Role of rare earth magnetism on structural and magnetic properties of HoMn_1-xCr_xO₃

The effect of doping Cr on the structural and magnetic properties has been investigated in a series of polycrystalline compounds HoMn_1-xCr_xO₃ (0≤x≤ 1). Orthorhombic HoMnO₃ exhibits E-type magnetic ordering and the polarization observed in this compound is attributed to exchange-striction. Doping hex-HoMnO₃ with non JT Cr³⁺ causes it to crystallize in orthorhombic structure (Pnma) for x>0.125 and in a mixed phase of hexagonal and orthorhombic for lower Cr dopings. A decrease in the volume is observed with increasing x in agreement with the lower ionic radii of Cr³⁺ as compared to Mn³⁺. The structure changes from O (c3+ ions. The temperature dependence of magnetization (M(T) curves) is shown. For x>0.5 a ferro-paramagnetic transition is observed at T=140K. The field dependence of magnetization (M(H) curves) at 5K are shown in the inset. Magnetization curves do not show any hysteresis at room temperature but at lower temperatures (∼ 5K), a small hysteresis curve is obtained, with increasing ferromagnetic moment as a function of x, indicating weak ferromagnetic nature of the compound. The neutron diffraction pattern at 6K for a few representative samples is shown. For x<0.375 no magnetic contribution is observed, indicating these samples are paramagnetic down to 6K. For x = 0.375 and 0.5 superlattice reflections (110) and (011) are observed at 6K and the magnetic structure is described by Γ₄ (FyGz) representation with moment on Mn/Cr site 2.3μ_B and 1.9μ_B for x=0.5 and 0.375 respectively. No evidence of Ho moment is observed for these samples. A sharp rise in some magnetic reflections for x=0.75-1, observed below approximately 30K, indicates the existence of induced magnetic ordering of the Ho³⁺ moments. The magnetic structure of these samples was fitted with Γ₂ IR (G_yF_z for Cr and C_xF_z for Ho). The moments are ∼ 2.6 μB at Ho site and ∼ 2.6 μB at Cr/Mn site for x=0.75, 0.875, 1 samples at 6K. The phase diagram for the entire series is shown. (author)