Strong correlation between structure and magnetic ordering in tetragonally distorted off-stoichiometric spinels Mn1.15Co1.85O4 and Mn1.17Co1.60Cu0.23O4

Abstract

We report a systematic study on the structural and magnetic properties of off-stoichiometric polycrystalline bulk spinels ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$ using neutron and x-ray diffraction, ferromagnetic resonance, and magnetic measurements. Both compounds show a weak tetragonal distortion with $c/a<1$, where the crystal structure could be refined in the tetragonal space group $I{4}_{1}/amd$. Both ${\mathrm{Co}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ ions are located at the tetrahedral $A$ site, and ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Co}}^{3+}$ at the octahedral $B$ site. Ferrimagnetic (FI) ordering of ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$ sets in below 184 and 164 K, respectively. Magnetic structure analysis revealed that the ferrimagnetically coupled ${A}^{2+}$- and ${B}^{3+}$-site moments are aligned parallel to the tetragonal $c$ axis. Additionally, a noncollinear antiferromagnetic order appears in the $ab$ plane, where the moments point along [110] and $[1\overline{1}0]$. The net magnetic moment [$2{\ensuremath{\mu}}_{\mathrm{FI}}({\mathrm{Mn}}_{B}$/${\mathrm{Co}}_{B})\ensuremath{-}{\ensuremath{\mu}}_{\mathrm{FI}}({\mathrm{Co}}_{A}$)] of ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ obtained from neutron data varies between 0.88--1.08 ${\ensuremath{\mu}}_{B}$ which is in good agreement with $M=0.89\text{--}1.13$ ${\ensuremath{\mu}}_{B}$ as determined from magnetization measurements. However, for the Cu-containing compound a larger discrepancy in the magnetic moment was observed between the neutron data (1.89--1.92 ${\ensuremath{\mu}}_{B}$) and low-temperature ($T=1.9$ K) field-dependent ($H=90$ kOe) magnetization data ($0.97\text{--}1.21$ ${\ensuremath{\mu}}_{B}$). From the three-sublattice model we obtained canting angles ${28}^{\ensuremath{\circ}}$ and ${25}^{\ensuremath{\circ}}$ for ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_{4}$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_{4}$, respectively. Both the bulk systems exhibit high magnetocrystalline anisotropy (${K}_{\text{U}}\ensuremath{\sim}9\ifmmode\times\else\texttimes\fi{}{10}^{5}$ and $7.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ erg/${\mathrm{cm}}^{3}$) and a field-induced transition (${H}_{\text{D}}$) across 4.0 kOe due to the domain reorientation. Temperature ($1.9\text{--}350$ K) and field ($\ifmmode\pm\else\textpm\fi{}90$ kOe) dependence of magnetization data confirms the high-spin ($S=\frac{3}{2}$ and $S=2$) ground-state configuration for both the divalent Co and trivalent Mn.