Study of EuAs3 near the magnetic phase transition

A neutron diffraction Study of EuAs3 near the magnetic phase transition was performed successfully of D15. Below 10K EuAs3 is antiferromagnetically ordered. In the range between 10 and 11K an incommensurate antiferromagnetic phase has been found whose periodicity changes continuously with temperature. The figure below (Fig.1) shows the intensity variation for the (101) reflection along with its satellites. Since the satellites are observed only near magnetic reflections of the commensurate antiferromagnetic phase, the incommensurate phase is antiferromagnetic with a continuously changing propagation vector.

click on figure for more details (Fig.1)

Fig.1: Temperature variations of the intensities of the - 101 magnetic reflexion and its satellites in EuAs3.

The possibility of an incommensurate structure in the antiferromagnet YMn2 has been investigated and further measurements have also been carried out on the Gd-Y system both at the Gd and Y rich end of the series (shown below. Fig.2). The magnetic structures of transition metal oxides have been extensively studied, but until a few years ago the magnetic structure of CuO was unknown. Availability of mm sized single crystals allowed the detailed determination of the low temperature antiferromagnetic structure.

click on figure for more details (Fig.2)

Fig.2: Diffracted intensity along the 10l direction versus temperature for Gd0.62Y0.38. The outer pair of satellites is due to the inplane component of the basal-plane helix, the inner pair to an out-of-plane oscillation of the spin vector.

In the figure below (Fig.3) dependence of the intensity of the (0.5, 0, -0.5) reflection given by this structure is plotted as a function of temperature. The intensity vanishes abruptly at Tc=213K, 17° below the Neel temperature deduced from susceptibility measurements. We have found that in the region between 213K and 230K an incommensurate antiferromagnetic phase occurs which is characterised by a propagation vector torr: -0.506a* + 0.483c*. This propagation does not change significantly in the temperature range 213 to 230K.

click on figure for more details (Fig.3)

Temperature dependence of the magnetic scattering in CuO. The triangles represent measurements of the (0.5, 0, -0.5) reflexion and the circles measurements of the (0.506, 0, -0.483) reflexion on twice the relative scale.

The intensities of the magnetic reflections lead to a model in which the couplings between moments within a unit cell are essentially the same as for the commensurate phase but in which the spin direction rotates in the a c plane so that the angle between spins separated by a vector torr if 2KT.torr. The temperature dependence of the (0.506, 0, -0.483) satellite intensity is also plotted in figure 3. The intensity has been multiplied by a factor 2 to compensate for the a c phase orientation of the moment. The plotted intensities are therefore proportional to the mean square value of the moment on the same arbitrary scale for both phases.

Last updated by Andrew Crowe on 09/02/1996