Nuclear relaxation study of the randomly diluted antiferromagnets MnxZn1-xF2 and FexZn1-xF2

摘要

The nuclear spin‐lattice (1/T1) and spin‐spin (1/T2) relaxation rates have been measured for 19F nuclei with no magnetic near neighbors (F0) in diluted antiferromagnetic MnxZn1-xF2 and FexZn1-x F2, for x between 0.1 and 0.8. For T≫Tn, 1/T1 in FexZn1-xF2 varies nearly as x1/2. This result indicates that the relaxation process is dominated by the electron‐nuclear dipole interaction with electronic spins in exchange‐coupled clusters. In contrast, 1/T1 in MnxZn1-xF2 is independent of x, for x greater than 0.5. As x decreases, 1/T1 increases until a maximum is observed at x≃0.1. The departure from x1/2 behavior is found to be proportional to x(1-x)8, indicating that the exchange‐isolated Mn electronic spins dominate the nuclear relaxation for x less than 0.4. The absence of the exchange‐isolated electronic spin contribution 1/T1 in FexZn1-xF2 is attributed to the rapid electronic spin‐lattice relaxation rate (τsl)-1 of the Fe2+ ion at elevated temperatures. However, this contribution has been observed at 77 K where the dependence of 1/T1 on x is virtually identical to that observed in the Mn‐Zn system. The temperature dependence of 1/T1 in FexZn1-xF2 exhibits a critical‐like divergence near the Néel temperature, for x above the percolation limit.