Magnetoresistance of magnetite point contacts and nanoconstrictions

Abstract

Point contacts of magnetite exhibit large, reversible increases of conductance by a factor of up to 5 in the presence of a small magnetic field (<10?mT), provided the conductance is less than the quantum of conductance G0 (12?900??)?1. Smaller effects are found in (La0.7Sr0.3)MnO3 and Co2Cr0.6Fe0.4Al. Comparable effects observed in magnetite nanoconstrictions milled using a focused-ion beam in thin films deposited on oxide substrates are only reversible on a time scale of hours. High-resistance nanogaps show evidence of field emission beyond a voltage threshold, which is itself field dependent. The results are discussed in terms of spin-polarized transport, magnetostriction, magnetic dipole strain, and Fowler?Nordheim tunneling.