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This paper presents a study of coherent dynamic nuclear polarization (DNP) using frequency swept pulses at 94 GHz which optimize the polarization transfer efficiency. Accordingly, an enhancement ℇ∼496 was observed using 10mM trityl-OX063 as the polarizing agent in a standard d8-glycerol:D2O:H2O :: 6:3:1 glassing matrix at 70K. At present this is the largest DNP enhancement reported at this microwave frequency and temperature. Furthermore, the frequency swept pulses enhance the nuclear magnetic resonance (NMR) signal and reduce the recycle delay, accelerating the NMR signal acquisition.

A general theory is presented describing the photo-excitation dynamics of transient, paramagnetic triplet states of aromatic molecules including their absorption properties in a molecular host crystal when irradiated by a pulsed laser beam. It is applied to the system of a pentacene doped naphthalene single crystal, where the photo-excitation of pentacene results in a highly non-equilibrium population of intermediate electronic triplet states. These paramagnetic states can be addressed to align the proton spins of the naphthalene host at moderate experimental conditions using dynamic nuclear polarisation (DNP). The photo-excitation profile and the resulting triplet state distribution, which is determined by the material and the laser parameters, leads to a corresponding proton polarisation profile. Measurements of the resulting polarisation inhomogeneity by spatially resolved neutron transmission confirm the theory. The theoretical model then allows the extraction of crucial parameters that enable quantitative investigations of the dynamics of spin systems. For applications of triplet DNP with pentacene-doped naphthalene crystals, polarisation maximum, homogeneity and build-up rate are key factors, and the theory allows the identification of optimal experimental parameters depending on the priorities placed on these factors.

This paper presents a theory describing the dynamic nuclear polarization (DNP) process associated with an arbitrary frequency swept microwave pulse. The theory is utilized to explain the integrated solid effect (ISE) as well as the newly discovered stretched solid effect (SSE) and adiabatic solid effect (ASE). It is verified with experiments performed at 9.4 GHz (0.34 T) on single crystals of naphthalene doped with pentacene-d(14). It is shown that the SSE and ASE can be more efficient than the ISE. Furthermore, the theory predicts that the efficiency of the SSE improves at high magnetic fields, where the EPR line width is small compared to the nuclear Larmor frequency. In addition, we show that the ISE, SSE, and ASE are based on similar physical principles and we suggest definitions to distinguish among them.

Nuclear spin hyperpolarization provides a promising route to overcome the challenges imposed by the limited sensitivity of nuclear magnetic resonance. Here we demonstrate that dissolution of spin-polarized pentacene-doped naphthalene crystals enables transfer of polarization to target molecules via intermolecular cross relaxation at room temperature and moderate magnetic fields (1.45$\,$T). This makes it possible to exploit the high spin polarization of optically polarized crystals while mitigating the challenges of its transfer to external nuclei, particularly of the large distances and prohibitively weak coupling between source and target nuclei across solid-solid or solid-liquid interfaces. With this method, here we inject the highly polarized mixture into a benchtop NMR spectrometer and observe the polarization dynamics for target $^1$H nuclei. Although the spectra are radiation damped due to the high naphthalene magnetization, we describe a procedure to process the data in order to obtain more conventional NMR spectra, and extract the target nuclei polarization. With the entire process occurring on a timescale of one minute, we observe NMR signals enhanced by factors between -200 and -1730 at 1.45$\,$T for a range of small molecules., 8 pages, 4 figures

The spectrum of the electron spin-spin interactions largely determines which mechanism is responsible for the growth of the nuclear spin polarization in dynamic nuclear polarization (DNP). When electron spin-spin interactions are weak and their spectrum is narrow, the solid effect (SE) dominates the process. When they are stronger, the cross effect (CE) and thermal mixing (TM) come into play. Then a narrow spectrum favours the CE—that is an exchange of electron Zeeman energy with the nuclear spins—and a broad spectrum also TM—that is an exchange of electron spin-spin interaction energy with the nuclear spins. Moreover, the spectrum of the electron spin-spin interactions critically determines the rate of spectral diffusion of electron spin polarization across the electron spin resonance (ESR) line, and the associated conversion of electron Zeeman energy into electron spin-spin interaction energy. This way electron spin-spin interactions indirectly influence the DNP process. The present work describes Monte Carlo simulations of the spectrum of these interactions for approximately spherical radicals in glasses and analytical approximations of the simulation results. As an example application expressions for the relative strengths of the energy flows due to the CE and TM are derived.

We report a transportable neutron spin filter based on the strong spin dependence of neutron proton scattering . The necessary high proton polarization is created by dynamic nuclear polarization (DNP) that uses short-lived optically excited triplet states instead of stable radicals, which eliminates the main path of nuclear spin lattice relaxation . The polarization can be preserved for many days under moderate experimental conditions and allows to realize a transportable spin filter which is conveniently polarized in the laboratory and then operated on a neutron beam line with practically frozen polarization maintained with a minimum of equipment. Using triplet DNP bulk proton spins in a pentacene doped single crystal of naphthalene have been polarized to a record value of 80% at a field of 0.36 T using a simple helium flow cryostat for cooling. This highly polarized sample can be transported in the compact cryostat without loss of polarization to a neutron beam line where it serves as a spin filter for polarization analysis in a neutron scattering experiment. Relaxation times of T 1 ∼ 800 h are achieved at a field of 20 mT and the polarization can be reversed by adiabatic fast passage with an efficiency of above 99%.

The Dzyaloshinskii-Moriya interaction (DMI) is believed to be operative in low-symmetry crystal structures lacking space-inversion symmetry. However, already in 1963, Arrott pointed out that even in a high-symmetry lattice, where the DMI would normally vanish, this interaction is present in the vicinity of any lattice defect. Based on these considerations and recent theoretical work, first experimental studies of the impact of the DMI on the spin-polarized magnetic small-angle neutron scattering (SANS) of polycrystalline magnets exhibiting a large density of microstructural defects have been performed. They demonstrated that an asymmetry in the difference between the two polarized SANS cross sections is induced by the DMI in nanocrystalline terbium and holmium as well as in mechanically-deformed microcrystalline cobalt. Here, we present a more complicated case, the nanocrystalline magnetically-textured soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), where the interface between the FeSi nanoparticles and the amorphous magnetic matrix serves as the defect. The SANS cross section exhibits the polarization-dependent asymmetric term originating from the DMI. The effect has a magnetic field dependence and is less pronounced at higher fields until it eventually vanishes at full saturation. The result supports the generic relevance of the DMI for the magnetic structure of defect-rich ferromagnets. Furthermore, it shows that polarized SANS is a particularly powerful tool for investigating defect-induced DMI, which is a consequence of the unique dependence of the SANS cross section on the chiral interactions.

The antisymmetric Dzyaloshinskii-Moriya interaction (DMI) plays a decisive role for the stabilization and control of chirality of skyrmion textures in various magnetic systems exhibiting a noncentrosymmetric crystal structure. A less studied aspect of the DMI is that this interaction is believed to be operative in the vicinity of lattice imperfections in crystalline magnetic materials, due to the local structural inversion symmetry breaking. If this scenario leads to an effect of sizable magnitude, it implies that the DMI introduces chirality into a very large class of magnetic materials---defect-rich systems such as polycrystalline magnets. Here, we show experimentally that the microstructural-defect-induced DMI gives rise to a polarization-dependent asymmetric term in the small-angle neutron scattering (SANS) cross section of polycrystalline ferromagnets with a centrosymmetric crystal structure. The results are supported by theoretical predictions using the continuum theory of micromagnetics. This effect, conjectured already by Arrott in 1963, is demonstrated for nanocrystalline terbium and holmium (with a large grain-boundary density), and for mechanically-deformed microcrystalline cobalt (with a large dislocation density). Analysis of the scattering asymmetry allows one to determine the defect-induced DMI constant, $D = 0.45 \pm 0.07 \, \mathrm{mJ/m^2}$ for Tb at $100 \, \mathrm{K}$. Our study proves the generic relevance of the DMI for the magnetic microstructure of defect-rich ferromagnets with vanishing intrinsic DMI. Polarized SANS is decisive for disclosing the signature of the defect-induced DMI, which is related to the unique dependence of the polarized SANS cross section on the chiral interactions. The findings open up the way to study defect-induced skyrmionic magnetization textures in disordered materials.

We present the status of development of a novel neutron spin filter based on the strong spin dependence of the neutron scattering on protons. Using photo-excited triplet states for the dynamic nuclear polarization (DNP) process, proton polarization values of 80% can now be achieved in pentacene doped single crystals of naphthalene at a field of 0.36 T in a simple helium flow cryostat. Careful sample preparation lead to extremely long polarization decay times under moderate conditions that allowed us to develop a transportable device. The filter is polarized in the lab under well controlled conditions and is then transferred to the neutron beam line where it can be operated during several days with almost constant polarization while requiring only a minimum of equipment. We describe briefly the main features of the spin filter and its use as a spin analyzer in a small-angle neutron scattering (SANS) experiment probing the magnetic structure of a nanocrystalline soft magnetic material. Furthermore the procedure for performing the background and spin leakage corrections of the neutron data is outlined in detail.

The use of polarized protons as a broad-band neutron spin filter is an attractive alternative to the well-established neutron polarization techniques, namely polarized 3He gas and super mirrors, since the spin-dependent neutron proton scattering cross-section is large in a broad wavelength range. We have developed a novel neutron spin filter where we create the necessary large proton polarization in a solid with a recent method of dynamic nuclear polarization (DNP) that uses photo-excited triplet states. This requires only moderate experimental means and allows a compact design. In order to quantify the efficiency of the spin filter, we have measured the relevant spin-dependent and spin-independent terms of the neutron scattering cross-section of a naphthalene single crystal. The data allows to estimate the triplet spin filter performance over a broad wavelength range. With the recently achieved proton polarization of 80% the triplet filter compares well with a state of the art 3He filter.