Scientific Highlight 07/2011
The rational design of the spintronic devices was recently proposed by us based on theoretical electronic structure engineering methods. It concerns the basic elements of spintronics, such as the so-called TMR (based on tunelling magnetoresistance effect) or GMR (giant magnetoresistance) junctions used to switch on and off the electric current by applying small magnetic fields. The operating efficiency directly depends on spin-polarization of the current (i.e. the domination of electrons with a certain spin orientation). The ideal situation would imply the fully spin-polarized case, i.e. when all moving electrons have the same spin. This in turn requires the injector and acceptor electrodes made out of the so-called half-metallic ferromagnetic materials, i.e. conducting only certainly spin-orientated electrons and insulating the other ones. The electrodes are separated by the non-magnetic spacer, which can be a semiconductor (TMR junction) or metal (GMR junction). The injected electrons tunnel (or flow) through the spacer without changing their spin state, and depending on the magnetic orientation of the acceptor, either flow through it (low resistance) or get reflected (high resistance). In real-life implementations various mechanisms strongly reduce the efficiency of spintronic elements by destroying the spin-polarization of the current. Large losses of spin-polarization typically occur at the half-metallic/semiconductor interfaces and are often due to mechanical and chemical incompatibilities of the stacking materials. Our scheme finds the efficient solution to this problem by building the junctions out of only Heusler materials. This diverse class of compounds is able to provide thermally stable half-metallic ferromagnets as well as non-magnetic semiconductors within similar crystal structures. The latter property is important in order to avoid mechanical tenses and related interface defects. The chemical compatibility is another critical issue which is improved by stacking the half-metallic and semiconducting parts in such a way that the effective interfacial compound again corresponds to a certain stable half-metallic Heusler which exhibits the intermediate properties with respect to both electrode and spacer parts. Using these rules, we have veryfied the possibility to design the spin-polarization lossless TMR junctions for the number of known Heusler materials. Published as: S. Chadov et al.; "Efficient Spin Injector Scheme Based on Heusler Materials " in Phys. Rev. Lett. 107 (2011) 036402.
