Colossal magnetoresistance of oxide spinels, CoxMn3-xO4

CoxMn3-xO4 (x = 0.6 to 2) spinels exhibit a transition between para- to fer rimagnetic phase below 200 K. This transition is accompanied by a change in slope of the electrical resistance from negative to positive temperature c oefficient (NTC to PTC) although associated with large resistivity of simil ar to 10(9) Omega cm. Under the external magnetic field, the electrical res istivity decreases by more than one order of magnitude around the transitio n temperature. The magnetoresistance (MR) coefficient maximizes around 0.8. The resistivity ratio, rho/rho(H = 0), does not saturate with magnetic fie ld, confirming that the large MR is the result of colossal magnetoresistanc e (CMR). Doping with Li or Cu decreases the room temperature resistivity as a result of the increase in hole concentration. However, Cu decreases the MR because addition of copper ions reduces the exchange interactions. Analy ses of the complex impedance spectra show that the grain boundary resistanc e R-gb increases with decrease in temperature and the large resistivity may be due to the trapping of charge carriers by the oxygen vacancies. Charge trapping at the defect centres is also indicated from the variable maximum in the rho-T curves of the same specimen (x = 0.6) annealed in different p( O2) at elevated temperatures. (C) 1999 Elsevier Science B.V. All rights res erved.