P. A. Joy

Effect of mechanical milling on the structural, magnetic and dielectric properties of coprecipitated ultrafine zinc ferrite

Abstract Nanosized ZnFe 2 O 4 particles containing traces of α-Fe 2 O 3 by intent were produced by low temperature chemical coprecipitation methods. These particles were subjected to high-energy ball milling. These were then characterised using X-ray diffraction, magnetisation and dielectric studies. The effect of milling on zinc ferrite particles have been studied with a view to ascertaining the anomalous behaviour of these materials in the nanoregime. X-ray diffraction and magnetisation studies carried out show that these particles are associated with strains and it is the surface effects that contribute to the magnetisation. Hematite percentage, probably due to decomposition of zinc ferrite, increases with milling. Dielectric behaviour of these particles is due to interfacial polarisation as proposed by Koops. Also the defects caused by the milling produce traps in the surface layer contributes to dielectric permittivity via spin polarised electron tunnelling between grains. The ionic mechanism is enhanced in dielectrics with the rise in temperature which results in the increase of dielectric permittivity with temperature.

Structural, magnetic and electrical properties of the sol-gel prepared Li0.5Fe2.5O4 fine particles

Fine particles of lithium ferrite were synthesized by the sol-gel method. By subsequent heat treatment at different temperatures, lithium ferrites of different grain sizes were prepared. A structural characterization of all the samples was conducted by the x-ray diffraction technique. A grain size of around 12 nm was observed for Li0.5Fe2.5O4 obtained through the sol-gel method. Magnetic properties of lithium ferrite nanoparticles with grain size ranging from 12 to 32 nm were studied. Magnetization measurements showed that Li0.5Fe2.5O4 fine particles exhibit a deviation from the predicted magnetic behaviour. The as-prepared sample of lithium ferrite showed a maximum saturation magnetization of 75 emu g−1. Variation of coercivity is attributed to the transition from multi-domain to single domain nature. Dielectric permittivity and ac conductivity of all the samples were evaluated as a function of frequency, temperature and grain size. Variation of permittivity and ac conductivity with frequency reveals that the dispersion is due to the Maxwell–Wagner type interfacial polarization.

The relationship between field-cooled and zero-field-cooled susceptibilities of some ordered magnetic systems

Analysis of the irreversible field-cooled (FC) and the zero-field-cooled (ZFC) magnetic susceptibilities of one ferrimagnetic and three ferromagnetic systems, measured at different applied magnetic fields, shows that the irreversibility indicated by the difference between the FC and the ZFC susceptibilities arises from magnetic anisotropy. The two susceptibilities are related to each other through the coercivity which is a measure of the anisotropy. The ZFC susceptibility can be calculated from the FC susceptibility (or vice versa) and the coercivity.

Finite size effects on the electrical properties of sol?gel synthesized CoFe2O4 powders: deviation from Maxwell?Wagner theory and evidence of surface polarization effects

Fine particles of cobalt ferrite were synthesized by the sol?gel method. Subsequent heat treatment at different temperatures yielded cobalt ferrites having different grain sizes. X-ray diffraction studies were carried out to elucidate the structure of all the samples. Dielectric permittivity and ac conductivity of all the samples were evaluated as a function of frequency, temperature and grain size. The variation of permittivity and ac conductivity with frequency reveals that the dispersion is due to Maxwell?Wagner type interfacial polarization in general, with a noted variation from the expected behaviour for the cold synthesized samples. High permittivity and conductivity for small grains were explained on the basis of the correlated barrier-hopping model.

Highly sensitive and fast responding CO sensor based on Co3O4 nanorods

Co(3)O(4) nanorods (diameters approximately 6-8 nm and lengths approximately 20-30 nm) were synthesized for the first time through a simple co-precipitation/digestion method by calcination of cobalt hydroxyl carbonate in air and their CO gas sensing properties were investigated. The Co(3)O(4) nanorods exhibited outstanding gas sensing characteristics such as, higher gas response (approximately 6.55-50 ppm CO gas at 250 degrees C), extremely rapid response (approximately 3-4s), fast recovery (approximately 5-6s), excellent repeatability, good selectivity and lower operating temperature (approximately 250 degrees C). Furthermore, the Co(3)O(4) nanorods are able to detect up to 5 ppm for CO with reasonable sensitivity (approximately 3.32) at an operating temperature 250 degrees C and they can be reliably used to monitor the concentration of CO over the range (5-50 ppm). The experimental results clearly demonstrate the potential of using the Co(3)O(4) nanorods as sensing material in the fabrication of CO sensors. Plausible CO sensing mechanism of the Co(3)O(4) nanorods is also discussed.

Ferromagnetism induced by hydrogen in polycrystalline nonmagnetic Zn0.95Co0.05O

Polycrystalline Zn0.95Co0.05O is found to be paramagnetic at room temperature and down to 12K. Optical measurements prove the incorporation of Co2+ ions inside the ZnO lattice in the tetrahedral site. When the paramagnetic sample is heated in Ar∕H2 at 1125K for 2h, ferromagnetism with a very high value of magnetization is observed at room temperature. Hydrogen reduction does not affect the substituted Co2+ ions inside the wurtzite crystal lattice as evidenced from optical studies. X-ray diffraction studies show the presence of Co metal after the reduction process which is the origin of room temperature ferromagnetism.

Origin of high room temperature ferromagnetic moment of nanocrystalline multiferroic BiFeO3

Single phase nanocrystalline BiFeO3 of average crystallite size ∼25 nm with very high magnetization at room temperature is synthesized by an autocombustion method. Magnetic measurements above room temperature show deviation between field cooled and zero field cooled magnetization below 645 K, the Neel temperature (TN) of the bulk material, indicating intrinsic nature of ferromagnetism. However, observation of a broad magnetic transition above TN of BiFeO3 and extended up to 800 K suggests the presence of Fe3O4 as a possible magnetic impurity phase. Evidence for the presence of Fe3O4 is obtained from detailed analysis of the powder x-ray diffraction pattern.

Magnetic and magnetostrictive properties of manganese substituted cobalt ferrite

The magnetic and magnetostrictive properties of polycrystalline Co1−xMnxFe2O4 (0 ≤ x ≤ 0.4) have been studied. Although the Curie temperature decreases continuously with increasing concentration of Mn, the magnetization remains high up to x = 0.3 and unexpectedly low coercivity is observed for this composition showing an unusual magnetostrictive behaviour. This composition shows a relatively larger magnetostriction at low fields. Moreover, the strain derivative which is the slope of the magnetostriction curve at low magnetic fields is almost doubled and the field at which maximum magnetostriction is observed is reduced to almost half for 30% of Mn substitution. The results show that x ≈ 0.3 in Co1−xMnxFe2O4 is an optimum composition with superior magnetostrictive properties for many applications.

Enhanced magnetostrictive properties of Mn substituted cobalt ferrite Co1.2Fe1.8O4

The effect of substitution of Fe and Co by Mn on the magnetostrictive properties of the cobalt ferrite Co1.2Fe1.8O4 has been studied and compared. The studies on Co1.2−xMnxFe1.8O4 and Co1.2Fe1.8−xMnxO4 (0⩽x⩽0.4) showed a marked dependence of magnetostriction on the substitution of both Fe and Co by Mn. Higher magnetostriction is obtained for the substitution of Co by Mn whereas the magnetostriction is reduced substantially on the substitution of Fe by Mn. Higher magnetostriction at low magnetic fields and enhancement of the strain ratio have been observed for the substitution of Co by Mn.

Origin of the cluster-glass-like magnetic properties of the ferromagnetic system

The magnetic behaviour of La 0.5 Sr 0.5 CoO 3 at low magnetic fields has been studied by ac susceptibility, and field cooled (FC) and zero field cooled (ZFC) magnetization measurements. The cluster-glass-like magnetic behaviour of the compound is found to originate from its magnetocrystalline anisotropy as similar properties are observed for ferromagnetic systems also. The cluster glass freezing temperature and its magnetic field dependence, the irreversibility between the FC and ZFC magnetization curves. the shape of the low-field susceptibility curves, etc are related to the magnitude and temperature variation of the coercivity (H t ) which is a measure of the anisotropy, and the ratio H A /H C where H A is the applied magnetic field.