Investigations of weak and dilute magnetic behaviour in organic and inorganic systems

Abstract

Muon spin relaxation is an ideal tool with which to study dilute magnetic systems, coupled with tailored bulk magnetic susceptibility measurements it is possible to examine previously unobserved magnetic exchange interactions. Investigations into non-stoichiometric LaCo03 reveals evidence of magnetic excitons in transition metal oxides for the first time. Moreover, data is presented that supports the concept of the defect driven excitons interacting with the stoichiometric LaCo03 which is known to undergo a thermally driven spin state transition. The data suggest the occurrence of more than one possible magnetic interaction of the excitons. Hole doped La1-xSrxCo03 is of interest as it is known to be magnetically and electronically phase separated; by a direct analogy with magnetic excitons it is suggested that the Sr rich ferromagnetic clusters interact with the pure LaCo03 below the metal insulator transition (x = 0.18 ) . It is suggested that it is this interaction observed for the first time that enables the rich phase diagram of La1_xSrxCo03 . The persistent photoconductivity effect on the spin glass transition in the elilute magnetic semiconductor Ccl0.85Mn0.15 Te:In has been investigated using low temperature magnetic susceptibility measurements and for the first time muon spectroscopy. Muon measurements on an Al eloped sample clearly show the spin glass transition, however the presence of the DX centre, which causes PPC when doping with In donors, perturbs the muon response. Particular attention is paid to possibility of the DX centre trapping muonium and preventing the detection of the spin glass transition. PPC does not induce a change in the muon response, however continuous illumination of the sample allows the observation of the spin glass transition, suggesting the presence of multiple DX centres, moreover the centre is found to be diamagnetic. The search for magnetic ordering at room temperature in an organic material has generally neglected polymers. PANiCNQ combines a fully conjugated nitrogen containing backbone with molecular charge transfer side groups. This combination gives rise to a stable polymer with a high density of localised spins, which are expected to give rise to coupling. Magnetic measurements suggest that the polymer is ferri- or ferro- magnetic with a Curie temperature of over 350 K, and a maximum saturation magnetization of 0.1 JT-1 kg-1 . Magnetic force microscopy images support this picture of room temperature magnetic order by providing evidence for domain wall formation and motion.