Models of degeneracy breaking in pyrochlore antiferromagnets

Motivated by the low-temperature magnetization curves of several spinel chromites, we theoretically study classical mechanisms of degeneracy lifting in pyrochlore antiferromagnets. Our main focus is on the coupling of spin exchange to lattice distortions. Prior work [Penc , Phys. Rev. Lett.93, 197203 (2004)] has demonstrated that such coupling leads to a robust magnetization plateau at half the saturation moment per spin, in agreement with experiment. We show that a simple Einstein model incorporating local site distortions generates magnetic order on the plateau, and highlight the distinct predictions of this model from that in Penc The plateau magnetic order in this model is universal, in the sense that systems could have different magnetic ordered states in the zero field region, while still displaying the same order in the plateau regime. We also consider the complementary degeneracy-lifting effects of further-neighbor exchange interactions. We discuss the implications for transitions off the plateau at both the high-field and low-field end, as well as at fields close to the saturation value. We predict that under certain circumstances there is spontaneous uniform XY magnetization (transverse to the field) for field values just above the plateau. These features may be tested in experiments. While selecting a unique magnetic order in the half-magnetization plateau, at zero magnetic field the Einstein model retains a thermodynamic degeneracy, though significantly reduced compared to the pure Heisenberg antiferromagnet.