Heusler Alloy films for spintronic devices

Abstract

This chapter reviews the requirements for the Heusler-alloy films to be used in spintronic devices. Four key requirements are identified to be large giant magnetoresistance (GMR), large tunnelling magnetoresistance (TMR), large spintransfer torque and fast spin resonance. These requirements can be achieved by utilising the fundamental properties of the Heusler alloys, such as atomic substitution, generalised Slater-Pauling behaviour, crystalline ordering, half-metallicity, low damping constant, high Curie temperature, good latticematching and large activation volume. To date the main obstacles for the Heusler-alloy films to be used in spintronic devices are their (i) high crystallisation temperature, (ii) interfacial atomic disordering and (iii) small activation volume. Here, we have investigated these properties for both epitaxial and polycrystalline films and have found a favourable crystallisation orientation to lower the ordering temperature by inducing a two-dimensional growth. We have demonstrated the effect of interfacial dusting to maintain the crystalline ordering from atomic diffusion by annealing. We have also established that the above requirements can be controlled by the competition between the structural and magnetic volume, the latter of which can be defined as activation volume. In all cases, the polycrystalline films have found to be advantageous over the epitaxial ones due to their strain-free growth with controlled grain size. We anticipate that the optimised polycrystalline films can be used in the next generation hard disk read heads and magnetic random access memory cells.