Giant Magnetostrictive, Spring Magnet Type Multilayers and Torsion Based Microactuators
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571 Mat. Res. Soc. Symp. Proc. Vol. 459 01997 Materials Research Society
THEORY One possibilty to reduce the saturation fields while keeping the total magnetostriction sufficiently high, is to increase the saturation magnetisation Ms. Giant magnetostrictive materials used up to now have rather low magnetisation due to their ferrimagnetic nature (often even sperimagnetic). For the composition of interest Tb M2 [1, 5], the Tb moments dominate and so an increase in the transition metal component will only further reduce the magnetisation at room temperature. The increase of the Tb concentration will decrease the ordering temperature and for this reason it is not possible to increase the magnetisation (at room temperature) with simple alloying. However we can use multilayer systems to exchange couple two magnetic materials having different magnetic characteristics in a similar way to "spring magnets". There is one material (Tb-M) which shows a large room temperature magnetostriction but however a low magnetisation. The other one (Fe-Co) is magnetically soft with a very high magnetisation. When the individual layer thickness is smaller than the magnetic exchange length, domain walls can not be formed at the interfaces at room temperature. The whole system is entirely coupled to behave like a one layer system with its properties corresponding to an average of the characteristics of the two types of layers. In this case the saturation field is reduced while keeping relatively large values of magnetostriction. For example, consider a very simple model having a multilayer system with layers of equal thickness. Also we suppose perfect coupling with high exchange stiffness so that we have a uniform magnetisation direction throughout the whole thickness. The soft material has a high magnetisation Mshi, and zero magnetostriction and no anisotropy Xhi = Khi = 0. The magnetostrictive layer has a magnetisation MSMS, a magnetostriction XMS and an anisotropy KMS. Typically Mshi = 5 MsmS so that the multilayer has got a magnetisation of 3 MSMS. The anisotropy is KMS/2 - so that the resulting saturation field of the mulitlayer is Hsmulti = KMS/3 MSMS = HsMS/6. As for the magnetostriction of the multilayer we get XMS/2. Thus the a ratio (X/H) is 3 times higher for the multilayer than for the simple magnetostrictive alloy. The advantage of these composite multilayer materials over the normal homogenous alloys, is the fact that in each layer of the soft material, the magnetisations are very large and this can not be achieved in R based alloys at room temperature. In addition to this, large interface anisotropies can be developped and become very important increasing the magnetostriction performences as shown below. So the critical design criteria are the individual layer thickness, the interface quality and the composition of each layer. RESULTS We have studied the multilayer system (Fe 0 .65 Co 0 .35 / Tbl_x (Fel-y Coy )x ) *n where we have varied the individual layer thicknesses tFeCo, tTbCo, the compositions x and y and the number o
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