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)

Abstract

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 $\approx T_G\approx 12$ K and -400 K $\gtrsim {\Theta }_{CW}\gtrsim -500$ K. At high-$T$ the EPR intensity follows the $\chi (T)$ 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 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 $=3/2$%) spin-coupled pairs with an exchange coupling of $|J/k|\approx$ 50 K. The appearance of small resonance modes for $T\lesssim 17$ 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.

17 pages, 8 figures, 1 Table. Submitted to Physical Review B

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• Properties of a classical spin liquid: the Heisenberg pyrochlore antiferromagnet
• Pyrochlore Antiferromagnet: A Three-Dimensional Quantum Spin Liquid
• Local spin resonance and spin-Peierls-like phase transition in a geometrically frustrated antiferromagnet
• Quantum tetrahedral mean field theory of the magnetic susceptibility for the pyrochlore lattice

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