Structure and bi-magnetism of nanocomposites and nanoalloys synthesized by reduction of (Co, Ni) Fe2O4 nano-ferrites.
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Date
2014
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Abstract
Structural and magnetic properties of CoFe2O4, NiFe2O4 nano-ferrites, CoFe2O4
/CoFe2, NiFe2O4/NiFe bi-magnetic nanocomposites and CoFe2, NiFe alloys were
studied. The nano-ferrites were synthesised using the glycol-thermal method at 200
C for 6 hours. The nanocomposites and alloys were produced by the reduction of
the ferrites using di erent amounts of activated charcoal. The reduction reaction was
performed at 900 C for 3 hours in high purity
owing argon atmosphere. Complete
reduction yields the alloy phases while partial reduction produces the nanocomposites.
The samples were studied using X-ray di raction (XRD), high resolution
transmission electron microscopy (HRTEM), high resolution scanning electron microscopy
(HRSEM), 57Fe M ossbauer spectroscopy, LakeShore vibrating sample magnetometer
and a mini cryogen free system. XRD measurements of the nano-ferrites
showed a single phase spinel structure with average crystallite sizes of about 10 nm.
the CoFe2 alloy was obtained at an activated charcoal to ferrite molar ratio nC = 6
while the NiFe alloy was obtained at nC = 5. XRD results of the reduced samples
of NiFe2O4 showed the coexistence of bcc -Fe and fcc
-Fe lattice structures for
NiFe alloy. The HRTEM and HRSEM measurements of all the reduced samples
showed clear di erences between the morphology of the parent nano-ferrites and
the reduced samples. 57Fe M ossbauer spectra measurements of the reduced samples
showed the transformation of the spinel structure to the alloy phase. The presence
of mixed phases in the nanocomposites was revealed. A small amount of mixed
phase in CoFe2 and NiFe alloys which was not detected by XRD was revealed by
M ossbauer measurement for nC = 6 and nC = 5 respectively. The M ossbauer ts
showed that Fe3+ was reduced to Fe2+ with lower magnetic hyper ne elds observed
in the reduced samples. High eld (50 kOe) magnetization measurements at room
temperature for CoFe2O4/CoFe2 nanocomposite show signi cant enhancement of
saturation magnetization from 63 emu/g of the parent nano-ferrite to 221 emu/g at
nC = 5. In the NiFe2O4/NiFe nanocomposite the in magnetization increased from 57
emu/g to 141 emu/g at nC = 6. Low temperature measurements performed on the
nanocomposites exhibited higher magnetizations. The coercivity of the fully reduced
samples of the nano-ferrites were observed to be less dependent on temperature.
Description
M. Sc. University of KwaZulu-Natal, Durban 2014.
Keywords
Nanocomposites (Materials), Ferrites (Magnetic materials), Ferrimagnetism., Ferromagnetism., Magnetism., Alloys., Theses--Physics.