In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions

Due to finite size effects, such as the high surface-to-volume ratio and different crystal structures, magnetic nanoparticles are found to exhibit interesting and considerably different magnetic properties than those found in their corresponding bulk materials. These nanoparticles can be synthesized in several ways (e.g., chemical and physical) with controllable sizes enabling their comparison to biological organisms from cells (10–100 μm), viruses, genes, down to proteins (3–50 nm). The optimization of the nanoparticles’ size, size distribution, agglomeration, coating, and shapes along with their unique magnetic properties prompted the application of nanoparticles of this type in diverse fields. Biomedicine is one of these fields where intensive research is currently being conducted. In this review, we will discuss the magnetic properties of nanoparticles which are directly related to their applications in biomedicine. We will focus mainly on surface effects and ferrite nanoparticles, and on one diagnostic application of magnetic nanoparticles as magnetic resonance imaging contrast agents.

https://www.mdpi.com/1422-0067/14/11/21266/pdf

Magnetic Nanoparticles: Surface Effects and Properties Related to Biomedicine Applications

Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route

Nanocrystalline ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ spinel has been synthesized with various grain sizes by high-energy ball milling. From the high-field magnetization studies and extended x-ray-absorption fine-structure, and M\"ossbauer measurements in an external magnetic field of 5 T applied parallel to the direction of gamma rays, we could observe that ${\mathrm{Ni}}^{2+}$ ions occupy tetrahedral sites on grain-size reduction due to milling. The ${\mathrm{Fe}}^{3+}$ spins have a canted structure and the canting angle increases with grain-size reduction. It is possible that the core ${\mathrm{Fe}}^{3+}$ spins are also canted because of the magnetocrystalline anisotropy introduced by the occupancy of the ${\mathrm{Ni}}^{2+}$ ions in the tetrahedral sites.

Mixed spinel structure in nanocrystalline NiFe 2 O 4

Electrical conductivity and dielectric measurements have been performed for nanocrystalline NiFe2O4 spinel for four different average grain sizes, ranging from 8 to 97 nm. The activation energy for the grain and grain boundary conduction and its variation with grain size have been reported in this paper. The conduction mechanism is found to be due to the hopping of both electrons and holes. The high-temperature conductivity shows a change of slope at about 500 K for grain sizes of 8 and 12 nm and this is attributed to the hole hopping in tetrahedral sites of NiFe2O4. Since the activation energy for the dielectric relaxation is found to be almost equal to that of the dc conductivity, the mechanism of electrical conduction must be the same as that of the dielectric polarization. The real part e' of the dielectric constant and the dielectric loss tanδ for the 8 and 12 nm grain size samples are about two orders of magnitude smaller than those of the bulk NiFe2O4. The anomalous frequency dependence of e' has been explained on the basis of hopping of both electrons and holes. The electrical modulus analysis shows the non-Debye nature of the nanocrystalline nickel ferrite.

https://iopscience.iop.org/article/10.1088/0953-8984/14/12/311/pdf

Electrical conductivity and dielectric behaviour of nanocrystalline NiFe2O4 spinel

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.

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

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...

A. Narayanasamy

Papers

Mixed spinel structure in nanocrystalline NiFe 2 O 4

Electrical conductivity and dielectric behaviour of nanocrystalline NiFe2O4 spinel

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

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

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