Correlating magnetic and structural properties of full Heusler Co2FeSi films

The half-metallic properties of Heusler alloys make them ideal spin injectors for numerous spintronic applications. In particular, Co2FeAl1−xSix alloys are very attractive for current perpendicular-to-plane giant magnetoresistance (CPP-GMR), tunnel magnetoresistance (TMR) and in lateral spin-valve (LSV) devices due to their high spin polarisation (SP) and high Curie temperature (Tc). However, half-metallicity can be significantly reduced due to structural disorder, defects or interfacial effects.

The work in this thesis had two main objectives. The first was building a bespoke molecular beam epitaxy (MBE) system for the growth of Co2FeAl(1−x)Six Heusler films. The second, was optimizing the growth conditions for Co2FeSi (CFS) thin films on technologically relevant substrates, namely Si(111) and MgO(001). With that aim, the effect of stoichiometry and growth temperature on the structural and magnetic properties of CFS has been studied. The CFS films grown in this work exhibit properties comparable to those reported in the literature for high quality Heusler films used in spintronic devices. Lastly, the effect of a NiO overlayer on the magnetocrystalline anisotropy (MCA) of CFS is discussed.

Non-annealed CFS films are partially L21 ordered when grown on Si(111). However, even at temperatures <130C, interfacial effects switch the magnetic easy axis (EA) from [10-1] to [1-10] in CFS/Si(111). HAADF-STEM images suggest that the CFS/Si interface has suffered some changes although further studies are required to characterize them. Contrary to CFS/Si(111), the growth temperature has to be increased to 350C for CFS films to show some degree of L21 on MgO(001). Whereas uncapped CFS/MgO(001) exhibits a cubic MCA with Kc>0, NiO-capped CFS/MgO(001) samples have a Kc<0, which brings the EA along <110>.