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

在磁共振光谱学中，分子和材料的结构和动力学是通过与未配对电子和某些原子核的自旋有关的磁矩来分析的。关于样品及其环境的信息可以从耦合常数和相干相互作用中获得，这些耦合常数和相干相互作用负责光谱中的线位置和模式，以及由非相干随机过程引起的弛豫瞬态或线宽度，这些过程导致外部激发后的热平衡。电子顺磁共振（EPR）光谱是一种研究电子自旋的技术，样品通常是多晶或玻璃状固体。因为各种相互作用依赖于方向，所以弛豫时间也依赖于方向。因此，松弛不能用单指数衰减函数精确地建模。为了利用弛豫时间来表征样品的动力学及其与环境的相互作用，有必要对弛豫进行瞬态测量。脉冲EPR技术在研究慢弛豫样品方面是非常强大的。然而，对于大多数金属离子化合物来说，它们构成了顺磁样品的很大一部分，快速的横向弛豫阻止了回波的形成。这些样品只能在低温下进行研究，导致弛豫时间的温度依赖性只能在有限的温度范围内使用。对于EPR的纵向检测，采用轴线平行于外磁场的线圈来测量纵向自旋磁化强度的变化。这样的线圈不会从垂直于用来激发电子自旋的外场的振荡磁场中接收信号。因此，即使在样品辐照时，也可以监测纵向磁化强度的变化。在这个项目中，一个LOD EPR探头优化用于测量快速纵向弛豫瞬态。通过仔细表征探针的传递函数，可以对信号进行反求，从而得到感应信号的磁化瞬态。这样就可以在单次重复实验中测量完整的纵向弛豫瞬态，而不是在脉冲EPR中常见的逐点采集。这有助于新的多维实验，其中弛豫时间与共振频率等相关。为了充分利用现有数据，必须编制分析程序来获得松弛时间分布。下一步，将开展实验来研究顺磁掺杂材料中顺磁畴和有序磁畴之间的相互作用。在过渡金属黄钾铁矾中，根据金属阳离子的不同，磁序可以在铁磁性、反铁磁性和受挫反铁磁性之间变化。我们将通过一个新的实验来研究这些材料，在一个微波饱和脉冲之后的纵向弛豫，以及在一个小环路实验中对场跳变的响应。这种类型的实验，结合传统的EPR实验，将使我们能够识别与微波场相互作用的磁相。最后，我们将研究不同磁相之间的相互作用，特别是在接近磁相转变的温度范围内，这些磁相预计会共存。

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.