Magnetic properis of Cr2AlC(Mn) thin films
Final Report Abstract
The major objective of this project was to correlate a magnetic signal and the aging condition of (Cr1-xMnx)2AlC thin film to allow for non-destructive tracking of the oxidation progress. It was previously predicted that for Mn concentrations above 10 at.% the Curie temperature would be increased above room temperature and hence allow these properties to be measured at room temperature. Within this project (Cr1-xMnx)2AlC thin films with Mn concentrations ranging from 0 to 6.4 at.% were deposited by high power impulse magnetron sputtering. High resolution transmission electron microscopy (HRTEM) studies on Cr2AlC revealed the occurrence of nanometer scale voids at interfaces between MAX phase and a disordered solid solution (Cr,Al) XCY phase. Structural analysis of (Cr1-xMnx)2AlC thin films by atom probe tomography and HRTEM revealed impurity phases on the nm-scale that could not be resolved by X-ray diffraction (XRD) techniques. Hence, the results suggest the need for high resolution probes for the assessment of phase purity. Furthermore, due to the limited solubility of Mn in (Cr1-xMnx)2AlC, it was not possible to synthesize phase pure MAX phase thin films with Mn concentrations above 10 at.%. Despite these limitations a continuous change in the magnetization of the as-deposited coatings with increasing Mn content was observed, where both the Curie-Weiss and Pauli-paramagnetic terms of the coatings increased. No ordering transitions were observed and hence ferromagnetism could not be substantiated, which ran contrary to a core-idea of the project. In addition, the Pauli-paramagnetism showed some temperature dependence which indicates some co-operative phenomenon in the “free” charge carriers. Electrical measurements, which were not original planned in this work, showed charge doping effects, where the resistivity dropped with increasing Mn content. The change in the number of conduction electrons derived from the Hall measurements correlated well with the change in Mn content. The magnetoresistance of the sample was small, but changed from negative to positive as the temperature decreased. The annealing of the coatings was observed to cause the formation of ferromagnetic Mn 3O4 in addition to the oxides and carbides expected for non-Mn containing Cr2AlC, as observed by both XRD and magnetic measurements. Hence, due to the observed limitation in Mn solubility in (Cr1-xMnx)2AlC and the lack of ferromagnetism in synthesized coatings, it was not possible to track the ageing condition of these coatings by measurements of the magnetic signal.
Publications
- Nanometer-scale 3D defects in Cr2AlC thin films, Scientific Reports 7 (2017) 1-8
Y.T. Chen, D. Music, L. Shang, J. Mayer, J.M. Schneider
(See online at https://doi.org/10.1038/s41598-017-01196-3)
