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Erschienen in:
01.12.2016 | Research Paper
The magnetization reversal in CoFe2O4/CoFe2 granular systems
Erschienen in: Journal of Nanoparticle Research | Ausgabe 12/2016
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Abstract
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 CoFe2O4/CoFe2 composites with different relative content of CoFe2. 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 CoFe2, 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 CoFe2, 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 CoFe2O4.
Graphical abstract
CoFe2O4/CoFe2 composites with different relative content of CoFe2 were prepared by reducing CoFe2O4 in H2 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 CoFe2O4, exchange–spring occurring at the interface of CoFe2O4 and CoFe2 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 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 CoFe2O4.
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