Main Text:Following the recent isolation of monolayer CrI3 1 , there has been a surge of new twodimensional van der Waals magnetic materials 2-12 , whose incorporation in van der Waals heterostructures offers a new platform for spintronics 5-8 , proximity magnetism 13 , and quantum spin liquids 14 . A primary question in this burgeoning field is how exfoliating crystals to the few-layer limit influences their magnetism. Studies on CrI3 have shown a different magnetic ground state for ultrathin exfoliated films 1,5,6 but the origin is not yet understood. Here, we use electron tunneling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order, finding a ten-fold enhancement in the interlayer exchange compared to bulk crystals. Moreover, temperatureand polarization-dependent Raman spectroscopy reveal that the crystallographic phase transition of bulk crystals does not occur in exfoliated films. This results in a different low temperature stacking order and, we hypothesize, increased interlayer exchange. Our study provides new insight into the connection between stacking order and interlayer interactions in novel two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets, such as RuCl3 14 .A key family of van der Waals magnets is the layered transition metal trihalides, which have been studied for decades as prototypical magnetic insulators 15-17 and as a platform for quasitwo-dimensional magnetism [18][19][20] . In the chromium trihalides, the Cr atoms are arranged in a honeycomb structure, with each Cr atom surrounded by six halide atoms in an octahedral geometry (Fig. 1a). The bulk crystals undergo a crystallographic phase transition from a monoclinic phase (space group C2/m) at room temperature to a rhombohedral phase (space group R3 " ) at low temperatures (below about 240 K for CrCl3 21 ). While the intralayer lattice spacings are largely

We present details of synthesis optimization and physical properties of nearly single phase carbon doped MgB 2 with a nominal stoichiometry of Mg(B 0.8 C 0.2 ) 2 synthesized from magnesium and boron carbide (B 4 C) as starting materials. The superconducting transition temperature is ≈ 22 K (≈ 17 K lower than in pure MgB 2 ). The temperature dependence of the upper critical field is steeper than in pure MgB 2 with H c2 (10K) ≈ 9 T. Temperature dependent specific heat data taken in different applied magnetic fields suggest that the two-gap nature of superconductivity is still preserved for carbon doped MgB 2 even with such a heavily suppressed transition temperature. In addition, the anisotropy ratio of the upper critical field for T /T c ≈ 2 3 is γ ≈ 2. This value is distinct from 1 (isotropic) and also distinct from 6 (the value found for pure MgB 2 ).