In-plane anisotropy of electrical resistivity was studied in samples of the hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ in the composition range $0.21 leq x leq 0.26$ where anisotropy changes sign. Low-temperature ($sim$20~K) irradiation with relativistic
2.5 MeV electrons was used to control the level of disorder and residual resistivity of the samples. Modification of the stress-detwinning technique enabled measurements of the same samples before and after irradiation, leading to conclusion of anisotropic character of predominantly inelastic scattering processes. Our main finding is that the resistivity anisotropy is of the same sign irrespective of residual resistivity, and remains the same in the orthorhombic $C_2$ phase above the re-entrant tetragonal transition. Unusual $T$-linear dependence of the anisotropy $Delta rho equiv rho_a(T)-rho_b(T)$ is found in pristine samples with $x=$0.213 and $x=$0.219, without similar signatures in either $rho_a(T)$ or $rho_b(T)$. We show that this feature can be reproduced by a phenomenological model of R.~M.~Fernandes {it et al.} Phys. Rev. Lett. {bf 107},217002 (2011). We speculate that onset of fluctuations of nematic order on approaching the instability towards the re-entrant tetragonal phase contributes to this unusual dependence.
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. Her
e we report the discovery of ferromagnetism in a layered van der Waals semiconductor, VI3, which is based on honeycomb vanadium layers separated by an iodine-iodine van der Waals gap. It has a BiI3-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. VI3 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.
