  The magnetic and structural properties of polycrystalline Co$_{4-x}$ Ni$_x$
Nb$_2$ O$_9$ (x=1,2) have been investigated by neutron powder diffraction,
magnetization and heat capacity measurements, and density functional theory
(DFT) calculations. For x=1, the compound crystallizes in the trigonal
P$\bar{3}$c1 space group. Below T$_N$ = 31 K it develops a weakly non-collinear
antiferromagnetig structure with magnetic moments in the ab-plane. The compound
with x=2 has crystal structure of the orthorhombic Pbcn space group and shows a
hard ferrimagnetic behavior below T$_C$ =47 K. For this compound a weakly
non-collinear ferrimagnetic structure with two possible configurations in ab
plane was derived from ND study. By calculating magnetic anisotropy energy via
DFT, the ground-state magnetic configuration was determined for this compound.
The heat capacity study in magnetic fields up to 140 kOe provide further
information on the magnetic structure of the compounds.
