High-resolution time of flight neutron diffraction and magnetization studies of spin reorientation and polar transitions in SmCrO3

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 Sm-149 in natural Sm based compounds, correct magnetic structure determination of SmCrO3 through neutron diffraction measurements has been a challenge. In the present study we investigate the pre- and post-SRPT spin configurations in well characterized SmCrO3 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 = 192 K, 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 = 192 K the Cr3+ and Sm3+ moments order in a Pb'n'm:Gamma(4) (G(x) , A(y) , F-z; F-Z(R)) spin configuration with their tiny ferromagnetic components F-z and (R)(Z), giving rise to weak ferromagnetism. Below 37 K the Pb'n'm:Gamma(4) (G(x), A(y), F-z; F-Z(R)) configuration transforms to Pbn'm': Gamma(2) (F-x, C-y, G(z); F-x(R), C-y(R)) as a result of continuous rotation of Cr3+ moments, while approaching SRPT below T-N1 . At still lower temperatures the Pbn'm': Gamma(2) (F-x, C-y, G(z);F-x(R), C-y(R)) phase transforms to polar phases, either the P2(1)'2(1)'2(1) : Gamma(26)(C-x, G(y), F-z;c(x)(R), A(y)(R), F-z(R)) or the Pn'a2(1)' : Gamma(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 < 4 K through Sm3+ -Sm3+ direct interaction. Our result of the transformation of SmCrO3 from Gamma(4) to Gamma(2) below SRPT is in contradiction with the Gamma(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 Gamma(2) spin configuration to be of lower energy as compared to that of the F l . The role of magnetocrystalline anisotropy in the occurrence of SRPT has been discussed.