Magnetoresistance of nanocontacts with constrained magnetic domain walls

Spin-dependent transport was studied in single contacted granular nanowires composed of carbon encapsulated magnetic nanoparticles embedded in a Co or permalloy matrix. A large variety of magnetoresistance (MR) signals were measured. The ratio of the maximum variation of the resistance as a function of the magnetic field over the minimum resistance ΔR/R_min ranges from a few to 10 000%. Characterization of the samples was performed, including hysteresis loops as a function of the amplitude and angle of the applied magnetic field, temperature, as well as I-V characteristics and relaxation time. The huge MR signals are probably associated with a reversible mechanical change of the structure of the matrix close to a conduction channel. When this huge magnetomechanical effect is avoided, a change of the transport regime is observed, and ballistic MR of some tens of percents can be measured.