The intrinsic magnetic moment of a neutron, combined with its charge neutrality, is a unique property which allows the investigation of magnetic phenomena in matter. Here we present how the ...utilization of a cold polarized neutron beam in neutron grating interferometry enables the visualization and characterization of magnetic properties on a microscopic scale in macroscopic samples. The measured signal originates from the phase shift induced by the magnetic potential. Our method enables the detection of previously inaccessible magnetic field gradients, in the order of T cm
, extending the probed range by an order of magnitude. We visualize and quantify the phase shift induced by a well-defined square shaped uniaxial magnetic field and validate our experimental findings with theoretical calculations based on Hall probe measurements of the magnetic field distribution. This allows us to further extend our studies to investigations of inhomogeneous and anisotropic magnetic field distribution.
It has been known for decades that a ferromagnetic sample can depolarize a transmitted neutron beam. This effect was used and developed into the neutron-depolarization technique to investigate the ...magnetic structure of ferromagnetic materials. Since the polarization evolves continuously as the neutrons move through the sample, the initial spin states on scattering will be different at different depths within the sample. This leads to a contamination of the measured spin-dependent neutron-scattering intensities by the other spin-dependent cross sections. The effect has rarely been considered in polarized neutron-scattering experiments even though it has a crucial impact on the observable signal. A model is proposed to describe the depolarization of a neutron beam traversing a ferromagnetic sample, provide the procedure for data correction and give guidelines to choose the optimum sample thickness. It is experimentally verified for a small-angle neutron-scattering geometry with samples of the nanocristalline soft-magnet Vitroperm (Fe
Si
B
Nb
Cu
). The model is general enough to be adapted to other types of neutron-diffraction experiments and sample geometries.
Hyperpolarization of nuclear spins enhances nuclear magnetic resonance signals, which play a key role for imaging and spectroscopy in the natural and life sciences. This signal amplification unlocks ...previously inaccessible techniques, such as metabolic imaging of cancer cells. In this paper, electron spins from the photoexcited triplet state of pentacene-doped naphthalene crystals are used to polarize surrounding protons. As existing strategies are rendered less effective by experimental constraints, they are replaced with optimal control pulses designed with redcrab. In contrast to previous optimal control approaches, which consider one or two effective nuclei, this closed-loop optimization is macroscopic. A 26% improvement in signal and 15% faster polarization rate are observed. Additionally, a strategy called autonomously optimized repeated linear sweep (ARISE) is introduced to efficiently tailor existing hyperpolarization sequences in the presence of experimental uncertainty to enhance their performance. ARISE is expected to be broadly applicable in many experimental settings.
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
3
He 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
3
He filter.
Relaxation of nuclear dipolar energy Steiner, Jakob M.; Hautle, Patrick; Wenckebach, W. Tom
Journal of magnetic resonance (1997),
December 2021, 2021-12-00, Letnik:
333
Journal Article
Recenzirano
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•New mechanism for relaxation of nuclear dipolar energy by electron spins.•Explains ultrafast relaxation at low temperature and high magnetic field.•Extends to dipolar relaxation by ...photo-excited triplet states.•Explains single dipolar energy reservoir for multiple spin species.•Can be extended to DNP samples with polarization and relaxation agents.
Under typical conditions for dynamic nuclear polarization (DNP)—temperature about 1 K or below and magnetic field about 3 T or higher—the polarization agent causes nuclear dipolar order to relax up to four orders of magnitude faster than nuclear polarization. However, as far as we know, this ultra-fast dipolar relaxation has thus far not been explained in a satisfactory way.
We report similar ultra-fast dipolar relaxation of proton spins in naphthalene due to the photo-excited triplet spin of pentacene and propose a three-step mechanism that explains such ultra-fast dipolar relaxation by ground state electron spins as well as by photo-excited triplet spins: nuclear spin diffusion transfers nuclear dipolar order—that is nuclear dipolar energy—spatially to near the electron spins. Flip-flop transitions between nuclear spins near the electron spins convert this dipolar energy into electron-nuclear interaction energy. Finally electron spin–lattice relaxation or decay of the triplet spin transfers the latter type of energy to the lattice. We will show that this mechanism quantitatively explains the observed dipolar relaxation rate.
The proposed mechanism is expected to contribute to dipolar relaxation in any spin system containing more than one spin species. It tends to create a stationary state, in which all dipolar interactions are combined in a single energy reservoir described by a single spin temperature. As an example we suggest that the addition of a relaxation agent in samples used for DNP may significantly accelerate the relaxation of the dipolar energy of the polarization agent, and as a result could possibly reduce the contribution of thermal mixing (TM) to DNP.
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.
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