Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

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

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

https://www.andrew.cmu.edu/user/dl0p/laughlin/pdf/PMS.pdf

Amorphous and nanocrystalline materials for applications as soft magnets

The present work describes a synthesis and characterization strategy employed to study the magnetic anisotropic properties of a diluted nanoparticulate system. The system under analysis is composed of monodisperse and highly crystalline 16 nm Co0.5Fe2.5O4 nanoparticles (NPs), homogenously dispersed in 1-octadecene. Owing to the liquid nature of the matrix at room temperature, the relative orientation of the nanoparticle easy axis can be controlled by an external magnetic field, enabling us to measure how the magnetic properties are modified by the alignment of the particles within the sample. In turn, by employing this strategy, we have found a significant hardness and squareness enhancement of the hysteresis loop in the magnetically oriented system, with the coercive field reaching a value as high as 30.2 kOe at low temperatures. In addition, the magnetic behavior associated with the system under study was supported by additional magnetic measurements, which were ascribed to different events expected to take place throughout the sample characterization, such as the melting process of the 1-octadecene matrix or the NP relaxation under the Brownian mechanism at high temperatures.

Significant coercivity enhancement at low temperatures in magnetically oriented cobalt ferrite nanoparticles

Metastable COxCU100−x alloys with extended cobalt solubility (5≤x≤20), which can not be obtained by equilibrium methods, were prepared by melt spinning. The separation of the cobalt rich phase upon heating was monitored by X-ray diffraction and calorimetric measurements. The volume fraction of cobalt rich particles can not be determined using X-ray diffraction if the cobalt rich particle size is below a critical value. The separation of the cobalt rich phase seems to be a three step process including nucleation, growth, and coarsening. The non-isothermal kinetics of nucleation and growth of the cobalt rich phase were analysed in terms of a modified Kissinger model. The heat release during phase separation is closely related to the amount of cobalt dissolved in the copper matrix.MST/3258

Cobalt rich phase formation in metastable Cu–Co alloys by means of melt spinning

The structural evolution of the nanocrystalline material obtained from the amorphous compound Fe73.5CuNb3Si13.5B9 was investigated by small-angle X-ray scattering. Four sets of amorphous metallic ribbons were produced by melt spinning using different quenching rates. Samples of each set were annealed above the crystallization temperature; X-ray diffraction analysis showed the formation of an ordered Fe–Si solid solution with average grain sizes ranging from 9 to 11 nm. Small-angle scattering curves of the samples in the initial amorphous states revealed different intensities for different quenching rates, caused by the presence of heterogeneities with dimensions larger than those of the crystallites formed after thermal treatment. In addition, scattering measurements carried out in situ during isothermal annealing showed an intensity evolution as a function of time, attributed to electron-density contrast variation caused by the atomic diffusion process that occurs during crystallization.

Marcelo Knobel

Papers

Significant coercivity enhancement at low temperatures in magnetically oriented cobalt ferrite nanoparticles

Cobalt rich phase formation in metastable Cu–Co alloys by means of melt spinning

Small-Angle X-ray Scattering Study of Nanocrystalline and Amorphous States of the Fe73.5CuNb3Si13.5B9 Alloy

A New Molecular Magnetic Semiconductor Based On Tetrathiafulvalene (ttf) And Oxamato Ligand (opba): [ttf]2[cu(opba)]·h2o

Determination of the Quenched-In State of Amorphous Ribbons Applying the Law of Approach to Saturation