Ion-beam induced changes in magnetic and microstructural properties of thin iron films

Changes in magnetic and structural properties of 60–82 nm iron films induced by heavy-ion implantation were studied using the magneto-optical Kerr effect, M?ssbauer spectroscopy, Rutherford backscattering spectroscopy, X-ray diffraction, and X-ray absorption fine structure. The influence of ion-beam parameters (ion mass, fluence) and of sample parameters (external magnetic field and stress during implantation) were investigated. The Fe films, some of them containing a thin ⁵⁷Fe marker layer for M?ssbauer spectroscopy, were deposited on Si(100) substrates, by electron-beam and effusion-cell evaporation. The films were irradiated with ²⁰Ne, ⁵⁶Fe, ⁸⁶Kr and ¹³²Xe ions at energies chosen so that the implantation profiles peaked near the middle of the Fe films. The as-deposited films were magnetically isotropic and had a high coercivity. After ion implantation, the coercivity decreased and magnetic anisotropy developed. Both changes correlated with a decrease in the internal film stress. External mechanical stress applied during the irradiation had hardly any influence on the magnetic texture, opposite to an external magnetic field applied during or before ion implantation. The results are compared with those obtained for ion-irradiated polycrystalline Ni films and epitaxial Fe films and discussed with respect to the role of radiation-induced extended defects as pinning centers.     

57Fe Mössbauer study of Fe nitrides

Summary Magnetic properties of Fe nitrides have been re-examined by⁵⁷Fe M?ssbauer spectroscopy. Hyperfine magnetic fields for α″-Fe₁₆N₂ are 30, 31 and 39T at 298K, but the averaged hyperfine field is 33T and nearly equal to the value of pure α-Fe. σ-Fe₂ N is an antiferromagnet below 9K having a small magnetic moment less than 0.1 μ_B, although γ′-Fe₄N and ε-Fe_3–2N are ferromagnets. ZnS-type FeN is non-magnetic at 4.2K. M?ssbauer spectra obtained from NaCl-type FeN are complex and some Fe atoms in this nitride show a surprisingly large hyperfine magnetic field of 49T. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

Spin dynamics in (( BEDO-TTF )6[ M (CN)6](H3O,CH3CN)2 ( M = Fe, Cr) crystals

Layered single crystals of the (BEDO-TTF)₆[M(CN)₆](H₃O,CH₃CN)₂ (M = Fe, Cr) compounds with alternating conducting layers of BEDO-TTF and [M(CN)₆](H₃O,CH₃CN)₂ are studied. The contributions to the magnetic susceptibility from charge carriers in BEDO-TTF layers and from the subsystem of localized magnetic moments of iron (or chromium) transition metal complexes are separated for both compounds under investigation. It is revealed that the crystals with [Fe(CN))₆]³⁻ anions at a temperature of ∼80 K and the crystals with [Cr(CN))₆]³⁻ anions at ∼30 K undergo magnetic transitions which are accompanied by drastic changes in the parameters of the EPR lines associated with the BEDO-TTF layers and the subsystem of localized spins of transition metal complexes. It is established that the presence of the BEDO-TTF layers in the structure affects the magnetic properties of iron and chromium hexacyanide complexes. Original Russian Text ? R.B. Morgunov, E.V. Kurganova, T.G. Prokhorova, E.B. Yagubskiĭ, S.V. Simonov, R.P. Shibaeva, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 4, pp. 657–663.     

Ab initio calculations of interfacial magnetism in Fe/Mo superlattices

Ab initio calculations have been performed on Fe/Mo(1 0 0) superlattices in order to study the interfacial magnetic properties and layer thickness effect on the magnetic moments. In most cases, the magnetic moments of interfacial Fe monolayers are always smaller than those of the inner layers, and the induced magnetic moments of interfacial Mo monolayers oriented in the opposite direction. Calculation results show that the Fe layers are ferromagnetic when n = 3. As the thickness of the Mo layers increases, the influence of the Mo layer increases and the magnetic state of the Fe layer gradually changes into an antiferromagnetic or non-magnetic state. The change of magnetic moments of Fe/Mo superlattices is in agreement with the experimentally observed oscillation periods.     

Scattering of conduction electrons in Fe(t x , Å)/Cr(10 Å) superlattices with ultrathin iron layers

IR magnetoreflection spectra, diagonal σ _xx and off-diagonal σ _xy components of the effective optical conductivity tensor, and magnetic properties of Fe(t _x , Å)/Cr(10 Å) superlattices have been studied. The abrupt decrease in the amplitude of dissipative function ?ωImσ _xy (ω) (ω is the cyclic frequency of light wave) in the superlattices with ultrathin Fe layers (t _Fe = 3.2, 2.6, 2.1 Å) has been analyzed. It has been found that the magnetorefractive effect in nanostructures with ultrathin iron layers is due to scattering of conduction electrons by magnetic interfacial layers formed in the Cr matrix with complete consumption of deposited iron atoms. The parameters of the interfacial scattering of electrons in the spin-up (└) and spin-down (┌) conduction channels have been discussed.     

Optical and magneto-optical properties of Fe/Cu multilayered films: Influence of the modulation period and the bcc-fcc phase transition in iron

The optical and magneto-optical properties of multilayered film samples of the Fe/Cu system prepared by high-frequency sputtering on an Si(100) substrate are studied by ellipsometry and by measuring the equatorial Kerr effect (the δ _p effect) in the spectral range 0.25–7 μm. The optical characteristics, the plasma frequency ω _p and the relaxation frequency γ ₀ of the conduction electrons, and the δ _p effect are found as functions of the modulation period D=12.5–100 Å. Anomalous behavior of the optical and magneto-optical characteristics is discovered in short-period Fe/Cu structures. The results are discussed within a phenomenological theory of optical and magneto-optical properties for layered structures. Several factors, such as the indirect exchange interaction between the iron layers, the presence of a transition layer on the internal boundaries, the possible “magnetizing” of copper, and the formation of an fcc iron phase in the thin layers, are taken into account in the analysis of the experimental data. Zh. éksp. Teor. Fiz. 112, 1694–1709 (November 1997)     

The interaction of Fe on MgO(1 0 0) surfaces

The atomic interaction and magnetic properties of ultrathin Fe films grown on cleaved and polished MgO(1 0 0) surfaces were studied by conversion electron Mössbauer spectroscopy (CEMS). ⁵⁷Fe layers were deposited as probe atoms in different layer positions in 10 ML thick Fe films. Fe layers of different thicknesses were formed on polished and cleaved substrate surfaces at RT deposition. The analysis of the spectra showed no Fe-O^2- interaction in MgO/Fe interface. FeO phase formation was excluded. The Mössbauer spectrum of 5 ML ⁵⁷Fe sample showed enhanced internal magnetic field at 80 K. No interdiffusion of ⁵⁷Fe and ⁵⁶Fe atoms was observed between the layers at room temperature.     

Inhomogeneous ferromagnetic resonance modes in [Fe/Cr]n superlattices with a high biquadratic exchange constant

In a set of [Fe/Cr]_n superlattices, magnetization curves and spectra of ferromagnetic resonance under an in-plane magnetic field have been studied at room temperature. Along with the acoustic branch, several additional branches have been observed in resonance spectra. Resonance spectra have been calculated analytically for a structure with an infinite number of layers and numerically for finite numbers of layers in real samples using a model of biquadratic exchange taking account of the fourth-order magnetic anisotropy. A possibility of describing both static and resonance properties of the system in terms of this model has been demonstrated. Zh. éksp. Teor. Fiz. 116, 1817–1833 (November 1999)     

Mössbauer investigation of the Fe/Cu interfaces in BCC Fe/Cu/Fe(0 0 1) structures

A RHEED study shows quasi layer-by-layer growth in BCC Fe/Cu/Fe(0 0 1) structures. The BCC stacking of Cu layers is maintained up to a critical thickness of 11 layers. The different iron sites at the Fe/Cu interfaces can be identified by Mössbauer spectroscopy from the distinct values measured for the magnetic hyperfine fields H_hf and isomer shifts at the ⁵⁷Fe nuclei. This makes it possible to determine the concentration of ⁵⁷Fe atoms in the different iron sites. The roughness of the Cu on Fe interface estimated from the Mössbauer study is more pronounced than that estimated from a RHEED study of the structure. The growth of Fe on Cu produces CuFe alloy layers at the Cu/Fe interface.     

Fe–Ag granular multilayers and heterostructures studied in applied magnetic field

An (0.2 nm ⁵⁷Fe / 2.6 nm Ag)₇₅ granular multilayer sample and heterostructures with additional continuous Fe layers in different sequences were studied in magnetic field applied at different temperatures. The broadening of the superparamagnetic lines was found to be very similar for the three samples in applied fields both parallel and perpendicular to the sample plane. While the layer sequence has no significant effect on the superparamagnetic properties, the continuous magnetic layers follow a different approach to saturation in perpendicular magnetic fields.     

Point defects in FeAl

Summary Point defects in annealed B2-phase FeAl samples in the range 47–53 at.% Fe were studied using⁵⁷Fe M?ssbauer spectroscopy. Spectra were analyzed using local environment models according to which point defects in atomic shells close to probe atoms induce shifts in the nuclear monopole interaction. For well-annealed samples, better results were obtained assuming only the presence of Fe_Al antisite and V_Fe vacancy defects, and not of Al_Fe antisite defects. Monopole interactions of⁵⁷Fe probes on the Fe and Al sublattices having no defects in the first two shells were about +0.27 and −0.03 mm s⁻¹, respectively, with respect to Fe in alpha-Fe metal. The shifts induced by Fe_Al and V_Fe defects in the first shells of Fe probes on the Fe and Al sublattices were −0.15 and −0.24 mm s⁻¹, respectively, and, in the second shells, +0.06 and +0.011 mm s⁻¹. In addition to structural defects needed to accommodate deviations from stoichiometry, annealed samples were found to contain several percents of Fe_Al and V_Fe defects due to lattice disorder, with greater disorder in Fe-deficient alloys. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

57Fe Mössbauer study of ternary alloy systems Fe(Sb1−x Te x )2 for 0.5≤x≤1.0 at 4.2K

Summary We report on low-temperature (4.2 K)⁵⁷Fe M?ssbauer studies of the ternary alloy system Fe(Sb_1−x Te _x )₂ for 0.5≤x≤1.0. The M?ssbauer spectra are electric-quadrupole doublets for all the specimens, indicating a non-magnetic character. The quadrupole splitting decreases with the increase in tellurium concentration. FeTe₂ has also been studied in external magnetic fields up to 6T. These experiments show that the electric-field gradient in this alloy is negative and has an asymmetry parameter of η ≈0.7. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

Iron transfer between ferritin molecules

When ferritin solutions containing 8–12⁵⁷Fe atoms/molecule are mixed after iron loading, with ferritin containing a small⁵⁶Fe core (⁵⁶Fe-ferritins) or with an equal volume of NaCl solution (controls) and the mixtures are frozen in liquid nitrogen (LN) at specified times, the spectra of the⁵⁶Fe-ferritins show that almost all⁵⁷Fe is attached to large clusters, whereas in the controls all⁵⁷Fe is present as isolated atoma,⁵⁷Fe dimers or small clusters and no large⁵⁷Fe containing clusters are observed. This shows that⁵⁷Fe present as solitary or dimer species in ferritin migrates to ferritin molecules containing larger⁵⁶Fe clusters.     

Morin transition suppression in Polycrystalline 57Hematite (α-Fe2O3) exposed to 56Fe(II)

We used the isotope selectivity of ⁵⁷Fe Mössbauer spectroscopy to investigate changes in the magnetic properties of polycrystalline hematite exposed to ferrous iron (Fe(II)). We found that sorption of ⁵⁶Fe(II), followed by interfacial electron exchange, alters the bulk magnetic properties of ⁵⁷hematite. After reaction with ⁵⁶Fe(II), we observed partial suppression of the Morin transition of ⁵⁷hematite to below 13 K. This is significantly lower than the Morin temperature (T _M) of ～230 K measured for isotopically enriched polycrystalline ⁵⁷hematite, as well as the T _M of 264?±?2 K reported for normal polycrystalline hematite.     

Magnetic properties of Eu-Sc-Fe garnets studied with the Mössbauer effect

Magnetic properties of the garnet system Eu_3-ySc_2+yFe₃O₁₂ for 0 ? y ? 0.5 have been studied with ⁵⁷Fe and ¹⁵¹ Eu Mössbauer spectroscopy. The system is shown to have a ferrimagnetic structure. Both, the ordering temperatures and the hyperfine magnetic fields at the ⁵⁷Fe nuclei are found to increase with an increase in y. The iron magnetic moments at the octahedral and the tetrahedral sites are non-collinear. The average canting angles at these sites are determined and used for the calculation of the magnetic moments. Good agreement is obtained between the calculated and the experimental moment for the y = 0.0 sample. The determined magnetic properties are consistent with the recently proposed cation distribution in this system.     

Mössbauer study of iron in sp-metals and semimetals

Bi, In, Pb, Sb, α- and β-Sn were implanted with⁵⁷Co and⁵⁷Fe. The results of the analysis suggested strongly hybridized bonds between the iron and the host atoms. An upturn of the isomer shift (δ) values relative to the δ values in hosts with completed d-shell was not justified. Charge states of Fe²⁺ or Fe⁺ also appeared. The possible role of the native surface oxide layers in the formation of these states was investigated.     

Magnetism of ferriprotoporphyrin IX monomers and dimers

The SQUID and the ⁵⁷Fe Mössbauer spectroscopy studies of the magnetic properties of monomeric and dimeric forms of iron porphyrin were performed between 2 and 305 K. The effective magnetic relaxation rate of the Fe atoms in iron porphyrin monomers exhibits complex temperature dependence, resulting from the competing spin-spin and spin-lattice relaxation processes. The dimerization of iron porphyrin dramatically speeds up the magnetic relaxation. The Fe-Fe antiferromagnetic exchange coupling constant in Fe-O-Fe dimer is J≈−110 cm⁻¹. The complementary application of SQUID and the Mössbauer spectroscopy is proposed as a new precise quantitative analytical methodology for monitoring of the aggregation process of iron porphyrin.     

Structure and composition of iron-containing langmuir-blodgett films

Summary The composition of differently prepared Langmuir-Blodgett multilayers of ferric arachidate has been investigated by means of absorption M?ssbauer spectroscopy and heavy-ion-induced mass spectrometry. We found that the iron concentration in qualitatively good films is 3.5 times higher than the stoichiometric value. Antiferromagnetic ordering in this two-dimensional model substance can be observed when the Fe−Fe distance in the layers is about 0.3 nm or below. The area density of the iron atoms in the planes decreases with decreasingpH values of the subphase which also leads to a disappearance of the long-range magnetic ordering and to a decrease of the Debye temperature of the films which is linearly dependent on the iron concentration in the films. Paper presented at the ICAME-95, Rimini, 10–16 September 1995     

Thickness dependence of the magnetic anisotropy of Fe layers separated by Al

Magnetic multilayers of ⁵⁷Fe with nominal thickness, T _nom, between 0.4 and 1.0 nm separated by 3.0 nm Al spacer layers were prepared by alternate deposition of the constituents in high vacuum. The samples were investigated at 4.2 K in external magnetic field. A fraction of Fe atoms corresponding to about 0.3 nm equivalent Fe-thickness was found to mix into the Al spacer. The extremely strong magnetic anisotropy observed for T _nom < 0.8 nm is attributed to Fe layers of approximately two atomic planes thick. The anisotropy decreases considerably after the building up of the third Fe atomic layer starts at T _nom = 0.8 nm, but full saturation was not achieved even for T _nom = 1 nm and 3 T magnetic field applied perpendicularly to the sample plane.     

NMR study of the intermetallic Laves alloys LuFe2 and LuFe1.96Al0.04

NMR spectra of ⁵⁷Fe and ²⁷Al in LuFe₂ and LuFe_0.96Al_0.04 alloys were measured by the spin echo method at 4.2 K. The main contributions to the hyperfine fields on ⁵⁷Fe nuclei are estimated, along with the magnetic moments on the Fe and Lu atoms in LuFe₂. Fiz. Tverd. Tela (St. Petersburg) 41, 1142–1144 (July 1999)