J. Englich

Anisotropy of hyperfine field in diamagnetically substituted YIG

Abstract The results of the nuclear magnetic resonance of 57 Fe in Al-, Ga- and In-substituted Y 3 Fe 5 O 12 garnets are reported. When diamagnetic Ga 3+ or Al 3+ ions are substituted for tetrahedral Fe 3+ ions, four satellite lines in the NMR spectrum of octahedral Fe 3+ ions appear. The substitution of In 3+ ions for octahedral Fe 3+ ions leads to three satellite lines in the spectrum of tetrahedral Fe 3+ ions. The hyperfine fields of these satellites are explained using the independent bond model for the hyperfine interaction. Suprisingly large change of the anisotropic part of the hyperfine field relative to the isotropic one was found.

DEFECTS IN FERRITES STUDIED BY 57FE NMR

Presence of defects in ferrites gives rise to a modification of the hyperfine field on nuclei of those iron ions, which are situated in the neighborhood of the defect. If the concentration of defects is small, satellite structure of the NMR spectra of 57Fe nuclei is observed. Analysis of this satellite structure provides valuable information. In particular concentration of the defect, its distribution over various sublattices, magnetic moment and the valency may be obtained. In the present paper we describe how the analysis of the satellite structure is performed, summarize the requirements on the NMR experiment and illustrate the method using several examples of defects in the yttrium iron garnet.

NMR spectra of57Fe in hexagonal ferrites with magnetoplumbite structure

The NMR spectra of57Fe in domains of MFe12O19 (M=Ba, Sr, Pb) were measured by spin echo technique at 4.2 K. The change of the heavy ion causes frequency shifts of lines corresponding to Fe3+ ions in 2b and 4f2 sites while leaving other lines essentially unchanged; the significant role of different Fe3+−M2+ bonding was found. The dipolar broadening of lines in BaFe12O19 caused by random and static displacement of bipyramidal Fe3+ ions from the mirror plane is calculated and the results are compared with the experiment.

On the transferred hyperfine interaction in substituted yig

NMR of57Fe is studied in a number of (MxY3−x) Fe5O12 garnets for small concentrations of M (M is either trivalent RE ion −Ho 3+, Gd 3+, Nd3+, Pr 3+, La 3+ or Bi 3+ ion). Beside the main resonance lines, the satellites were observed, which correspond to those Fe, in vicinity of which the impurity M is located. After correcting for the dipolar field, the field corresponding to the change of the transferred hyperfine interaction in M3+−O2−-Fe3+ vs. Y3+−O2−-Fe3+ triad was deduced from the satellites splitting. The analysis of the results indicates that the observed change in the transferred hyperfine field is mainly connected with the transfer of electrons between M3+ and Fe3+ ions and not with the local deformation around the impurity.

NMR of 57Fe above the Verwey transition in Al-substituted magnetite

Abstract Temperature dependence of 57 Fe NMR was measured above the Verwey transition in three Fe 3− x Al x O 4 ( x =0, 0.005, 0.03) single crystals. The satellite lines corresponding to Fe (A) which have a single Al (B) nearest neighbor were detected in the NMR spectra of x =0.005 and 0.03 compounds. Temperature dependence of NMR linewidth for x =0.03 indicates that the electron density becomes inhomogeneous as T → T V .

Applicability of NMR to impurities detection in YIG thin films

Abstract An impurity in the vicinity of the 57 Fe nucleus induces a specific structure of satellite lines in the NMR spectrum, the intensities of which are proportional to the impurity concentration. Recently achieved sensitivity makes it possible to detect as little as 0.02% impurity. Spectra of nominally pure YIG epitaxial films, containing Pb, Ga, Al, Si, Pt, Ca impurities and ‘antisite’ defects are presented to illustrate the approach.

Defects in yttrium-iron garnet studied by NMR on iron nuclei

Abstract The NMR spectra of 57 Fe in nominally pure YIG were measured and a rich satellite structure caused by defects was found. We identified the satellites caused by antisite defects (yttrium on iron sites, and vice versa) and satellites caused by Pb 2+ impurities. From amplitudes of the satellites the concentration of the antisite defects was determined.

NMR study of La3+ substituted hexagonal ferrites with magnetoplumbite structure

Abstract Domain 57 Fe NMR spectra of La 3+ substituted hexagonal ferrites Pb 1− x La x Fe 12 O 19 and Ba 1− x La x Fe 12 O 19 have been measured at 4.2 K using the spin echo technique. Besides the five main lines, the satellite lines caused by the substitution were observed. The assignment of the satellite lines to the Fe 3+ sublattices was made using the dependence of the NMR spectrum on the external magnetic field. The experimental results are explained assuming the existence of Fe 2+ ions in the 2a sublattice.

Anisotropy of the Hyperfine Field on 57Fe in Pure and Substituted Yttrium Iron Garnet

Using NMR, anisotropy of the hyperfine field on 57Fe nuclei was measured in Y3Fe5O12 with nonmagnetic, trivalent substitutions on d (Ga3+) and a (Y3+ - yttrium antisite defect) sites which are occupied by ferric ions in a pure, stoichiometric system. The measurements were performed by the spin-echo method at liquid helium temperature. Thorough analysis of the hyperfine field anisotropy on the 57Fe nuclei in an ideal environment and in an environment where one of Fe3+ nearest cation neighbours is replaced by the nonmagnetic impurity is given. When the Fe3+ on which the NMR is measured is on the a site and the impurity is Ga3+(d), the results may be interpreted in terms of the superposition model. On the other hand, the results for Fe3+ on the d site and Y3+(a) are in clear disagreement with the predictions of this model.

Influence of impurities and defects on the nuclear relaxation in YIG films

Abstract The relaxation of 57 Fe nuclei in yttrium iron garnet (YIG) is very sensitive to the way in which charge compensation is achieved in these magnetic insulators. Charge compensation becomes necessary either if nontrivalent impurities are incorporated in the garnet structure or in the presence of intrinsic defects. From measurements of the nuclear relaxation times T 1 and T 2 in epitaxial YIG films doped with small amounts of Ca, Si or Pb, we conclude that the relaxation behavior at low temperatures depends critically on the presence of certain intrinsic defects, namely Fe 2+ . These ions also increase the optical absorption in the near infrared.