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Browsing Physics by Subject "Alloys--Magnetic properties."

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    Mossbauer spectroscopy and x-ray diffraction study of (Cd, Zn) substituted mixed ferrites.
    (2002) Msomi, Justice Zakhele.; Moyo, Thomas.
    The study of magnetic properties and structures of Zn and Cd substituted mixed ferrites has been carried out using Mossbauer spectroscopy and X-ray diffraction on powdered samples at about 300 K. Two series of mixed ferrites, (Cd, Zn)xCol-xFe2-xAlxO4 and (Cd, Zn)xCo0.9Fe1.7-xTi0.4O 4 (where 0≤ x≤1.0) were synthesized. In the former series the effect of simultaneous site dilution by Zn or Cd and Ai atoms on tetrahedral (A) and octahedral (B) sites is investigated and in the latter the effect of single site dilution by Zn or Cd is also studied. The Mossbauer spectra show that the compounds transform with increase in x from ordered to disordered magnetic states. Systematic decrease in the hyperfine fields indicating weakening of the magnetic coupling with an increase in x is observed. We report the presence of a cross over effect with respect to the hyperfine fields on A and B sites at low concentration of diamagnetic ions in the simultaneously diluted series. Both series of compounds show no significant changes in isomer shifts with x. Differences in the evolution of Fe atoms on A and B sites between Zn and Cd based compounds are observed. The results of our analysis show that Zn and Cd ions occupy both tetrahedral and octahedral sites. The spinel structure of the compounds studied in this work is confirmed by X-ray diffraction (XRD). The lattice parameters derived from XRD show systematic change with x consistent with Vergard's law. In Cd based samples an increase of the lattice parameter with x is explained on the basis of the size difference of the cations involved. However, an anomalous behavior of the lattice parameter is observed in Zn based samples. The average grain sizes of the samples were determined from the line width of the (311) XRD intensity lines using the Scherrer formula. These vary between 50 nm and 70 nm for all the samples except for Zn and Al substituted samples which show a systematic anomalous reduction for x ≥ 0.4 in grain size. The porosity, x-ray and bulk densities of the samples are also presented.
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    Synthesis, structural and magnetic properties of bulk and nanosized (Zn, Cd, Cu)0.5Ni0.5Fe2o4 and NiFe204 ferrites
    (2007) Msomi, Justice Zakhele
    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.