Frequency dependence of Néel temperature in CaMnO3−δ ceramics: Synthesized by two different methods

• CaMnO3−δ (δ=0.02) powders were synthesized by sol–gel and solid-state reaction methods.
• characterizations of the samples were preformed employing XRD, SEM and TEM techniques.
• The orthorhombic phase of CaMnO3−δ starts to form at calcination temperature of 800 °C, in both procedures.
• The frequency dependence of Néel temperature and hysteresis loop were observed in both types of the specimens.

Calcium manganese oxide CaMnO3−δ (δ=0.02) nanopowders were synthesized by two different methods; sol–gel and solid-state reaction. X-ray diffraction patterns indicated that both types of the prepared powders have orthorhombic symmetry structures at room temperature. Further characterizations of the samples were preformed employing SEM and TEM techniques. At low magnetic fields, the phase transition temperature from antiferromagnetic to paramagnetic was found to be slightly higher in the sintered O-ring shape specimens made from powders synthesized by sol–gel method. This result can be described in relation to Mn3+–O–Mn4+ double-exchange interaction and also electron hopping mechanism which occur more in these specimens than those made from solid-state reaction powders. The frequency dependence of Néel temperature and the shape of the hysteresis loops, observed in both types of the prepared ceramics specimens, can be attributed to the reduction of relaxation time which occurs with increasing the frequency.