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.