Spin Sorting: Apparent longitudinal relaxation without spin transitions

Nuclear spins experience forces in the presence of a magnetic field gradient. The forces cause the spin-up and spin-down nuclei to move in opposite directions, resulting in a flow of longitudinal magnetization. The effect can generate local longitudinal spin magnetization though it does not involve transitions (flipping) of spins. This phenomenon, spin sorting, competes with real spin-lattice relaxation and is generally not observable when T1 is short. We present our calculations of the longitudinal magnetization of diffusing spins with long T1 in magnetic field gradients. We show that the longitudinal spin magnetization due to spin sorting can be dominant at short times in such a system. Experimental confirmation is provided by observation of sorting-induced negative spin magnetization in 3He gas. We show how this phenomenon can potentially be used to generate nuclear magnetizations larger than thermal equilibrium.