Etude par diffraction d'electrons lents, microscopie electronique en transmission et effet Mossbauer a electrons de conversion de couches minces epitaxiques Fe/Cu et Cu/Fe/Cu deposees sur supports de NaCl

The $^{57}\mathrm{Fe}$ M\"ossbauer effect on 16.8-at.% Fe-Au displays the magnetic double transition from paramagnetism to ferromagnetism at ${T}_{c}=165$ and from ferromagnetism to spinglass-like behavior at ${T}_{f}\ensuremath{\sim}45$ K. Below ${T}_{f}$ the magnetic state is characterized by freezing of Fe spin components which are transverse with respect to a strong applied dc field.

Mössbauer spectra of amorphous Fe4Ni74Si9 B13 indicate a spin-glass freezing temperature Tf = (11.o ± o.5)K. For T < Tf the measured average hyperfine field Hint as a function of temperature T follows the relation Hint = Hsat. (1−T/Tf)β with β = o.4o ± o.o5 and a saturation hyperfine field Hsat = 26okOe. For T > Tf quadrupole-split spectra were observed (ΔEO = o.45 mm/s); it was found by application of magnetic fields that rapidly relaxing superparamagnetic clusters exist far above Tf. The average cluster moment was estimated to be roughly 7o μB.

The Mgssbauer effect on very thin 5 7 ~ e films of various thicknesses d vacuum deposited on polycrysta1;ine Cu and Ag substrates shows a well-resolved magnetic hyperfine pattern for d 5 30 A. For d 7 18 A superparamagnetism was observed. The spectra at 4.2 K of films with d 7 10 1 indicate a distribution of hyperfine fields with a low-field component and a large-field ferrowgnetic component. A perpendicular magnetic anisotropy has been observed for these ultra-thin films. The Mzssbauer effect offers a unique possibility to study microscopically via the hyperfine field (h.f.) the magnetic properties of Fe atoms located at a solid interface /I/. The present paper reports some of our Mzssbauer results on very thin 5 7 ~ e films embedded between Cu or Ag layers. 5 7 ~ e films with average thickness dof 30,18, 10, 5 and 2 have been vacuum deposited between 300 1 thick polycrystalline layers of Cu or Ag and used as Mgssbauer absorbers. Our multi-layer samples (with about 15 5 7 ~ e layers) were prepared by alternately evaporating 5 7 ~ e and Cu (or Ag) onto a room-temperature (R.T.) A1 foil in an oil-free UHV system with %lo-' torr during evaporation (base pressure %5 x 10-'"0rr). Thickness and evaporation rates (%0.5 A/s) were monitored with a quartz crystal oscillator. The Mzssbauer y-ray direction was normal to the film plane. Results obtained for Fe films on Cu will be discussed first. At R.T. and below (Fig. 1 a) our 30 1 thick Fe films showed a well-resolved magnetic hiperfine pattern typical for ferromagnetic a-Fe. We conclude that these films were fairly continuous and homogeneous'. The h . f . value at R.T. was only 315 + 3 kOe, i.e. smaller than for bulk a-Fe at R.T., while at 4.2 K the h.f. of 30 Fe on Cu and that of bulk a-Fe were found to be equal. This can be explained by a reduction in the Curie temperature for this film. A small positive isomer shift of +0.03 2 0.01 mm/s (relative to bulk a-Fe at R.T., as all the isomer shift values given in this paper) indicates a slight decrease in s-electron density at theFe nucleus compared to bulk a-Fe.

The influence of X-rays irradiation at 4.2 K on the charge state of iron impurities in LiNbO3+ has been studied by Mossbauer spectroscopy. By the irradiation Fe3+ impurities can be converted to Fe2+. A reconversion is possible by annealing at temperatures ≥ 160 K for a sufficiently long time.

Thin films (∽ 1000 Å thick) of highly supersaturated bcc FeCu alloys (0–26.4 at % Cu) have been prepared by simultaneous vapor deposition of Fe and Cu and investigated by the Mössbauer effect. The magnetic-hyperfine split spectra showed broadening of the outer lines due to the different hyperfine fields at Fe atoms with different local environment. For low Cu content the relative decrease of the hyperfine field at first and second neighbor Fe atoms and the isomer shift per nearest-neighbor Cu atom was obtained. The concentration and temperature dependence of the average hyperfine field is discussed.

57Fe Mössbauer experiments were performed to study the magnetic properties of an iron surface which was selectively enriched by vacuum deposition of a thin (≈5 Å) 57Fe layer and subsequently protected by a thick Cu film. The spectra at 4.2 K show that all Fe atoms near the surface are ferromagnetic, but the reduced hyperfine field suggests a partial decrease of the local magnetic moment.

Thin iron films (∼18 Å thick and 90% enriched in 57Fe) were prepared on (001) Cu single crystal substrates. By using Mössbauer spectroscopy it was found that antiferromagnetic ordering begins at around 80 K ± 10 K with average hyperfine fields of about 14–20 kOe at 4.2 K. Additional proof for the existence of antiferromagnetism has been obtained by measurements in a longitudinal external magnetic field. The apparent discrepancy in the literature of ferro- and antiferromagnetic ordering in fcc Fe films is discussed.

The sensitivity of conversion electron Mossbauer spectroscopy has been investi- gated and it is demonstrated that a monolayer of Fe57 corresponding to w 1.8 X 1015 atoms give an effect of about 1.8 % assuming the spectrum consisting of only one resonance line with natural line width. 1. Introduction. - The question concerning the sensitivity of conversion electron Mijssbauer spectro- scopy (CEMS) has been asked at various occasions. The usefullness of this method in the field of adsorption and catalysis depends on the possibility to distinguish the spectral contribution of surface atoms. Baverstam et al. (l) tried to separate in a CEM- spectrum the contribution of the various surface layers through computer deconvolution techniques and found that a depth selectivity of about 20 monolayers in the case of natural iron can be achieved with CEMS. In our approach to this problem thin Fe57 films, down to a few monolayers in average thickness, are evaporated onto several substrates and investigated by measuring the 7.3 keV conversion eiectrons and the 5.4 keV Auger electrons.

Abstract The Mossbauer effect at dilute Fe3+ and Fe2+ impurities in LiNbO3 and LiTaO3 has been measured. Lattice site substitution and the charge compensating process are discussed.

A quadrupole Fe57 polarimeter consisting of single crystals of LiNbO3 : CoJ7 as source (polarizer) and of Fee03 (siderite) as absorber (analyzer) is described. The quadrupole interactions of the two materials are nearly equal in magnitude but opposite in sign and in addition the asymmetry parameter q ss 0. The polarization dependence in Mossbauer spectroscopy has been worked out in a number of theoretical treatise [I-101 and various techniques have been used to carry out polarimetry experiments, mostly based on transversely magnetized Co57 a-Fe sources [lo-271 and/or by selective excitation of nuclear sublevels [28] or filter techniques [13, 29-32]. These methods have disadvantages and difficulties are encountered in the analysis of the spectra. In the former case by the multiplicity resulting from the six linearly polarized source lines and in the latter cases by the inefficiency of the selective excitation or of the filters in terms of intensity, degree of polarization and non-Lorentzian lines. Sources exhibiting quadrupole hyperfine interaction will circumvent some of these problems. The usefulness of a quadrupole split Co57 source for a polarimeter depends on the fulfillment of the following conditions : 1. The material can be grown as single crystal and cutting of thin plates in various directions is

The hyperfine field distribution has been measured for Fe - 28 per cent Ni - 3 per cent C alloy at 22 K with a longitudinal external field. Relative Mössbauer line intensities indicate that the effective hyperfine field at Fe57 nuclei lower than 30–40 kOe tends to align perpendicular with a 50 kOe external field which is explicable only by the model of coexistence of antiferromagnetic and ferromagnetic Fe states.

A simple electron-beam heated furnace is described which allows the controlled evaporation of small quantities of isotopes, particularly of 57Fe, for thin film deposition.

The isomeric states ${\mathrm{Sn}}^{119m}$ and ${\mathrm{Te}}^{125m}$ make the compound SnTe particularly attractive for M\"ossbauer studies since both SnTe-source and -absorber atoms are in the same chemical environment and are natural constituents rather than impurity atoms. Measurements on both isotopes in SnTe which has the NaCl structure offer the opportunity to test theoretical predictions for the respective mean-square atomic displacements in a cubic diatomic lattice. Using a $\mathrm{Sn}{\mathrm{Te}}^{125m}$ source and a SnTe absorber the recoilless fraction at ${\mathrm{Te}}^{125}$ nuclei has been determined to be 0.464 \ifmmode\pm\else\textpm\fi{} 0.036 and 0.240 \ifmmode\pm\else\textpm\fi{} 0.030 at 4.2 and 77 K, respectively, and a force-constant value of 0.43 \ifmmode\pm\else\textpm\fi{} 0.03 mdyn/\AA{} has been obtained for the Te atom. The mean-square displacement $〈{x}^{2}〉$ of ${\mathrm{Te}}^{125}$ nuclei as a function of temperature is well represented by the Debye model with ${\ensuremath{\Theta}}_{M}=141$ K. On the other hand, $〈{x}^{2}〉$ values for ${\mathrm{Sn}}^{119}$ nuclei in SnTe show that their temperature dependence cannot be described by the Debye approximation but indicates low-temperature anharmonicity. This deviation from the behavior of the quasi-Debye diatomic lattice can be understood qualitatively by the ionic size difference of the two components, and indicates that second-neighbor forces between Te atoms are important.

In ferromagnetically ordered cementite, Fe3C, the spin orientation was investigated by correlating the texture measured with X-rays and the relative line intensities of a Mössbauer hyperfine pattern. Formulae are derived to make this correlation method generally applicable. The spin in Fe3C was found to be oriented within a solid angel of 20° along the crystallographic c-axis. In ferromagnetisch geordnetem Zementit, Fe3C, werden die Spinorientierungen durch Verknüpfung der gemessenen Textur mit Röntgenstrahlen und den relativen Intensitäten des Mössbauer-Hyperfeindiagramms untersucht. Formeln werden abgeleitet, die diese Korrelationsmethode allgemein anwendbar machen. Es wurde gefunden, daß der Spin in Fe3C in einem Raumwinkel von 20° zur kirstallographischen c-Achse orientiert ist.