Conversion Electron Mossbauer Spectroscopy Research Articles

Mössbauer spectroscopy of interfaces in metals

Publications on the studies of interfaces in metals by the methods of Mossbauer (nuclear gammaresonance) spectroscopy are reviewed. Physical principles of the Mossbauer effect, various methods of nuclear gamma-resonance (NGR) spectroscopy and the Mossbauer spectra parameters are considered. The available results on grain boundary studies in coarse-grained and nanostructured materials and on interlayer interfaces in multilayers are analyzed. Capabilities of application of absorption and emission Mossbauer spectroscopy as well as of conversion electrons Mossbauer spectroscopy (CEMS) are discussed.

Anomalous Magnetization Behavior of Fe-N Films Deposited by Reactive Pulsed Laser Deposition

A series of Fe-N films with thickness of 200 nm have been prepared by reactive pulsed laser deposition (PLD) at 25°C and at 150°C in various nitrogen pressures. Both series of samples consist of mixtures of γ'-Fe 4N and α-Fe phase. The amount of α-Fe decreases with the increase of nitrogen pressure. Higher remnant ratio and lower coercivity are observed for the films deposited at 150°C compared to the films deposited at 25°C. An anomalously large magnetization is observed for both series of films deposited at a nitrogen pressure of 8 mTorr. The average magnetization is as high as or even larger than that of α-Fe. Conversion electron Mossbauer spectroscopy demonstrated that the highest-magnetization film consists mixtures of γ'-Fe4N and α-Fe phase at a mole ratio of 1:6. The origin of this intriguing high moment is ascribed to a certain lattice constant of the α-Fe which is in a heterogeneous state with γ'-Fe4N for films deposited at a certain nitrogen pressure.

Galvanostatic charge–discharge tests, 57Fe and 119Sn Mössbauer and XRD measurements on novel Sn-Ni-Fe electrodeposits

Novel Sn–Ni–Fe ternary alloys were successfully deposited by pulse plating technique from an electrolyte based on sodium gluconate which acts as a complexing agent. XRD results revealed the predominantly amorphous character for the majority of the deposits. 57Fe and 119Sn conversion electron Mossbauer spectroscopy indicated the formation of a paramagnetic amorphous alloy using a short on-pulse duration and where the Fe content was less than 22 wt.%. Galvanostatic charge–discharge tests of the novel Sn-Ni-Fe deposits were carried out in a model Li-ion cell and indicated that the Sn-Ni-Fe alloys have potential as an electrode material.

Phase Composition and Magnetic Properties of Nanoperm Thin Films Doped with Yttrium

Thin lms of Nanoperm alloys with partial substitution of iron by yttrium are the subject of investigations. The attention is xed on the in uence of yttrium substitution on structure and magnetic characteristics of the samples. The e ect of lm thickness is also considered. Conversion electron Mossbauer spectroscopy and magneto-optic Kerr e ect were used to derive hyper ne parameters and coercive eld, respectively. Coexistence of crystalline bcc-Fe phase and an amorphous one with diverse proportion was stated in most of the samples. It was found that surface and interface regions contribute to the magnetic properties of lms.

Mössbauer spectroscopy and magnetization of ordered arrays of ultrathin FePt nanodisks with perpendicular magnetisation

The purpose of this communication is to show that arrays of FePt with perpendicular anisotropy can be studied using energy-selective conversion electron Mossbauer spectroscopy. Magnetic nanostructures have been in the focus of technology and science for many years. One of the economically and comparably scientifically most demanding application of these structures is in storage devices such as hard-disks and MRAMs. The FePt nanodisks presented here show a combination of material and magnetic properties which enable the fabrication of highly ordered nanostructures.

Growth and electronic and magnetic structure of iron oxide films on Pt(111)

Ultrathin (111)-oriented polar iron oxide films were grown on a Pt(111) single crystal either by the reactive deposition of iron or oxidation of metallic iron monolayers. These films were characterized using low energy electron diffraction, scanning tunneling microscopy and conversion electron Mossbauer spectroscopy. The reactive deposition of Fe led to the island growth of Fe3O4, in which the electronic and magnetic properties of the bulk material were modulated by superparamagnetic size effects for thicknesses below 2 nm, revealing specific surface and interface features. In contrast, the oxide films with FeO stoichiometry, which could be stabilized as thick as 4 nm under special preparation conditions, had electronic and magnetic properties that were very different from their bulk counterpart, w\"ustite. Unusual long range magnetic order appeared at room temperature for thicknesses between three and ten monolayers, the appearance of which requires severe structural modification from the rock-salt structure.

Application of Mössbauer spectroscopy in magnetism

An overview is provided on our recent work that applies 57Fe Mossbauer spectroscopy to specific problems in nanomagnetism. 57Fe conversion electron Mossbauer spectroscopy (CEMS) in conjunction with the 57Fe probe layer technique as well as 57Fe nuclear resonant scattering (NRS) were employed for the study of various nanoscale layered systems: (i) metastable fct-Fe; a strongly enhanced hyperfine magnetic field Bhf of ∼39 T at 25 K was observed in ultrahigh vacuum (UHV) on uncoated three-monolayers thick epitaxial face-centered tetragonal (fct) 57Fe(110) ultrathin films grown by molecular-beam epitaxy (MBE) on vicinal Pd(110) substrates; this indicates the presence of enhanced Fe local moments, μFe, as predicted theoretically; (ii) Fe spin structure; by applying magnetic fields, the Fe spin structure during magnetization reversal in layered (Sm–Co)/Fe exchange spring magnets and in exchange-biased Fe/MnF2 bilayers was proven to be non-collinear and depth-dependent; (iii) ferromagnet/semiconductor interfaces for electrical spin injection; CEMS was used as a diagnostic tool for the investigation of magnetism at the buried interface of Fe electrical contacts on the clean surface of GaAs(001) and GaAs(001)-based spin light-emitting diodes (spin LED) with in-plane or out-of-plane Fe spin orientation; the measured rather large average hyperfine field of ∼27 T at 295 K and the distribution of hyperfine magnetic fields, P(Bhf), provide evidence for the absence of magnetically “dead” layers and the existence of relatively large Fe moments (μFe ∼ 1.8 μB) at the ferromagnet/semiconductor interface. - Finally, a short outlook is given for potential applications of Mossbauer spectroscopy on topical subjects of nanomagnetism/spintronics.

Noncollinear Fe spin structure in (Sm-Co)/Fe exchange-spring bilayers: Layer-resolved57Fe Mössbauer spectroscopy and electronic structure calculations

Magnetization reversal in nanoscale (Sm-Co)/Fe (hard/soft) bilayer exchange-spring magnets with in-plane uniaxial magnetic anisotropy was investigated by magnetometry, conversion-electron M\ossbauer spectroscopy (CEMS) and atomistic Fe spin-structure calculations. Magnetization loops along the easy direction exhibit signatures typical of exchange-spring magnets. In-field CEMS at inclined \ensuremath{\gamma}-ray incidence onto thin (2 nm) ${}^{57}$Fe probe layers embedded at various depths in the 20-nm-thick natural (soft) Fe layer provides depth-dependent information (via the line-intensity ratio ${R}_{23}$ as a function of the applied field $H$) about the in-plane rotation of Fe spins. A minimum in the ${R}_{23}$-vs-$H$ dependence at (${H}_{\mathrm{min}}$, ${R}_{\mathrm{min}}$) determines the field where Fe magnetic moments roughly adopt an average perpendicular orientation during their reversal from positive to negative easy-axis orientation. A monotonic decrease of ${H}_{\mathrm{min}}$ with distance from the hard/soft interface is observed. Rotation of Fe spins takes place even in the interface region in applied fields far below the field of irreversible switching, ${H}_{\mathrm{irr}}$, of the hard phase. Formation of an Fe-Co alloy is detected in the interface region. For comparison, the noncollinear Fe spin structure during reversal and the resulting ${R}_{23}$ ratio were obtained by electronic-structure calculations based on a quantum-mechanical Hamiltonian for itinerant electrons. The coupling at the hard/soft interface is described by the uniaxial exchange-anisotropy field, ${h}_{\mathrm{int}}$, as a parameter. Our calculated ${R}_{23}$ ratios as a function of the (reduced) applied field $h$ exhibit similar features as observed in the experiment, in particular a minimum at (${h}_{\mathrm{min}}$, ${R}_{\mathrm{min}}$). ${R}_{\mathrm{min}}$ is found to increase with ${h}_{\mathrm{int}}$, thus providing a measure of the interface coupling. Evidence is provided for the existence of fluctuations of the interface coupling. The calculations also show that the Fe spin spiral formed during reversal is highly inhomogeneous. In general, our simulation of the Fe spin structure is applicable for the interpretation of experimental results on layered exchange-spring magnets.

Layer-by-layer epitaxial growth of polar FeO(111) thin films on MgO(111)

We report on the structural properties of epitaxial FeO layers grown by molecular beam epitaxy on MgO(111). The successful stabilization of polar FeO films as thick as 16 monolayers (ML), obtained by deposition and subsequent oxidation of single Fe layers, is presented. FeO/MgO(111) thin films were chemically and structurally characterized using low-energy electron diffraction, Auger electron spectroscopy and conversion electron Mössbauer spectroscopy (CEMS). Detailed in situ CEMS measurements as a function of the film thickness demonstrated a size-effect-induced evolution of the hyperfine parameters, with the thickest film exhibiting the bulk-wüstite hyperfine pattern. Ex situ CEMS investigation confirmed existence of magnetic ordering of the wüstite thin film phase at liquid nitrogen temperature.

Tunneling Magnetoresistance (TMR) on Fe-Al<sub>2</sub>O<sub>3</sub> Nano Granular Film Growth by Helicon Plasma Sputtering

Fe-Al 2 O 3 nanogranular thin film by helicon plasma sputtering with the variation of Fe content from 0.1 to 0.7 volume fraction have been prepared. The magnetic and magnetoresistance properties were investigated by a Vibrating Sample Magnetometer (VSM) and a Four Point Probe (FPP). The Rutherford BackScattering (RBS) was performed with the SIMNRA software. Conversion Electron Mossbauer Spectroscopy (CEMS) study was also performed to estimate the fraction of Fe and α-Fe 2 O 3 in the granular film. The results suggested that the percolation concentration occured at 0.55 Fe volume fractions, with the maximum Magnetoresistance (MR) ratio of 3%. The present MR ratio that was lower than the previous results may be related to the existence of α-Fe 2 O 3 phase.

57Fe conversion electron Mössbauer study of oxidized iron-nickel alloy pins

Iron-nickel alloy pins were pretreated at 700°C and 500°C for 30 min in H2, and additionally heat-treated at 600°C for 30 min in N2 atmosphere, respectively. The oxide layers of these samples were compared with that of a non-preheated sample. Thin iron oxide layers on all samples were observed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and 57Fe conversion electron Mossbauer (CEM) spectrometry. The iron products of Fe3O4 and γ-Fe2O3 were determined among various heated samples.

CEMS study of corrosion products formed by NaCl aqueous solution

Conversion electron Mossbauer spectroscopy was used to study corrosion products by NaCl aqueous solution. A drop of the solution is put on an iron foil and the foil is left at RT. During the evaporation of the solution, corrosion products are formed. Conversion electron Mossbauer spectra were taken at temperatures between 15 K and room temperature (RT). In the Mossbauer spectra a ferric doublet is observed at RT, but sextets are found at 15 K. These results show that the corrosion product mainly consists of γ - FeOOH and a small amount of β - FeOOH is noticed. As NaCl concentration increases, the corrosion layer becomes thick and β- FeOOH / γ - FeOOH ratio increases slightly. Consequently, it has been concluded that the produced amount of β- FeOOH increases more rapidly than that of γ - FeOOH with increasing NaCl concentration.

Gas counter for low temperature Conversion Electron Mössbauer Spectroscopy experiments

The operation of a gas counter, designed for Conversion Electron Mossbauer Spectroscopy measurement at low temperature, has been investigated. The experimental setup is described and tested with two pure gases, He and Ne, and two mixtures, He-5%CH4 and He-5%N2. The impacts on the counter performances of the applied voltage, the gas composition and pressure as well as the gas renewing are investigated between 41 K and 300 K. This investigation is made using 119Sn Mossbauer source and metallic tin absorber. The appropriate operating conditions of the present counter have been established for temperatures down to 41 K for both pure gases, and 61 and 85 K for He-5%N2 and He-5%CH4 respectively.

Metallurgical phases and their magnetism at the interface of nanoscale MgB2/Fe layered structures

We report on the characterization of metallurgical phases and their magnetism at the interfaces of nanoscale MgB2/Fe layered structures. MgB2/57Fe multilayers with varying layer thicknesses were prepared by vacuum deposition and investigated, before and after annealing by electrical resistance measurements, x-ray diffraction and 57Fe conversion-electron Mössbauer spectroscopy (CEMS) down to 5 K. Interfacial Fe–B phases, such as Fe2B, were identified by CEMS. A superparamagnetic-to-ferromagnetic transition is observed with increasing 57Fe film thickness. Ultrahigh vacuum annealing at 500 °C of the multilayers leads to strong diffusion of Fe atoms into the boundary regions of the MgB2 layers. MgB2 in the as-grown multilayers is non-superconducting. Structural disorder and the effect of Fe interdiffusion contribute to the suppression of superconductivity in the MgB2 films of all the as-grown multilayers and the thinner annealed multilayers. However, an annealed MgB2/57Fe/MgB2 trilayer with thicker (500 Å) MgB2 layers is observed to be superconducting with an onset temperature of 25 K. At 5 K, the annealed trilayer can be conceived as being strongly chemically modulated, consisting of two partially Fe-doped superconducting MgB2 layers separated by an interdiffused weakly magnetic Fe–B interlayer, which is characterized by a low hyperfine magnetic field Bhf of ∼11 T. This chemically modulated layer structure of the trilayer after annealing was verified by Rutherford backscattering.

Mössbauer study of C18N/Fe Langmuir-Blodgett layers

Langmuir-Blodgett (LB) films of octadecanoyl hydroxamic acid (C18N) complexed with Fe3 + ions have been prepared at various subphase pH values. The LB films consisting of different number of layers were investigated by 57Fe conversion electron Mossbauer spectroscopy (CEM) at room temperature. The CEM detector contained a piece of α-iron, enriched with 57Fe, using as an internal standard. The Mossbauer pattern of the C18N/Fe LB films is a doublet with parameters δ = 0.35 mm/s and Δ = 0.74 mm/s. A gradual increase of the relative occurrence of the doublet compared to the sextet of the internal standard was observed with the increasing number of layers, indicating the nearly uniform distribution of Fe among the LB layers.

A CEMS search for precipitate formation in 57Fe implanted ZnO

Conversion electron Mossbauer Spectroscopy measurements have been made on ZnO single crystals implanted with 60 keV 57Fe to 4 and 8 at.% peak concentrations, and annealed up to 800°C. The spectra show quite strong changes with annealing, but no evidence of magnetic components, thus precluding the formation of large sized precipitates or secondary phases. Above an annealing temperature of 650°C, the dominant spectral component is a doublet with hyperfine parameters typical of Fe3 + , which is attributed to Fe3 + ions in nano-precipitates ∼5 nm in size.

Tuning exchange spring effects in FePt/Fe(Co) magnetic bilayers

Structural and magnetic properties of exchange spring magnets consisting of hard magnetic (FePt) and soft magnetic (Fe and Co) bilayers, prepared by ion beam sputtering method are studied via X-ray diffraction (XRD), magneto-optic Kerr effect (MOKE) and vibrating sample magnetometry (VSM). Thin tracer layers of 57Fe were introduced in the soft layer in order to observe the Fe spin structure and interfacial diffusion by Conversion Electron Mössbauer Spectroscopy (CEMS). The observed in-plane exchange spring behavior extends also to the magnetic hard layer, whose switching field can be tuned in an unexpected manner via the top soft magnetic layer. To explain the observed phenomenon it is suggested that the increased switching field, found in the system with a Co/Fe bilayer acting as a single soft magnetic layer, is compatible with a peculiar behavior of the stiffness coefficient of the heterogeneous soft magnetic layer. According to this observation, possibilities to maximize the exchange spring effects via suitably chosen non-homogeneous soft magnetic layers are open.

Relaxation scenarios in Fe–Pd and Fe–Pd–Cu ferromagnetic shape memory splats: Short range order and microstructure

Fe–Pd-based ferromagnetic shape memory alloys are based on metastable random solid solutions in the austenite and martensite phases, which require preparation from the equilibrium γ phase by rapid cooling. Employing splats, we explore the impact of annealing treatment on the disordered solid solution, phase formation and lattice defects using a combined conversion electron Mossbauer spectroscopy – X-ray diffraction study. We discuss indications of short range ordering tendencies as well as the impact of splat microstructure and defects on phase formation.

Mössbauer spectroscopic study of iron–nickel nitrides thin films prepared by ion beam assisted deposition

In this paper we have shown the feasibility of ion beam assisted deposition to produce iron–nickel nitride thin films of various compositions. We have carried out their compositional and structural characterization by means of Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and integral conversion electron Mossbauer spectroscopy (ICEMS). The results have shown that Ni is incorporated within the films and that the temperature of the substrate has a large influence both in the phase composition obtained as well as in the degree of crystallinity of the thin films produced.

Surface magnetic canting of iron films

The magnetization direction of $\ensuremath{\alpha}$-Fe thin films was investigated by means of quantum-beat spectroscopy using nuclear resonant scattering in the glancing-incidence regime and conversion-electron M\"ossbauer spectroscopy (CEMS). The CEMS results show that the mean magnetization of the films is in plane. Quantum-beat data, on the other hand, reveal that perpendicular components are present in the magnetization at the film surfaces, indicating that the magnetization cants from the in-plane to the perpendicular direction near the surfaces.