  Magnetic properties of heterometallic warwickites $MgFeBO_4$,
$Mg_{0.5}Co_{0.5}FeBO_4$, and $CoFeBO_4$ are presented, highlighting the effect
of Co substitution on the magnetic properties of these compounds. The analysis
of magnetization and heat capacity data has shown that these compounds exhibit
a spin-glass transition below $T_{SG}$=10, 20 and 22 K, respectively. Using
zero field ac susceptibility as entanglement witness we find that the low
dimensional magnetic behavior above $T_{SG}$ show quantum entanglement behavior
$\chi(T)\varpropto T^{-\alpha(T)}$ up to $T_E$ = 130K. The $\alpha$ parameters
have been deduced as a function of temperature and Co, indicating the existence
of random singlet phase in this temperature region. Above $T_E$ the
paramagnetism is interpreted in terms of non-entangled spins giving rise to
Curie-Weiss paramagnetism. The different intra- and inter-ribbon exchange
interaction pathways have been calculated within a simple indirect coupling
model. It is determined that the triangular motifs in the warwickite structure,
together with the competing interactions, induce frustration. The spin-glass
character is explained in terms of the substitutional disorder of the Mg, Fe
and Co atoms at the two available crystallographic sites, and the frustration
induced by the competing interactions. The Co substitution induces uniaxial
anisotropy along the b axis, increases the absolute magnetization and increases
the spin-glass freezing temperature. The entanglement behavior is supported in
the intermediate phase irrespective of the introduction of anisotropy by the Co
substitution.
