  Two channels of the sd exchange interaction are considered in magnetic
junctions. The first channel describes the interaction of transversal spins
with the lattice magnetization. The second one describes the interaction of
longitudinal spins with magnetization. We show the longitudinal channel leads
to a number of significant effects: 1) drastic lowering of the current
instability threshold down to three (or even more) orders of magnitude; 2)
creation of large enough distortion of equilibrium due to current driven spin
injection leading to inversion of energy spin subband populations and
laser-like instability in THz frequency range at room temperature. External
magnetic field may tend to lower additionally the instability threshold due to
the proximity effect of purely magnetic reorientation phase transition. This
effect demonstrates the new properties: the giant magnetoresistance (GMR)
becomes strongly current dependent and the exchange switching becomes of very
low threshold. We derived some matching condition that should be satisfied to
achieve high spin injection level. Some characteristic quantities were appeared
in the condition. We investigated also the junctions having variable lateral
dimensions of the layers, for example, a ferromagnetic rod contacting with a
very thin ferromagnetic film. Large enhancement of the current density may
appear near the contact region leading to the spin injection luminescence.
