Two-dimensional (2D) materials are promising candidates for next-generation electronic devices. In this regime, insulating 2D ferromagnets, which remain rare, are of special importance due to their potential for enabling new device architectures. Here we report the discovery of ferromagnetism in a layered van der Waals semiconductor, VI 3 , which is based on honeycomb vanadium layers separated by an iodine-iodine van der Waals gap. It has a BiI 3 -type structure (R-3, No.148) at room temperature, and our experimental evidence suggests that it may undergo a subtle structural phase transition at 78 K. VI 3 becomes ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly shows ferromagnetic domains, which can be manipulated by the applied external magnetic field. The optical band gap determined by reflectance measurements is 0.6 eV, and the material is highly resistive.

The in-plane resistivity anisotropy is studied in strain-detwinned single crystals of FeSe. In contrast to other iron-based superconductors, FeSe does not develop long-range magnetic order below the nematic/structural transition at Ts ≈90 K. This allows for the disentanglement of the contributions to the resistivity anisotropy due to nematic and magnetic orders. Comparing direct transport and elastoresistivity measurements, we extract the intrinsic resistivity anisotropy of strainfree samples. The anisotropy peaks slightly below Ts and decreases to nearly zero on cooling down to the superconducting transition. This behavior is consistent with a scenario in which the in-plane resistivity anisotropy in FeSe is dominated by inelastic scattering by anisotropic spin fluctuations.PACS numbers: 74.70. Xa, 74.25.Ld Electronic nematicity has emerged as a key concept in iron-based superconductors since the observation of inplane resistivity anisotropy in stress-detwinned crystals of Co-doped BaFe 2 As 2 [1, 2]. The fact that the resistivity anisotropy is much larger than what is expected from the small lattice distortion led to the proposal that the tetragonal-to-orthorhombic transition in the iron pnictides is driven not by phonons, but by an electronic nematic phase. Subsequent experiments revealed an intricate dependence of the resistivity anisotropy on doping (a sign change between electron-and hole-doped materials [2-6]), and disorder [7,8], sparking hot debates about its microscopic origins (see Refs. [9 and 10] for reviews).Electronic contributions involved in the in-plane resistivity anisotropy [10] can be separated into the Drude weight and/or of the scattering rate anisotropies. Fermisurface anisotropies arising, for instance, from the ferroorbital order triggered at the nematic transition, affect mostly the Drude weight [11][12][13]. Anisotropic scattering, can be due to elastic processes, such as the development of local magnetic order around an impurity [14,15], or inelastic processes, such as the scattering of electrons by anisotropic magnetic fluctuations [16,17] known to exist below T s [18]. Recent stress-dependent optical reflectivity studies in Co-doped BaFe 2 As 2 point to a dominant effect of the Drude weight [19,20]. However, stripe magnetic order appearing at the magnetic transition severely complicates the analysis. This is because the magnetic state breaks tetragonal symmetry leading to an anisotropic reconstruction of the Fermi surface [7,21] and to the appearance of "Dirac cones" [22], which may dramatically alter the resistivity anisotropy [23]. Disentangling these contributions is fundamental to reveal the origin of the resistivity anisotropy and, consequently, of the nematic state.In this context, the stoichiometric FeSe [24] is an ideal system. It is rather clean (residual resistivity ratios as high as 50 [25]) and its orthorhombic/nematic phase transition at T s ≈ 90 K is not accompanied by a longrange magnetic order [26] eliminating effects of Fermi surface folding.In this Letter we rep...

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