Magnetoresistance of ferromagnetic nanostructures

We have studied the magnetoresistance behavior in thin films and very narrow wires of ferromagnetic structures composed of Ni, Co, Ni81Fe 19 and Ni90Fe10. The experiments showed that the transport measurements are very convenient and sensitive in probing the magnetization configuration of these nanostructures. We were able to explain some of the observations by using anisotropic magnetoresistance (AMR) in conjunction with the sample geometry. However, we also found that the magnetoresistance behavior in very narrow wires can be quite complicated especially when the measurement current was parallel to the applied magnetic field and cannot always be explained using an AMR based model. The measurements with very narrow wires of Ni, Ni 81Fe19 and Ni90Fe10 showed that the resistance of a domain wall is negative; that is, the resistance of a wire containing a domain wall is lower than the resistance without the wall. We devised new experimental techniques to probe the behavior of domain walls, including nucleation, movement and annihilation of a wall. For Co wires we found that the magnetoresistance behavior is rather different from Ni, Ni 19Fe19 and Ni90Fe10. In many Co wires, we found that the strong magnetocrystalline anisotropy based AMR effect overwhelms the domain wall resistance. However, in some many other samples the anisotropy was evidently not strong enough, hence they were used to study the domain wall resistance. The measurements done with these Co wires also provided us with strong experimental evidence for a negative wall resistance