Structural Inhomogeneities and Suppressed Magnetostructural Coupling in Mn-Substituted GeCo2O4

A comprehensive study of the Ge_1–xMn_xCo₂O₄ (GMCO) system was conducted using neutron powder diffraction (NPD), X-ray diffraction (XRD), scanning electron microscopy, magnetometry, and heat capacity measurements. Comparative analysis with GeCo₂O₄ (GCO) highlights the influence of Mn substitution on crystal and magnetic structures at low temperatures. Surprisingly, phase separation is observed in GMCO with a targeted nominal composition with Ge/Mn close to 50:50. Scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis reveals pronounced compositional inhomogeneities, which are not evident in the XRD data. The GMCO sample predominantly consists of a Mn-rich primary phase with approximate stoichiometry Mn_0.74Ge_0.18Co₂O₄, along with a minor Ge-rich secondary phase of composition Ge_0.91Mn_0.18Co₂O₄. Although both GCO and GMCO crystallize in cubic symmetry at room temperature, a substantial difference in low-temperature structural properties has been observed. Magnetic and heat capacity data indicate ferrimagnetic ordering in Mn_0.74Ge_0.18Co₂O₄ near T_c = 108 K, while Ge_0.91Mn_0.18Co₂O₄ exhibits antiferromagnetic order at T_N = 22 K. Analysis of heat capacity data reveals that the estimated magnetic entropy amounts to only 56% of the theoretical value expected in GMCO. A collinear ferrimagnetic arrangement is observed in Mn_0.74Ge_0.18Co₂O₄ below the magnetic ordering temperature, characterized by antiparallel spins of Mn at the A site and Co at the B site along the c-direction. At 5 K, the refined magnetic moments are 2.31(3) μ_B for Mn_A and 1.82(3) μ_B for Co_B in the Mn_0.74Ge_0.18Co₂O₄ phase. The magnetic structure at 5 K in Ge_0.91Mn_0.18Co₂O₄ is identical to the antiferromagnetic structure of the parent compound GeCo₂O₄. The refined value of the Co_B moment in this phase at 5 K is 2.53(3) μ_B.