CaMn$_2$Al$_{10}$: itinerant Mn magnetism on the verge of ferromagnetic order

We report the discovery of \CMA, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of $0.83\,\mathrm{\mu_B}$/Mn, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only $\approx 9$ \% of $R \mathrm{ln}\,2$. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structure, the magnetic susceptibility $\chi$ is anisotropic below 20 K, with a maximum ratio of $\chi_{[010]}/\chi_{[001]}\approx 3.5$. A strong power-law divergence $\chi(T)\sim T^{-1.2}$ below 20 K implies incipient ferromagnetic order, and an Arrott plot analysis of the magnetization suggests a vanishingly low Curie temperature $T_C\sim 0$. Our experiments indicate that \CMA~is a rare example of a Mn-based weak itinerant magnet that is poised on the verge of ferromagnetic order.