NMR and Mössbauer study of spin-glass behavior in $\mathrm{Fe}{\mathrm{Al}}_2$

We have used NMR and Mössbauer spectroscopy to investigate the magnetism and spin-glass behavior in $\mathrm{Fe}{\mathrm{Al}}_2$. The results show that the observed behavior can be interpreted as local-moment magnetism residing on the Fe sites. An increase in ${}^{57}\mathrm{Fe}$ Mössbauer magnetic hyperfine field was observed at the $35\text{−}\mathrm{K}$ spin-glass freezing temperature, with a reduced low-temperature value consistent with unquenched fluctuations in the spin-glass state. However, the ${}^{27}\mathrm{Al}$ NMR shifts and spin-lattice relaxation behavior are consistent with transfer hyperfine interactions to Fe local moments, with a magnitude of approximately $2.5{\mu }_B$. This value is consistent with the bulk magnetization, but surprising compared to other Fe aluminides, given the small $\mathrm{Fe}-\mathrm{Fe}$ coordination number and associated weakening expected for the magnetic moment.

Figure 1

$\mathrm{Fe}{\mathrm{Al}}_2$ structure. Filled circles: Fe (5/cell); open circles: Al (10/cell); dot-filled circles: mixed-occupancy sites (3/cell). (a) (021) plane, with frame showing a $3\times 3$ set of triclinic unit cells. (b) View along (101) showing Fe pairs/triads. This Fe-containing layer alternates with an Al-only layer. (c) Rotated view of layer pictured in (b).

Figure 2

Powder x-ray results for $\mathrm{Fe}{\mathrm{Al}}_2$, with results of refinement and difference plot. The vertical marks are fitted reflections.

Figure 3

${}^{57}\mathrm{Fe}$ Mössbauer spectra for $\mathrm{Fe}{\mathrm{Al}}_2$, with theoretical fits described in the text plotted as solid curves.

Figure 4

Reduced hyperfine field vs reduced temperature, from the Mössbauer fitting for $\mathrm{Fe}{\mathrm{Al}}_2$. The dashed curve is a fit according to ${(1-T∕T_c)}^{\beta }$, with $\beta =0.35$, $T_c=35.6\mathrm{K}$, and ${\mu }_o{H}_{\mathit{hf}}\left(0\right)=6.1\mathrm{T}$. The inset shows the full temperature dependence.

Figure 5

${}^{27}\mathrm{Al}$ NMR spectra for $\mathrm{Fe}{\mathrm{Al}}_2$. The data are offset vertically for clarity.

Figure 6

${}^{27}\mathrm{Al}$ center-of-mass NMR shifts vs temperature. Dashed curve: Curie-Weiss fit with $\theta =-38\mathrm{K}$.

Figure 7

$\mathrm{Fe}{\mathrm{Al}}_2$ paramagnetic moment per Fe obtained from the magnetization curve, plotted vs magnitude of effective on-site Al moment obtained from the mean NMR shift. The dashed curve is a least-squares linear fit.

Figure 8

${}^{27}\mathrm{Al}$ NMR FWHM linewidths for $\mathrm{Fe}{\mathrm{Al}}_2$. The dashed curve is a Curie-Weiss fit with $\theta =-38\mathrm{K}$.

Figure 9

${}^{27}\mathrm{Al}$ NMR spin-lattice relaxation rate vs $T$ in $\mathrm{Fe}{\mathrm{Al}}_2$. The dashed horizontal line shows $T$-independent local-moment behavior at high temperatures. The solid curve is a guide to the eye.