Spin-charge coupling in lightly doped Nd _2-xCe _x-CuO ₄

We use neutron scattering to study the influence of a magnetic field on spin structures of Nd ₂CuO ₄. On cooling from room temperature, Nd ₂CuO ₄ goes through a series of antiferromagnetic (AF) phase transitions with different noncollinear spin structures. While a c-axis aligned magnetic field does not alter the basic zero-field noncollinear spin structures, a field parallel to the CuO ₂ plane can transform the noncollinear structure to a collinear one ("spin-flop" transition), induce magnetic disorder along the c axis, and cause hysteresis in the AF phase transitions. By comparing these results directly to the magnetoresistance (MR) measurements of Nd _1.975Ce _0.025CuO ₄, which has essentially the same AF structures as Nd ₂CuO ₄, we find that a magnetic-field-induced spin-flop transition, AF phase hysteresis, and spin c-axis disorder all affect the transport properties of the material. Our results thus provide direct evidence for the existence of a strong spin-charge coupling in electron-doped copper oxides.