Nuclear Magnetic Resonance in FeAl and CoAl

We have investigated the ${\mathrm{Al}}^{27}$ nuclear magnetic resonance in ${\mathrm{Ni}}_3$Al, NiAl, FeAl, and both the ${\mathrm{Al}}^{27}$ and ${\mathrm{Co}}^{59}$ resonances in CoAl. The cobalt resonance in CoAl exhibits a weakly temperature-dependent, positive shift. This shift (≈1.5%) is too large to be accounted for solely by the hyperfine field from conduction electrons polarized by the external magnetic field, and orbital paramagnetic effects appear to be the dominant factor, core polarization playing a relatively minor role. The aluminum Knight shift in CoAl is small (0.014%) and temperature-independent. This is to be contrasted with aluminum in FeAl which exhibits a large, negative, temperature-dependent shift (-0.38% at 293°K). It is shown that both the large aluminum Knight shift in FeAl and the small aluminum Knight shift in CoAl are consistent with the predictions of the Ruderman-Kittel-Yosida (RKY) theory. However, it is now believed that the small shift observed in CoAl results from a lack of $s$ character in the conduction-electron wave functions rather than from a node anticipated in the conduction-electron polarization. The temperature dependence of the resonance in FeAl can also be accounted for by the RKY mechanism if it is assumed that the temperature dependence of the magnetic susceptibility is associated with disorder in the material. This assumption is necessary because the Knight shift is not linearly related to the bulk susceptibility of the sample. The aluminum linewidth in FeAl increases as the temperature is lowered. At room temperature the linewidth is independent of magnetic field but greater than the calculated dipolar linewidth. At 77 and 4.2°K the linewidth increases with increasing magnetic field. This effect is attributed mainly to inhomogeneous Knight-shift broadening, although inhomogeneous magnetization broadening also contributes. A similar situation is observed in CoAl. At room temperature the cobalt and aluminum resonances have essentially the same width. The linewidths are independent of magnetic field but greater than the dipolar values. As the temperature is lowered the linewidths increase and become magnetic-field-dependent. The cobalt resonance broadens more severely than the aluminum resonance. It is believed that inhomogeneous Knight-shift broadening and inhomogeneous magnetization broadening determine the cobalt linewidth at low temperatures. The aluminum nuclei in CoAl do not exhibit appreciable hyperfine coupling with the conduction electrons, so that only inhomogeneous magnetization broadening contributes to the linewidth.