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SIGNIFICANCE OF SHAPE ANISOTROPY FOR NANOPARTICLES

Introduction

Over the last decade a growing interest in the filed of fundamental and applied research has been stimulated by the new category of materials shaped by nanostructures. A great deal of physical and chemical techniques such as colloidal solutions (Bréchignac et al. 2008) has been proven extremely effective for controlling not only the size but also the shape of developed nanoparticles. Shape anisotropy, sometimes also referred to as magnetic dipolar anisotropy (Bland, 2002), is mediated by dipolar long-range interaction where its contribution is dependent on the shape of a given sample (Bland, 2002). Magnetic dipole-dipole interactions exhibit an important and well known feature that, regardless of their weakness with respect to exchange coupling, plays a vital role in magnetic systems. The long-range characteristic is used to determine the ground state, excitation spectrum, and breaking of a bulk sample in several magnetic domains, in such magnetic system (Politi & Pini, 1998). The distinctiveness of such dipolar interaction in three dimensions in expounded by the fact that the shape anisotropy is always present (Politi & Pini, 1998) which is independent of the size of the sample. Shape of hysteresis loops is strongly affected by magnetic anisotropy which also controls the coercivity and remanence (Liu at el, 2008). Consequently, taking infinite sample is meaningless without specifying the limiting shape of the sample (Politi & Pini, 1998).

The shape anisotropy due to such magnetic dipole-dipole interactions allows an in-plane magnetization without constituting any discriminatory direction within the film. It is mainly for these reasons that shape anisotropy becomes significant in thin films and often forms in-plane alignment of moments (Bland, 2002) (Politi & Pini, 1998) and as shape anisotropy is exploited in the design of most magnetic materials of commercial importance, therefore shape anisotropy is of considerable practical significance (Bréchignac et al. 2008) (He & Chen, 2007) (Chang et al, 2006).

Lu et al (1997) have demonstrated the control of magnetic response by using the shape anisotropy and have discussed the electrode layers magnetic coupling. It has been shown by Lu et al (1997) that the shape anisotropy is useful in controlling the response properties of magnetic tunnel junctions by varying the junction shape as represented in fig.3. The figure demonstrates the shape anisotropy of the top electrode in the used devices is more important than the intrinsic anisotropy induced during film deposition.

 

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