Strains magnetostrictive

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress (14,15). This hysteresis can be caused by motion of ferroelastic domain walls. This behavior is more compHcated and complex near a martensitic phase transformation. At this transformation, both crystal stmctural changes iaduced by mechanical stress and by domain wall motion occur. Martensitic shape memory alloys have broad, diffuse phase transformations and coexisting high and low temperature phases. The domain wall movements disappear with fully transformation to the high temperature austentic (paraelastic) phase. 

Oriented In-Plane Texture. In this kind of film the properties (H and in the various in-plane directions (texture and nontexture directions) are different. The texture of the film can be supported by the texture of the substrate and the crystal lattice can be smaller in the texture direction than in the transverse direction. This can be the source for strain-induced magnetic anisotropy (magnetostriction). It is also found that the crystal is aligned in the texture direction (92). 

Due to their high piezoelectric response, electrostriction in ferroelectrics, induced by an applied electric field, can be used as strain-inducing components (just as ferromagnetic materials can be exploited for their magnetostriction). Thus barium... 

Fig. I. Schematic representation of the phenomena of magnetostriction. The surrounding atoms, schematised as positive charges, are displaced from their initial symmetrical position (open circles) to their final strained positions (black circles) due to the electrostatic interactions with the aspherical electron distribution.

For an isotropic material, the space spanned by the five remaining strain components cannot be reduced further. Consequently, there are only two magnetostriction modes. The energy density can be written down directly, in principle for either irreducible representation separately ... 

For uniaxial (hexagonal) symmetry the 6 strain components are subdivided in two (invariant) one-dimensional subsets (indicated by the superscript a, and subscripts 1 and 2 for the volume dilatation and the axial deformation, respectively), and two different two-dimensional subsets, indicated by y for deformations in the (hexagonal) plane, and by e for skew deformations. These modes are also depicted in fig. 3. In this case, the magnetostriction can be expressed as... 

Various experimental methods have been developed for investigating the magnetoelastic properties of thin films and nanoscale magnetic systems. In the following subsections, we discuss the most important ones (i) the magnetoelastic cantilever, (ii) strain induced anisotropy, (iii) magnetostriction in spin valves, (iv) strain modulated ferromagnetic resonance, (v) secondary-electron spin-polarisation, and (vi) strain-induced anisotropy due to the spontaneous strains. 

For technical applications (sensors), Hristoforou et al. (1998) have developed an interesting method to determine the field-dependence of the magnetostrictive strain, based on measuring the delay due to Bloch-wall motion. 

Investigations on SmFeB/TbFeB multilayers have shown that strain and stress are transferred effectively at the interface (Shima et al. 1997). In these SmFeBA bFeB multilayers, the thickness of the layers was varied, and it was found that the magnetostriction is sensitively affected by Young s modulus, the Poisson ratio and the thickness of the constituent layers. 

Optical microactuators like a micromirror for scanning applications have been proposed by Orsier et al. (1996). A two-dimensional bimorph (Si 4- magnetostrictive film) is driven remotely by two differently oriented magnetic fields working at different frequencies in order to obtain the bending and torsional vibrations due to the magnetostrictive strain. 

Equation (13) shows that the complete temperature and field dependence of the strains can be calculated from static correlation functions (J Jj )7-,h (y, y — 1.2,3 label the cartesian components of the angular momentum J) where O7- h denote thermal expectation values (Callen and Callen 1965). As already mentioned above, a mean field theory may be used to evaluate (13) and calculate the magnetostriction. 

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress. This hysteresis can be caused by motion of fenoelastic domain walls. Tins behavioi is mote complicated and complex near a martensitic phase transformation. 

The practical application of ultrasonics requires effective transducers to change electrical energy into mechanical vibrations and vice versa. Transducers are usually piezoelectric, ferroelectric, or magnetostrictive. The application of a voltage across a piezoelectric crystal causes it to deform with an amplitude of deformation proportional to the voltage. Reversal of the voltage causes reversal of the mechanical strain. Quartz and synthetic ceramic materials are used. 

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