Dipolar nuclear polarization via the spin diffusion of a dipole order

Three nested spherical sections: (a) the innermost sphere represents the electron spin; spherical section in the middle part represents the nuclear spins, closest to the electron (nuclear spins inside the spin-diffusion barrier); and outer spherical section represents the nuclear spins in bulk. The electron polarization is transferred to the spins inside the spin-diffusion barrier. Then the Zeeman order is transferred to the dipolar order of spins inside the spin-diffusion barrier. Cooling of the dipolar reservoir of the bulk spins propagates throughout the bulk nuclei via homonuclear spin diffusion. The enhanced polarization of the bulk spins is then realized by converting the dipole order of the bulk spins to the Zeeman order. [Display omitted] •Transfer of the paramagnetic impurity polarization to nuclei in bulk is studied.•The spin diffusion of dipole energy is used.•Restriction of the diffusion barrier is removed.•Gain of the nuclei polarization of a 1H nuclear spin reaches 220, 13C ~ 850, and 15N ~ 2130. We propose transfer of the paramagnetic impurity (PI) polarization to nuclei in bulk, outside the diffusion barrier, by using dipolar system of the nuclear spins. The transfer can overcome influence of the diffusion barrier and is proposed to be implemented in four stages. At the first stage, transition of the Zeeman PI order to the Zeeman order of nuclear spins inside the spin-diffusion barrier is occurred. During the second stage the Zeeman order of both the nuclear spins inside the barrier and the nuclear spins in bulk is transferred into the nuclear dipolar spin order. As a result, the nuclear dipolar spin reservoir inside the barrier acquires a lower spin temperature, and thus a gradient of the spin temperature of the nuclear dipolar spin system is created. Since the external magnetic field and the magnetic field created by PIs do not effect on the dipole-dipole interaction between the nuclear spins, the dipolar reservoir is common for all nuclear spins, both inside and outside the diffusion barrier. Restriction of the diffusion barrier is removed and the spin diffusion of the dipole energy and transfer of the spin dipolar order to bulk spins occurs without obstacles (the third stage). At the last stage, to register an NMR signal, the dipolar order of the bulk spins is transferred into the Zeeman order of these spins. Estimations show that enhancement of the polarization can reaches in the case of a 1H nuclear spin, ~220, for 13C ~850, and for 1