Development of Triplet Dynamic Nuclear Polarization for Polarization Analysis in Small-Angle Neutron Scattering

We present the status of the development of a novel neutron spin filter based on the dynamic nuclear polarization (DNP) of protons in a naphthalene single crystal that uses highly polarized optically excited triplet states of pentacene as the polarizing agent (PA). The filter is applied as spin analyzer in small-angle neutron scattering (SANS) experiments to study magnetism.
In order to improve the spin filter performance, a better understanding of the electron polarization creation was essential. For this purpose, careful light absorption measurements have been performed an a theory developed to describe the light propagation and absorption in the biaxial anisotropic absorptive pentacene:naphthalene single crystal and the subsequent triplet production of pentacene molecules. The DNP build-up in a crystal of given size can now be simulated and optimized by the proper choice of the experimental parameters, e.g. the type of excitation light source or the pentacene dopant concentration. As a result 80% proton polarization can now be routinely achieved, close to the theoretical maximum, with extremely long relaxation times. This significantly improved the figure of merit of the spin filter and furthermore allowed to implement a new scheme of filter operation that greatly facilitates its operation in the environment of a large-scale neutron scattering facility. We have made the device transportable, i.e. the filter is conveniently polarized under optimum conditions in the laboratory and then transferred to the neutron beam line where it can be operated during several days with practically frozen polarization while requiring only a minimum of equipment. This saves cost on instrumentation, beam time and work.
These improvement allowed to apply the spin filter as neutron polarization analyzer in a series of polarized SANS experiments to study magnetism on the nano-scale. These studies focused on an exotic and elusive physical phenomena – the defect-induced Dzyaloshinskii- Moriya interaction (DMI) in a nanocrystalline two-phase alloy Fe73Si16B7Nb3Cu1. An asymmetric signal is observed in the difference between the two spin-flip cross sections, which is a key signature directly related to the DMI. The result supports the generic relevance of the DMI for the magnetic structure of defect-rich ferromagnets.
Two additional studies are presented that do not directly relate to the spin filter subject but further exploit the unique properties of highly polarized proton spin systems with extremely long relaxation times that now can be prepared as a result of all the optimized processes. The first addresses a fundamental issue of DNP that storage and transport of hyperpolarized samples is severely restricted. A procedure and equipment is presented to transport polarized samples over long distance and provide hyperpolarized nuclear spins to users that are not in the possession of DNP equipment. The second studies the long-range nuclear magnetic ordering (ferromagnetic or antiferromagnetic) that is created by adiabatic demagnetization in the rotating frame. We focus on the antiferromagnetic nuclear spin configuration of hydrogen nuclei in a naphthalene single crystal, similar to the antiferromagnetic structure of electron spins ordered by the Heisenberg exchange interaction.