Interacting magnetic nanoparticles in discontinuous Co80Fe20/Al2O3 multilayers and in granular FeCl2-Fe heterostructures : = Wechselwirkende magnetische Nanopartikel in diskontinuierlichen Co80Fe20/Al2O3 Viellagenschichten und granularen FeCl2-Fe Heterostrukturen

Nanometer scale magnetic materials have gained widespread interest both technologically and scientifically because of the novel effects arising in connection with the reduction of their spatial extension. New experimental techniques have made it possible to prepare and investigate magnetic systems on a nanometer scale. This leads to a growing theoretical interest to understand the properties of nanoscale magnetic systems. Especially, the dynamic behavior of an assembly of magnetic nanoparticles is a subject of considerable current investigation. The aims of this experimental work can be divided into two parts. First, we investigate the magnetic properties of an ensemble of interacting nanoparticles embedded in an insulating matrix. The system is prepared as a discontinuous-metal-insulator multilayer [Co80Fe20(tn)/Al2O3(3nm)]10, where tn corresponds to the nominal thickness of CoFe layer. The CoFe forms well-separated and quasi-spherical nanoparticles in the Al2O3 matrix. The magnetic properties are investigatic criticality studies evidence spin glasslike cooperative freezing of magnetic moments m = 1000mB ("superspins") at low temperatures in the nanoparticle system with tn = 0.9 nm. Non-equilibrium collective phenomena such as aging, memory, and rejuvenation are observed in the superspin glass phase. On the other hand, nanoparticle sytems with tn = 1.3 and 1.4 nm reveal pertinent features of a superferromagnetic state. This is evidenced by field dependence of ac-susceptibility in the tn = 1.3 nm system and by a Cole-Cole analysis of the ac-scusceptibilty in the tn = 1.4 nm system. Second, we investigate the properties of a granular system consisting of ferromagnetic nanometric Fe particles in an antiferromagnetic FeCl2 matrix. In this system the nanoparticle-matrix interaction is effective. Apart from direct exchange coupling at the interface between the Fe granules and the Fe2+-ions of FeCl2 matrix, the dipolar stray-fields of the granules play a key role in the magnetic properties of the system. Giant metamagntic moments containing Fe granules as nucleation cores are observed under the combined effects of these two mechanisms.



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