The magnetization reversal in CoFe2O4/CoFe2 granular systems

• Published in: Journal of nanoparticle research : an interdisciplinary forum for nanoscale science and technology 2016-12, Vol.18 (12), Article 383
• Main Authors: Jin, J., Sun, X., Wang, M., Ding, Z.L., Ma, Y.Q.
• Format: Article
• Language: English
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Inorganic Chemistry; Lasers; Materials Science; Nanotechnology; Optical Devices; Optics; Photonics; Physical Chemistry; Research Paper
• ISSN: 1388-0764; 1572-896X
• DOI: 10.1007/s11051-016-3690-9

The temperature-dependent field cooling (FC) and zero-field cooling (ZFC) magnetizations, i.e., M FC and M ZFC , measured under different magnetic fields from 500 Oe to 20 kOe have been investigated on two exchange–spring CoFe 2 O 4 /CoFe 2 composites with different relative content of CoFe 2 . Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at a field-dependent irreversible temperature T irr . For the sample with less CoFe 2 , the curves of − d ( M FC  −  M ZFC )/ dT versus temperature T exhibit a broad peak at an intermediate temperature T 2 below T irr , and the moments are suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the − d ( M FC  −  M ZFC )/ dT curves of the sample with more CoFe 2 , besides a broad peat at an intermediate temperature T 2 , a rapid rise around the low temperature T 1 ~15 K is observed, below which the moments are suggested to fully freeze. Increase of magnetic field from 2 kOe leads to the shift of T 2 and T irr towards a lower temperature, and the shift of T 2 is attributable to the moment reversal of CoFe 2 O 4 . Graphical abstract CoFe 2 O 4 /CoFe 2 composites with different relative content of CoFe 2 were prepared by reducing CoFe 2 O 4 in H 2 for 4 h (S4H) and 8 h (S8H). The temperature-dependent FC and ZFC magnetizations, i.e., M FC and M ZFC , under different magnetic fields from 500 Oe to 20 kOe have been investigated. Two samples exhibit different magnetization reversal behaviors. With decreasing temperature, a progressive freezing of the moments in two composites occurs at field-dependent irreversible temperature T irr . For the S4H sample, the curves of − d ( M FC  −  M ZFC )/ dT versus temperature T exhibit a broad and field-dependent relaxing peak at T 2 below T irr (figure a), and the moments were suggested not to fully freeze till the lowest measuring temperature 10 K. However, for the S8H sample, it exhibits the reentrant spin-glass state around 50 K, as evidenced by a peak in the M FC curve (inset in figure b) and as a result of the cooperative effects of the random anisotropy of CoFe 2 O 4 , exchange–spring occurring at the interface of CoFe 2 O 4 and CoFe 2 together with the inter-particle dipolar interaction (figure c); in − d ( M FC  −  M ZFC )/ dT curves, besides a broad relaxing peat at T 2 , a rapid rise around the low-temperature T 1 ~15 K is observed, below wh