Condensed-Matter Physics & Materials Science Seminar

"In situ Measurements of Quantum Conductance and Magnetoresistance in Nickel Nanocontacts"

Presented by Cheol-Soo Yang, Univ. of Massachusetts - Amherst

Thursday, January 26, 2006, 1:30 pm — Small seminar room, Bldg. 510

Magnetoresistance (MR) properties of nickel nanocontacts in the ballistic
quantum regime are investigated in situ during closure and opening of
electrochemically grown planar electrodes. 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 e^2/h and
stabilized conductance plateau were observed during both forming and opening
of the contact. Statistical investigation revealed that an external magnetic
field enhanced the occurrence of odd integer plateau in magnetic contacts
conductance values, indicating a lifting of the spin degeneracy.

We investigated magnetoresistance properties under varying external field
and for several orientations between field and electrodes. The magnitude of
conductance change when sweeping the magnetic field is of the order of one
quantum conductance e^2/h for conductance values spanning 1 - 100 quanta.
The relative orientation of electric current and applied magnetic field
changes the magnetoresistance sign, with symmetry properties reminiscent of
bulk anisotropy magnetoresistance. Ex situ investigations of samples of
higher conductance values, of the order of 1000 quanta, unambiguously show
the analogy with bulk anisotropy magnetoresistance.

With limited mechanical artifacts in our samples, 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%.

Hosted by: Myron Strongin

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