Antiferromagnetism in Cr${}_3$Al and relation to semiconducting behavior

Antiferromagnetism and chemical ordering have both been previously suggested as causes of the observed semiconductorlike behavior in Cr${}_3$Al. Two films of Cr${}_3$Al(001)/MgO(001) were grown under different conditions to achieve different types of chemical ordering and electronic properties: one $X$-phase structure (semiconducting) and one $C$11${}_b$ structure (metallic). The films were investigated by x-ray and neutron diffraction. Both films show commensurate antiferromagnetic order, with a high Néel temperature greater than 578 K, showing that the antiferromagnetism in Cr${}_3$Al is quite robust. Density-functional theory calculations were performed and it was shown that the well-known antiferromagnetic pseudogap in the density of states occurs for all types of chemical ordering considered. The conclusion of these studies is that the antiferromagnetism causes a pseudogap in the density of states, which is a necessary condition for the semiconductorlike transport behavior; however, that antiferromagnetism is seen in both metallic and semiconducting Cr${}_3$Al samples shows that antiferromagnetism is not a sufficient condition for semiconducting behavior. Chemical ordering is equally important.

Figure 1

(a) Commensurate spin-density wave (simple antiferromagnetism) on the bcc lattice. (b) Experimental setup for x-ray and neutron-diffraction experiments.

Figure 2

Characterization of the $X$-phase Cr${}_3$Al 0.95-$\mu$m film used for neutron diffraction, deposited at 300${}^{\circ }$C. (a) The $\theta -2\theta$ XRD scan. Solid circles indicate diffraction peaks from the sample, while open circles indicate diffraction peaks from the substrate. An azimuthal $\phi$ scan (not shown) showed fourfold symmetry, indicating epitaxy. (b) The $X$-phase Cr${}_3$Al superstructure. Left: rhombohedral structure showing surrounding bcc environment. Right: rhomboid primitive cell. Black arrows are primitive lattice vectors. (c) Resistivity measured by the van der Pauw technique.

Figure 3

Characterization of the $C$11${}_b$ Cr${}_3$Al 1-$\mu$m film used for neutron diffraction, deposited at 600${}^{\circ }$C. (a) The $\theta -2\theta$ XRD scan. Solid circles indicate diffraction peaks from the sample, while open circles indicate diffraction peaks from the substrate. An azimuthal $\phi$ scan (not shown) showed fourfold symmetry, indicating epitaxy. (b) The $C$11${}_b$ Cr${}_3$Al superstructure. (c) Resistivity measured by the van der Pauw technique.

Figure 4

(a) and (c) Antiferromagnetic (001) and (b) and (d) structural (002) neutron-diffraction intensities observed in the (a) and (b) $X$-phase and (c) and (d) $C$11${}_b$ Cr${}_3$Al films. The lines are Gaussian fits to the data to determine the integrated peak intensity.

Figure 5

Magnetic moment on Cr atom vs temperature from neutron diffraction of the $X$-phase and $C$11${}_b$ Cr${}_3$Al films.

Figure 6

Theoretical density of states of Cr, Cr${}_2$Al, and Cr${}_3$Al with four different types of chemical ordering. The calculations compare magnetic and nonmagnetic cases.

Figure 7

Band structure for Cr${}_3$Al with $X$-phase ordering for both the antiferromagnetic and nonmagnetic cases.