Determination of the absolute magnetic moment direction in Cr2O3 using generalized polarization analysis

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Abstract
Generalized polarization analysis, in which all three components of polarization of both incident and scattered neutron beams are determined, can be used to probe fine details of magnetic structure. For instance, in certain antiferromagnetic crystals such as Cr2O3 neutrons scattered from reflections containing both nuclear and magnetic components have the direction of their polarization rotated in the plane containing the scattering vector as was demonstrated by Alperin [International Conference on Magnetism (Moscow, 1973)]. The sense of the rotation depends on whether the moments point towards the center of symmetry or away from it. The two corresponding configurations are distinguishable because the coordinating octahedra of oxygen ions are not centrosymmetric and should therefore have different energies. We have used a new technique which exploits the Meissner effect to obtain magnetic isolation to measure the rotation of the neutron polarization by the 102 and 1̄04 (hexagonal) reflections of Cr2O3 between 7 and 320 K. The results show that the lower‐energy configuration is the one in which the chromium moments point towards the smaller of the two equilateral triangles of oxygen neighbors. The measured depolarizations were found not to be consistent with a simple domain model.