Tuning transport properties of Cr2AlC conducting ceramic through point defects

Abstract

Nano-lamellar alloys known as MAX phases integrate ceramic structural properties with metallic conduction. Here we induce point defects in the prototype Cr${}_2$${}_2$AlC system thereby locally perturbing the lamellar structure and track the changes to the magnetic and electron transport behaviour. Systematic defect generation is achieved by the irradiation of energetic Co${}^{+}$${}^{+}$, Cr${}^{+}$${}^{+}$ as well as Ar${}^{+}$${}^{+}$ ions at fluences ranging from 10¹² – 10¹⁵ ions$\cdot$$\cdot$cm⁻². The magnetic behaviour is shown to consist of contributions from $J=\frac{1}{2}$$J=\frac{1}{2}$ quantum spins as well as $J=\infty$$J=\infty$ classical cluster-like behavior containing $\sim 80$$\sim 80$ spins. Both these magnetic defect types contribute to the transport properties, where the $J=\frac{1}{2}$$J=\frac{1}{2}$ spins give rise to the Kondo effect with a characteristic temperature, $T_K\sim 5$$T_K\sim 5$ K. Kondo defects are present in the as-prepared alloy as well as post-irradiation. A low-field magnetoresistive switching is observed post-irradiation, showing a temperature dependence that is consistent with polaron hopping within defect clusters. The enhanced magnetization as well as spin-scattering occur regardless of the irradiated species, proving that the source of these effects are the displaced Cr atoms of the precursor alloy. These results demonstrate the electronic tunability of MAX phases making them promising materials for spin-transport devices.