Characterization ferromagnetic materials

At high temperatures, ferroelectric materials transform to the paraelectric state (where dipoles are randomly oriented), ferromagnetic materials to the paramagnetic state, and ferroelastic materials to the twin-free normal state. The transitions are characterized through order parameters (Rao Rao, 1978). These order parameters are characteristic properties parametrized in such a way that the resulting quantity is unity for the ferroic state at a temperature sufficiently below the transition temperature, and is zero in the nonferroic phase beyond the transition temperature. Polarization, magnetization and strain are the proper order parameters for the ferroelectric. 

The peculiar property of ferrogel magneto-elasticity is the hysteresis that characterizes the extension-contraction process. Despite its irreversible nature, this process is time-independent under ordinary conditions. It must be emphasized that the observed hysteresis phenomena is not a consequence of the well-known magnetic hysteresis of ferromagnetic materials, since according to our measurements the ferrogels exhibit no magnetic hysteresis at all... 

Nitroxides are persistent free radicals [1] which can often be isolated and handled as kinetically stable species. Nitroxides react rapidly with carbon free-radical intermediates [2] with well-characterized rate constants [3], and can thus be used as kinetic and mechanistic probes, as well as to trap carbon radicals in synthetic processes. They are easily oxidized or reduced, and thus have a rich redox chemistry that has been utilized for a variety of oxidations. As nitroxides have an unpaired electron, they are paramagnetic and thus ESR active, making them valuable as spin labels for biomolecules [4] and as spin traps for transient radicals [5]. In addition, nitroxides have been developed as organic ferromagnetic materials [6]. The synthesis of nitroxides has been reviewed in 1994 [7]. This review will focus on the synthetic applications of nitroxides. 

In all cases, the same physical quantity is used to characterize the magnetostrictive properties of a ferromagnetic material, it is the dimensionless coefficient of linear change also called fractional change in length, denoted X, expressed in pm/m or ppm at a given magnetic field, and defined as follows ... 

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