Magnetotransport mechanisms in strongly underdoped YBa{sub 2}Cu{sub 3}O{sub x} single crystals

We report magnetoresistivity measurements on strongly underdoped YBa{sub 2}Cu{sub 3}O{sub x} (x=6.25 and 6.36) single crystals in applied magnetic fields H{parallel} c axis. We identify two different contributions to both in-plane {Delta}{rho}{sub ab}/{rho}{sub ab} and out-of-plane {Delta}{rho}{sub c}/{rho}{sub c} magnetoresistivities. The first contribution has the same sign as the temperature coefficient of the resistivity {partial_derivative} ln {rho}{sub i}/{partial_derivative}T (i={l_brace}c,ab{r_brace}). This contribution reflects the incoherent nature of the out-of-plane transport. The second contribution is positive, quadratic in field, with an onset temperature that correlates to the antiferromagnetic ordering.