High-resolution time of flight neutron diffraction and magnetization studies of spin reorientation and polar transitions in SmCrO_3

摘要

Rare-earth chromates have always been of interest due to temperature-induced magnetization reversal and spin-reorientation phase transitions (SRPTs). In orthochromates containing magnetic rare earths, the spin configuration is supposed to undergo a characteristic changeover across the SRPT followed by an independent ordering of rare-earth moments leading to polar order. However, due to the presence of nearly 14% of highly neutron-absorbing isotope ~(149)Sm in natural Sm based compounds, correct magnetic structure determination of SmCrO_3 through neutron diffraction measurements has been a challenge. In the present study we investigate the pre- and post-SRPT spin configurations in well characterized SmCrO_3 through time of flight neutron diffraction measurements carried out in zero field at the high-resolution high-flux WISH beam line of ISIS, in the United Kingdom. Magnetization measurement shows a canted antiferromagnetic phase transition at T_(N1) = 192K, giving rise to a weak ferromagnetism, which undergoes a SRPT at 37 K. Rietveld analysis of the neutron powder diffraction data shows that below T_(N1) = 192K the Cr~(3+) and Sm~(3+) moments order in a Pb′n′m:Γ_4(G_x, A_y F_Z; F_Z~R) spin configuration with their tiny ferromagnetic components F_Z and F_Z~R , giving rise to weak ferromagnetism. Below 37 K the Pb′n′m:Γ_4(G_x, A_y F_Z; F_Z~R) configuration transforms to Pb′n′m:Γ_2(F_x, C_y G_Z; F_x~R,C_y~R) as a result of continuous rotation of Cr~(3+) moments, while approaching SRPT below T_(N1·) At still lower temperatures the Pbn′m′ :Γ_2(F_x, C_y G_Z; F_x~R,C_y~R) phase transforms to polar phases, either the P2′_12′_12_1 : Γ_(26)(C_x, G_y, F_z;C_x~R, A_y~R, F_z~R) or the Pn′a2′_1 :Γ_(27)(F_x, C_y, G_z; F_x~R, C_y~R, G_z~R) phase, as a result of independent antiferromagnetic ordering of Sm~(3+) moments at T_(N2) ＜ 4K through Sm~(3+) -Sm~(3+) direct interaction. Our result of the transformation of SmCrO_3 from Γ_4 to Γ_2 below SRPT is in contradiction with the Γ_1(A_x, G_y,C_Z;C_z~R) spin configuration as reported in Tripathi et al. [Phys. Rev. B 96, 174421 (2017)]. This issue has been independently settled through ground-state energy calculation using spin-dependent density functional theory confirming the Γ_2 spin configuration to be of lower energy as compared to that of the Γ_1. The role of magnetocrystalline anisotropy in the occurrence of SRPT has been discussed.