H. Lütgemeier

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.

NMR and NQR investigation of the magnetic order of Cu in the 123 compounds REBa2Cu3Oy

In the 123 compounds antiferromagnetic order at the Cu(2) sites appears either at low oxygen concentration or for RE = Pr. In both cases no ordered magnetic moments exist at the Cu(1) sites. The charge distribution at the Cu(1) sites is not changed by the replacement of Y by Pr.

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.

Site preference of Bi in YIG:Bi thin films determined by NMR of 57 Fe

Abstract NMR of57Fe in Y3-xBixFe5O12 garnet thin films with [111] orientation was measured at liquid-helium temperature by the spin-echo method. The satellite lines corresponding to those Fe3+ ions in the vicinity of which one yttrium ion was replaced by bismuth ion were observed in the NMR spectra. Using the superposition model for the transferred hyperfine field a correspondence was established between the satellite lines and the specific dodecahedral sites occupied by Bi3+ ions. By an analysis of the intensity of the satellite lines the fractions of Bi3+ on different dodecahedral sites were then determined.

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.

57Fe NMR in BaFe2-W hexagonal ferrite

Abstract 57 Fe NMR spectra from magnetic domains of BaFe 2 2+ Fe 16 3+ O 27 hexaferrite single crystal were measured at 4.2K using the conventional spin echo technique. External magnetic fields up to 3 T parallel and perpendicular to the hexagonal axis were applied to separate the overlapped NMR lines and to find their correspondence to the seven nonequivalent lattice sites occupied by Fe 3+ ions. A pronounced satellite structure of lines from tetrahedral sites was found. These results were analyzed with respect to the distribution of Fe 2+ ions among the iron sites.

Hyperfine field anisotropy on iron nuclei in barium hexaferrite

Abstract The NMR spectra of 57Fe nuclei in a barium hexaferrite single crystal were measured by spin-echo technique at 4.2 K. An external magnetic field of 1.4 and 2.7 T was applied in several different directions in the hexagonal plane in order to detect the influence of the hyperfine anisotropy on the resonance line positions. The number of observed NMR lines was found to agree with our prediction, based on the site symmetry analysis. The dependences of the NMR frequencies on the magnetization direction were fitted to determine (after subtracting the dipolar field anisotropy) the hyperfine field anisotropy tensors for each iron site.

Hyperfine field anisotropy in YIG with diamagnetic impurities

Abstract The 57 Fe NMR spectra of pure yttrium iron garnet, measured at 4.2 K, exhibit a well-resolved satellite structure caused by antisite defects (yttrium ions replacing iron on a sites). The analysis of the a site spectra, obtained in a magnetic field of 0.75 T applied in several directions in the (1 1 0) plane, yielded the values of the isotropic and anisotropic terms of the hyperfine field for four resolved satellite sets.

57Fe NMR study of magnetization processes in barium hexaferrites

Abstract NMR spectra of 57 Fe nuclei in single crystals of BaFe 12 O 19 and Ba(Fe 2− x Co x )Fe 16 O 27 ( x = 0, 0.8) were measured at 4.2 K. The field dependence of the resonant frequency at iron site 4f VI (R) in external magnetic fields parallel and perpendicular to the hexagonal axis was used to trace the magnetization processes. The change of the easy direction with Co content was determined from the results.