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Single-molecule and nanoparticle-based nanomagnets : a magneto-optical study

Rafael L. Novak - Laboratory of Molecular Magnetism Dipartimento di Chimica Fiorentino (Fi) – Italia


Current central issues in nanomagnetism are the understanding the novel magnetic properties of these small structures and its production for possible future applications in ultra high-density data storage and biomedicine. However, as the dimensions are reduced the magnetic characterization gets increasingly difficult, posing a serious challenge for understanding their properties. In this talk, the characterization of two different types of nanomagnets, single-molecule magnets and magnetic nanoparticles, by means of magneto-optical methods, will be presented. Single-Molecule Magnets (SMMs) are chemically synthesized molecules in which a combination of a high-spin ground state and strong uniaxial magnetic anisotropy leads to very slow relaxation of the magnetization at low enough temperatures and magnetic hysteresis of dynamic origin. The magnetic properties of nanoparticles depend mainly on the size-effects and the surface contributions. In both cases the magnetic state of the systems is described by a macro-spin and the study of the magnetization reversal process should be possible by means of MO techniques thanks to their high sensitivity.

 

In the first part the MO properties of Mn12 and Fe4 SMMs prepared with different nanostructures and dimensions, 3D composites formed by SMMs embedded in transparent polymeric films, 3D-2D multilayer Langmuir-Blodgett Films and Self-Assembled monolayers, are presented and discussed. In the second part the MO properties of nanocomposites containing magnetic alloy nanoparticles of composition CoxNi100-x are presented. In both nanomaterials the use of a magneto-optical probe evidenced a larger coercive field with respect to the magnetometric reference measurements. Ac-MO susceptibility measurements indicate a change in the attempt frequency of the reversal process.

 

In both types of nanomagnets the MO hysteresis loops are wavelength dependent and the reversal process detected by MO technique is different than the one recorded with magnetometric techniques. This is discussed considering the particular electronic structure for each case and, in particular for the metal nanoparticles, the effect of surface Plasmon excitations.

 

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