Disorder-induced ferrimagnetism in Mn_xCoGe films

We report on the observation of ferrimagnetism in Ni₂In-type Mn_xCoGe films grown by magnetron sputtering for 0.8 ≤ x ≤ 2.5. The ferrimagnetic behavior is observed in contrast to the ferromagnetism reported for bulk Ni₂In-type MnCoGe, which we attribute to the presence of defects. In the Mn-deficient samples, these defects arise from atomic disorder, whereas for the Mn-rich samples, the excess Mn is driven onto the Co or Ge sites. For lower values of x, the concentration of antisite defects increases and leads to a compensation point in the temperature-dependent magnetization (N or V-type ferrimagnetic behavior). Density-functional theory calculations explain the origin of the ferrimagnetism through antiferromagnetic coupling between Mn antisite defects and the Mn on the Mn-sites. Mean-field modeling supports the analysis and provides an estimate of the evolution of effective exchange parameters with Mn concentration.