We report comprehensive studies of the crystallographic, magnetic, and thermal properties of a spinel-type magnetically frustrated compound, $\mathrm{CoA}{\mathrm{l}}_2{\mathrm{O}}_4$, and a magnetically diluted system, $\mathrm{C}{\mathrm{o}}_{\text{1}\text{−}x}\mathrm{Z}{\mathrm{n}}_x\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4$. 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 $T=3.4\mathrm{K}$ 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 $x=0.6$. At $x\sim 0.6$, magnetic susceptibility and specific heat were described by temperature power laws, $\chi \sim C/T\sim T^{-\delta }$, 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 $0\le x\le 1$ for $\mathrm{C}{\mathrm{o}}_{\text{1}\text{−}x}\mathrm{Z}{\mathrm{n}}_x\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4$. The transition temperature of $\mathrm{CoA}{\mathrm{l}}_2{\mathrm{O}}_4$ determined from magnetic susceptibility measured under hydrostatic pressure increased with increasing pressure.

• Received 26 December 2014
• Revised 21 May 2015

DOI:https://doi.org/10.1103/PhysRevB.91.224412