Magnetic properties of the frustrated AFM spinel ZnCr_2O_4 and the spin-glass Zn_{1-x}Cd_xCr_2O_4 (x=0.05,0.10)

The $T$-dependence (2- 400 K) of the electron paramagnetic resonance (EPR), magnetic susceptibility, $\chi (T)$, and specific heat, $C_v(T)$, of the $normal$ antiferromagnetic (AFM) spinel ZnCr${}_2$O${}_4$ and the spin-glass (SG) Zn${}_{1-x}$Cd${}_x$Cr${}_2$O${}_4$ ($x=0.05,0.10$) is reported. These systems behave as a strongly frustrated AFM and SG with $$ $ \approx T_{G}\approx 12$Kand-400K$\gtrsim \Theta_{CW}\gtrsim -500$K\mathrm{.}Athigh-$T$theEPRintensityfollowsthe$\chi (T)$andthe$g$-valueis$T$-independent. The linewidth broadens as the temperature is lowered, suggesting the existence of short range AFM correlations in the paramagnetic phase. For ZnCr${}_2$O${}_4$ the EPR intensity and $\chi (T)$ decreases below 90 K and 50 K, respectively. These results are discussed in terms of nearest-neighbor Cr${}^{3+}$ (S $=3\mathrm{/}2$%) spin-coupled pairs with an exchange coupling of $| J/k| \approx $50K\mathrm{.}Theappearanceofsmallresonancemodesfor$T\lesssim 17$ K, the observation of a sharp drop in $\chi (T)$ and a strong peak in $C_v(T)$ at $T_N=12$ 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.