Developing New Methods to Measure Fast Longitudinal Magnetization Changes in Electron Paramagnetic Resonance

点击翻译按钮获取中文摘要

In magnetic resonance spectroscopy, the structure and dynamics of molecules and materials is analyzed via the magnetic moment associated with the spin of unpaired electrons and certain nuclei. Information about a sample and its environment can be obtained from coupling constants and coherent interactions that are responsible for the line positions and patterns in a spectrum, and from relaxation transients or linewidths caused by incoherent random processes that lead to a thermal equilibration following an external excitation. In electron paramagnetic resonance (EPR) spectroscopy, which is a technique to study electron spins, samples are very often either polycrystalline or glassy solids. Because various interactions are orientation dependent, so are the relaxation times. Thus relaxation cannot be modelled accurately using a mono-exponential decay function. In order to use relaxation times for characterizing the dynamics of a sample and its interactions with the environment, it is necessary to measure relaxation transiently. Pulse EPR techniques are very powerful in studying samples with slow relaxation. However, for most metal ion compounds, which make for a large fraction of paramagnetic samples, fast transverse relaxation prevents the formation of an echo. These samples can only be studied at cryogenic temperatures, causing the temperature dependence of relaxation times to be available only over a limited temperature range. For longitudinal detection (LOD) of EPR, a coil with its axis parallel to the external magnetic field is used to measure changes of the longitudinal spin magnetization. Such a coil does not pick up a signal from the oscillating magnetic field perpendicular to the external field that is used to excite the electron spins. Therefore it is possible to monitor changes of the longitudinal magnetization even while the sample is irradiated.In this project, a LOD EPR probe optimized for measuring fast longitudinal relaxation transients is being built. By carefully characterizing the transfer function of the probe, the signal can be inverted to obtain the magnetization transient that was inducing the signal. It then becomes possible to measure full longitudinal relaxation transients in a single repetition of an experiment instead of the point-by-point acquisition common in pulse EPR. This facilitates novel multi-dimensional experiments, where relaxation times are correlated with, for example, the resonance frequency. To take full advantage of the available data, analysis routines must be produced to obtain relaxation time distributions.In a next step, experiments will be developed to study interactions between paramagnetic and ordered magnetic domains in paramagnetically doped materials. In transition metal jarosites, the magnetic ordering can be varied between ferromagnetic, antiferromagnetic and frustrated antiferromagnetic, depending on the metal cation. We will study these materials by using a novel experiment to correlate longitudinal relaxation, following a microwave saturation pulse, and the response to a field jump in a minor loop experiment. This type of experiment, in combination with traditional EPR experiments, will allow us to identify the magnetic phases that interact with the microwave field. Eventually we will study interactions between different magnetic phases, which are expected to coexist especially in the temperature range close to a magnetic phase transition.