1. Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USA
2. NEC, 4 Independence Way, Princeton, New Jersey 08540, USA
3. National High Magnetic Field Facility, Florida State University, Tallahassee, Florida 32306, USA
4. Present address: Laboratoire L'eon Brillouin, CEA-CNRS, CE Saclay, 91191 Gif-Sur-Yvette , France.

Correspondence to: J. F. DiTusa¹ Correspondence and requests for materials should be addressed to J. F. D. (e-mail: Email: ditusa@phys.lsu.edu.)

The desire to maximize the sensitivity of read/write heads (and thus the information density) of magnetic storage devices has stimulated interest in the discovery and design of new magnetic materials exhibiting magnetoresistance. Recent discoveries include the 'colossal' magnetoresistance in the manganites^{1, 2, 3, 4} and the enhanced magnetoresistance in low-carrier-density ferromagnets^{4, 5, 6}. An important feature of these systems is that the electrons involved in electrical conduction are different from those responsible for the magnetism. The latter are localized and act as scattering sites for the mobile electrons, and it is the field tuning of the scattering strength that ultimately gives rise to the observed magnetoresistance. Here we argue that magnetoresistance can arise by a different mechanism in certain ferromagnets—quantum interference effects rather than simple scattering. The ferromagnets in question are disordered, low-carrier-density magnets where the same electrons are responsible for both the magnetic properties and electrical conduction. The resulting magnetoresistance is positive (that is, the resistance increases in response to an applied magnetic field) and only weakly temperature-dependent below the Curie point.