Magnetic field sensors perform according such material specific characteristics as magneto-resistance (MR) and resistance area product (RA) The MR-RA scaling behavior important for performance of HDD sensor, MRAM and spin torque oscillator devices. We consider fundamental aspects of MR-RA scaling problem for two classes of planar FM/NM/FM hetero-structures: (i) Fe/MgO/Fe tunneling junctions and (ii) all Heusler alloy giant-magneto-resistance (GMR) spin valves. In both cases we focus on the electronic structure contributions to RA(MR) arising from ferromagnet (FM), non-magnet (NM) and FM/NM interface states. We calculate MgO and CaO complex band structure and interface electronic states for (001) Fe/MgO/Fe using recently developed QSGW method. Experimental results for two types of non-magnetic spacers (i) elemental metal and (ii) non-magnetic Heusler alloy spacers deserve careful comparison. We investigate spin dependent interface scattering for (001) CMG/Ag/CMG (Case1) and (110) CMG/RCS/CMG (Case2) on the basis of microscopic transport simulations.