Nuclear Magnetic Relaxation in Magnetite

Abstract

The nuclear magnetic relaxation times in Fe₃O₄ have been measured. The experimental values of T₁ and T₂ range from 1.4 sec to 0.4 ms between 4.2° and 300°K. For the A ion, T₁ and T₂ are roughly equal, increasing with a magnetic field, above 120°K and show the minima at about 80° and 40°K, respectively, independent of the field. For the B ion, T₁ and T₂ are independent of the field with T₁⁄T₂≈2 from 130°K to 300°K. The results are explained semi-quantitatively by the effect of the electron migration in the B sites. Detailed calculations show: (1) T₁ and T₂ for the A ion are governed by the coupling of the nucleus with the migrating electrons through the dipolar interaction below 120°K and through the hyperfine and the A-B exchange interactions above 120°K; (2) the anisotropic and the isotropic hyperfine interactions give the essential contributions to T₁ and T₂ for the B ion, respectively.