| Summary: | The synthesis, structure and physical properties of a series of Mnx(Co, Mg)₁ˍxFe₂O₄,
(Mg, Sr)₀.₂ Mn₀.₁Co₀.₇Fe₂O₄ and Mg₀.₅Mn₀.₅(RE)₀.₁Fe₁.₉O₄ (where RE are rare earth
elements) nanoferrites have been studied. These compounds were synthesized at low
reaction temperature of about 200 ⁰C using the glycol-thermal method. The starting
materials were high-purity metal chlorides or nitrates which were precipitated by
NH₄OH and KOH respectively. In addition, MnxCo₁₋xFe₂O₄ (x = 0, 0.5 and 1)
samples were produced directly from high-purity metal oxides by high-energy ball
milling technique. Single-phase cubic spinel structure and nanoparticle structure of
the synthesized samples were confirmed by X-ray diffraction (XRD) and transmission
electron microscope (TEM). The results show that the produced powders of the asprepared
samples have average grain sizes ranging from 7 to 16 nm. Filtering the
precipitates by Whatman glass microfiber filters (GF/F) appears to be important in
obtaining the small particle sizes. We suspect higher stability of the MnxCo₁₋xFe₂O₄
at x = 0 and 0.5 where complete symmetry in the proportion of the atoms on
tetrahedral (A) and octahedral (B) sites would tend to favour larger nanoparticles.
The evolutions of the magnetic properties as a function of composition, annealing
temperature under air and argon atmospheres or measuring temperature have
been investigated by ⁵⁷Fe Mössbauer spectroscopy, vibration sample magnetometer
(VSM) and superconducting quantum interference device (SQUID). Significant
changes in magnetic properties are observed across the composition ranges studied.
The Mössbauer spectra indicate ferrimagnetic, superparamagnetic and paramagnetic
behaviours of the compounds. The results show evidence of transformation
from single-domain to multi-domain structure with thermal annealing in
our samples. Temperature dependence of magnetization shows differences between
field cooling (FC) and zero field cooling (ZFC) which we attribute to spin-freezing
and thermal relaxation for typical nanoparticles. Significant increase in coercive
field with reduction in measuring temperature is obtained in Co- based compounds.
Mn₀.₅Co₀.₅Fe₂O₄, Sr₀.₂Mn₀.₁Co₀.₇Fe₂O₄ and Mg₀.₂Mn₀.₁Co₀.₇Fe₂O₄ have large coercive
fields of 1.45, 3.02 and 10.70 kOe at 4 K compared to 0.17, 0.05 and 0.05 kOe
at room temperature respectively. Variation of coercive fields (Hc) with measuri
ing temperature for MnxCo₁₋xFe₂O₄ (x = 0.1 and 0.05), (Mg, Sr)₀.₂Mn₀.₁Co₀.₇Fe₂O₄
nanoferrites follow the Kneller's law for uniaxial non-interacting single domain particles
of the form Hc(T) = Hc(0)[1-( T/Tβ)α]. The observed temperature dependences
are consistent with α = 1/2. We also find evidence of the departure from this law
at lower temperature. The temperature dependence of the saturation magnetizations
were observed to vary with temperature according to the modified Bloch's
law Ms(T) = Ms(0)[1 - ( T/T₀)ᵝ] where β is at least 1.5. This is attributed to the
confinement effects of the spin-wave spectrum for magnetic clusters. The equation
appears to fit the saturation magnetization data over the entire temperature range
with values of β from 2.1 to 2.4 for the samples studied. These results are consistent
with the nanoparticle nature of the compounds.
In Mg₀.₅Mn₀.₅(RE)₀.₁Fe₁.₉O₄ nanoferrites, the grain sizes, lattice parameters and
saturation magnetizations increase with RE substitution which we attribute to larger
RE ions substituting smaller Fe ions. The results show evidence of superparamagnetic
behaviour of the nanoparticles. The highest grain size and magnetizations are
obtained for the Gd substituted sample. We find strong correlation between the saturation
magnetizations, grain sizes and microstrains with de Gennes factor G. The
correlation with grain sizes and microstrains appear to be unique and characteristic
of the nanoparticle nature of the compounds.
Bulk samples in the form of pellets were also produced from the as-prepared
samples of MnxCo₁₋xFe₂O₄ for resistivity measurements. The temperature dependence
of the electrical resistivity for samples sintered from 600 - 1100 ⁰C under
argon atmosphere were studied using the four-probe method from room temperature
to about 110 ⁰C. Two possible mechanisms for resistivity involving Tˉ¹ and
Tˉ¹/² dependences were investigated which we associated with semiconducting and
inter-grain conductivity respectively. The Tˉ¹/² dependence is found to fit the data
better and predicts higher activation energies. The resistivity was observed to be
sensitive to the surface of the pellet being probed and the annealing temperature. === Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2012.
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