Universität Osnabrück, Graduiertenkolleg Mikrostruktur oxidischer

GRADUIERTENKOLLEG MIKROSTRUKTUR OXIDISCHER KRISTALLE 15
Electron spectroscopy and magnetism: Investigations on molecular
magnets and rare-earth compounds
Start of the project: 01.01.1999
Dipl.-Phys Sorin Gheorghe Chiuzbăian
Supervisor: apl. Prof. Dr. Manfred Neumann
Summary:
The magnetic properties of matter are in a direct connection with the electronic structure of the materials. However,
this link is far from being completely understood. By investigating the electronic structure with spectroscopic
methods and performing magnetic measurements, we were able to achieve a better understanding of the
magnetic behaviour of the materials.
We have focused our attention on two classes of materials: molecular magnets containing 3d-transition metal
atoms and RM5 rare-earth compounds.
State of art:
In the recent years, special attention was paid in our group to the magnetic materials and to the spectroscopic
features of the spectra due to the magnetic properties [1]. Thus, in the case of transition metal oxides [2], rareearth
oxides [3] or Heusler alloys [4], it was possible to find reliable connections between the magnetic properties
and the spectral features of the materials. The actual studies are a natural enlargement of these interests.
The molecular magnets are a class of magnetic materials consisting of 3d-metal atoms trapped in cluster structures
inside of organic or inorganic molecules [5]. It is an attractive subject in understanding the magnetic interactions
at the microscopic level [6]. Experimental investigations of the electronic structure of the molecular
magnets are practically inexistent. They also can serve as an important achievement to our previous results on
transition metal-oxides due to their molecular structure which involves particular metal-ligand interactions. We
have investigated a ferric-wheel by using XPS and shown that the final state effects can play a more decisive
role in influencing the photoelectron spectra in comparison to the 3d-transition metals. In order to clarify the
electronic structure, we have performed very recently investigations with synchrotron radiation. The late results
are still under discussion. Additional molecules are under investigation.
We have investigated the La(Ni,Cu)5 series by using X-ray photoelectron spectroscopy (XPS) and performed
additional magnetic measurements in a wide temperature range. We have shown, for the first time, that these
materials have to be considered as non-saturated spin-fluctuations systems [7]. The LaNi5 compound was traditionally
considered to be an exchange-enhanced paramagnet [8,9].
We have investigated compounds in the Gd(Ni,Al)5 series and shown that the Ni 3d-band in these compounds is
sensitive to the local environment, showing a large variety of magnetic behaviour [10]. The magnetic properties
were discussed in terms of the spin-fluctuation theory [11].
Experimental results und discussions:
1. Molecular magnets
Ring–shaped clusters have gained a lot of attention as attractive models for one-dimensional magnetic materials.
The synthesis and magnetic properties for even-membered Fe(III) cyclic clusters containing 6-18 ions have been
reported ([12] and the references therein). Recently, new hexanuclear clusters containing an alkali ion (Li or Na)
were reported [13]. The alkali ion placed in the center of a hexagonal wheel formed by Fe(III) ions plays an
important role in stabilizing the structure and influences the Fe-O-Fe angles. The S = 5/2 spins are coupled antiferromagnetically
in the sense of a negative coupling constant . The magnetic coupling results in a total cancellation
of the spins and gives a Stot = 0 ground state.