Synthesis and characterization of Ga-containing MAX phase thin films

Abstract: The study of magnetic Mn+1AXn (MAX) phases (n = 1 − 3, M – a transition metal, A – an A group element, X – C or N) is a recently established research area, fuelled by theoretical predictions and first confirmed experimentally through alloying of Mn into the well-known Cr2AlC and Cr2GeC. Theoretical phase stability investigations suggested a new magnetic MAX phase, Mn2GaC, containing Ga which is liquid close to room temperature. Hence, alternative routes for MAX phase synthesis were needed, motivating a further development of magnetron sputtering from liquid targets.In this thesis, (Cr1-xMnx)2GaC 0 ≤ x ≤ 1  MAX phase thin films have been synthesized from elemental and/or compound targets, using ultra high vacuum magnetron sputtering. Initial thin film synthesis of Cr2GaC was performed using elemental targets, including liquid Ga. Process optimization ensured optimal target size and crucible geometry for containing the Ga. Films were deposited at 650 °C on MgO(111) substrates. X-ray diffraction and transmission electron microscopy confirms the growth of epitaxial Cr2GaC MAX phase with minor inclusions of Cr3Ga.To explore the magnetic characteristics upon Mn alloying, synthesis of (Cr0.5Mn0.5)2GaC thin films was performed from elemental Ga and C and a composite Cr/Mn target of 1:1 composition. Films were deposited on MgO(111), Al2O3(0001) (with or without NbN seed layer), and 4° off-cut 4H-SiC(0001) substrates. The films are smooth and of high structural quality as confirmed by X-ray diffraction and transmission electron microscopy. The film composition measured by high resolution energy dispersive X-ray spectroscopy confirms a composition corresponding to (Cr0.5Mn0.5)2GaC. The magnetic response, as measured with vibrating sample magnetometry, displays a ferromagnetic component, however, the temperature dependence of the magnetic moments and saturation fields suggests competing magnetic interaction and possible non-collinear magnetic ordering.Finally, inspired by theoretical predictions, a new member of the MAX phase family, Mn2GaC, was synthesized. This is the first MAX phase containing Mn as a sole M element. X-ray diffraction and transmission electron microscopy confirms the characteristic MAX phase structure with a 2:1:1 composition. Theoretical work suggests that the magnetic ground state is almost degenerate between ferromagnetic and anti-ferromagnetic. Vibrating sample magnetometry shows ferromagnetic response with a transition temperature Tc of 230 K. However, also for this phase, complex magnetism is suggested. Altogether, the results indicate a new family of magnetic nanolaminates with a rich variation of magnetic ground states.