Magnetic structure evolution and magnetoelastic coupling across the spin reorientation transition in TmCrO 3

We present a comprehensive study of the magnetic structure evolution, employing bulk magnetization and neutron powder diffraction (NPD) measurements down to 3.5 K, across the spin reorientation transition in orthorhombic (Pnma) ${\mathrm{TmCrO}}_{3}$. Magnetic susceptibility reveals canted antiferromagnetic (CAFM) ordering at ${T}_{N}=125\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, followed by the magnetization reversal, showing minima between the two compensation points ${T}_{\mathrm{comp}1}$ and ${T}_{\mathrm{comp}2}$. Heat capacity shows a \ensuremath{\lambda}-transition at ${T}_{N}$, associated with long-range antiferromagnetic order of Cr, followed by a broad feature at \ensuremath{\sim} 9 K, arising due to Schottky-type anomaly. Magnetic refinement of NPD data establishes the $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}a$: \ensuremath{\Gamma}2(${C}_{x},{G}_{y};{C}_{x}^{Tm},\phantom{\rule{4pt}{0ex}}{F}_{z}^{Tm}$) CAFM structure below ${T}_{N}$. A gradual change in magnetic structure occurs during the spin-reorientation (SRO) transition below 30 K. During this, the magnetic structure gradually changes from $P{n}^{\ensuremath{'}}{m}^{\ensuremath{'}}a$ (\ensuremath{\Gamma}2) to $P{n}^{\ensuremath{'}}m{a}^{\ensuremath{'}}$ (\ensuremath{\Gamma}4): ${A}_{x},{F}_{y},\phantom{\rule{4pt}{0ex}}{G}_{z};{F}_{y}^{Tm}$. Below 30 K, however, neither \ensuremath{\Gamma}2 nor \ensuremath{\Gamma}4 alone adequately fit the intensity of magnetic reflections. A satisfactory refinement could only be achieved in the monoclinic subgroup $P{{2}^{\ensuremath{'}}}_{1}/{c}^{\ensuremath{'}}$ derived from a combination of \ensuremath{\Gamma}2 and \ensuremath{\Gamma}4. The observed gradual SRO of Tm and Cr moments is consistent with the magnetic symmetry $P{{2}^{\ensuremath{'}}}_{1}/{c}^{\ensuremath{'}}$: ${C}_{x},{G}_{y},\phantom{\rule{4pt}{0ex}}{G}_{z};{C}_{x}^{Tm},\phantom{\rule{4pt}{0ex}}{F}_{y}^{Tm},{F}_{z}^{Tm}$. Furthermore, the ordered moments Cr and Tm in ${\mathrm{TmCrO}}_{3}$ exhibit a complex, nonmonotonic temperature dependence as expected during SRO. Anomalies in lattice parameters suggest magnetoelastic coupling, linking structural distortions to the SRO.