Magnetic properties of the frustrated antiferromagnetic spinelZnCr2O4and the spin-glassZn1−xCdxCr2O4(x=0.05,0.10)

The T dependence $\left(2\mathrm{}-400\hspace{0.5em}\mathrm{K}\right)$ of the electron paramagnetic resonance (EPR), magnetic susceptibility $\chi \mathrm{}\left(T\right),$ and specific heat $C_v\mathrm{}\left(T\right)\mathrm{}$ of the normal antiferromagnetic (AFM) spinel ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ and the spin-glass (SG) ${\mathrm{Zn}}_{1-x}{\mathrm{Cd}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ $\mathrm{}(x=0.05,0.10\mathrm{})\mathrm{}$ are reported. These systems behave as a strongly frustrated AFM and SG with $T_N\approx T_G\approx 12\hspace{0.5em}\mathrm{K}$ and $-400\hspace{0.5em}\mathrm{K}\gtrsim {\Theta }_{\mathrm{CW}}\gtrsim -500\hspace{0.5em}\mathrm{K}.$ At high-T the EPR intensity follows the $\chi \mathrm{}\left(T\right)\mathrm{}$ and the g value is T independent. The linewidth broadens as the temperature is lowered, suggesting the existence of short range AFM correlations in the paramagnetic phase. For ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ the EPR intensity and $\chi \mathrm{}\left(T\right)\mathrm{}$ decreases below 90 and $50\hspace{0.5em}\mathrm{K},$ respectively. These results are discussed in terms of both nearest-neighbor ${\mathrm{Cr}}^{3+}$ $\mathrm{}(S=3/2\mathrm{})\mathrm{}$ spin-coupled pairs and spin-coupled tetrahedral clusters with an exchange coupling of $|J/k|\approx 35-45\hspace{0.5em}\mathrm{K}.$ The appearance of small resonance modes for $T\lesssim 17\hspace{0.5em}\mathrm{K},$ the observation of a sharp drop in $\chi \mathrm{}\left(T\right)\mathrm{}$ and a strong peak in $C_v\mathrm{}\left(T\right)\mathrm{}$ at $T_N=12\mathrm{}\hspace{0.5em}\mathrm{K}$ confirms, as previously reported, the existence of long range AFM correlations in the low-T phase. A comparison with recent neutron diffraction experiments, that found a near dispersionless excitation at 4.5 meV for $T\lesssim T_N$ and a continuous gapless spectrum for $T\gtrsim T_N,$ is also given.