H. Štěpánková

Dielectric, magnetic, and lattice dynamics properties of Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22: Comparison of ceramics and single crystals

We prepared multiferroic Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22 ceramics and compared their magnetic and dielectric properties with single crystal. Magnetic susceptibility and microwave resonance measurement revealed magnetic phase transition at TC=312 K, similar as in single crystal. Ferroelectric (FE) phase can be induced by external magnetic field in all investigated samples and the phase diagram in ceramics qualitatively resembles that of the single crystal. The range of magnetic fields, where the FE phase is induced, broadens after annealing of single crystal. Ceramics quenched after sintering exhibit several orders of magnitude lower conductivity than the single crystal. Heavily damped magnetic resonance was discovered in terahertz spectra at 10 K and its frequency softens below 5 GHz near TC. Number and symmetry of observed infrared (IR) and Raman active phonons correspond to paraelectric phase with D3d5 hexagonal structure. No evidence for a structural phase transition was found in the IR and Ram...

Bentonite/Iron Oxide Composites: Preparation and Characterization by Hyperfine Methods

Bentonite/iron oxide system is prepared by isothermal calcination of powder composed of bentonite clay and precursor containing ferric acetate. This preparation technique enables one to get the composite material directly, that is, iron oxide particles embedded in a bentonite matrix. Calcination temperature is varied from 320°C to 700°C. The resulting series of samples is characterized by local methods based on hyperfine interactions: 57Fe nuclear magnetic resonance (NMR) and the Mössbauer spectroscopy. The results show that the phase composition changes significantly in dependence on calcination temperature. The amount of maghemite phase rapidly increases up to °C and decreases abruptly for higher than 460°C.

NMR study of Ti-Co, Ti-Mg and Ti substituted hexagonal ferrites with magnetoplumbite structure

Abstract Domain 57 Fe NMR spectra of Ti-Co, Ti-Mg and Ti substituted hexagonal ferrites have been measured at 4.2 K using the spin echo technique. Characteristic satellite lines corresponding to the substitution in the vicinity of the resonating nucleus appeared in the NMR spectra. The dependence of the relative intensities of the NMR lines on the amount of the substitution is determined and the preference of the substitutions for the different crystallographic sites is discussed.

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.

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.

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.

57Fe NMR spectra in domain walls of hexagonal ferrites with magnetoplumbite structure

The domain wall NMR spectra of57Fe were measured on polycrystalline samples of BaFe12O19 and SrFe12O19 at 4·2 K. We have calculated the anisotropy of the hyperfine field dipolar component. The measured NMR spectra were interpreted supposing that the hyperfine field anisotropy is caused only by the dipolar field anisotropy.

Magnetically induced structural reorientation in magnetite studied by nuclear magnetic resonance

Phenomenon related to low symmetry phase of magnetite Fe3O4 below the Verwey transition is the switching of magnetic easy axis by external magnetic field connected with a structural transition. Results of nuclear magnetic resonance (NMR) studies of axis switching are presented, preceded by careful characterization by magnetic measurements. We detect changes in the F57e NMR spectra that evidence the structural transition interrelated with the axis switching. We also observe the switching process in time and analyze the dynamics of the phenomenon. Phenomenological approach according to Kolmogorov–Johnson–Mehl–Avrami is employed to find a link between the experimental data from magnetization measurements and NMR, showing that the effect observed by these two methods has the same origin. Additionally, NMR identifies separately the unswitched and switched phase during the switching process.

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.

Hyperfine fields and superexchange in yttrium-iron garnet

Abstract Supertransferred hyperfine fields are deduced from the NMR spectra of 57Fe in substituted YIG. These fields, obtained for several different Fe3+-Fe3+ pairs, are then compared with corresponding Fe3+-Fe3+ exchange integrals.