  Using theoretical arguments, we show that, in order to exploit half-metallic
ferromagnets in tunneling magnetoresistance (TMR) junctions, it is crucial to
eliminate interface states at the Fermi level within the half-metallic gap;
contrary to this, no such problem arises in giant magnetoresistance elements.
Moreover, based on an a priori understanding of the electronic structure, we
propose an antiferromagnetically coupled TMR element, in which interface states
are eliminated, as a paradigm of materials design from first principles. Our
conclusions are supported by ab-initio calculations.
