Incommensurate Spin Density Waves in Iron Aluminides

Neutron diffraction in Fe(Al) reveals incommensurate spin density waves (SDWs) in alloys known to be spin glasses. The wave vectors for crystals of Fe(34Al), Fe(40Al), and Fe(43Al) show $n$ varying from 11 to 6 for $\overrightarrow{q}=2\pi (h\pm 1/n,k\pm 1/n,l\pm 1/n)/a_0$, where $(h,k,l)$ and $a_0$ characterize the parent bcc lattice of the CsCl structure. The magnetic reflections are present far above the spin-glass freezing temperatures. These SDWs keep the spins on nearest-neighbor Fe atoms close to parallel, in contrast with SDWs in Cr, which keep nearest-neighbor spins close to antiparallel.

Figure 1

Magnetization patterns with cubic symmetry for (a) ${\mathrm{F}\mathrm{e}}_3\mathrm{A}\mathrm{l}$ and (b) ${\mathrm{F}\mathrm{e}}_9{\mathrm{A}\mathrm{l}}_7$. The green and red atoms are Fe(I) sites with opposite spin directions. The gold atoms are Al. The grey atoms sit in positions of magnetic frustration. In (a) these are Fe(II) and in (b) they are Al. The first three layers of the 128-atom cubic unit cells are shown in perspective.

Figure 2

Magnetization patterns with cubic symmetry for ${\mathrm{F}\mathrm{e}}_9{\mathrm{A}\mathrm{l}}_7$ with the magnetic unit cell doubled in each direction to form a structure that would produce magnetic scattering related to that observed in Fe(43Al) . The first three layers of the 1024-atom unit cell are shown with the coding of Fig. 1.

Figure 3

Neutron scattering intensity for (a) polycrystalline Fe(40Al) at 120 and 1.7 K with their difference shown in the inset and (b) single crystal Fe(40Al) at 12 K shown as contours about the $(\frac{1}{7},\frac{1}{7},\frac{1}{7})$ peak.

Figure 4

Temperature dependence of the SDW amplitudes of Fe(Al) single crystals [14]. The black arrows are the freezing temperatures from Ref. 8. Fitting with Eq. (1) yields the solid curves. The red arrows are the $T_{\Lambda }$ from the fits.

Figure 5

The reciprocal lattice spots in Fe(40Al).