6 1Ni Mössbauer spectroscopy of magnetic hyperfine interaction in nickel spinels

Abstract

The sign and magnitude of the magnetic hyperfine field in the Ni spinels NiFe2O4, NiCr2O4, NiFe0.3Cr1.7O4, NiRh2O4, and GeNi2O4 were determined by means of 61Ni Mössbauer spectroscopy in external magnetic fields. The hyperfine fields of Ni2+ ions in octahedral lattice sites are negative and of the order of −10 T, and positive for Ni2+ at tetrahedral sites ranging from +2.5 T in NiRh2O4 up to +63 T in NiFe0.3Cr1.7O4. The sign and magnitude of the hyperfine fields are explained in the frame of ligand field theory including the effects of 3d–4p orbital mixing.

Applications of 61Ni Mössbauer spectroscopy

“…The calculations also agree with earlier Mossbauer measurements (3 1) indicating that the spins of the Ni2+ ions are perpendicular to the c axis of the tetragonally distorted spinel structure. The canting angle of the moments on the Ni2+ ions within the plane, as determined from Mossbauer measurements, is 42" + 8" relative to the direction of the applied field (29); this agrees with results obtained by neutron diffraction. ~i~+ occupies only the tetrahedrally coordinated A sites in NiCr204, but as iron is substituted for chromium in the spinel structure, Ni2+ occupies progressively more of the octahedrally coordinated B sites.…”

“…The sign of the hyperfine field at the A site is found to be positive; whereas the sign of the field at the B site is found to be negative (29). The large positive field at the A site results from unquenched orbital angular momentum, similar to NiCr204.…”

“…The sign was first concluded to be negative from 6 1~i measurements in an applied field (27) based on the proposed spin structure of Bertaut and Dulac (28). Later work combining the results of applied-field measurements and ligand-field calculations, however, indicate that the sign of the field is positive (29), and that the dominant magnetic moment is associated with the A-site ions as proposed by McGuire and Greenwald (30), not the B-site ions as proposed by Bertaut and Dulac. Ligand-field calculations have confirmed (29) that the cause of the large, hyperfine magnetic field in NiCr204 is a large, positive contribution from the orbital angular momentum.…”

“…6 1~i Mossbauer measurements of the spinels NiRh204 and GeNi204 have also been made (29). NiRh204 is isostructural with NiCr204 but exhibits a considerably smaller hyperfine Can.…”

Mössbauer study of spin structure transformation from an incommensurate to a commensurate state

“…At this temperature a crystallograpic change from a cubic to tetragonal(c/a <1) transition occurs [6][7][8].…”