Structure and magnetic properties ofLaMn1−xMgxO3compounds

Abstract

We substitute $\mathrm{Mg}$ for $\mathrm{Mn}$ in the ${\mathrm{LaMnO}}_{3}$ compound. ${\mathrm{LaMn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{O}}_{3}$ single phases can be formed up to $x=0.5$. Structural and electronic properties were studied by means of x-ray diffraction and x-ray absorption spectroscopy. A double perovskite is observed for $x\ensuremath{\geqslant}0.4$ whereas no long $\mathrm{Mn}∕\mathrm{Mg}$ ordering is detected for $x\ensuremath{\leqslant}0.3$. The former samples also show a structural transition from rhombohedral to monoclinic with decreasing temperature. The local structure around $\mathrm{Mn}$ evidences the absence of Jahn-Teller distortion since $x=0.1$. The samples with low $\mathrm{Mg}$ content have a ferromagnetic ground state while samples with $x\ensuremath{\geqslant}0.3$ undergo a spin-glass transition at low temperatures. The frequency dependence of the freezing temperature follows a Vogel-Fulcher law and it is similar to canonical spin-glasses. The high temperature ac susceptibility of all samples obeys the Curie-Weiss law but the effective paramagnetic moments do not agree with a spin-only contribution for $x\ensuremath{\leqslant}0.4$. Finally, we have demonstrated that $\mathrm{Mg}$ does not replace $\mathrm{La}$ in the perovskite lattice by studying selected ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{MnO}}_{3}$ samples. It always enters onto the perovskite B-site.