Magnetoresistance of mechanically stable Co nanoconstrictions

Abstract

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 ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ 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 $500\phantom{\rule{0.3em}{0ex}}\ensuremath{\Omega}$. After ion milling these contacts, resistances as high as $20\phantom{\rule{0.3em}{0ex}}\mathrm{k}\ensuremath{\Omega}$ may be obtained. The second technique consists of Co deposition through a $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ 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 $\mathrm{k}\ensuremath{\Omega}$ 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.