Negative hopping magnetoresistance and dimensional crossover in lightly doped cuprate superconductors

We show that, due to the weak ferromagnetism of La2-xSrxCuO4, an external magnetic field leads to a dimensional crossover from two to three dimensions for the in-plane transport. The crossover results in an increase of the hole's localization length and hence in a dramatic negative magnetoresistance in the variable range hopping regime. This mechanism quantitatively explains puzzling experimental data on the negative magnetoresistance in the Neel phase of La2-xSrxCuO4.