  Here we present a comparative study of the magnetic and crystal chemical
properties of two Co2+ containing garnets, NaCa2Co2V3O12 and CaY2Co2Ge3O12.
Both phases exhibit the onset of antiferromagnetic order at 8K and 6K
respectively, as well as field-induced transitions in their magnetization at 1T
and around 11 T. We find these field-dependent transitions correspond to
quantum critical points that result in the suppression of antiferromagnetic
order and that these transitions can be clearly seen using magnetocapacitance
measurements. Finally, we perform detailed crystal chemistry analyses and
complimentary density functional theory calculations to show that changes in
the local environment of the Co-ions are responsible for differences in the two
magnetic structures and their respective properties.
