Role of the crystal electric field on the two magnetic transitions in the orthorhombic YbMnO3 perovskite

In orthorhombic ${\mathrm{LuMnO}}_{3}$ perovskite with nonmagnetic ${\mathrm{Lu}}^{+3}$ ions ($4{f}^{14}5{d}^{0}6{s}^{0}$) there exists one magnetic transition of ${\mathrm{Mn}}^{+3}$ ions at ${T}_{N}\ensuremath{\approx}35.5$ K. In ${\mathrm{YbMnO}}_{3}$ perovskite with ${\mathrm{Yb}}^{+3}$ ions of one electron less ($4{f}^{13}5{d}^{0}6{s}^{0}$), two magnetic transitions are observed at ${T}_{N}\ensuremath{\approx}43$ K and $T\ensuremath{\approx}3$--4 K, respectively. While the transition at 43 K in ${\mathrm{YbMnO}}_{3}$ was attributed to the same magnetic ordering of ${\mathrm{Mn}}^{+3}$ moments as ${\mathrm{LuMnO}}_{3}$ at 35.5 K, the nature of the magnetic transition at 3--4 K is still unresolved. Although the bump feature in magnetic susceptibility hints at a possible antiferromagnetic ordering of ${\mathrm{Yb}}^{+3}$ ions, no clear magnetic ordering of ${\mathrm{Yb}}^{+3}$ was determined in either neutron scattering diffraction or in M\"ossbauer spectra of iron-doped ${\mathrm{YbMnO}}_{3}$. To explain the amplitude-modulated nature of the low magnetic anomaly, we propose in this paper that a gradual quenching of the orbital moment of ${\mathrm{Yb}}^{+3}$ ions below a critical temperature is a possible mechanism. The small crystal field splitting of $4f$ electrons can survive only at extremely low temperature because of thermal fluctuation. In addition, we show, like in ${\mathrm{LuMnO}}_{3}$, that ${\mathrm{YbMnO}}_{3}$ takes an E-type antiferromagnetic ground state for its Mn ions and can be tuned into ferromagnetic state when (110)-oriented ${\mathrm{YAlO}}_{3}$ is used as a substrate.