Transport properties of systematically disordered Cr2AlC films

Nano-lamellar composite materials, known as MAX-phases, can possess a combination of ceramic and metallic properties. A prototype compound is Cr2AlC, formed from a unit cell of Cr2C sandwiched between atomic planes of Al, thereby imparting a good electrical conductivity, as well as mechanical stability, radiation and oxidation resistance [1, 2]. These properties rely on the lamellar structure of the compound, and systematic introduction of defects, such as displacing or doping atoms within the layers, has the potential to tune electron transport and modify magnetic properties [3]. An ideal tool for defect implantation is ion-irradiation, available both in the form of a broad-beam for wafer-scale processing as well as focused ion-beams for device prototyping. Here we observe the modifications to the structural, transport and magnetic behavior of 500 nm thick Cr2AlC after irradiation with Co+ ions, and Ar+ noble gas ions as control. The films were irradiated with 450 keV of Co+ ions at fluences varying from 5E12 to 5E15 ions.cm-2, and the control samples with 400 keV Ar+ ions keeping the sample fluences. Structural analysis using XRD shows that ion-irradiation induces a suppression of the 0002 reflection, indicating a gradual decay of the nano-lamellar structure. Increasing ion-fluence also leads to an increase of the saturation magnetization at 1.5 K, whereby both Ar+ and Co+ cause an increased magnetization, respectively to 150 and 190 kA.m-1, for the highest fluences used. Large variations of the transport properties are observed. Magnetoresistance (MR) in the non-irradiated sample shows a classical B2 dependency, even up to high temperatures. At Co+ fluences of 5E13 ions.cm-2 the MR shows a two orders of magnitude increase, up to 3% (10 T) at 100 K. A similar effect also occurs for 5E12 ions.cm-2 Ar+ irradiated films, however with a smaller MR-increase. It appears that resistivity increases and the residual resistance ratio reduces with increasing fluence due to the introduction of disorder. These results show that ion irradiation induces significant changes in the transport properties of MAX phase materials, that will be further investigated. The systematic disordering of nano-laminated MAX phase films may therefore reveal interesting disorder and spin-related transport phenomena.

Funding by the Deutsche Forschungsgemeinschaft (DFG) - Project number 456078299 is acknowledged. Ion-irradiation has been performed at the Ion Beam Centre of the HZDR.
[1] A. S. Ingason, M. Dahlqvist, J. Rosen, Magnetic MAX phases from theory and experiments; a review; J. Phys.: Condens. Matter 28, 433003, (2016). [2] M. W. Barsoum, MAX Phases: Properties of Machinable Ternary Carbides and Nitrides; Weinheim: Willey-VCH (2013). [3] C. Wang, T. Yang, C. L. Tracy, C. Lu, H. Zhang, Y.-J. Hu, L. Wang, L. Qi, L. Gu, Q. Huang, J.Zhang, J. Wang, J. Xue, R. C. Ewing, Y. Wang, Disorder in Mn+1AXn phases at the atomic scale, Nature Communications 10, 622 (2019).