Magnetic structure and magnetic properties of the spinel solid solutions ZnCr_2xAl_2-2xS₄(0.85⩽x⩽1). I. Neutron diffraction study

Abstract

The spinel solid solutions ZnCr 2x Al 2-2x S 4 have been studied by means of neutron diffraction in the concentration range 0.85<or=x<or=1. For the pure compound (x=1), several powder samples with different heat treatments and a single crystal were studied. At T N (x=1)=15.5, T N (x=0.9)=14.5 and T N (x=0.85)=14K a helical structure with k 1 =(0, 0, 0.79) and m k1 perpendicular to (001) is built up in a second-order phase transition ( beta =0.3). At T 0 =12K, a first-order transition takes place and the helical structure begins to transform into two collinear structures corresponding to k 2 =( 1 / 2 , 1 / 2 , 0), m k2 //(110) and k 3 =(1, 1 / 2 , 0); m k3 //(001). At low temperature, the three phases coexist in different percentages depending on the number of sulphur vacancies. This mixed phase results from a competition between the ferromagnetic first- and third-nearest-neighbour couplings (j 1 approximately=2K and J 3 approximately=1K, respectively) and the antiferromagnetic second-nearest-neighbour interaction (J 2 approximately=-1K).