Improved magnetic performance at low and high temperatures in non-exchange-coupling CoFe2O4/CoFe2 nanocomposites

摘要

The dispersive and uniform CoFe2O4 nanoparticles were synthesized by thermal decomposition of metal organic salt in organic solvent with high boiling point, and subsequently reduced in H-2 ambient for 1.5, 4 and 8 h to obtain the CoFe2O4/CoFe2 composite. The M(H) loops of all composites were measured at 10, 50, 100, 150, 200, 250, 300 and 390 K, respectively. The jumps around the zero magnetic field in M(H) loops below 200 K signify that no exchange-coupling takes place between soft and hard species. At 10 K, the maximum H-c, and M-r/M-s ratio are 16 kOe and 0.81 for the sample reduced for 1.5 h, the much larger value than that previously observed. When the measuring temperature increases, the H-c value monotonously decreases to 1.8, 1.7, 1.2 kOe, and correspondingly the M-r/M-s ratio decreases to 0.55, 0.45 and 0.40 at T = 300 K for samples reduced for 1.5, 4 and 8 h, respectively; these higher values at room temperature has not been obtained previously in CoFe2O4/CoFe2 composites as far as we know. We suggest that it is important to keep high anisotropy and weak dipolar interaction for improving H-c and M-r/M-s and that the M-r/M-s will be further improved if the exchange coupling was really realized between soft and hard magnets. Further investigations are in process. (C) 2015 Elsevier B.V. All rights reserved.