Ferromagnetic substances

Domb 0 and Sykes M F 1957 Specific heat of a ferromagnetic Substance above the Ourie point Phys. Rev. 129 567... 

Other. Alkali chiorochromate compounds, including cesium chiorochromate, CsCrCl, are ferromagnetic substances being studied for potential apphcation in optically-read computer memory devices. Cesium has also been used in vapor glow lamps (44), vapor rectifiers, and high energy lasers (qv)... 

Permeability ( l), which is often used, albeit imprecisely, in referring to ferromagnetic substances, is the ratio of the magnetic-flux density to the magnetic-field density. 

Above a specific temperature, the Curie temperature, a ferroelectric substance becomes paraelectric since the thermal vibrations counteract the orientation of the dipoles. The coordinated orientation of the dipoles taking place during the ferroelectric polarization is a cooperative phenomenon. This behavior is similar to that of ferromagnetic substances, which is the reason for its name the effect has to do nothing with iron (it is also called seignette or rochelle electricity). 

Phase separations and boundaries may occur in many systems in alloys, in ferromagnetic substances, in solutions. The basic mechanism can be understood within a simple model, which is known as the lattice gas model. We present a simple version adapted to liquid-liquid interfaces. 

As suggested earlier, high-spin molecules that have thousands of parallel spins would correspond to single domain particles of ferromagnetic substances. In a-iron, for example, a microcrystalline particle of radius 200 pm contains ca. 5000 spins and is considered to form a single domain structure. 

The values for ferromagnetic substances are large and vary with the field strength. See also Magnetic Susceptibility... 

FIGURE 9.2 Variation of magnetic susceptibility with temperature for (a) a paramagnetic substance, (b) a ferromagnetic substance, and (c) an antiferromagnetic substance. 

The usefulness of a particular ferromagnetic substance depends on factors such as the size of the magnetisation produced, how easily the solid can be magnetised and demagnetised and how readily it responds to an applied field. The number of unpaired electrons will determine the maximum field, but the other factors depend on the structure of the solid and the impurities it contains, as discussed next. 

This method is based on the Villari effect applying a uniaxial stress to a ferromagnetic substance induces a magnetoelastic anisotropy which may modify all the parameters of its magnetisation curve, e.g. magnetic susceptibility, coercive force, and so on. Some experimental techniques to measure the strain-induced anisotropy are discussed shortly below. 

Values of the low-temperature saturation magnetic moment of ferromagnetic substances represent the maximum component of the atomic magnetic moment in the field direction for example, for spin alone the value in Bohr magnetons is 2S, whereas the magnetic moment obtained from the paramagnetic susceptibility is 2 /S(S + 1). 

Fig. 11.57 Plot of the reciprocal of magnetic susceptibility vs. temperature for three, magnetic behaviors (a) itie Curie law (b) the Curie-Weiss law for a ferromagnetic substance with Curie temperature, Tc, (c) the Curie-Weiss law for an antiferromagnetic, substance with Neel temperature, T. ...

CURIE, PIERRE (1859-1906) CURIE, MARIE (1867-1934). Pierre Cume was bom and raised in Paris. With his brother. Jacques, he studied crystals and in 1880 discovered piezoelectricity. Piezoelectricity is the production of an electric charge by pressure on certain crystals. Pierre became director at the School of Industrial Physics and Chemistry in Paris where he worked for 22 years. His doctoral thesis on magnetism led lo his discovery, the Curie point, a temperature at which ferromagnetic substances lose their magnetism. 

CURIE-WEISS LAW. The transition from ferromagnetic to paramagnetic properties, which occurs in iron and other ferromagnetic substances at the Curie point, is accompanied by a change in the relationship of Ihe magnetic susceptibility lo the temperature. P. Curie stated in 1895 that above this point the susceptibility varies inversely as the absolute temperature. But this was found in be not generally true, and was modified in 1907 by P. Weiss to stare that the susceptibility uf a paramagnetic substance above the Curie point varies inversely as the excess of the temperature above that point. At or below the Curie point, the Curie-Weiss law does not hold. 

The substances which are strongly attracted by magnetic field are called ferromagnetic substances. These substances show permanent magnetism even in the absence of magnetic field. Some examples of ferromagnetic solids are iron, cobalt, nickel and CrO. 

Exogenous contrast agents based on paramagnetic or ferromagnetic substances may be administered to alter the natural relaxation times in order to highlight features of interest. 

A cluster of atoms in a ferromagnetic substance, all of which align in the same direction in the presence of an external magnetic field. 

Figure 8.2. Different types of magnetic behavior (a) a typicai M-H curve (both axes in Tesias) of a paramagnetic substance (b) a diamagnetic substance and (c) a ferromagnetic substance (note the change in the scaie of the M axis). An isotropic singie crystai, or poiycrystai with random crystaiiite orientation, is assumed.

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