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

Understanding the correlation between magnetic properties and nanostructure involves collaborative efforts between chemists, physicists, and materials scientists to study both fundamental properties and potential applications. This article introduces a classification of nanostructure morphology according to the mechanism responsible for the magnetic properties. The fundamental magnetic properties of interest and the theoretical frameworks developed to model these properties are summarized. Common chemical and physical techniques for the fabrication of magnetic nanostructures are surveyed, followed by some examples of recent investigations of magnetic systems with structure on the nanometer scale. The article concludes with a brief discussion of some promising experimental techniques in synthesis and measurements.

Magnetic Properties of Nanostructured Materials

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

Amorphous and nanocrystalline materials for applications as soft magnets

Inspired by the natural design of bacterial flagella, we report artificial bacterial flagella (ABF) that have a comparable shape and size to their organic counterparts and can swim in a controllable fashion using weak applied magnetic fields. The helical swimmer consists of a helical tail resembling the dimensions of a natural flagellum and a thin soft-magnetic “head” on one end. The swimming locomotion of ABF is precisely controlled by three orthogonal electromagnetic coil pairs. Microsphere manipulation is performed, and the thrust force generated by an ABF is analyzed. ABF swimmers represent the first demonstration of microscopic artificial swimmers that use helical propulsion. Self-propelled devices such as these are of interest in fundamental research and for biomedical applications.

Artificial bacterial flagella: Fabrication and magnetic control

In the past 20 years, there has been a surge in research on the magnetocaloric response of materials, due mainly to the possibility of applying this effect for magnetic refrigeration close to room temperature. This review is devoted to the main families of materials suitable for this application and to the procedures proposed to predict their response. Apart from the possible technological applications, we also discuss the use of magnetocaloric characterization to gain fundamental insight into the nature of the underlying phase transition.

The Magnetocaloric Effect and Magnetic Refrigeration Near Room Temperature: Materials and Models

A new method was developed to measure the saturation magnetostriction of a thin amorphous ribbon. It is based on the use of small-angle magnetization rotation to measure the change in anisotropy field caused by the tensile stress. Measurements have been performed on Metglas 2826, 2605, and Co-Si-B amorphous ribbons. The maximum experimental error of the measurement was estimated to be about ±5 percent. It is shown that the sensitivity of the method depends on the shape anisotropy field. The estimated sensitivity was about 2 × 10-7for Metglas 2826 ribbon with 1.8 mm × 40 μm × 12 cm in dimensions.

/pdf/measurement-of-saturation-magnetostriction-of-a-thin-1djsgxyda3.pdf

Measurement of saturation magnetostriction of a thin amorphous ribbon by means of small-angle magnetization rotation

The effect of intergrain exchange and magnetostatic interactions on remanence and coercivity is calculated numerically for an isotropic Nd-Fe-B model magnet composed of 8000 grains. The intergrain exchange interaction affects the magnetizing process markedly when the grain size L is small, but the magnetostatic interaction does not have substantial effects. The calculated remanence which takes the intergrain exchange interaction into account increases with decreasing L from Ms/2 (Ms: the saturation magnetization) to Ms and agrees roughly with the experimental results. The calculated coercivity decreases with decreasing L from 0.48HA (HA: the anisotropy field) to 0. Our results suggest that the large remanence above Ms/2 observed experimentally in isotropic Nd-Fe-B magnets can be explained by the intergrain exchange interaction between Nd2Fe14B grains.

http://iopscience.iop.org/article/10.1143/JJAP.31.1347/pdf

Effect of Intergrain Exchange Interaction on Magnetic Properties in Isotropic Nd–Fe–B Magnets

The structural origin of magnetic anisotropy in Fe–Si–B–Nb–Cu alloys annealed under a tensile stress of 200 MPa is studied by transmission x-ray diffraction. The diffraction peak of the (310) plane, whose normal vector is parallel to the tensile direction (ribbon direction), appears at a lower angle than the one perpendicular to it by about 0.1°. This indicates that the spacing of the (310) plane normal to the tensile direction is about 0.2% larger than the one parallel to it. This is direct evidence for the structural origin of the stress-induced magnetic anisotropy of nanocrystalline soft magnetic alloys.

Direct evidence for structural origin of stress-induced magnetic anisotropy in Fe–Si–B–Nb–Cu nanocrystalline alloys

Interior permanent magnet motors are widely used in various fields. However, in high-speed operations, it is important to decrease the eddy current loss in the permanent magnet. Therefore, we propose to divide the permanent magnet. This paper describes a practical technique to analyze the eddy current in the divided magnet. The effect of division of permanent magnet on eddy current loss is also quantitatively clarified.

3-D eddy current analysis in permanent magnet of interior permanent magnet motors

The dependence of the structural anisotropy of Fe-Si-B-Nb-Cu alloy on the applied stress during annealing has been studied by transmission x-ray diffraction. After crystallizing under stress, the Fe-Si nanocrystals show anisotropy in the lattice spacing of the (620) planes. Their elongations are proportional to the applied stress and show a linear correlation with the magnetic anisotropy energy, Ku. These results indicate that Ku originates from a magnetoelastic effect due to an elastic elongation of the Fe-Si phase constrained by the surrounding amorphous phase.

/pdf/origin-of-the-magnetic-anisotropy-induced-by-stress-1r1rabieul.pdf

Hirotoshi Fukunaga

Papers

Measurement of saturation magnetostriction of a thin amorphous ribbon by means of small-angle magnetization rotation

Effect of Intergrain Exchange Interaction on Magnetic Properties in Isotropic Nd–Fe–B Magnets

Direct evidence for structural origin of stress-induced magnetic anisotropy in Fe–Si–B–Nb–Cu nanocrystalline alloys

3-D eddy current analysis in permanent magnet of interior permanent magnet motors

Origin of the magnetic anisotropy induced by stress annealing in Fe-based nanocrystalline alloy