In addition, the efficiency of the field in aligning the moments is opposed by the randomizing effects of temperature. This results in a temperature dependent susceptibility, known as the Curie Law. At normal temperatures and in moderate fields, the paramagnetic susceptibility is small but larger than the diamagnetic contribution. Under these conditions, paramagnetic susceptibility is proportional to the total iron content. Many iron bearing minerals are paramagnetic at room temperature. The paramagnetism of the matrix minerals in natural samples can be significant if the concentration of magnetite is very small.
In this case, a paramagnetic correction may be needed. When you think of magnetic materials, you probably think of iron, nickel or magnetite.
Unlike paramagnetic materials, the atomic moments in these materials exhibit very strong interactions. These interactions are produced by electronic exchange forces and result in a parallel or antiparallel alignment of atomic moments. Exchange forces are very large, equivalent to a field on the order of Tesla, or approximately a million times the strength of the earth's field.
The exchange force is a quantum mechanical phenomenon due to the relative orientation of the spins of two electron. Ferromagnetic materials exhibit parallel alignment of moments resulting in large net magnetization even in the absence of a magnetic field.
The spontaneous magnetization is the net magnetization that exists inside a uniformly magnetized microscopic volume in the absence of a field. The magnitude of this magnetization, at 0 K, is dependent on the spin magnetic moments of electrons. A related term is the saturation magnetization which we can measure in the laboratory. The saturation magnetization is the maximum induced magnetic moment that can be obtained in a magnetic field H sat ; beyond this field no further increase in magnetization occurs.
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The difference between spontaneous magnetization and the saturation magnetization has to do with magnetic domains more about domains later. Saturation magnetization is an intrinsic property, independent of particle size but dependent on temperature. There is a big difference between paramagnetic and ferromagnetic susceptibility. As compared to paramagnetic materials, the magnetization in ferromagnetic materials is saturated in moderate magnetic fields and at high room-temperature temperatures:.
Handbook of Magnetic Materials, Volume 27 - 1st Edition
Even though electronic exchange forces in ferromagnets are very large, thermal energy eventually overcomes the exchange and produces a randomizing effect. This occurs at a particular temperature called the Curie temperature T C. Below the Curie temperature, the ferromagnet is ordered and above it, disordered. The saturation magnetization goes to zero at the Curie temperature. A typical plot of magnetization vs temperature for magnetite is shown below. The Curie temperature is also an intrinsic property and is a diagnostic parameter that can be used for mineral identification.
However, it is not foolproof because different magnetic minerals, in principle, can have the same Curie temperature. In addition to the Curie temperature and saturation magnetization, ferromagnets can retain a memory of an applied field once it is removed. This behavior is called hysteresis and a plot of the variation of magnetization with magnetic field is called a hysteresis loop. Another hysteresis property is the coercivity of remanence Hr. This is the reverse field which, when applied and then removed, reduces the saturation remanence to zero.
It is always larger than the coercive force. The various hysteresis parameters are not solely intrinsic properties but are dependent on grain size, domain state, stresses, and temperature. Because hysteresis parameters are dependent on grain size, they are useful for magnetic grain sizing of natural samples. In ionic compounds, such as oxides, more complex forms of magnetic ordering can occur as a result of the crystal structure.
One type of magnetic ordering is call ferrimagnetism. A simple representation of the magnetic spins in a ferrimagnetic oxide is shown here. The magnetic structure is composed of two magnetic sublattices called A and B separated by oxygens. Vol 2. Characterization and Simulation. Vol 3. Fabrication and Processing.
Vol 4. Yi Liu , D. Sellmyer , Daisuke Shindo.
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In December , the world's first commercial magnetic levitation super-train went into operation in Shanghai. Now consumers are enjoying 50 GB hard drives compared to 0. Achievements in magnetic materials research have made dreams of a few decades ago reality. The objective of the four volume reference, Handbook of Advanced Magnetic Materials, is to provide a comprehensive review of recent progress in magnetic materials research.
Each chapter will have an introduction to give a clear definition of basic and important concepts of the topic. The details of the topic are then elucidated theoretically and experimentally. New ideas for further advancement are then discussed. Sufficient references are also included for those who wish to read the original work.
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In the last decade, one of the most significant thrust areas of materials research has been nanostructured magnetic materials. There are several critical sizes that control the behavior of a magnetic material, and size effects become especially critical when dimensions approach a few nanometers, where quantum phenomena appear. The first volume of the book, Nanostructured Advanced Magnetic Materials, has therefore been devoted to the recent development of nanostructured magnetic materials, emphasizing size effects.
Magnetic Properties: From Traditional to Spintronic
Our understanding of magnetism has advanced with the establishment of the theory of atomic magnetic moments and itinerant magnetism. Simulation is a powerful tool for exploration and explanation of properties of various magnetic materials. Simulation also provides insight for further development of new materials. Naturally, before any simulation can be started, a model must be constructed.
This requires that the material be well characterized. Therefore the second volume, Characterization and Simulation provides a comprehensive review of both experimental methods and simulation techniques for the characterization of magnetic materials. After an introduction, each section gives a detailed description of the method and the following sections provide examples and results of the method.
- Handbook of Magnetic Materials, Volume 3 - 1st Edition.
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Finally further development of the method will be discussed. The success of each type of magnetic material depends on its properties and cost which are directly related to its fabrication process.