Néel to spin-glass-like phase transition versus dilution in geometrically frustrated Zn Cr2-2x Ga2x O4

Zn Cr2 O4 undergoes a first order spin-Peierls-like phase transition at 12.5 K from a cubic spin liquid phase to a tetragonal Néel state.. Using powder diffraction and single crystal polarized neutron scattering, we determined the complex spin structure of the Néel phase. This phase consisted of several magnetic domains with different characteristic wave vectors. This indicates that the tetragonal phase of Zn Cr2-2x Ga2x O4 is very close to a critical point surrounded by many different Néel states. We have also studied, using elastic and inelastic neutron scattering techniques, the effect of nonmagnetic dilution on magnetic correlations in Zn Cr2-2x Ga2x O4 (x=0.05 and 0.3). For x=0.05, the magnetic correlations do not change qualitatively from those in the pure material, except that the phase transition becomes second order. For x=0.3, the spin-spin correlations become short range. Interestingly, the spatial correlations of the frozen spins in the x=0.3 material are the same as those of the fluctuating moments in the pure and the weakly diluted materials.