From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions

We fabricate ferromagnetic constrictions whose cross section can be reduced gradually from 100 $\times $ 30 nm$^{2}$ to the atomic scale and eventually to the tunneling regime by means of electromigration. The contacts are attached to non-magnetic substrates and are measured at 4.2 K, so they are much more mechanically stable than previous room-temperature studies. We measure magnetoresistances (MR) $<$ 3{\%} for contacts $<$ 400 $\Omega $, consistent with previous experiments. As the contact diameter is reduced in the range 400 $\Omega $ - 25 k$\Omega $, we observe reproducible non-monotonic changes in the MR. We find first a minimum in the MR and sometimes a change in sign to small negative values, and then a strongly increasing positive MR as the contact approaches the atomic scale ($\sim $ 25 k$\Omega )$. For near-atomic-sized constrictions the maximum MR is 80{\%}. Measurements as a function of the direction of applied magnetic field allow us to separate the contribution of anisotropic magnetoresistance from the MR due to a domain wall. In the tunneling regime the MR fluctuates over a wide range, -10{\%} to 85{\%}, even for small changes in the atomic structure in a single device.