The Mn66Ga34 alloy at this particular composition is known to be zero alloy in which the linear combination of the two neutron scattering lengths weighted by the atomic compositions vanish. Thus for this specific concentration, the effect of the partial structure factors SNN and SNC is cancelled by a weighted term, which value is zero. Then the measured total structure factor S(q) gives directly the concentration-concentration structure factor SCC(q). We present here the first experimental results of neutron diffraction on the Mn66Ga34 "null matrix alloy" at 1050 °C. The main peak of the experimental SCC(q) gives a strong evidence of a hetero-atomic chemical order in this coordinated alloy. This order also appears in real space radial distribution function which is calculated by the Fourier transform of the structure factor. The degree of hetero-coordination is discussed together with other manganese-polyvalent alloys. However manganese also shows abnormal magnetic scattering in the alloy structure factor which must be corrected. This correction gives an experimental information on the mean effective spin of manganese in this liquid alloy. We present the first critical theoretical calculations of the magnetic correction factor in Mn-Ga zero-alloy based on our accurate experimental measurements of SCC(q).
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