Dilute magnetism and vibrational entropy in ${\text{Fe}}_2{\text{Al}}_5$

${\text{Fe}}_2{\text{Al}}_5$ contains a Fe-Al matrix through which are threaded disordered one-dimensional chains of overlapping Al sites. We report magnetic, nuclear-magnetic-resonance (NMR), and specific-heat measurements addressing its magnetic and vibrational properties. The Curie-type susceptibility is found to be due to dilute moments, likely due to wrong-site Fe atoms. ${}^{27}\text{A}\text{l}$ NMR shift and spin-lattice relaxation measurements confirm these to be indirectly coupled through a Ruderman-Kittel-Kasuya-Yoshida-type interaction. Specific-heat results indicate a large density of low-energy vibrational modes. These excitations generate a linear-$T$ contribution to the specific heat, which however freezes out below about 10 K. These results are attributed to the presence of anharmonic vibrational modes associated with the disordered structural chains.

Figure 1

(Color online) ${\text{Fe}}_2{\text{Al}}_5$ orthorhombic structure, showing four adjacent unit cells. Larger spheres denote Al, and smaller spheres Fe, as labeled. Undulating chains of overlapping Al(2) and Al(3) sites are shown here as if completely occupied.

Figure 2

(Color online) Powder x-ray results for ${\text{Fe}}_2{\text{Al}}_5$, with results of refinement and difference plot.

Figure 3

(Color online) ${\text{Fe}}_2{\text{Al}}_5$ dc susceptibility $\left[\chi \left(T\right)\right]$ per mol Fe in an applied field of 1000 Oe. Inset: ${\left(\chi -{\chi }_o\right)}^{-1}$ vs temperature. Solid curves: Curie-Weiss fit.

Figure 4

(Color online) ${\text{Fe}}_2{\text{Al}}_5$ magnetization per mol Fe. (a) Measurements at 150 K with Curie-Weiss fit described in the text. (b) 2.0 K, with saturation fit described in text.

Figure 5

(Color online) ${}^{27}\text{A}\text{l}$ NMR line shapes for ${\text{Fe}}_2{\text{Al}}_5$. (a) 295 K measurement, with calculated quadrupole powder pattern for Al(1) sites (upper curve) and difference curve attributed to Al(2) and Al(3) sites (lower curve). Inset: lower-field measurement, with curves calculated using same parameters. (b) 4.2 K measurement.

Figure 6

${}^{27}\text{A}\text{l}$ MAS-NMR spectrum for ${\text{Fe}}_2{\text{Al}}_5$, in 9.4 T. Spinning $\text{rate}=12\text{kHz}$. Shift reference: ${\text{Al}}_2{\text{O}}_3$.

Figure 7

(Color online) ${}^{27}\text{A}\text{l}$ NMR line shifts and widths, plotted with fitted Curie-Weiss curves described in text.

Figure 8

(Color online) Temperature dependence of relaxation rates for ${}^{27}\text{A}\text{l}$, measured at 8.75 T. Solid line: fit to $\text{Korringa}+\text{paramagnetic}$ relaxation. Upper curve is guide to the eyes. Inset: low-temperature portion of data.

Figure 9

(Color online) $C/T$ vs $T^2$ for ${\text{Fe}}_2{\text{Al}}_5$ between 0 and 30 K, for samples and fields as indicated. Calibration is per mole formula unit, ${\text{Fe}}_2{\text{Al}}_5$. Solid lines correspond to the fitted parameters described in the text.

Figure 10

(Color online) Debye temperature extracted from ${\text{Fe}}_2{\text{Al}}_5$ specific heat, for $B=0$ and 6 T. Solid curve: calculated after removal of $\gamma T$ as described in text (6 T data). Inset: specific heat measured in $B=0$ and 6 T, per mole formula unit.

Figure 11

(Color online) Specific-heat difference for 0 and 6 T data, with calculated $B=6\text{T}$ multilevel Schottky anomaly for fitted density of paramagnetic defects. Normalization is per mole formula unit.

Figure 12

(Color online) Reduced $C/T$ with $\beta T^2$ and magnetic Schottky terms removed (left scale). Also plotted: entropy obtained from reduced $C/T$ (right scale). Units are per mole formula unit.