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From the history of the development of permanent magnetic materials, at the end of nineteenth Century the use of carbon steel, magnetic energy product (BH) max (a measure of permanent magnet storage physical quantity of the magnetic energy density (1MGOe) deficiency, while the foreign high Optoma trillion) mass production of Nd-Fe-B permanent magnetic materials, magnetic energy product has reached more than 50MGOe. In this century, the remanence Br of the material has been improved very little, and the improvement of the energy product is due to the improvement of the coercive force Hc. The coercivity increased, mainly due to that understanding of the nature and high magnet ocrystal line anisotropy compounds, and preparation technology progress. At the beginning of twentieth Century, people mainly use carbon steel, tungsten steel, chromium and cobalt for permanent magnetic materials. At the end of the 1930s, the successful development of AlNiCo permanent magnetic materials made it possible for the large-scale application of permanent magnet materials. In 50s, the emergence of barium ferrite not only reduced the cost of permanent magnets, but also widened the application range of permanent magnets to the high frequency field. By 60s, the emergence of rare earth cobalt permanent magnets has opened up a new era for the application of permanent magnets.

A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, inductors, magnetic recording heads, and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrites. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material. The magnetic field is often created by a coil of wire around the core that carries a current. The presence of the core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core. The use of a magnetic core can enormously concentrate the strength and increase the effect of magnetic fields produced by electric currents and permanent magnets. The properties of a device will depend crucially on the following factors: the geometry of the magnetic core. the amount of air gap in the magnetic circuit. the properties of the core material (especially permeability and hysteresis). the operating temperature of the core. whether the core is laminated to reduce eddy currents. In many applications it is undesirable for the core to retain magnetization when the applied field is removed. This property, called hysteresis can cause energy losses in applications such as transformers. Therefore, 'soft' magnetic materials with low hysteresis, such as silicon steel, rather than the 'hard' magnetic materials used for permanent magnets, are usually used in cores. https://en.wikipedia.org/wiki/Magnetic_core

A ferrite is a type of ceramic compound composed of iron oxide (Fe2O3) combined chemically with one or more additional metallic elements.[1] They are both electrically nonconductive and ferrimagnetic, meaning they can be magnetized or attracted to a magnet. Ferrites can be divided into two families based on their magnetic coercivity, their resistance to being demagnetized. Hard ferrites have high coercivity; they are difficult to demagnetize. They are used to make magnets, for devices such as refrigerator magnets, loudspeakers and small electric motors. Soft ferrites have low coercivity. They are used in the electronics industry to make ferrite cores for inductors and transformers, and in various microwave components. Yogoro Kato and Takeshi Takei of the Tokyo Institute of Technology invented ferrite in 1930.[2] Composition and properties Ferrites are usually non-conductive ferrimagnetic ceramic compounds derived from iron oxides such as hematite (Fe2O3) or magnetite (Fe3O4) as well as oxides of other metals. Ferrites are, like most of the other ceramics, hard and brittle. Many ferrites are spinels with the formula AB2O4, where A and B represent various metal cations, usually including iron Fe. Spinel ferrites usually adopt a crystal motif consisting of cubic close-packed (fcc) oxides (O2−) with A cations occupying one eighth of the tetrahedral holes and B cations occupying half of the octahedral holes. If one eighth of the tetrahedral holes are occupied by B cation, then one fourth of the octahedral sites are occupied by A cation and the other one fourth by B cation and it's called the inverse spinel structure. It's also possible to have mixed structure spinel ferrites with formula [M2+1-δFe3+δ][M2+δFe3+2-δ]O4 where δ is the degree of inversion. The magnetic material known as "ZnFe" has the formula ZnFe2O4, with Fe3+ occupying the octahedral sites and Zn2+ occupy the tetrahedral sites, it's an example of normal structure spinel ferrite.[3][page needed] Some ferrites have hexagonal crystal structure, like Barium and Strontium ferrites BaFe12O19 (BaO:6Fe2O3) and SrFe12O19 (SrO:6Fe2O3). [4] In terms of their magnetic properties, the different ferrites are often classified as "soft" or "hard", which refers to their low or high magnetic coercivity, as follows. https://en.wikipedia.org/wiki/Ferrite_%28magnet%29

Chemistry, engineering and materials Filter (aquarium), critical components of both freshwater and marine aquaria Filter (chemistry), a device (usually a membrane or layer) that is designed to physically block certain objects or substances while letting others through Filter paper, a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air Air filter, a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air Oil filter, a filter to remove contaminants from engine oil, transmission oil, lubricating oil, or hydraulic oil Pneumatic filter, a device which removes contaminants from a compressed air stream Water filter, removes impurities from water by means of a fine physical barrier, a chemical process or a biological process. Filters cleanse water to various extents for irrigation, drinking water, aquariums, and pools Optics and photography In optics and photography, a device to remove, or to enhance, certain ranges of wavelengths (colors) of light. See: Filter (optics), including types: Interference filter Dichroic filter Photographic filter, a camera accessory consisting of an optical filter that can be inserted in the optical path Infrared cut-off filter, designed to reflect or block mid-infrared wavelengths while passing visible light H-alpha filter, a specific red visible spectral line created by hydrogen with a wavelength of 6562.8 Å Chelsea filter Astronomical filter, a telescope accessory used to enhance the details of celestial objects. Turbulence modeling In turbulence modeling, a procedure to dampen or remove the highest frequencies of the flow: Filter (large eddy simulation), in the context of large eddy simulation (LES) is a mathematical operation intended to remove a range of small scales from the solution to the Navier-Stokes equations Signal processing In signal processing, especially electronics, an algorithm or device for removing part(s) of a signal: Filter (signal processing), especially, Electronic filter, an electronic circuit which processes signals, for example to remove unwanted frequency components Digital filter, a system that performs mathematical operations to reduce or enhance certain aspects of a signal Analogue filter, a basic building block of signal processing much used in electronics. Computing In computing: Filter (higher-order function), a higher-order function that processes a data structure (typically a list) in some order to produce a new data structure containing exactly those elements of the original data structure for which a given predicate returns the boolean value true A program to process a data stream: Filter (software), a computer program to process a data stream Filter (Unix), a program that gets most of its data from its standard input (the main input stream) and writes its main results to its standard output (the main output stream) Filter (video) Email filter Internet filter Wordfilter, a script typically used on Internet forums or chat rooms that automatically scans users' posts or comments as they are submitted and automatically changes or censors particular words or phrases BPF filter expression used in the qualification of network data DSL filter, a low-pass filter installed between analog devices and a telephone line Philosophy Category-based filtering of perception - according to (objectivist interpretations of), Kant Other In mathematics. See Filter (mathematics), a special subset of a partially ordered set Filtering problem (stochastic processes), a mathematical model for a number of filtering problems in signal processing and the like In psychology, a type of learning blockage. See Affective filter, an impediment to learning or acquisition caused by negative emotional ("affective") responses to one's environment. It is a hypothesis of second language acquisition theory, and a field of interest in educational psychology In transportation. See Lane splitting, a practice that cyclists use to pass slow or stopped congested traffic https://en.wikipedia.org/wiki/Filter