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Nanomagnetism

The characteristic length scale of magnetism is given by the exchange length, which is typically of the order of a few nanometers. The magnetic properties of objects with dimensions comparable to the exchange length differ from those of macroscopic magnets. The local atomic symmetry given by the crystal or the molecule structure and finite-size effects due to the increased influence of surfaces and interfaces dominate the behavior and give rise to novel phenomena only present on the nanoscale. They comprise superparamagnetism, novel spin structures, special short-range coupling mechanisms, interaction between electrical current and magnetization (magnetotransport, spin-transfer torque) as well as magnetization tunneling.

Magnetic Nanoparticles

Elements of the 4d series of the periodic table such as Ru, Rh, and Pd are “almost ferromagnetic”. That means, their electronic structure is similar to the ones of the conventional ferromagnets Fe, Co, and Ni. It is suggested by theoretical investigations, that small modifications of the electronic density of states are sufficient to switch on ferromagnetism in the 4d elements.

In the case of nanoparticles, the low coordination number of surface atoms may already lead to the desired change in the electronic properties. Furthermore, these properties can be additionally tailored e.g. by charge transfer to/from organic molecules attached to the surface. This targeted manipulation plays a crucial role in understanding the fundamentals of ferromagnetism and the development of new nanoscale ferromagnets.

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Beyond charge transfer effects, also external stress and strain caused by a matrix hosting the 4d nanoparticles and the lattice expansion of Pd upon loading with hydrogen will be studied in order to gain a more detailed picture.

The nanoparticles will be either synthesised by a wet-chemical method or prepared starting from polyoxypalladates in collaboration with the group of Prof. Kögerler (RWTH Aachen). Using this molecular approach, it is possible to tailor highly symmetric Pd clusters.

Both magnetic and electronic properties will be monitored by x-ray absorption spectroscopy and its associated circular dichroism. Since this method is element-specific, the mutual influence of nanoparticles, ligands and substrate or hosting matrix can be studied.

 

Magnetic Molecules

DoubleDecker_jpgDouble-decker Phthalocyanine molecule

Magnetic molecules form a link between the field of spintronics and molecular electronics. They offer unique properties like metallic or semiconducting behavior, high magnetic moments, size-induced quantum effects (Kondo effect, Berry phase interference, quantum tunneling of magnetization, etc.) as well as manifold ways of functionalization leading to electric, magnetic, optical, and chemical sensitivities or selective reactions with their environment.

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Dr. Frank Matthes
Dr. Daniel E. Bürgler

  

Micromagnetic Simulations

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The particular properties of magnetic micro- and nanostructures result from of a complicated interplay between several energy terms. A detailed description of the magnetic structure is a prerequisite for the understanding of magnetization processes in magnetic particles.

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Elements of the 4d series of the periodic table like Ru, Rh, and Pd are “almost ferromagnetic”. That means, their electronic structure is similar to the ones of the conventional ferromagnets Fe, Co, and Ni. It is suggested by theoretical investigations, that small modifications of the electronic density of states are sufficient to switch on ferromagnetism in the 4d elements.

For the case of nanoparticles, the low coordination number of surface atoms may already lead to the desired change in the electronic properties. Furthermore, these properties can be additionally tailored e.g. by charge transfer to/from organic molecules attached to the surface. This targeted manipulation plays a crucial role in understanding the fundamentals of ferromagnetism and the development of new nanoscale ferromagnets.

 

-> more

 

Beyond charge transfer effects, also external stress and strain caused by a matrix hosting the 4d nanoparticles and the lattice expansion of Pd upon loading with hydrogen will be studied in order to gain a more detailed picture.

The nanoparticles will be either synthesised by a wet-chemical method or prepared starting from polyoxypalladates in collaboration with the group of Prof. Kögerler (RWTH Aachen). Using this molecular approach, it is possible to tailor highly symmetric Pd clusters.

Both magnetic and electronic properties will be monitored by x-ray absorption spectroscopy and its associated circular dichroism. Since this method is element-specific, the mutual influence of nanoparticles, ligands and substrate or hosting matrix can be studied.


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