27Al Knight Shift and Hyperfine Interaction in GdAl2

A new ²⁷Al NMR has been observed in the paramagnetic state of GdAl₂ between 350° and 575°K. The ²⁷Al NMR linewidth δH was found to be strongly temperature‐dependent. Data are presented which suggest that the previously reported ²⁷Al NMR in GdAl₂ is probably due to a small amount of some other Gd–Al compound. The ²⁷Al Knight shift is temperature‐dependent, varying from about −6% to −3% between 350° and 575°K, respectively. The temperature‐dependent part of the Knight shift obeys a Curie‐Weiss law, i.e., K(T) α (T−θ)⁻¹, with θ≃150°K. The temperature‐independent part of the Knight shift is consistent with the value measured for the nonmagnetic isomorph LaAl₂. Extrapolating the Knight shift data to 300°K and combining K(300) with the room‐temperature susceptibility χ(300) yields an effective ²⁷Al hyperfine field H^hf≃−6.8 kOe per Bohr magneton. An ²⁷Al hyperfine field in the ferromagnetic state is thus calculated to be ²⁷H≃−46 kOe in good agreement with experiment. The s‐f exchange energy is calculated to be ${\mathcal{J}}_{\mathrm{sf}}\text{=−0.13 eV}$ . The ²⁷Al nuclear quadrupolar coupling constant was measured to be (e²qQ/h)≃4.27 Mc/sec.