  We report comprehensive studies of the crystallographic,magnetic, and thermal
properties of a spinel-type magnetically frustrated compound, CoAl2O4, and a
magnetically diluted system,Co1-xZnxAl2O4. These studies revealed the effects
of dilution and disorder when the tetrahedral magnetic Co ion was replaced by
the nonmagnetic Zn ion. Low-temperature anomalies were observed in magnetic
susceptibility at x < 0.6. A multicritical point was apparent at T = 3.4 K and
x = 0.12, where the antiferromagnetic, spin glass-like, and paramagnetic phases
met. At that point, the quenched ferromagnetic component induced by a magnetic
field during cooling was sharply enhanced and was observable below x = 0.6.
Around x = 0.6, magnetic susceptibility and specific heat were described by
temperature power laws, chi ~ C/T ~ T-d, in accord with the site percolation
threshold of the diamond lattice. This behavior is reminiscent of a quantum
critical singularity. We propose an x-temperature phase diagram in the range 0
< x < 1 for Co1-xZnxAl2O4. The transition temperature of CoAl2O4 determined
from magnetic susceptibility measured under hydrostatic pressure increased with
increasing pressure.
