Ferrimagnetism in Fe-rich NbFe${}_2$

We report a study of the spin moment in single crystal Nb${}_{(1-y)}$Fe${}_{(2+y)}$ with $y=0.015$ using spin-dependent Compton scattering in conjunction with ab initio electronic structure calculations. From the experiment, we find that the total spin moment is $0.245\pm 0.004{\mu }_B$. Comparison of the measured spin density with theoretical results from linearized muffin-tin orbital calculations determines there to be a ferrimagnetic arrangement of Fe moments, with the 2$a$ Fe site aligned antiparallel to the bulk moment. There is a small moment on the Nb site, also antiparallel to the net moment. The orbital moment has been determined to be $0.02\pm 0.02{\mu }_B$.

Figure 1

Phase diagram of Nb${}_{1-y}$Fe${}_{2+y}$, adapted from.[8] The $\times$ marks the composition and temperature for the experimental measurement reported in this paper.

Figure 2

Bulk dc magnetization of the Nb${}_{0.985}$Fe${}_{2.015}$ single crystal at 13 K. The inset shows the zero-field ac susceptibility.

Figure 3

C14 Laves phase of NbFe${}_2$ in the ferrimagnetic state predicted by our LMTO calculation. The arrows on the atomic sites indicate the relative sizes and directions of the respective moments.

Figure 4

MCPs produced from the LMTO calculation for (a) the ferrimagnetic and (b) the ferromagnetic states of NbFe${}_2$ with the scattering vector along the $c$ axis. The different contributions to the MCPs have been resolved onto each inequivalent atomic site, and also the interference term between sites.

Figure 5

Experimental Nb${}_{0.985}$Fe${}_{2.015}$ magnetic spin momentum density (black points) projected along the [0001] crystal direction, plotted together with LMTO ferrimagnetic calculation (red dashed line) and ferromagnetic calculation (blue solid line). Both the calculated LMTO profiles have been normalized to the same total spin moment (area). The inset shows difference plots for the experimental data minus each theoretical profile, again with the ferrimagnetic (ferromagnetic) case plotted as red squares (blue points).