Effect of rare earth dopants on the magnetic ordering of frustrated h-YMnO3

Abstract

In this report the combined effects of chemical pressure and R–Mn interaction on the structural and magnetic properties of frustrated h-YMnO3 have been investigated. Towards this, neutron powder diffraction and magnetization measurements were carried out on isostructural compounds Y1−xRxMnO3(R=Yb, Er, Tb, Ho; x≤0.2) with hexagonal structure (P63cm space group). The dopants are evenly distributed between the two Yttrium sites. The unit cell volume shows a linear increase with average A-site ionic radii, <rA>. The average apical <Mn–O>a and planar <Mn–O>p bond lengths are found to increase with <rA>. The tilting angle of the MnO5 polyhedron decreases linearly with increase in <rA>, whereas the buckling angle remains constant. No significant change in TN (within 10K) is observed on doping. The temperature variation of the volume indicates an anomalous reduction in volume at TN which is found to be correlated with the square of the antiferromagnetic Mn moment. A spin reorientation behavior (evident from a change in the irreducible representation (IR) Γ3 to Γ4) is observed on decreasing <rA> from 1.019Å (Y) to 1.012Å (Yb) similar to that reported in external pressure studies on YMnO3. Additional interaction between the doped R and Mn influences the magnetic structure in the case of Ho and Tb doped samples. With Ho doping at Y site, the magnetic structure is described by IR Γ3 alone for 5K≤T<35K and a mixture of Γ3 and Γ4 for T≥35K. However, in Tb doped sample, the magnetic structure is better described by Γ4Tb,Mn IR with additional moment on Tb. The frustration parameter, f reduces from 6 to 1 in the doped samples. A combination of chemical pressure effect and magnetic coupling between the magnetic R ion and Mn moments thus describes the magnetic structures and relieves the frustration effects inherent to the quasi-two dimensional Mn moment ordering.