effect of their 3D counterparts are hardly approachable in these “testing grounds”. Through this talk, I will prove to you that our recently discovered 2D magnetic van der Waals crystal Cr2Ge2Te6 constitutes an unprecedentedly ideal material platform that hosts intrinsic 2D ferromagnetism. The close-to-ideal 2D Heisenberg ferromagnet allows us to access a critical physical regime, in which intrinsic magnetic anisotropy is so tiny that a small external magnetic field can substantially modify the spin wave excitation spectrum and effectively control Curie temperatures. Furthermore, the observed prominent dimensionality effect highlights the significant interlayer magnetic coupling despite of the van der Waals spacing. Finally, I will present our recent advance in pushing the boundary of Curie temperatures to make an emergent 2D magnetic van der Waals crystal suitable for room temperature applications. Relevant work on spin-valley and spin-lattice coupling in magnetic van der Waals crystals and heterostructures will also be discussed. I will conclude the talk with outlooks for this emerging field.