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

A facile thermal decomposing method has been developed for the fabrication of Co(x)P nanostructures with controlled size, phase, and shape (e.g., Co(2)P rod and spheres, CoP hollow and solid particles). An amorphous carbon layer could be introduced by the carbonization of organic surfactants from the precursors. The electrochemical performance of typical CoP and Co(2)P samples as anode materials has been investigated and the CoP hollow nanoparticle with carbon coating layer depicts good capacity retention and high rate capability (e.g., specific capacity of 630 mA h g(-1) at 0.2 C after 100 cycles, and a reversible capacity of 256 mA h g(-1) can be achieved at a high current rate of 5 C).

Synthesis of cobalt phosphides and their application as anodes for lithium ion batteries

Microelectromechanical systems (MEMS) technology allows the integration of magnetic field sensors with electronic components, which presents important advantages such as small size, light weight, minimum power consumption, low cost, better sensitivity and high resolution. We present a discussion and review of resonant magnetic field sensors based on MEMS technology. In practice, these sensors exploit the Lorentz force in order to detect external magnetic fields through the displacement of resonant structures, which are measured with optical, capacitive, and piezoresistive sensing techniques. From these, the optical sensing presents immunity to electromagnetic interference (EMI) and reduces the read-out electronic complexity. Moreover, piezoresistive sensing requires an easy fabrication process as well as a standard packaging. A description of the operation mechanisms, advantages and drawbacks of each sensor is considered. MEMS magnetic field sensors are a potential alternative for numerous applications, including the automotive industry, military, medical, telecommunications, oceanographic, spatial, and environment science. In addition, future markets will need the development of several sensors on a single chip for measuring different parameters such as the magnetic field, pressure, temperature and acceleration.

Resonant Magnetic Field Sensors Based On MEMS Technology.

Particulate composites of lead–zirconate–titanate (PZT) and NiFe2O4 were prepared using conventional ceramic processing. The measured magnetoelectric (ME) response demonstrated strong dependence on the volume fraction of NiFe2O4, the magnetic field, and the angle between the magnetic field and polarization in the ceramics. A large ME voltage coefficient of about 80 mV cm−1 Oe−1 was observed for 0.32NiFe2O4/0.68PZT composite ceramic. In particular, at low magnetic fields, the ceramics were found to have a large ME response, linearly varying with both dc and ac magnetic fields.

https://iopscience.iop.org/article/10.1088/0022-3727/37/6/002/pdf

Magnetic-dielectric properties of NiFe2O4/PZT particulate composites

We report the structural evolution and the diffusion processes which occur during the phase transformation of nanoparticles (NPs), e-Co to Co2P to CoP, from a reaction with tri-n-octylphosphine (TOP). Extended X-ray absorption fine structure (EXAFS) investigations were used to elucidate the changes in the local structure of cobalt atoms which occur as the chemical transformation progresses. The lack of long-range order, spread in interatomic distances, and overall increase in mean-square disorder compared with bulk structure reveal the decrease in the NP’s structural order compared with bulk structure, which contributes to their deviation from bulk-like behavior. Results from EXAFS show both the Co2P and CoP phases contain excess Co. Results from EXAFS, transmission electron microscopy, X-ray diffraction, and density functional theory calculations reveal that the inward diffusion of phosphorus is more favorable at the beginning of the transformation from e-Co to Co2P by forming an amorphous Co-P shell, while retaining a crystalline cobalt core. When the major phase of the sample turns to Co2P, the diffusion processes reverse and cobalt atom out-diffusion is favored, leaving a hollow void, characteristic of the nanoscale Kirkendall effect. For the transformation from Co2P to CoP theory predicts an outward diffusion of cobalt while the anion lattice remains intact. In real samples, however, the Co-rich nanoparticles continue Kirkendall hollowing. Knowledge about the transformation method and structural properties provides a means to tailor the synthesis and composition of the NPs to facilitate their use in applications.

/pdf/the-structural-evolution-and-diffusion-during-the-chemical-jpkho50ekp.pdf

The structural evolution and diffusion during the chemical transformation from cobalt to cobalt phosphide nanoparticles

We have combined the printed circuit board (PCB) technology with the electrodeposition of amorphous ferromagnetic Co–P alloys to produce a new 2D fluxgate sensor. The fabrication procedure avoids the use of glue to integrate the core in the device, which improves its performance and makes the production process more reproducible. Choosing properly the frequency and amplitude of the driving current, high sensitivity (160 V/T) or a large linear range (0–250 μT) can be achieved. The use of electrodeposited amorphous Co–P opens new perspectives to improve the performances of inductive microdevices as fluxgates, inductors and transformers.

Planar fluxgate sensor with an electrodeposited amorphous core

Magnetic properties of CoP alloys electrodeposited at room temperature

Transversal magnetic field effect on Fe40Ni40P14B6 amorphous alloys

The origin of anisotropy induced by local laser annealing has been studied in amorphous ferromagnetic materials with different magnetostriction constants. The results have been explained using a theoretical model based on the assumption that the origin of the induced anisotropies are internal stresses produced by inhomogeneous heat flows during the local laser annealing. The induced anisotropies are not uniform, which produces a magnetostatic energy that can modify the induced magnetoelastic anisotropy. This effect can be used to induce different anisotropies by changing the shape of the local laser annealing.

/pdf/induced-anisotropies-in-ferromagnetic-amorphous-ribbons-1jsdrmr69e.pdf

Induced anisotropies in ferromagnetic amorphous ribbons locally annealed by laser.

The influence of compressive, tensile, and bending stresses on the magnetization processes, anisotropy energy, and remanence of an as-quenched amorphous Fe 40 Ni 40 P 14 B 6 ribbon has been studied. The magnetostriction constant has been obtained from the variation of the susceptibility with the applied compressive stress. To better understand the results, domain structures have been observed by using the Bitter technique. Our results show that the Metglas ribbon has a central region with compressive stress, while the edge regions have tensile stress. So, the influence of these regions on the magnetization processes will be very different according to the applied stress.

C. Aroca

Papers

Planar fluxgate sensor with an electrodeposited amorphous core

Magnetic properties of CoP alloys electrodeposited at room temperature

Transversal magnetic field effect on Fe40Ni40P14B6 amorphous alloys

Induced anisotropies in ferromagnetic amorphous ribbons locally annealed by laser.

Magnetoelastic effects in amorphous Fe 40 Ni 40 P 14 B 6 alloys