A Density Functional Theory Study of Magnetism Near Quantum Criticality

The transition metal intermetallic compound NbFe2 displays a magnetic quantum critical point very near stoichiometry, unlike other Fe-based intermetallics, and no field or pressure tuning is required. In this compound we obtain an obvious candidate for the origin of quantum criticality: an accidental Fermi surface "hot stripe" centered on a point of vanishing quasiparticle velocity on the Fermi surface at an unconventional band critical point (uBCP) of NbFe2. Around this uBCP the dispersion is cubic (epsilon k -- epsilonF ∝ k3x ) in one direction in the hexagonal basal plane and has a saddle point character in the orthogonal ky, kz plane; both aspects have significant consequences. At such a uBCP Moriya's theory of weak magnetism breaks down due to divergent contributions to the dynamic bare susceptibility from the uBCP, both at Q → 0 and at momenta spanning the uBCPs. These results are reminiscent of an earlier suggestion that anomalously low Fermi velocities are an essential aspect of the incipient or weak ferromagnetism of TiBe2, and strongly support the viewpoint that, for some quantum critical points, the mechanism may be identifiable in the underlying (mean field) electronic structure.;Next, we study YMn2 in the cubic Laves phase (C15). With its highly frustrated pyrochlore type sublattice of Mn sites, it is one of a small but growing class of ordered magnets that lie close to a quantum critical point at stoichiometry. Its ground state displays long-spiral helical magnetic order that is highly sensitive to volume, disappearing due to the substitution of 3% Sc for the larger Y atom (chemical pressure), or by application of just 0.4 GPa pressure. The large change of volume (5%) upon ordering (T N = 100 K) argues for itinerant magnetism. In recent years there have been developments in modeling magnetic fluctuations in itinerant magnets near the ordering point. We extend earlier results of Terao and Yamada on the first principles based energetics versus volume, and quantify the sensitivity of the magnetic state to pressure.;Finally, we examine a tertiary nitride compound, Fe3Mo 3N, which demonstrates quantum critical behavior and weak magnetism, with a structure consisting of a geometrically frustrated stella quadrangula lattice. Neutron scattering reveals antiferromagnetic ordering, but a 14 T magnetic field induces a ferromagnetic state, as does substitution of 5% Co on the Fe site. We present the energetics of a transition between these states with density functional based fixed spin moment studies. As the fixed spin moment in calculation is reduced, the ferromagnetic state switches to a ferrimagnetic state, approximately 0.2 eV higher in energy per formula unit.