Charge ordering melting in CaMnO3-δ single crystals with ordered oxygen vacancies

The magnetization curves of CaMnO3-delta (delta=0.20 and 0.25) single crystals in pulsed magnetic fields up to 350 kOe were studied, and the threshold increase of magnetization at H-cr similar to 50 kOe and T=4.2 K was discovered. This anomaly is explained by melting of the charge ordering, which is associated with oxygen vacancy ordering. The jump at the metamagneticlike transition is equal to 0.02 mu(B)/Mn; no hysteresis is evidenced in the field-up and field-down processes. The magnetization dependence versus magnetic field is almost linear at T=77 K. The results obtained are explained assuming high stability of the antiferromagnetic G-type charge ordering phase at T=4.2 K and its destabilization under magnetic field or temperature increase because of the double-exchange reinforcement between Mn3+ and Mn4+ ions. The oxygen vacancy ordering can be considered as an additional degree of freedom in manganites.