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 $¹⁴⁹\mathrm{Sm}$ in natural Sm based compounds, correct magnetic structure determination of ${\mathrm{SmCrO}}₃$ through neutron diffraction measurements has been a challenge. In the present study we investigate the pre- and post-SRPT spin configurations in well characterized ${\mathrm{SmCrO}}₃$ 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 ${T}_N1=192\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, giving rise to a weak ferromagnetism, which undergoes a SRPT at 37 K. Rietveld analysis of the neutron powder diffraction data shows that below ${T}_N1=192\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ the ${\mathrm{Cr}}³⁺$ and ${\mathrm{Sm}}³⁺$ moments order in a $P{b}^{\ensuremath{'}}{n}^{\ensuremath{'}}m$:${\mathrm{\ensuremath{\Gamma}}}₄$(${\mathbit{G}}_{\mathbit{x}},{A}_y,{F}_Z;{F}_Z^R$) spin configuration with their tiny ferromagnetic components ${F}_Z$ and $\phantom{\rule{0.28em}{0ex}}{F}_Z^R,$ giving rise to weak ferromagnetism. Below 37 K the $P{b}^{\ensuremath{'}}{n}^{\ensuremath{'}}m$:${\mathrm{\ensuremath{\Gamma}}}₄$(${\mathbit{G}}_{\mathbit{x}},{A}_y,{F}_Z;{F}_Z^R$) configuration transforms to $Pb{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}:{\mathrm{\ensuremath{\Gamma}}}₂$(${F}_x,{C}_y,{\mathbit{G}}_{\mathbit{Z}};{F}_x^R,\phantom{\rule{0.16em}{0ex}}{\mathbit{C}}_{\mathbit{y}}^{\mathbit{R}}$) as a result of continuous rotation of ${\mathrm{Cr}}³⁺$ moments, while approaching SRPT below ${T}_N1$. At still lower temperatures the $Pb{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}:{\mathrm{\ensuremath{\Gamma}}}₂$(${F}_x,{C}_y,{\mathbit{G}}_{\mathbit{Z}};{F}_x^R,\phantom{\rule{0.16em}{0ex}}{\mathbit{C}}_{\mathbit{y}}^{\mathbit{R}}$) phase transforms to polar phases, either the $P{2}₁^{\ensuremath{'}}{2}₁^{\ensuremath{'}}{2}₁:{\mathrm{\ensuremath{\Gamma}}}₂₆$(${C}_x,{\mathbit{G}}_{\mathbit{y}},{F}_z;{\mathbit{C}}_{\mathbit{x}}^{\mathbit{R}},\phantom{\rule{0.16em}{0ex}}{A}_y^R,\phantom{\rule{0.16em}{0ex}}{F}_z^R$) or the $P{n}^{\ensuremath{'}}a{2}₁^{\ensuremath{'}}:{\mathrm{\ensuremath{\Gamma}}}₂₇$(${F}_x,{C}_y,{\mathbit{G}}_{\mathbit{z}};{F}_x^R,\phantom{\rule{0.16em}{0ex}}{\mathbit{C}}_{\mathbit{y}}^{\mathbit{R}},{G}_z^R$) phase, as a result of independent antiferromagnetic ordering of ${\mathrm{Sm}}³⁺$ moments at ${T}_N2<4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ through ${\mathrm{Sm}}³⁺\text{\ensuremath{-}}{\mathrm{Sm}}³⁺$ direct interaction. Our result of the transformation of ${\mathrm{SmCrO}}₃$ from ${\mathrm{\ensuremath{\Gamma}}}₄$ to ${\mathrm{\ensuremath{\Gamma}}}₂$ below SRPT is in contradiction with the ${\mathrm{\ensuremath{\Gamma}}}₁$(${A}_x,{\mathbit{G}}_{\mathbit{y}},{C}_Z;{C}_z^R$) 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 ${\mathrm{\ensuremath{\Gamma}}}₂$ spin configuration to be of lower energy as compared to that of the ${\mathrm{\ensuremath{\Gamma}}}₁$. The role of magnetocrystalline anisotropy in the occurrence of SRPT has been discussed.