Zastosowanie techniki μSR w badaniach własnosci magnetycznych wybranych magnetyków molekularnych

Abstract

The subject of this paper is an application of the muon spin research (μSR) in study of critical properties of molecule-based magnetic materials, particularly the novel cyano-bridged molecular networks. Among several elementary particles commonly used in condensed matter science, the positive muon is fast establishing itself alongside the electron, the positron and the neutron. Although it would be an exaggeration to describe the muon as a universal microscopic probe, but only a few restrictions exist as to the suitability of the examined material. Research involving muons provides a wealth of structural and dynamical information on the atomic scale on metallic systems, on magnetic materials, on semiconductors and on insulators, including organic and molecular materials. μSR spectroscopy makes use of implanted muons to probe properties of matter at the microscopic level. According to one of the earliest definition of μSR - appeared on the cover of the first issue of the μSR Newsletter in 1974 -: “μSR stands for Muon Spin Relaxation, Rotation, Resonance, Research or what have you”. Generally speaking, the abbreviation covers any study of the interactions of the muon’s magnetic moment with its surrounding following implantation in matter of choice. μSR is a relatively new nuclear method, that can be classified in between NMR and diffraction techniques. .However, a key difference is the fact that in μSR one does not relay on internal nuclear spins, making use of the muon’s spin instead. Moreover, no radio-frequency technique alignment of the probing spin is required. Another clear distinction between the μSR technique and those involving neutrons or X-rays is that scattering is not involved. Neutron diffraction techniques, for example, use the change in energy and/or momentum of a scattered neutron to deduce the sample properties. In contrast to the neutron diffraction techniques, the implanted muons are not diffracted but remain in a sample until the time of their natural decay. A careful analysis of the decay positrons provides information about the interaction between the implanted muon and environment it is probing. The [M(CN)8]n− complexes, are universal building blocks for a moleculebased magnets, leading to various spatial structures, depending on the surrounding ligands and the choice of the metal ions. With these complexes many novel, functional magnetic compounds of different network dimensionality and unique physical properties have been recently developed. Research on functionality of organic and metalorganic systems receives still growing attention, since in case of molecular material it is possible to design it’s properties through careful preparation of the chemical synthesis path. Molecular magnets predominantly belong to the class of compounds involving well localized magnetic moments. Unprecedented properties of novel molecule-based magnetic materials are due to interplay of unique molecular network architectures and magnetic anisotropy induced by different coordination patterns. Such features together with the fact that the nature and symmetry of magnetic interactions is encrypted in the critical behaviour makes them a perfect testing ground of the existing theoretical spin models. The μSR experimental method allows to study magnetic properties of such materials in zero applied field. Therefore it is perfectly suited to study magnetic fluctuations and spin dynamics in the vicinity of phase transition, supplying complete set of static and dynamic critical exponents. Present dissertation concerns study of critical behaviour for six molecular magnets based on [M(CN)8]n− building blocks (M = WV,MoV, NbIV) and d-electron spin centres such as CuII,MnII, FeII. First part of this monograph provides a brief descrption of muon as a elementary particle and it’s interaction in condensed matter. The second part contains general introduction to the μSR experimental technique itself as well as it’s aplication in study of critical properties of magnetic materials. The second part was also meant to serve as a basic handbook, for students and possible new users, wishing to utylize this experimantal method in their research. Therefore author tried to make it possibly comprehensive. The third part of this monography is devoted to analysis of μSR data obtained for six compounds exhibiting different dimensionalities of crystall and magnetic structures. The fourth and last part of this paper summarizes main conclusions presented in this habilitation thesis. This dissertation intends to be an example of application of the μSR technique in study of critical phenomena in molecule-based magnetic materials. The author’s intention was, that this work could be a handy guide for those, (students, researchers) wishing to use μSR method for the first time.