Spin filtering and an enhanced regime of giant magnetoresistance

A new regime of magnetoresistance (MR) in systems composed of magnetic layers separated by non-magnetic modulated structures is studied. The ability to tailor the electronic structure of superlatticed systems enables one to engineer contrasting spin-dependent transport properties, enhancing the magnetoresistance ratios. When the system acts as a spin filter the MR ratios reach values many orders of magnitude larger than those of conventional giant magnetoresistance. Rather than a mere enhancement of the magnetoresistance ratios, this new regime involves ingenious combinations of spin-polarized currents. Moreover, it results from a magnetic-field-induced metal-insulator transition along the modulation direction, characterizing a highly anisotropic transport behaviour. The existence of an insulating phase circumvents the experimental challenge of probing excessively small resistances in the current-perpendicular-to-plane (CPP) geometry of magnetoresistance. A picture in terms of the hulk Fermi surfaces of the constituent materials emerges and provides general guidelines on how to achieve this regime.