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

We report a systematic study on the structural and magnetic properties of off-stoichiometric polycrystalline bulk spinels Mn1.15Co1.85O4 and Mn1.17Co1.60Cu0.23O4 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 I4(1)/amd. Both Co2+ and Cu2+ ions are located at the tetrahedral A site, and Mn3+ and Co3+ at the octahedral B site. Ferrimagnetic (FI) ordering of Mn1.15Co1.85O4 and Mn1.17Co1.60Cu0.23O4 sets in below 184 and 164 K, respectively. Magnetic structure analysis revealed that the ferrimagnetically coupled A(2+)- and B3+-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 (1) over bar0] . The net magnetic moment [2 mu(FI)(Mn-B/Co-B) - mu(FI)(Co-A)] of Mn1.15Co1.85O4 obtained from neutron data varies between 0.88-1.08 p,B which is in good agreement with M = 0.89-1.13 p,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 mu B ) and low-temperature (T = 1.9 K) field-dependent (H = 90 kOe) magnetization data (0.97-1.21 mu B ). From the three-sublattice model we obtained canting angles 28 degrees and 25 degrees for Mn1.15Co1.85O4 and Mn1.17Co1.60Cu0.23O4, respectively. Both the bulk systems exhibit high magnetocrystalline anisotropy (K-u similar to 9 x 10(5) and 7.5 x 10(5) erg/cm(3)) and a field-induced transition (H-D) across 4.0 kOe due to the domain reorientation. Temperature (1.9-350 K) and field (+/- 90 kOe) dependence of magnetization data confirms the high-spin (S = 3/2 and S = 2) ground-state configuration for both the divalent Co and trivalent Mn.