Tailoring Magnetic Properties of MAX Phases, a Theoretical Investigation of (Cr₂Ti)AlC₂ and Cr₂AlC

Abstract

(Cr 2 Ti)AlC 2 is a newly discovered MAX phase with ordered occupations of Ti and Cr atoms on M sites. The Cr-containing MAX phase is expected showing magnetic property, which provides functional applications in spintronics and as self-monitoring smart coating. The magnetic states of (Cr 2 Ti)AlC 2 are predicted by GGA and GGA + U methods and compared to those of Cr 2 AlC. The ground states are predicted as FM or AFM-XX configurations depending on the calculation methods. Analysis of the electronic structure shows that the magnetic moments mainly originate from the net spins of Cr 3d valence electrons, whereas the contribution of other atoms is negligible. The calculated magnetic moments of Cr atoms in (Cr 2 Ti)AlC 2 are higher than those in Cr 2 AlC due to the larger distance between the out-plane Cr atoms separated by the intercalated nonmagnetic Ti-C slab. This work provides an insight on tailoring magnetic properties of MAX phases by modifying the crystal structure.

Experimental and DFT investigation of (Cr,Ti)3AlC2 MAX phases stability

“…In agreement with Dahlqvist and Rosen [19], we find that the in-plane antiferromagnetic ordering type XX (following the nomenclature of Wang et al [53]) is the most favorable spin state, closely followed by simple ferromagnetic ordering with a difference of only 1meV. Note that this energy difference is much below the expected accuracy of point defect calculations within DFT (see methodology section, and the effect of on-site Coulomb correction discussed by Wang et al [53]), therefore all defect calculations were performed with the ferromagnetic ordering only, thus allowing greater flexibility on the size of supercell adopted. As discussed in the previous section, if the phase forms by progressive additions of Cr into the Ti3AlC2 structure, then it is expected to retain the ordered, layered structure of (Cr2/3Ti1/3)3AlC2.…”

“…Only the lowest energy magnetic configuration is reported for each composition in Figure 4 as these best represent reallife materials. However, changes in magnetic ordering are known to affect the lattice parameters of these materials significantly, as thoroughly investigated by Wang et al [53]. Notably, the change in lattice parameters between ferromagnetic and non-magnetic Cr3AlC2 was of the same order as the change in lattice parameter [23,24].…”

“…It has been shown that (Cr2/3Ti1/3)3AlC2 exhibits multiple metastable spin states [19,53], therefore all previously reported spin states were considered and the reaction energies of equation 4b are presented in Table 2. In agreement with Dahlqvist and Rosen [19], we find that the in-plane antiferromagnetic ordering type XX (following the nomenclature of Wang et al [53]) is the most favorable spin state, closely followed by simple ferromagnetic ordering with a difference of only 1meV. Note that this energy difference is much below the expected accuracy of point defect calculations within DFT (see methodology section, and the effect of on-site Coulomb correction discussed by Wang et al [53]), therefore all defect calculations were performed with the ferromagnetic ordering only, thus allowing greater flexibility on the size of supercell adopted.…”

Magnetic moment collapse induced axial alternative compressibility of Cr2TiAlC2 at 420 GPa from first principle

“…A value of about 0.8  μ B is observed in present FM Cr 2 TiAlC 2 . The difference between the present FM Cr 2 TiAlC 2 and AFM Cr 2 AlC is about 0.13  μ B , whereas they are nearly identical in a recent calculation38, with values of 0.99  μ B in FM Cr 2 TiAlC 2 and 1.0  μ B in AFM Cr 2 AlC, respectively. The slab of Ti-C between the two nearest Cr 2 AlC stack blocks in unit cell of Cr 2 TiAlC 2 possibly affect the atomic moment as the Ti atom could strongly stabilize the unavailable Cr 3 AlC 2 39.…”

Newly Synthesized Ta‐Based MAX Phase (Ta1−xHfx)4AlC3 and (Ta1−xHfx)4Al0.5Sn0.5C3 (0 ≤ x ≤ 0.25) Solid Solutions: Unravelling the Mechanical, Electronic, and Thermodynamic Properties

“…Such outstanding combinations put MAX phases into an extraordinary class of materials [ 2 ] and make them potential candidates for use in a long list of applications, such as in sensors, electrical contacts, and especially in high‐temperature engineering, superconductivity, fuel cells, the nuclear industry, and spintronics. [ 3–9 ] The laminated structure containing A layers (Sn, Al, Ge, etc.) in between the M n + 1 X n sheets (e.g., TaC) is the key factor for the hybrid properties of metals and ceramics [ 10,11 ] in MAX compounds.…”