Quantum criticality in magnetically unstable 4f- and 3d-electron metals

Abstract

Quantum phase transitions and quantum critical behavior can be observed in magnetically unstable 4f- and 3d-electron systems. In contrast to classical phase transitions, which are induced by temperature changes, quantum phase transitions arise as a result of the variation of a non-thermal control parameter. A great number of complex magnetic ground states are observed close to such quantum phase transitions, which are not fully understood yet. Therefore, we performed stoichiometry studies of CePt3B1-xSix, Nb1-yFe2+y and Ta(Fe1-xVx)2. During the determination of the magnetic ground state properties of the observed materials studies were performed on a macroscopic scale by measurements of magnetization, resistivity and specific heat and on a microscopic scale by muon spin rotation (µSR) and 57Fe Mossbauer spectroscopy. Combining these different experimental methods has led to comprehensive insights into the static and dynamic magnetic properties and allows the construction of magnetic phase diagrams. For low silicon concentration, the non-centrosymmetric system CePt3B1-xSix shows physical properties similar to those of CePt3B: an antiferromagnetic ground state and for lower temperatures a weakly ferromagnetic phase. Increasing silicon content leads to a continuous decrease of the antiferromagnetic phase, while the weakly ferromagnetic ordering is completely suppressed for a critical Si concentration. Further, the heavy-fermion system CePt3Si exhibits an additional unconventional superconducting phase. Moreover, the magnetic ground state properties of the isostructural C14 Laves phase systems Nb1-yFe2+y and Ta(Fe1-xVx)2 are investigated in this thesis. Slight changes in stoichiometry lead to complex magnetic phase diagrams in both systems, with evidence for quantum critical behavior at the transition from a ferromagnetic ordering to a spin density wave (SDW) ordering. Detailed µSR experiments confirm the rare case of a SDW modulated phase in Nb1-yFe2+y, which is surrounded by ferromagnetic phases and a quantum critical point. Furthermore, the magnetic properties exhibit quantum critical behavior for critical concentrations. 57Fe Mossbauer spectroscopy also identified Ta(Fe1-xVx)2 as a system with a SDW modulated phase. Further, a ferromagnetic instability for x = 0.02 in Ta(Fe1-xVx)2 is observed by means of magnetization studies. This indicates a quantum critical behavior similar to the behavior of Nb1-yFe2+y.