A. Narayanasamy

Magnetic properties of nanostructured ferrimagnetic zinc ferrite

Nanostructured ZnFe2O4 ferrites with different grain sizes were prepared by high energy ball milling for various milling times. Both the average grain size and the root mean square strain were estimated from the x-ray diffraction line broadening. The lattice parameter initially decreases slightly with milling and it increases with further milling. The magnetization is found to increase as the grain size decreases and its large value is attributed to the cation inversion associated with grain size reduction. The Fe-57 Mossbauer spectra were recorded at 300 K and 77 K for the samples with grain sizes of 22 and 11 nm. There is no evidence for the presence of the Fe2+ charge state. At 77 K the Mossbauer spectra consist of a magnetically ordered component along with a doublet due to the superparamagnetic behaviour of small crystalline grains with the superparamagnetic component decreasing with grain size reduction. At 4.2 K the sample with 11 nm grain size displays a magnetically blocked state as revealed by the Mossbauer spectrum. The Mossbauer spectrum of this sample recorded at 10 K in an external magnetic field of 6 T applied parallel to the direction of gamma rays clearly shows ferrimagnetic ordering of the sample. Also, the sample exhibits spin canting with a large canting angle, maybe due to a spin-glass-like surface layer or grain boundary anisotropies in the material.

Unusually high coercivity and critical single-domain size of nearly monodispersed CoFe2O4 nanoparticles

Nearly monodispersed CoFe2O4 nanoparticles with average sizes between 8 and 100 nm were synthesized by using seed-mediated growth dominant coprecipitation and modified oxidation methods. X-ray diffraction and Mossbauer spectroscopy analyses confirmed the spinel phase and a stoichiometric composition of (Co0.25Fe0.75)[Co0.75Fe1.25]O4 for powders with different particle diameters. Rotational hysteresis loss (Wr) analysis showed an average switching field (Hp) of 17 kOe and a magnetic anisotropy field (Hk) of 38 kOe for the 40 nm CoFe2O4 particles. The corresponding magnetocrystalline anisotropy energy constant (K) was about 5.1×106 erg/cc. The Hc and Hp results suggest that the critical single-domain size of CoFe2O4 is about 40 nm. The room temperature coercivity (Hc) of the 40 nm CoFe2O4 particles is found to be as high as 4.65 kOe.

Grain size effect on the Néel temperature and magnetic properties of nanocrystalline NiFe2O4 spinel

Nanocrystalline NiFe2O4 spinel ferrites with various grain sizes have been synthesized by ball milling the bulk NiFe2O4. The average grain sizes were estimated from the X-ray line broadening of the (3 1 1) reflection. The Neel temperatures of NiFe2O4 for various grain sizes were determined by magneto thermogravimetric method. The magnetic behaviour has been explained by combining the effects of changes in cation distribution on milling and finite size scaling. The shift in B-H loops has been correlated to the surface spin effects. The high coercivities observed here may be due to high anisotropies of the milled samples. The Hopkinson peak observed just below the Neel temperature has been explained by the mathematical formalism given by the Stoner Wohlfarth model.

Influence of grain size and structural changes on the electrical properties of nanocrystalline zinc ferrite

The electrical conductivity and dielectric properties of nanocrystalline zinc ferrite with various grain sizes ranging from 7 to 115 nm have been studied over a wide frequency range 1 Hz–15 MHz by impedance measurements in the temperature range of 300–650 K. The effect of grain size on the conductivity has been investigated by nonlinear least-squares fitting of the impedance data. The conduction mechanism is found to be due to the hopping of both electrons and holes. The activation energies have been obtained from Arrhenius plots of the grain and the grain boundary conductions and their values are characteristic of the hopping of charge carriers. As the grain size decreases, the conductivity also decreases along with a small increase in the activation energy. The real part e′ of the dielectric constant exhibits anomalous behavior, except for the 115 nm grain size sample, which has been accounted for based on the presence of both types of charge carriers. The dielectric loss factor also decreases as the gr...

On the magnetic properties of ultra-fine zinc ferrites

Zinc ferrite belongs to the class of normal spinels where it is assumed to have a cation distribution of Zn2+(Fe3+)2(O2−)4, and it is purported to be showing zero net magnetisation. However, there have been recent reports suggesting that zinc ferrite exhibits anomaly in its magnetisation. Zinc ferrite samples have been prepared by two different routes and have been analysed using low energy ion scattering, Mossbauer spectroscopy and magnetic measurements. The results indicate that zinc occupies octahedral sites, contrary to the earlier belief that zinc occupies only the tetrahedral sites in a normal spinel. The amount of zinc on the B site increases with decrease in particle size. The LEIS results together with the Mossbauer results and the magnetic measurements lead to the conclusion that zinc occupies the B site and the magnetisation exhibited by ultrafine particles of zinc is due to short range ordering.

Néel temperature enhancement in nanostructured nickel zinc ferrite

The Neel temperature of Ni0.5Zn0.5Fe2O4 spinel ferrite increases significantly from 538 K in the bulk state to 592 K when the grain size is reduced to 16 nm by milling in a high-energy ball mill. This has been attributed to an increase in the AB superexchange interaction strength due to a possible enhancement in the magnetic ion concentration in the A-site on milling, as is evident from extended x-ray absorption fine structure and in-field Mossbauer measurements. (c) 2005 American Institute of Physics.

Grain size effect on the dielectric behavior of nanostructured Ni0.5Zn0.5Fe2O4

The dielectric properties of nanocrystalline Ni0.5Zn0.5Fe2O4 spinel ferrite with various grain sizes, obtained by mechanically milling, have been studied using impedance measurements in the frequency range from 1 Hz to 10 MHz and in the temperature range from 300 to 823 K. The effect of milling, frequency, and temperature on the dielectric properties of nanocrystalline Ni-Zn ferrite is discussed. The real part of dielectric constant (e′) for the 14 nm grain size sample is found to be about an order of magnitude smaller than that of the bulk nickel zinc ferrite. The anomalous frequency dependence of e′ has been explained on the basis of hopping of both electrons and holes. The unusual increase in dielectric loss with milling is because of the increase in electrical conductivity due to oxygen vacancies introduced upon milling. The electrical modulus has been fitted to a stretched exponential function and the results clearly reveal the presence of the non-Debye type of dielectric relaxation in this material....

Irregular distribution of metal ions in ferrites prepared by co-precipitation technique structure analysis of Mn–Zn ferrite using extended X-ray absorption fine structure

Abstract The tetrahedral/octahedral site occupancy of non-magnetic zinc ion, added to maximize the net magnetic moment of mixed ferrites has been found to depend on the method of preparation. In this paper, we qualitatively analyze the metal ion distribution in Mn–Zn ferrite particles prepared by co-precipitation and ceramic methods using extended X-ray absorption fine structure (EXAFS) technique. The results suggest that the differences observed in the magnetic properties of the samples prepared by different methods are not only due to the difference in particle size but also due to the difference in cation distribution. The difference in cation distributions between ferrites of similar composition prepared differently has been found to depend on the crystal field stability energies of the metal ion of interest and associated cations.

Electrical and magnetic properties of chemically derived nanocrystalline cobalt ferrite

Nanocrystalline cobalt ferrite particles of 8nm grain size were synthesized by coprecipitation technique and subsequently suitably heat treated to obtain higher grain sizes. The experimentally observed changes in the dc electrical conductivity and Curie temperature with heat treatment have been attributed to the changes in the cation distributions as obtained from the Mossbauer and extended x-ray absorption fine structure (EXAFS) measurements and to the grain size. The activation energies for conduction as determined from the Arrhenius plots suggest that the conductivity is due to hopping of both electrons and holes. The observed decrease in conductivity when the grain size is increased from 8to92nm is clearly due to the predominant effect of migration of some of the Fe3+ ions from octahedral to tetrahedral sites, as is evident from in-field Mossbauer and EXAFS measurements. But the higher conductivity of the 102 and 123nm particles compared to that of the 92nm particles is attributed to the higher grain ...

Structure and soft magnetic properties of Finemet alloys

Abstract The soft magnetic properties of Fe73.5Cu1Nb3Si13.5B9 (Finemet) alloys prepared by mechanical alloying were studied using various experimental techniques including Mossbauer spectroscopy. The study includes the influence of the milling atmosphere. The results are compared with mechanically alloyed Fe–Si alloys as well as with melt-spun Finemet alloys. The coercivity of the mechanically alloyed powder is much larger than that of the melt-spun ribbons, though the saturation magnetisation is the same. This is ascribed to the absence of a grain boundary amorphous ferromagnetic phase resulting in a weakening of the exchange coupling between the nanograins. This study suggests that the interfacial component in Finemet alloys plays a crucial role in achieving good soft magnetic properties. The critical grain size for single domain particle was found to be 10 nm. For very small grain sizes, the existence of superparamagnetism was also studied using Fe-57 Mossbauer spectroscopy.