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

• The dispersive and uniform CoFe2O4 nanoparticles were synthesized.
• High magnetic parameters were obtained for CoFe2O4/CoFe2.
• Strategy to improve magnetic performance of CoFe2O4/CoFe2 was suggested.

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 H2 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 Hc and Mr/Ms 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 Hc value monotonously decreases to 1.8, 1.7, 1.2 kOe, and correspondingly the Mr/Ms 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 Hc and Mr/Ms and that the Mr/Ms will be further improved if the exchange coupling was really realized between soft and hard magnets. Further investigations are in process.

Numerous efforts were made to improve the magnetic performance of the CoFe2O4 (hard)/CoFe2 (soft) composite. Unfortunately almost all reported Mr/Ms values are smaller than 0.5. Based on the analyses of our previous work and the results in literature reported by other groups, it can be suggested that the severely agglomerated nanoparticles result in the strong interparticle dipolar interaction, which suppresses the Mr/Ms ratio. In this work the dispersive and uniform CoFe2O4 nanoparticles were synthesized, as shown in the inset of figure (a), and subsequently reduced in H2 ambient for 90, 240 and 480 min to obtain the CoFe2O4/CoFe2 composites. The reduction reaction was performed at relatively low temperature of 300 °C in order to weaken as possibly the aggregating degree.The M(H) loops of all samples were measured in temperature region 10–390 K, and figure (a) only shows the loop at 10 K for the sample reduced for 90 min. Some results obtained from the loops are as follows: (1) At 10 K, the maximum Hc and Mr/Ms ratio are 16 kOe and 0.81, the much larger value than that previously observed. (2) 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. (3) At 300 K, the curves of Hc, Ms and Mr/Ms versus reduction time for samples in references and in our work are plotted in figures (b)–(d), where we can see that the higher Hc and Mr/Ms values were obtained in our samples. So we confirm that it is important to keep high anisotropy and weak dipolar interaction for improving Hc and Mr/Ms and that the Mr/Ms will be further improved if the exchange coupling was really realized between soft and hard magnets. Further investigations are in process.Figure optionsDownload as PowerPoint slide