| Magnetism is an inherently quantum effect, and has been of great interest both scientifically and technologically. In our work, we studied the effects of tunneling in a ferromagnetic system, the magnetic tunnel junction; and an antiferromagnetic phase transition of the 2D Kondo lattice system.;The spin polarization of the tunneling current in magnetic tunnel junctions is closely related to that of the density of states of the Co leads near E F. Experiments have shown that the spin polarized tunneling current is positive, as opposed to the bulk DOS at EF. Using density functional theory calculations, we show that an Al-rich interface with disorder can lead to a positive spin polarization near the interface. This gives a spin-dependent effective mass, and the spin polarized tunneling current calculated is positive. The voltage dependence of the junction magnetoresistance obtained also shows good agreement with experiments.;The antiferromagnetic QPT of Kondo lattice systems has typically been understood using the Hertz-Millis approach, within which the Kondo interaction merely gives rise to Landau damping of the critical spin-wave fluctuations. Using a non-linear sigma model, we have studied the antiferromagnetic quantum phase transition of a 2D Kondo-Heisenberg square lattice. A renormalization group analysis of the competing Kondo and RKKY interactions gives a new quantum critical point (QCP) that is strongly affected by Kondo fluctuations. Near this QCP, the spin-wave velocity scales logarithmically, i.e. breakdown of hydrodynamic behavior, and the spin-wave is logarithmically frozen out. The RG results also allow us to propose a new phase diagram near the antiferromagnetic fixed point of this 2D Kondo lattice model; and allow us to study the effects of the critical spin-waves on the fermions near the QCP. |
