Fyzikální ústav Akademie věd ČR

Seminar Tuesday, 16/12/2014 15:00 - 16:00

Speakers: Peter Mohn (Department of Applied Physics, Vienna University of Technology)
Place: Na Slovance, main lecture hall
Presented in English
Organisers: Department of Condensed Matter Theory

Abstract: The exploration of magnetic states of matter is one of the key topics at the CMS Vienna. To present an overview on the diversity of our recent investigations I will report on 3 different approaches namely p-element substitutions and doping in TiO2, a quantum monte carlo (QMC) study of the low temperature NMR line broadening in doped SrCu₂O₃ and a cluster expansion simulation of the magnetic structure of fcc (fct) Fe.
• Doping TiO₂ with carbon or nitrogen leads to magnetic moments on the impurities when substituting for oxygen. C and N on an interstitial site form CO or NO molecular like dimmers, where the frequency of the stretching mode compares well with that of the free molecule.
• We wrote a QMC code based on statistic series expansion and studied the spin ladder compound SrCu₂O₃ where the magnetic moments on the Cu ions form an anti-ferromagnetic chain. Substituting one Cu by a non-magnetic ion like Zn lifts the perfect compensation of the AF order, leading to a distributed polarization around this impurity. Against the physical intuition it is found that the NMR lines for this state broaden when the temperature is lowered. It is shown, that the interaction between adjacent ladders is responsible for this effect.
• For fcc and tetragonal distorted fct iron a large number of magnetic configurations as a function of crystal structural parameters are studied by means of density functional calculations. These results were used for a cluster expansion technique which was applied to six different volumes for more than 90 000 collinear spin structures at each volume. For a slight tetragonal distortion we find a new equilibrium structure consisting of two atoms with antiferromagnetically ordered local moments of ±1.8 μB and of two atoms with zero local moment.