We present a study of the synthesis, structural and magnetic properties of
bulk and nanosized (Zn, Cd, Cu)0:5Ni0:5Fe2O4 and NiFe2O4 compounds. The
e®ects of electronic con¯guration and atomic sizes of Zn, Cd, Cu and Ni on
the magnetic properties of the ferrites are the primary focus of the study.
Di®erent synthesis routes, preparation conditions and how they a®ect single
phase formation are explored.
The synthesis was undertaken by solid{state reaction, combustion, hydrothermal
and glycothermal techniques. The structure determination was by Xray
di®raction. The magnetic measurements were performed using MÄossbauer
spectroscopy (from 79 K to about 850 K) and a vibrating sample magnetometer
(at about 300 K). The bulk densities of the sintered pellets were deduced
by Archimedes principle. The bulk oxides were produced by solid{state reaction
and combustion techniques. Fine powders with grain sizes of about 10
nm were produced from bulk compounds by a Retsch planetary ball mill and
by the hydrothermal and glycothermal processes. The e®ects of the applied
pressure used to make pellets (related to green density of the raw pellets) and
the sintering temperature on the properties were investigated.
An anomalous variation of bulk densities of (Zn, Cd)0:5Ni0:5Fe2O4 oxides
with increase in pelletizing pressure was observed which appears to suggest evidence
for trapped porosity. Di®erent states of pelletizing the samples appear
to be related to a systematic change of the hyper¯ne ¯eld distributions derived
from the MÄossbauer spectra. The temperature dependence of the magnetic hyper
¯ne ¯elds at tetrahedral (A) and octahedral (B) sites were observed to vary
with temperature according to the equations Bhf (T) = Bhf (0)[1 ¡ (T=TC)n]¯n
where n = 1 (based on the Landau{Ginzburg theory) and n = 2 (based on the
Stoner theory). The equation Bhf (T) = Bhf (0)[1¡(T=TC)2]¯2 appears to ¯t the
hyper¯ne ¯eld data over a wider temperature range. The Zn{ and Cd{based
oxides were found to be ferrimagnetic with Curie temperature TC = 548 § 3 K (measured by zero velocity technique). The Cu{based compound exhibited antiferromagnetic
behavior with a magnetic transition temperature of 825 § 3 K.
The di®erence in behavior between Zn{, Cd{ and Cu{based compounds is due
to di®erence in electronic con¯guration and atomic or ionic sizes. The stronger
magnetic coupling between spins in the Cu{based sample can be explained by
the presence of RKKY interactions in addition to superexchange interactions.
The larger ionic size for Cd appears to favour smaller grain sizes in Cd{based
oxides. An anomalous increase in TC is obtained in the Zn0:5Ni0:5Fe2O4 compound
with reduction in grain size. This increase in TC is attributed to a
distribution of Zn ions on both A and B sites.
The MÄossbauer spectra of the milled nanosized samples show a combination
of ferrimagnetic and paramagnetic behavior. The coercive ¯eld (HC) at room
temperature was found to increase with reduction in grain size (G) according to
the equation HC = am+bm=G, which is consistent with multidomain particles.
With further reduction in grain sizes, the coercive ¯eld reduced according to the
equation HC = as ¡bs=G2. This equation is associated with the onset of single
domain particles. The samples produced by hydrothermal and glycothermal
processes show evidence of transformation from single domain to multidomain
structure with increasing sintering temperature.
The ease of single{phase formation in the compounds studied is shown to
depend on the technique used to prepare the samples. Single phase formation of
the spinel structure was easier to achieve in samples prepared by wet chemical
methods because lower sintering temperatures (T < 1000 oC) were required. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2007.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/447 |
| Date | January 2007 |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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