Dynamics of complex magnetic systems

Sammanfattning: Dynamic magnetic properties of disordered and frustrated magnetic systems have beenexperimentally investigated utilising SQUID (Superconducting Quantum Interference Device)magnetometry. The studied systems are spin glasses, reentrant ferromagnets, reentrantantiferromagnets and diluted Ising antiferromagnets.Dynamics of spin glasses has been investigated in ac-susceptibility, zero-field-cooled andfield-cooled relaxation experiments. A real space droplet model is used to decipher the observed behaviour, including an intriguing memory effect.It is found that the intrinsic dynamic behaviour of reentrant ferromagnets is accessible byprobing with low enough magnetic fields. The ferromagnetic phase shows a chaotic response totemperature and magnetic field perturbations. An overlap length scale is established, similar to what is found in ordinary spin glasses. The dynamics of the ferromagnetic phase also shows distinct dissimilarities with the low temperature reentrant spin glass phase which displays archetypal spin glass dynamics. Dynamic scaling suggests that a finite phase transition temperature, TRSG, exists in the standard reentrant ferromagnet (Fe0.20Ni0.80)75P16B6Al3.The reentrant Ising antiferromagnet Fe0.35Mn0.65TiO3, shows a weak ageing behaviour but no indications of criticality are found at low temperatures. Instead the slowing down of the relaxation times obey a pure Arrhenius law. A dynamic H-T phase diagram is determined from acsusceptibility measurements in different applied dc-fields.The diluted Ising antiferromagnetic system FexZn1-xF2 of composition at and slightly above the percolation threshold is examined. The percolating sample shows a slowing down of the relaxation times according to a pure Arrhenius law at low temperatures. No indications of a finite temperature spin glass phase transition are observed. Frustration is present in the sample as indicated by a weak ageing behaviour. The sample of higher concentration also shows a slowing down of the dynamics according to a pure Arrhenius law in zero dc-field at low temperatures. In an applied dc-field of relevant magnitude the system displays typical and well established random field effects.