Electronic Transport in Itinerant Metamagnets with Strong Static Disorder

Abstract

We report on electronic transport in nearly magnetic conductors with strong structural disorder. The initial motivation for this work was a large positive magnetoresistance (MR) found in magnetically ordered ground state of (Y1-xGdx)Co2 alloys. This was a surprising result since a large positive MR is not expected in a system with strong static magnetic or structural disorder. Contemporary theory of magnetotransport and common sense agree that an external magnetic field should suppress magnetic fluctuations, resulting in a negative MR. On the contrary; a positive MR suggests that an external magnetic field enhances static magnetic disorder. It was shown that unusual MR of (Y1-xGdx)Co2 alloys is related to a combination of structural disorder and metamagnetic instability of itinerant Co-3d electrons. The new mechanism of MR is common of a broad class of materials featuring a static magnetic disorder and itinerant metamagnetism. Such systems display a number of unusual properties, among them strong pressure and magnetic field dependencies of resistivity and thermopower, Non-Fermi-Liquid (NFL) behavior of resistivity and, possibly, of thermopower. We review the relevant experimental data, mostly the properties of RCo2-based alloys, and discuss the theoretical model developed for the interpretation of the experimental results. This model includes new mechanism of magnetoresistivity in structurally disordered itinerant magnetic alloys.