Ce_{3-x}Mg_xCo₉: transformation of a Pauli paramagnet into a strong permanent magnet

We report on the synthesis of single crystalline and polycrystalline samples of Ce$_{3-x}$Mg$_x$Co$_9$ solid solution ($0\leq x \lesssim 1.4$) and characterization of their structural and magnetic properties. The crystal structure remains rhombohedral in the whole composition range and Mg partially replaces Ce in the 6\textit{c} site of the CeCo$_3$ structure. Ferromagnetism is induced by Mg substitutions starting as low as $x=0.18$ and reaching a Curie temperature as high as $450$\,K for $x=1.35$. Measurements on single crystals with $x=1.34$ and $T_\textrm{C}=440$\,K indicate an axial magnetic anisotropy with the anisotropy field of $6$\,T and a magnetization of $6$\,$\mu_B/$f.u. at $300$\,K. Coercicity is observed in the polycrystalline samples consistent with the observed axial magnetic anisotropy. Our discovery of ferromagnetism with large axial magnetic anisotropy induced by substituting a rare-earth element by Mg is a very promising result in the search of inexpensive permanent-magnet materials and suggests other non-magnetic phases, similar to CeCo$_3$, may also conceal nearby ferromagnetic phases.