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

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.

The most immediate impact of this project will be educational. Some of the proposed additions to the EPR spectrometer also enhance its usability for standard pulsed experiments. Several students from collaborating groups currently use the spectrometer for standard continuous wave EPR and for low-power pulse EPR experiments with longitudinal detection (LOD). Implementation of equally easy operation procedures also for standard pulse EPR and for the proposed new LOD techniques presented in this proposal reduces the threshold for students to learn how to perform such experiments themselves.
Further pathways to impact for this proposal include academic application of the presented methods within the EPR community, interdisciplinary academic dissemination, and significant long-term commercial potential. The transient measurement of relaxation times is an important issue in pulse EPR spectroscopy that can be performed only with considerable limitations using current methods. A reliable and robust method to characterize longitudinal relaxation of rapidly relaxing metal ions over a large temperature range would enable not only to study distances, but also dynamics of accordingly labelled compounds. Such a method is likely to gain a lot of traction in the EPR community.
Another impact of a strong pulse EPR research program would be the possibility for interdepartmental and interdisciplinary research. A major focus of the proposed research will be on developing reliable and robust protocols for the developed methods, which is a prerequisite for collaborations with application oriented groups.
The proposed experiments on materials with mixed magnetic phases could have a huge academic and also commercial impact. The possibility to alter the state of ordered magnetic domains by on-resonant irradiation of neighbouring paramagnetic domains would open up exciting possibilities for magnetic data storage, spintronics, or spin chemistry.
Commercially, I envisage a more long-term impact of the project. For one, the possibility to teach students and postdocs on pulse EPR methodology and applications strengthens the basis of this method in the UK. There are several world-leading companies providing components for EPR setups based in the UK, such as Oxford Instruments plc or Thomas Keating ltd. It is vital for a further development and improved competitiveness of these companies that students are educated to understand the technology and the requirements of the users. Additionally, the availability of transient EPR relaxation measurements that can be implemented with less cost and complexity will open new areas of application, thereby encouraging a more widespread adaptation of the technique.