Studies of ²⁷Al NMR in EuAl₄

EuAl₄ orders antiferromagnetically at T_N ≈ 16 K with an effective magnetic moment of 8.02 μ_B. In the paramagnetic phase, the magnetic susceptibility of EuAl₄ follows the Curie-Weiss law with a positive Curie-Weiss temperature θ_P +14 K. The antiferromagnetic state is changed into the field induced ferromagnetic state at a critical field H_c of approximately 2 T. In order to microscopically investigate the magnetic and electronic properties in EuAl₄, the NMR measurements of EuAl₄ have been carried out at temperatures between 2 and 300 K, applying an external magnetic field of approximately 6.5 T. The ²⁷Al NMR spectra corresponding to Al(I) and Al(II) sites are obtained. From the ²⁷Al NMR spectra, the isotropic part K_iso and anisotropic part K_aniso of Knight shift, and nuclear quadrupole frequncy ν_Q are obtained. The K_iso and K_aniso shift to negative side with decreasing temperature due to the RKKY interaction. These temperature dependences follow the Curie-Weiss law with θ_P +14 K, which is consistent with that of the magnetic susceptibility. From the K-χ plot, the values of the hyperfine fields H_hf-iso and H_hf-aniso are -3.231 and -0.162 kOe/μ_B for Al(I) site, and -1.823 and -0.264 kOe/μ_B for Al(II) site, respectively. The values of ν_Q of ²⁷Al nucleus for Al(I) and Al(II) sites are approximately 0.865 and 0.409 MHz, respectively. The nuclear relaxation time T₁ of ²⁷Al NMR for both sites is almost constant in the paramagnetic phase, while the value of 1/T₁ is abruptly decreased in the ordered ferromagnetic state.