  A new class of rare-earth-free permanent magnets is proposed. The parent
compound of this class is Co$_3$Mn$_2$Ge, and its discovery is the result of
first principles theory combined with experimental synthesis and
characterisation. The theory is based on a high-throughput/data-mining search
among materials listed in the ICSD database. From ab-initio theory of the
defect free material it is predicted that the saturation magnetization is 1.71
T, the uniaxial magnetocrystalline anisotropy is 1.44 MJ/m$^3$, and the Curie
temperature is 700 K. Co$_3$Mn$_2$Ge samples were then synthesized and
characterised with respect to structure and magnetism. The crystal structure
was found to be the MgZn$_2$-type, with partial disorder of Co and Ge on the
crystallographic lattice sites. From magnetization measurements a saturation
polarization of 0.86 T at 10 K was detected, together with a uniaxial
magnetocrystalline anisotropy constant of 1.18 MJ/m$^3$, and the Curie
temperature of $T_{\rm C}$ = 359 K. These magnetic properties make
Co$_3$Mn$_2$Ge a very promising material as a rare-earth free permanent magnet,
and since we can demonstrate that magnetism depends critically on the amount of
disorder of the Co and Ge atoms, a further improvement of the magnetism is
possible. From the theoretical works, a substitution of Ge by neighboring
elements suggest two other promising materials - Co$_3$Mn$_2$Al and
Co$_3$Mn$_2$Ga. We demonstrate here that the class of compounds based on
$T_3$Mn$_2$X (T = Co or alloys between Fe and Ni; X=Ge, Al or Ga) in the
MgZn$_2$ structure type, form a new class of rare-earth free permanent magnets
with very promising performance.
