  The structural and magnetic properties of Ge0.5Mn0.5Co2O4 (GMCO) have been
investigated in detail utilizing neutron powder diffraction (NPD), x-ray
diffraction (XRD), DC magnetometry, and heat capacity analysis and compared
with GeCo2O4. Despite both compounds exhibiting a cubic structure at room
temperature, a substantial difference on low temperature structural properties
have been observed for GMCO, indicating the effect of Mn substitution on
crystal structure. The magnetic and heat capacity data reveal a ferrimagnetic
ordering around 108 K in GMCO. A minor secondary phase is confirmed which
undergoes long range AFM ordering at further lower temperatures. This secondary
phase remains undetected in XRD due to identical lattice parameters.
Furthermore, the analysis of heat capacity data indicates a broadening of the
high-temperature transition, attributing to the short-range correlation
persisting up to higher temperatures. The estimated magnetic entropy amounts is
63% of the value expected for GMCO. The missing entropy is likely linked with
the short-range magnetic correlations persisting well above the transition
temperature. Cation distribution at the A and B sites has been estimated in
GMCO using NPD. Magnetic structures are also confirmed in the main phase as
well as in the secondary phase using NPD analysis. The high-temperature
transition corresponds to the ferrimagnetic ordering of A and B site cations in
the main phase. A collinear ferrimagnetic arrangement of A and B site spins
aligned parallel to c axis is observed. The average values of A and B site
moments in the ferrimagnetic phase at 5 K are 2.31(3) and 1.82(3)mB,
respectively, with the temperature dependence of moments following the expected
power law behavior. The low-temperature ordering is attributed to the
antiferromagnetic ordering of B site ions associated with the secondary phase,
something similar to GeCo2O4.
