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).
High-temperature series expansion of the spin correlation functions on the B-spinel lattice are computed to order 6 in for Heisenberg model having both nearest- and next-nearest-neighbour exchange integrals. The results are given for various neighbour correlations (up to the third). The behaviour with the temperature and the site dilution is presented. The obtained results provide a useful tool for a straightforward interpretation and understanding of experimental data. The approach is applied to the experimental results of the B-spinel in the dilution range . The critical temperature and the critical exponents for the susceptibility and the correlation length are deduced by applying the Padé approximant methods. The following estimates are obtained for the familiar critical exponents: and . These values are not sensitive to the dilution ratio x. The transition temperatures as a function of x obtained by the present theory are found to be in excellent agreement with the experimental ones.
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