Magnetic structure and magnetic properties of the spinel solid solutions ZnCr2xAl2-2xS4 (0.85?x?1). I. Neutron diffraction study

The spinel solid solutions ZnCr2xAl2-2xS4 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 TN(x=1)=15.5, TN(x=0.9)=14.5 and TN(x=0.85)=14K a helical structure with k1=(0, 0, 0.79) and mk1 perpendicular to (001) is built up in a second-order phase transition ( beta =0.3). At T0=12K, a first-order transition takes place and the helical structure begins to transform into two collinear structures corresponding to k2=(1/2, 1/2, 0), mk2//(110) and k3=(1, 1/2, 0); mk3//(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 (j1 approximately=2K and J3 approximately=1K, respectively) and the antiferromagnetic second-nearest-neighbour interaction (J2 approximately=-1K).