Magnetoresistance of mechanically stable Co nanoconstrictions

We have studied the resistivity and magnetoresistance of mechanically stable Co contacts of nanometer sizes made by electron (e)-beam lithography on Si, GaAs, and Al₂O₃ substrates. These constrictions were generated using two techniques. The first one uses conventional e-beam lithography to design fingers at different distances touching a perpendicular electrode. These contacts are generally in the tens of nanometers range with resistances as high as 5000.3em0exΩ. After ion milling these contacts, resistances as high as 200.3em0exkΩ may be obtained. The second technique consists of Co deposition through a 4000.3em0exnm hole made in a bilayer resist. The resistance in the ``current perpendicular to the plane'' geometry is monitored during deposition which is stopped when the desired resistance is obtained. Contacts in the kΩ range were thus fabricated between the bottom disk-like electrode and the top thin film. Magnetoresistance was measured in a wide range of applied magnetic fields and temperatures. Due to the large shape anisotropy difference between the electrodes, two well-defined coercive fields induce clear switching in the magnetization observable in the resistance. The magnetoresistances are in all cases below 1% and of varying signs. These effects are well within the range of the expected anisotropic magnetoresistance generated at the contacts or their vicinity.