Magnetoresistive effects $\left(R\right(0\left)-R\right(H\left)\right)/R\left(H\right)$ exceeding $500\%$ are found at room temperature in a field of 7 mT in nanocontacts between ${\mathrm{Fe}}_3{\mathrm{O}}_4$ crystallites. The shape of the $I\left(V\right)$ curve depends on field and the magnitude of the magnetoresistance is correlated with the resistance, the largest effects occurring when $R>\mathrm{}100\mathrm{}\mathrm{k}\Omega$. The explanation proposed involves hopping transport of spin-polarized electrons through a narrow domain wall pinned at the nanocontact; spin pressure on the domain wall pushes it out into the electrode, leading to the nonlinearity of the $I\left(V\right)$ characteristic. Application of current-induced wall motion in a simple fast-switching magnetic memory element is proposed.

• Received 16 January 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.026601