Fernanda A. Sampaio da Silva

A Simple Algorithm for the Calculation of Hysteresis for Isotropic NdFeB Magnets

The calculation of hysteresis curves with the Stoner-Wohlfarth model is somewhat laborious. For the Nd2Fe14B phase, the second order magnetocrystalline anisotropy (K2) constant is relevant, and this case only has been discussed in some studies. In this article, a simple algorithm for the calculation of the Stoner-Wohlfarth model for isotropic NdFeB magnets is described. This algorithm makes easier the modeling of the hysteresis curves of bonded melt-spun NdFeB magnets.

A Model for the Hysteresis Curves of Soft Magnetic Materials

A model, based on hyperbolic tangent curve, was found to be able to reproduce several different types of magnetization curves of soft magnetic materials. The model is especially suitable for curves with sigmoid shape. The model is compared with experimental data for bonded soft iron powders, and also other ferromagnetic materials (1045 steel). Three adjusting parameters were used in the original hyperbolic tangent equation. Each parameter has different physical meaning.

Particle Size Analysis of Fe3O4 Nanoparticles Coated by Polyethyleneglycol

Nanotechnology has been shown as an important tool for developing intelligent devices. In particular, magnetic nanoparticles have been studied due their applications in cancer treatments. However, nanoparticles need to be tightly controlled in relation to size, shape and coating. It makes particles suitable for in vivo applications. In this work, magnetite nanoparticles were used for particle size characterization. Fe3O4 crystals were coated by polyethyleneglycol (PEG 4000). Three different techniques were performed to obtain the nanoparticles average diameter: Rietveld Analysis, Scherrer Equation and Nanosight®. Results indicate good properties.

Stoner-Wohlfarth Model for Nanocrystalline Anisotropic Sm2Co17 Magnets

The Stoner-Wohlfarth model can be used for predicting hysteresis curves of either isotropic or anisotropic nanocrystalline Sm2Co17 type magnets. For isotropic magnets, with Mr/Ms=0.5, the Stoner-Wohlfarth model predicts coercive force of 48% of the anisotropy field. Here, Mr is remanence and Ms is magnetization of saturation. Two texture distributions were compared: Pearson VII and cosn. The calculations indicate that increasing the alignment degree of the grains, the coercivity increases. However, this increase of the coercive force is small. It is found that a very well aligned magnet, with Mr/Ms ratio of 0.96, presents coercivity only ~20% higher than that of the isotropic magnet.

Magnetic Domains Observation from Bitter Patterns of NdFeB Alloy

The Bitter method is an adequate method for domains observation, and depends on careful metallographic preparation and also on a proper magnetic colloid solution. In this study, Bitter patterns were obtained for a Nd-Fe-B as-cast alloy. The Nd2Fe14B grains show strong anisotropy during growing, and are larger for the direction perpendicular to the easy magnetization axis. The equilibrium distance between domain walls allows an estimate of intrinsic parameters of ferromagnetic phases as the domain wall energy and the single domain particle size.

Superparamagnetic Iron Oxide Nanoparticles for Magnetic Hipertermia: Synthesis, Surface Modification by Polyethylene Glycol and Characterization

Nowadays, superparamagnetic iron oxide nanoparticles are an important tool for cancer treatment, such as magnetic hyperthermia. The goal is heating diseased tissue and then tumor cells are destroyed. Magnetic nanoparticles are promising mainly because they have specific ability to reduce side effects. However, for in vivo applications, nanoparticles need to be coated by a biocompatible material. In this work, nanoparticles are coated by PEG (biocompatible polymer). Samples were produced by coprecipitation process. Information about particle size, magnetic properties and crystallinity were obtained.

EBSD Texture Analysis of NdFeB Magnets

The crystallographical texture is relevant information for NdFeB magnets, since the maximum energy product is directly related to orientation of the crystals. EBSD (Electron Backscattered Diffraction) is a very suitable tool for preferred orientation measurement of NdFeB magnets. The advantages of EBSD against X-ray Diffraction (XRD) pole figures for texture determination are discussed. EBSD identifies misaligned grains, and this is not feasible with XRD pole figures. EBSD is also helpful on the identification of oxides.

B-H Loop of Sintered Stainless Steel 410 Adjusted by Superellipse Model

B-H loop obtained from frequencies higher than 40 Hz can be satisfactorily adjusted by Superellipse model. Examples of application of the Superellipse model for sintered stainless steel 410 are presented. The parameters of model (four parameters) may have assigned physical meanings that reveal magnetic behavior, such as coercivity and remanence. Furthermore, they are directly related to the aesthetics of the B-H loop. The Superellipse model is very versatile, describing the most closed shapes (rods) and the more open (rectangular). The obtained parameter values reflect the magnetic variability of sample. In addition, by applying the model it was possible to analyze the magnetic and structural properties of a sintered stainless steel sample.

Study of Thermal Degradation of PEG/PVP Coating Adsorbed in Fe3O4 Nanoparticles

The nanoparticle materials have been intensively studied in recent decades aiming mainly at the medical fields. In particular, nanoparticles of magnetite, Fe3O4, because it has physical and magnetic properties compatible with in vivo applications. However, even if the magnetite has low toxicity, it is necessary that the particles are coated with biocompatible material. In this work, Fe3O4 nanoparticles coated by a polymer blend of polyethylene glycol / polyvinylpyrrolidone (type core / layer) were analyzed. It was performed to study the thermal degradation of the polymer coating layer aimed at understanding the nature of the forces involved in adsorption surface of the metallic core.

Hysteresis Modeling of NdFeB Magnets with High Nd

The Stoner-Wohlfarth hysteresis model can describe very well the hysteresis curves since the assumptions of the model are compatible with the studied samples. These conditions are (i) single domain particle size and (ii) inexistence of interactions between the particles. These conditions are obtained in Nd2Fe14B magnets with nanocrystalline grain size below 200 nm and with high Nd, which promotes a paramagnetic phase at the grain boundaries. In the present study, both isotropic and anisotropic NdFeB magnets are modeled.