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)

H. Martinho; N. O. Moreno; J. A. Sanjurjo; C. Rettori; A. J. Garcia-Adeva; D. L. Huber; S. B. Oseroff; W. Ratcliff; S. -W Cheong; P. G. Pagliuso; J. L. Sarrao; G. B. Martins

doi:10.1103/PhysRevB.64.024408

paper

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)

arXiv:cond-mat/0011171 · doi:10.1103/PhysRevB.64.024408

Abstract

The $T$ -dependence (2- 400 K) of the electron paramagnetic resonance (EPR), magnetic susceptibility, $χ (T)$ , and specific heat, $C_{v} (T)$ , of the $n or ma l$ 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 $≳ Θ_{C W} ≳ - 500$ K. At high- $T$ the EPR intensity follows the $χ (T)$ 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 $χ (T)$ 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 $T ≲ 17$ K, the observation of a sharp drop in $χ (T)$ 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 $T ≲ T_{N}$ and a continuous gapless spectrum for $T ≳ T_{N}$ , is also given.

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