Magnetic order in Mg doped YbMnO3 studied by neutron diffraction

We have studied the effect of Mg substitution in hexagonal YbMnO3 manganites by investigating the compounds Yb1-xMgxMnO3 (x = 0.0 and 0.05). The magnetic structure for both compounds has been determined from temperature dependent neutron diffraction (ND) experiments. The magnetic structure for YbMnO3 can be determined by considering Γ2 representation for Yb and Γ4 for Mn ions. The Néel temperature (TN) for YbMnO3 is ∼ 83 K and with 5% Mg doping, there is a marginal increase in TN to 86 K. The thermal evolution of ND patterns for both compounds and the temperature dependence of integrated intensities of Bragg peaks (101) and (102) is shown. We find that for x = 0.05, there is no change in the pristine magnetic structure. However, profound changes in the intensities of magnetic peaks are seen as a function of temperature. For both compounds, below their respective TN, the purely magnetic Bragg peak with index (101) develops. The temperature dependence of the (100) peak for both samples shows a sudden increase below 10 K which suggests the ordering of Yb ions. The (101) peak intensity abruptly increases below 83 K and 86 K for x = 0.0 and 0.05, respectively, indicating the ordering in Mn plane. Hence, it may be concluded that the variation in integrated intensities of peaks (100) and (101) is controlled by Yb and Mn ordering, while (102) Bragg peak has contribution from nuclear and magnetic intensities. Moreover, one observes a strong (101) peak in YbMnO3 with thermal evolution akin to that of the (100) peak in YMnO3. It may however be recalled that in YMnO3, the Mn order does not induce a strong (101) magnetic peak, hence in this case it means that Mn order is affected by Yb site and Mg doping will have influence on the magnetic order. (author)