Sustainable Heusler and Antiferromagnetic Thin Films for High Density Data Storage

We are reaching capacity limits in data storage devices, due to restrictions on further miniaturization. There are also sustainability concerns regarding long-term data storage solutions, specifically associated with the use of critical raw elements. The use of novel materials must be investigated to address both sustainability and capacity concerns in the field of spin transport electronics (spintronics). This thesis investigates thin films of Fe/Mn/Al based Heusler alloys and antiferromagnetic Mn$_3$$\it X$ ($\it X$ = Sn, Ga) as new materials for spintronic devices. Heusler alloys offer a very promising platform to tailor physical properties of materials through element compositional changes and substitution. In the first part of this thesis, both polycrystalline and epitaxial Fe/Mn/Al Heusler alloy thin films are investigated as potential sustainable electrodes for spin valves. 200-nm-thick Fe$_{2–x}$Mn$_{1+x}$Al ($\it x$ = –0.25, 0, 0.25) polycrystalline films (deposited on thermally oxidized Si-substrates) with intermixed antiferromagnetic and ferromagnetic phases were investigated as a single layer exchange biased system. The variation of Mn concentration determines the magnitude of the exchange bias effect, which can be either enhanced (in Fe$_{1.75}$Mn$_{1.25}$Al) or suppressed (in Fe$_{2.25}$Mn$_{0.75}$Al). An Fe-rich phase embedded in an Mn-rich microstructure was revealed to be associated with a ferromagnetic $\it L$2$_1$ phase and an antiferromagnetic $\it B$2 phase, respectively. Therefore, revealing that exchange coupling between these two phases is the cause of the exchange-bias effect. 20 nm – thick Fe$_{2–x}$Mn$_{1+x}$Al ($\it x$ = 0, 0.25, 0.5, 0.75, 1) epitaxial films are also investigated in this work. Films deposited on TiN buffer-layers show higher quality than those deposited directly onto MgO substrates, with $\it L$2$_1$ order verified by X-ray diffraction. SQUID magnetometry and X-ray magnetic dichroism have been utilized and show deviations from their predicted magnetic properties, which could possibly diminish their predicted half-metallic properties. In the second part of this thesis, the use of antiferromagnets, substituting conventional ferromagnets, for spintronic devices is explored. Antiferromagnetic materials offer higher stability than ferromagnetic materials, with higher packing density, due to minimal stray fields and insensitivity to external magnetic fields. Non-collinear antiferromagnetic $\it {D0}$$_{19}$ $\epsilon$-Mn$_{3}$$\it X$ ($\it X$ = Ga, Sn) thin films are investigated to this end. Structural characterization of the growth of the Ru-buffer (on $\it c$-Al$_{2}$O$_{3}$ substrates) and the subsequent deposition of Mn$_{3}$$\it X$ films are detailed. Epitaxial films together with smooth surfaces have been achieved. The growth parameters have been optimized for several compositions with unobserved interfacial diffusion. The non-collinear antiferromagnetic structure has been confirmed in both Mn$_{3}$Ga and Mn$_{3}$Sn by SQUID magnetometry. A small in-plane magnetic moment of ~12 kA m$^{–1}$ and ~9 kA m$^{–1}$ is observed for Mn$_{3}$Ga and Mn$_{3}$Sn respectively, with an insignificant out of plane contribution confirming bulk-like properties and paving the way for the generation of novel antiferromagnetic devices.This work was funded by the H2020-MSCA-ITN-2014 SELECTA (grant agreement no. 642642 of the European Commission