L. Bottyán

Calculation of nuclear resonant scattering spectra of magnetic multilayers

We report on calculations of the angle- and time-dependent photon reflectivity of multilayers using the technique of characteristic matrices. Spectra of56Fe/57Fe and Cr/Fe multilayers are calculated under various conditions. The parameters of the multilayers are optimized for suitable test samples of reflectometry measurements.

57Co emission studies of cupric oxide

Results on the incorporation, valence and spin states of Fe(Co) in CuO (with reference to similar studies on high temperature superconductors) and coupling of the Fe(Co) moment to the Cu magnetism in CuO are presented. Freshly prepared57Co: CuO shows two quadrupole doublets D1 and D2 withQ.S. of 2.49 and 1.52,I.S. of 0.35 and 0.70 mm/s and relative abundance of 74% and 26%, respectively at room temperature, the abundance being dependent on time in a sample exposed to ambient conditions and reaching 38 to 62% fifteen months after preparation. Below,TN=2251K, a typical combined magnetic-quadrupole interaction pattern is observed with a single saturation magnetic hfs of 25.6 T, central shift of 0.82 mm/s and a single |EQ|=1.62 mm/s at 4.2 K. External magnetic field spectra reveal an antiferromagnetic behaviour of the Fe(Co) ion. Temperature dependence of the magnetic hfs is fitted in the framework of the molecular field approximation allowing different spins and coupling constants for Cu and Fe(Co).

Structural and magnetic properties of two-dimensional oxalate-bridged bimetallic compounds

Abstract Single crystals of (cat)±[MnIIMIII(C2O4)3] ferromagnetic (MIII˭CrIII, (cat)± = (n-Pr)4, n-Bu(Ph)3P), and antiferromagnetic (MIII˭FeIII, (cat)±=(n-Bu)4N, (Ph)4P) compounds have been synthesized in order to further elucidate the correlations between their structural and magnetic properties. Single crystal X-ray structural as well as 57Fe Mossbauer studies are reported here. In all these compounds, assigned to a space group R3c. Z=6. alternating [MnIIMIII(C2O4)3]nn-2D honeycomb-like networks, comprise [MIII(C2O4)3]3− building units of both kinds of chirality. It has been established that previously reported crystal data for ((n-Bu)4N)[MnIIFeIII(C2O4) 3] (space group P63, Z=2)[1] represent a polymorph structure with similar metallo-oxalate layers containing [FeIII(C2O4)3]3− units of the same kind of chirality (Δ or Δ). An account for twinning effects in crystallization allow us to locate carbon atoms of the (cat)± in unit cell. Zero- and high-field 57Fe Mossbauer spectroscopy of the polycrystalline c...

Positron lifetime and Doppler studies of Co-Si alloys

A series of polycrystalline CoxSi100-x (x=0 to 100) samples were studied by positron lifetime and Doppler broadening methods and by X-ray diffractometry. Only one lifetime component could be resolved even in the case of two-phase samples, varying between 220 ps (Si) and 130 ps (Co). The parameter S and the lifetime show similar dependences on the composition. Both tau and S can be interpreted well in terms of the average electron density (including both core and valence electrons). Structural vacancies around CoSi2 can be excluded. In the range from Co to Co2Si tau and S saturate with values that can be expected for Co vacancies.

A common optical algorithm for the evaluation of specular spin polarized neutron and Mössbauer reflectivities

Using the general approach of Lax for multiple scattering of waves a 2x2 covariant expression for the reflectivity of polarized slow neutrons of a magnetic layer structure of arbitrary complexity is given including polarization effects of the external magnetic field. The present formalism is identical to the earlier published one for the (nuclear) resonant X-ray (Mossbauer) reflectivity and properly takes the effect of the external magnetic field of arbitrary direction on the neutron beam into account. The form of the reflectivity matrix allows for an efficient numerical calculation.

Layer magnetization canting in 57Fe/FeSi multilayer observed by synchrotron Mössbauer reflectometry

Synchrotron Mössbauer reflectometry and CEMS results on a [57Fe(2.55 nm)/FeSi\break(1.57 nm)]10 multilayer (ML) on a Zerodur substrate are reported. CEMS spectra are satisfactorily fitted by α‐Fe and an interface layer of random α‐(Fe, Si) alloy of 20% of the 57Fe layer thickness on both sides of the individual Fe layers. Kerr loops show a fully compensated AF magnetic layer structure. Prompt X‐ray reflectivity curves show the structural ML Bragg peak and Kiessig oscillations corresponding to a bilayer period and total film thickness of 4.12 and 41.2 nm, respectively. Grazing incidence nuclear resonant Θ–2Θ scans and time spectra (E = 14.413 keV, λ = 0.0860 nm) were recorded in different external magnetic fields (0 < Bext < 0.95 T) perpendicular to the scattering plane. The time integral delayed nuclear Θ–2Θ scans reveal the magnetic ML period doubling. With increasing transversal external magnetic field, the antiferromagnetic ML Bragg peak disappears due to Fe layer magnetization canting, the extent of which is calculated from the fit of the time spectra and the Θ–2Θ scans using an optical approach. In a weak external field the Fe layer magnetization directions are neither parallel with nor perpendicular to the external field. We suggest that the interlayer coupling in [Fe/FeSi]10 varies with the distance from the substrate and the ML consists of two magnetically distinct regions, being of ferromagnetic character near substrate and antiferromagnetic closer to the surface.

Synchrotron Mössbauer reflectometry

Grazing incidence nuclear resonant scattering of synchrotron radiation can be applied to perform depth-selective phase analysis and to determine the isotopic and magnetic structure of thin films and multilayers. Principles and recent experiments of this new kind of reflectometry are briefly reviewed. Methodological aspects are discussed. Model calculations demonstrate how the orientations of the sublattice magnetisation in ferro- and antiferromagnetic multilayers affect time-integral and time-differential spectra. Experimental examples show the efficiency of the method in investigating finite-stacking, in-plane and out-of-plane anisotropy and spin-flop effects in magnetic multilayers.

Step-induced canting of magnetization in Fe/Ag superlattices

We have investigated the reorientation of the easy axis of magnetization in (0 0 1) Fe/Ag superlattices using vibrating sample magnetometry, Mössbauer spectroscopy and nuclear resonance scattering of synchrotron radiation. Clear evidence is found that the Fe-layer magnetization can be oriented considerably out of the plane of the sample at room temperature, even for Fe-layer thicker than 6 ML at which the spin-reorientation transition usually occurs in Fe/Ag. The spin canting is attributed to frustration and a strong contribution of a step-induced anisotropy.

Conversion-electron Mössbauer study of sputtered Bi3Fe5O12

Temperature dependence of the magnetization measurements and Conversion Electron Mössbauer Spectroscopy (CEMS) have been performed on sputtered Bi3Fe5O12 (BiIG), Y3Fe5O12 (YIG) and (Bi, Y)IG films. Under the effect of the diamagnetic dilution of the diamagnetic yttrium sublattice the magnetization of BiIG is decreased with respect to YIG. The hyperfine field at the tetrahedral iron sites is increased, indicating a decrease in the intra-sublattice exchange interaction rather than the decrease of the tetrahedral iron moment.

Magnetism in some layer molecular magnets as revealed by Mössbauer spectroscopy

Abstract57Fe Mössbauer spectroscopy was used to explore magnetic properties of two 2D molecular ferrimagnets. In NPn4[FeIIFeIII(ox)3] (Pn = n-C5H11, (ox) =(C2O4)2−), the previously reported negative magnetization is shown here by external field studies to be due to a cross-over between FeIII and FeII- magnetizations. The form and parameters of the magnetic Hamiltonian describing the temperature dependence of both the FeIII hf-field and net magnetizations has been determined. In NBu4[MnIIFeIII(ox)3] (Bu = n-C4H9) soft XY-magnet the low temperature relaxation spectra are interpreted in terms of slowly varying classical magnetization-evolution at low temperature.