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In situ measurements of conductance quantization and magnetoresistance in nickel nanocontacts
We present an original method for fabricating magnetic electric contacts made of a few atoms. Controlled chemical environment and contact stability lasting for several tens of seconds allow systematic study of the electrical properties under external magnetic fields. The set-up was chosen for optimum mechanical stability, good control of the purity of the contacts and the surface composition of the electrodes, and compatibility with semiconductor large-scale processing. Controlled re-opening of the gap also provides a convenient method to make a pair of magnetic electrodes separated by a gap of a few nanometers only. ^ Quantum conductance (QC) steps at odd and even multiples of e2/h and stabilized conductance plateaus were observed during both forming and opening of the contacts. Statistical analysis of the data revealed that an external magnetic field enhanced the occurrence of odd integer plateaus in magnetic contacts conductance values, indicating a lifting of the spin degeneracy. ^ We investigated magnetoresistance properties under varying external fields and for several orientations between the field and the electrodes. The magnitude of conductance change when sweeping the magnetic field is of the order of one quantum conductance e2/h for conductance values spanning 1–100 quanta. The relative orientation of electric current and applied magnetic field changes the sign of magnetoresistance, with symmetry properties reminiscent of bulk anisotropic magnetoresistance. Ex situ investigations of samples of higher conductance values, of the order of 1000 quanta, unambiguously show the analogy with bulk anisotropic magnetoresistance. ^ Mechanical effects that can mimic the magnetoresistance in magnetic nanocontacts were discussed and compared in our sample geometry and other published results. A new design involving nickel base electrodes was used for further reducing mechanical effects and no difference in MR behavior was found, confirming limited mechanical artifacts in our results. Our data provided significant clarification on the dispute about the reality of very large MR in magnetic nanocontacts, and we showed that no colossal effects can be found for pure nanocontacts. The largest magnitude we observed was around 70%. ^
Engineering, Electronics and Electrical|Physics, Electricity and Magnetism|Physics, Condensed Matter
Yang, Cheol-Soo, "In situ measurements of conductance quantization and magnetoresistance in nickel nanocontacts" (2004). ETD collection for University of Nebraska - Lincoln. AAI3131571.