  Spinel chromites ACr$_2$O$_4$ with inherent magnetic geometrical frustration
usually exhibit a noncollinear ferrimagnetic ground state when A are magnetic
ions, with possibly crystallite-size dependent intriguing magnetic features.
Here, we report single-domain magnetic properties of ACr$_2$O$_4$ (A = Ni, Co,
and Mn) nanocrystals, with an average crystallite size of 18, 15 and 10 nm,
exhibiting an optical energy gap of 2.87, 3.05 and 2.9 eV, respectively. The
temperature dependence of magnetization indicates the main bulk magnetic
transitions with a commonly coexisting spin-glass-like state and finite-size
effects on the noncolinear ferrimagnetic transitions. An anomaly observed at Ts
= 15, 24 and 10 K is attributed to the bulk magnetic transition to a canted
antiferromagnetic state in NiCr$_2$O$_4$ and incommensurate spiral orders in
CoCr$_2$O$_4$ and MnCr$_2$O$_4$ NCs, respectively. A further bulk magnetic
transition to a commensurate spiral order is observed for CoCr$_2$O$_4$ NCs at
a lock-in temperature Tl = 5 K much lower than that reported using bulk
samples, while it is completely suppressed in the MnCr$_2$O$_4$ NCs.
Finite-size effects and single-domain magnetic behaviors indicated by anomalous
temperature-dependences of the coercive field and the hysteresis-loop
squareness, mainly driven by a magnetocrystalline anisotropy, are discussed in
comparison to results reported using bulk counterparts.
