Long-range ferromagnetic order in emerging two-dimensional atomic crystals, is significant in both fundamental science and technological applications. For the first time, we observed intrinsic ferromagnetism in pristine van der Waals layers Cr2Ge2Te6,1 via magneto-optic Kerr effect. The prominent thickness-dependent Curie temperatures reveal the strong dimensionality effect and the significant interlayer magnetic coupling across the van der Waals spacing. Remarkably, we showed that, due to the very small intrinsic magnetocrystalline anisotropy and the negligible spurious magnetic anisotropies, Cr2Ge2Te6 exhibits as the first close-to-ideal 2D Heisenberg ferromagnet. It therefore enables an unprecedented access to a regime in which a small magnetic field can effectively open up the spin wave excitation gap, thus substantially controlling Curie temperatures. The fundamental understanding in this work will shed lights on possible avenues of seeking high Curie temperature 2D magnets for practical applications.
 C. Gong et al. Nature 546, 265-269 (2017).