Non Collinear Moment in Single Domain Antiferromagnetic Manganese-copper

Diffuse neutron scattering measurements incorporating polarisation analysis have been made on a single domain single crystal of MnCu 90 at. % Mn at 5 K. The atoms are strongly clustered and the long range part of the magnetic defect is transverse to the antiferromagnetic direction. The face centred tetragonal form of manganese which can be stabilised at room temperature by the addition of small amounts of other elements, is the simplest of the antiferromagnetic structures occurring in the first transition metal series. The structure is one in which the atoms in (001) planes are ferromagnetically coupled and the moment of successive planes alternates along the tetragonal direction. The first alloy series to be studied with neutron diffraction was MnCu [I, 21. As well as establishing the magnetic structure, Bacon et al. [2] observed a diffuse peak near the (100) position in their powder diffraction patterns which they inferred was associated with the (001) reciprocal lattice point. More recently Cywinski et al. [3] showed using textured samples and polarisation analysis that the diffuse feature is indeed associated with the (OOl), magnetic, and most probably due to static magnetisation transverse to the direction of long range magnetic order. No energy analysis was made, and the static nature of the effect was inferred because the experiment was done at low temperatures and with neutrons with an energy less than the magnon band gap. The data reported here on a single crystal, largely single domain sample of MnCu containing 10 at. % Cu, were taken on the D7 diffuse scattering diffractometer at ILL. No energy analysis was done in the main experiment, but an initial check for a known magnon using time of flight analysis showed that at 5 K, with X = 4.85 a, magnon scattering should be very small. Data were collected over a wide area in the (110) and (001) planes but the data presented here are restricted to the region of the (001) reciprocal lattice point in the (110) plane. Using polarisation analysis three s e p arate cross-section could be isolated. These were nuclear defect scattering, the magnetic scattering from transverse moment plus one third of the nuclear spin incoherent scattering, and the longitudinal magnetic scattering with the other third of the nuclear spin incoherent scattering which gives rise to neutron spin flip [4]. The separation was achieved by changing the direction of the neutron polarisation with respect to the scattering vector. In the D7 configuration the polarisation was either vertical (perpendicular to the scattering vector) or at 35.5' to the horizontal. This latter angle was determined by assuming the (110) magnetic Bragg peak to be only due to magnetisation components parallel to the antiferromagnetic direction. Flipping ratios were obtained from scattering from quartz. Detector efficiencies were standardised from vanadium scattering, and the scattering from the crystal was corrected for absorption after subtracting the measured background. For this preliminary report, the nuclear defect crosssection was selected from the (110) scattering plane geometry in a band 0.13 reciprocal lattice units wide running out in the direction of the (001) reciprocal lattice position. This was fitted with Cowley [5] short range order parameters out to sixth neighbour. The parameters are shown in table I. The above is not a satisfactory fit because the atomic clustering is much longer ranged than to the sixth neighbour shell. This can be seen because of the large magnitudes of the outer Cowley parameters. Long range clustering was also seen in the nuclear data of [3]. The data was scaled according to the fitted value of the n independent part of the scattering to equal c (1 c) (bc, b ~ , , ) ~ with c the concentration. Figure 1 shows the data plotted as a function of the z component of n along with the fitted function. The same factor was also used to scale the magnetic data to obtain cross-sections. Table I. The fitted Cowley short range atomic order parameters. 'NOW at MASPEC, Parma, Italy. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988868 C8 164 JOURNAL DE PHYSIQUE Reduced Scattering Vector 001 direction Reciprocal Distance from 001 Position (A-') 12.5 10.05 e 7.54 a 5.0; 5 .?$ 2.5-