Fe Underlayer Research Articles

Asymmetric alloy formation at the Fe-on-Ti and Ti-on-Fe interfaces

The Fe-on-Ti and Ti-on-Fe interfaces were studied experimentally by Mössbauer spectroscopy (MS), transmission electron microscopy (TEM) and x-ray reflectometry (XRR) on Ti/Fe/Ti trilayers grown on Si(1 1 1) substrates by vacuum evaporation. The nanoscale structure and composition were explored in cross sections using TEM, the layer structure and the interface widths by specular x-ray reflectometry. MS was applied to identify the interface alloy phases and to determine the pure and alloyed Fe layer fractions. The experimental results were compared with molecular dynamics (MD) simulations of layer growth on Fe or Ti underlayers of different orientations. The concentration distributions provided by MD simulations show an asymmetry at the interfaces in the layer growth direction. The transition is atomically sharp at the Ti-on-Fe interface for the (0 0 1) and (1 1 0) crystallographic orientations of the Fe underlayer, while it spreads over a few atomic layers for Fe(1 1 1) underlayer and for all studied Ti underlayer orientations at the Fe-on-Ti interface. MS and XRR data on Ti/Fe/Ti trilayers confirm the asymmetry between the bottom and top Fe interface, but the inferred interface widths considerable exceed those deduced from the MD simulations.

Polycrystalline magnetite (Fe3O4) thin films from FeOx/Fe bilayers grown by pulsed laser depositions

The effect of heat treatment on the structural and magnetic properties of bilayered FeOx/Fe thin films grown by pulsed laser deposition on pre-oxidized amorphous SiO2/Si substrates has been studied. Post-deposition vacuum thermal annealing leads to remarkable changes in the structural and magnetic properties of the FeOx layer, which has been investigated by the combination of scanning electron and atomic force microscopy, Raman scattering spectroscopy and conversion electron Mössbauer spectroscopy. FeOx layer deposited at room temperature on top of the polycrystalline Fe underlayer is amorphous with the smooth surface (root mean square roughness ~0.5nm). Vacuum thermal annealing of the FeOx/Fe stack at T=500°C leads to the formation of a polycrystalline Fe3O4/Fe thin film structure with the magnetite nucleation centers of an average size of 100nm. The formation of the exact magnetite phase depends on the annealing conditions as shown by in-situ Raman and Mössbauer spectroscopy investigations.

Multilayer Ni/Fe thin films as oxygen evolution catalysts for solar fuel production

The slow kinetics and high overpotential of the oxygen evolution reaction is one of the main limiting factors to achieve the minimum required performances of the so-called photoelectrochemical water splitting systems. An oxygen evolution catalyst (OEC) becomes essential in order to perform this process with higher efficiency. Herein, we report the physical, optical and electrochemical characterization of multilayer Ni/Fe thin films as earth-abundant OEC, to avoid the use of platinum group metals (PGM). Uniform films of thicknesses ranging from 1 to 10 nm were fabricated by sequential and alternate thermal evaporation of Ni and Fe. It was found that the successive deposition allows the fabrication of a Ni terminated surface that does not need activation due to the Fe underlayer. The lowest overpotential achieved for NiFe was 370 mV at 10 mA cm−2 and a Tafel slope of 37 mV dec−1 with 1 nm thickness and 95% transmittance. Finally, NiFe OEC was implemented on top of Mo:BiVO4 photoanodes which resulted in a reduction of the open circuit potential of 0.2 V and up to five fold increase of the oxidation efficiency at 0.7 VRHE. The results presented facilitate the practical implementation of BiVO4 photoanodes in tandem configuration for bias free photoassisted water splitting.

Structure and magnetic properties of SmCo5/X (X=Fe, Fe-Co, Co) magnetic bilayer films on MgO(110) substrate

Sm17Co83 (at%) films are prepared on bcc-Cr(211), bcc-Fe(211), bcc-Fe65Co35(211), and hcp-Co(11(_)00) underlayers hetero-epitaxially grown on MgO(110) substrates at 500°C by using a UHV-MBE system. The growth behavior and the structures are investigated by in-situ reflection high-energy electron diffraction and X-ray diffraction. Sm-Co(11(_)00) single-crystal films of RT5 ordered structure with the c-axis lying in the film plane grow epitaxially on the underlayers. Small volumes of amorphous phases are interpreted to be involved in the films deposited on these underlayers. The crystal lattice of RT5 magnetic film is slightly deformed from the bulk hexagonal structure, which is caused possibly by the lattice mismatch with the underlayer. The order degree of film deposited on Cr underlayer is higher (S=0.77) than those prepared on Fe, Fe65Co35, and Co underlayers (S: 0.67–0.71). The films show in-plane uniaxial magnetic anisotropies reflecting the magnetocrystalline anisotropy of SmCo5 crystal.

Influence of Fe underlayers on stress evolution of Ti in Ti/Fe multilayers

A series of 40–2 nm bilayer spacing Ti/Fe multilayers were sputter-deposited. As the length scale of individual Ti layers equaled to 2 nm, Ti phase transforms from a hexagonal close packed (hcp)-to-body centered cubic (bcc) crystal structures for equal layer thicknesses in Ti/Fe multilayers. Further equal reductions in bilayer spacing to less than 1 nm resulted in an additional transformation from a crystalline to amorphous structure. Atom probe tomography reveals significant intermixing between layers which contributes to the observed phase transformations. Real-time, intrinsic growth stress measurements were also performed to relate the adatom mobility to these phase transformations. For the hcp Ti/bcc Fe multilayers of equivalent volume fractions, the multilayers undergo an overall tensile stress state to a compressive stress state with decreasing bilayer thickness for the multilayers. When the above phase transformations occurred, a modest reduction in the overall compressive stress of the multilayer was noted. Depending on the Fe thickness, the Ti growth was observed to be a tensile to compressive growth change to a purely compressive growth for thinner bilayer spacing. Fe retained a tensile growth stress regardless of the bilayer spacing studied.

Growth and Structural Characterisation of V/Fe Multilayers

The (110) oriented V/Fe multilayers were prepared at room temperature using UHV magnetron sputtering. As a substrate we have used Si(100) wafers with an oxidised surface. The surface chemical composition and the cleanness of all layers was checked in situ, immediately after deposition, transferring the samples to an UHV analysis chamber equipped with X-ray photoelectron spectroscopy. The structure of the multilayers has been studied ex situ by lowand high-angle X-ray di raction. The modulation wavelength was determined from the spacing between satellite peaks in the X-ray di raction patterns. Results were consistent with the values obtained from total thickness divided by the number of repetitions. Growth of the Fe (V) on 1.6 nm V (Fe) underlayer was studied by succesive deposition and X-ray photoelectron spectroscopy measurements starting from 0.2 nm of Fe (V) layer, respectively. From the exponential variation of the X-ray photoelectron spectroscopy Fe 2p and V 2p integral intensities with increasing layer thickness we conclude that the Fe and V sublayers grow homogeneously in the planar mode.

The effect of strain induced by Ag underlayer on saturation magnetization of partially ordered Fe16N2 thin films

Partially ordered Fe-N thin films were grown by a facing target sputtering process on the surface of a (001) Ag underlayer on MgO substrates. It was confirmed by x-ray diffraction that the Ag layer enlarged the in-plane lattice of the Fe-N thin films. Domains of the ordered α″-Fe16N2 phase within an epitaxial (001) α′-FexN phase were identified by electron diffraction and high-resolution aberration-corrected scanning transmission electron microscopy (STEM) methods. STEM dark-field and bright-field images showed the fully ordered structure of the α″-Fe16N2 at the atomic column level. High saturation magnetization(Ms) of 1890 emu/cc was obtained for α″-Fe16N2 on the Ag underlayer, while only 1500 emu/cc was measured for Fe-N on the Fe underlayer. The results are likely due to a tensile strain induced in the α″-Fe16N2 phase by the Ag structure at the interface.

Enhanced perpendicular magnetic anisotropy in Fe/Mn bilayers by incorporating ultrathin ferromagnetic underlayer through magnetic proximity effect

The perpendicular magnetic anisotropy(PMA) was shown to be established in ferromagnetic (FM)/fcc-Mn bilayers through the FM-antiferromagnetic (AFM) exchange coupling. We demonstrate here that such PMA can be further enhanced by incorporating an ultrathin Fe film as an underlayer. In a series of Fe/Mn bilayers, hysteresis loop measurement shows that the thickness of top Fe layer with PMA can be extended to a thicker range while an ultrathin Fe underlayer is inserted. Such enhancement of PMA is attributed to an increase of AFM ordering on the Mn film originated from the magnetic proximity effect with the Fe underlayer.

Nano-faceting of Cu capping layer grown on Fe/Si (111) and its effect on magnetic anisotropy

We represent a report on the growth and structure of Cu capping layer on ultrathin Fe films on Si (111) substrate as well as its effect on magnetic anisotropy. Cu grows as forming triangular-shaped pyramids with nano-facets and in epitaxial mode with Kurdjumov-Sachs orientation. Spin reorientation transition (SRT) from out-of-plane to in-plane of Fe films is induced by Cu capping, which is believed to be mainly affected by strain change in Fe films. Based on strain relief mechanism, rapid decrease in Cu critical thickness to induce SRT with increasing Fe underlayer thickness can be interpreted quite well.

Pulsed laser deposition of ultrathin BaTiO3/Fe bi-layers: Structural characterization and piezoelectric response

Ferroelectric ultrathin BaTiO3 (BTO) films in contact with Fe underlayer have been grown by pulsed laser deposition onto MgO(100) substrates in a single vacuum cycle. The structural properties of the composite system are investigated by Rutherford backscattering spectrometry/channeling and cross-sectional transmission electron microscopy. The BTO band gap is measured by in situ reflection electron energy loss spectroscopy to be Eg=4.1–4.3eV depending on the growth conditions. The ferroelectric nature of BTO (thickness down to 3nm) on top of Fe is demonstrated by piezoresponse force microscopy, which shows an out-of-plane piezoelectric coefficient of 17±4pm/V. The obtained results are promising in the view of integrating BTO/Fe stacks in functional ferroelectric tunnel junctions.

Fabrication of (001) Oriented TMR Film With Highly Ordered $L1_{0}$-Fe(Pd,Pt) Alloy Films by Using a Very Thin Fe Underlayer

Magnetic tunnel junctions with L10 -FePd storage layer which has low perpendicular magnetic anisotropy and lower ordering temperature compared with the case of L10-FePt films were fabricated with using a very thin Fe underlayer. The main results are as follows. 1) The long-range chemical order degree and the full width at half maximum of diffraction peak rocking curve for the FePd(002) for the L10-FePd film with a very thin Fe underlayer onto MgO(100) substrate were more than 0.9 and less than 2.0°. 2) The perpendicular coercive force and magnetic anisotropy field for the L10-FePd film with the very thin Fe underlayer were 300 Oe and 15 kOe. 3) The full width at half maximum of diffraction peak rocking curve for the Fe(Pd,Pt)(002) for the FePt/MgO/FePd film with a very thin Fe underlayer onto MgO(100) substrate was less than 2.0°. 4) The out-of-film-plane hysteresis loop of the FePt/MgO/FePd film with the very thin Fe underlayer shows a clear antiparallel state at the magnetic field from 500 to 1000 Oe between FePt reference layer and FePd storage layer. It is found that the very thin Fe underlayer is effective to obtain the FePt/MgO/FePd film with highly (001) orientation and highly L10 ordered structure.

Strongly in-plane magnetized FePt thin film on ultrathin Fe underlayer

The effect of ultrathin Fe underlayer on the strong in-plane magnetization of FePt magnetic thin film was investigated. This FePt thin film could be attained using the ultrathin Fe underlayer with 1nm thickness. The in-plane coercivity of FePt film with 20nm thickness grown on ultrathin Fe underlayer was high up to 7400Oe. However, its out-of-plane coercivity was extremely low to 350Oe compared to those of FePt thin films in other conventional studies. This result indicates that FePt thin film was strongly in-plane magnetized by ultrathin Fe underlayer. The strong ordering phase transformation kinetics and the high texturing to in-plane direction of the FePt thin film by ultrathin Fe underlayer were confirmed by Kinetics Monte Carlo (KMC) simulation and XRD measurement result, respectively. It is also supposed that they are associated with the reduction of an interface free energy between the film and the substrate with an introduction of ultrathin underlayer.

Interface structure of half-metallic Heusler alloyCo2MnSithin films facing an MgO tunnel barrier determined by x-ray magnetic circular dichroism

Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, we performed an element-specific investigation of the spin magnetic moments $({m}_{\text{spin}})$ of Mn and Co of ${\text{Co}}_{2}\text{MnSi}$ ultrathin layers grown on an Fe underlayer to stabilize the ferromagnetism and facing an MgO barrier. The experimentally observed dependence of the ${m}_{\text{spin}}$ values of Co and Mn on the ${\text{Co}}_{2}\text{MnSi}$ layer thickness in the ultrathin region is qualitatively in good agreement with the one theoretically obtained for a MnSi-terminated interface of the ${\text{Co}}_{2}\text{MnSi}$ layer facing an MgO barrier. This clarification would provide a strong basis for further advancement of applications of half-metallic Heusler alloy electrodes into spintronic devices.

Influences of underlayers on structure of TiO2 thin films prepared by radio frequency magnetron sputtering

In this study, the effects of several underlayers (Cu, Co, Ag and Fe) on the thin structure of TiO2 by RF magnetron sputtering were studied and compared. The results of X-ray diffraction (XRD) revealed that the Cu and Ag underlayer worsened the crystal quality of the TiO2 films. When the Fe underlayer was used, fabricated TiO2 film showed a mixture of rutile and anatase. The Co underlayer, on the other hand, induced TiO2 anatase polycrystalline structure. This study also compared the surfactant effect of a Ag layer on the TiO2 film structure. The results of XRD, X-ray photoelectron spectroscopy, and atomic force microscopy revealed that the Ag layer works as an effective surfactant for reducing the surface roughness and the rutile growth of TiO2 film on the Fe underlayer.

Effects of Pt and Fe underlayers on the microstructure and magnetization reversal of epitaxial FePt films for high areal density magnetic recording

The effects of underlayer on the microstructure and magnetic characterizations of epitaxial FePt films were studied. Enhanced out-of-plane coercivity and ordering were obtained in the Pt-underlayered FePt samples, which may be due to the proper lattice strain from the Pt underlayer. The larger lattice parameter of Pt may enlarge the a axis and reduce the c axis of the L10 FePt phase and result in low-temperature ordering. However, such an effect does not appear in Fe-underlayered samples because of the smaller lattice parameter and the considerable interdiffusion. The angular dependence coercivity revealed that the dominant reversal mechanism of the FePt films changed from domain-wall motion to rotation, which is caused by the formation of L10 island structures induced by the Pt underlayer. On the other hand, the Fe-underlayered FePt films showed a broad peak roughly at 45°, implying the tilt of the easy axis about 45° away from the film normal.

Enhancement of structural and magnetic properties in sputtered half-metallic Fe3O4 films

Polycrystalline Fe3O4 films grown on different thicknesses of Fe underlayers were prepared by dc reactive-magnetron sputtering in an Ar+O2 gas mixture and annealing under an infrared-lamp furnace system. Analyses of structural and magnetic properties revealed that a 15nm Fe underlayer can be oxidized to Fe3O4 by increasing the oxygen flow rates during initial Fe3O4 layer growth. These analyses also confirmed the stoichiometry and high-crystallographic quality of grown Fe3O4. Adopting this method, Fe3O4 films can be grown without any other elemental buffer layers.

Scanning tunneling microscopy observations of single-crystal Fe∕MgO∕Fe magnetic tunnel junctions

Scanning tunneling microscopy observations of a single-crystal Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junction which shows a large tunnel magnetoresistance effect were performed. Step-and-terrace structures of an epitaxial MgO barrier layer, which are very similar to surfaces of a bottom Fe underlayer, were observed. This indicates that the MgO terraces were grown flatly on each terrace of the Fe electrode. It is supposed that these atomically flat terraces enable a coherent tunneling of electrons through magnetic tunnel junctions.

Soft magnetic properties and microstructure of Fe65Co35 thin films with different underlayers

AbstractHigh‐moment soft magnetic Fe65Co35 thin films with well in‐plane uniaxial anisotropy were obtained by cosputtering on Co93Fe7, Ni80Fe20, Cu and Fe underlayers. The single‐layer Fe65Co35 film with thickness of 100 nm shows a magnetic anisotropy. The coercivity along easy axis H ce and that along hard axis H ch are equal to 3.3 kA/m and 2.1 kA/m, respectively. The soft magnetic properties of Fe65Co35 films were much improved by using an underlayer. The H ce and H ch of Fe65Co35 films with Ni80Fe20, Co93Fe7 and Cu underlayers can be reduced to 0.9–1.1 kA/m and 0.08–0.20 kA/m, respectively. For films with Fe underlayer, the H ce and H ch reduced to 1.1 kA/m and 0.6 kA/m, respectively. In the case of Co93Fe7, Ni80Fe20 and Fe underlayers, the coercivity of films did not depend remarkably on the thickness t of underlayer at t > 1 nm. While in the case of Cu underlayer, the coercivity of films changed remarkably with increasing underlayer thickness. It was shown that the texture of double‐layered underlayer/Fe65Co35 films is dependent on the material and the thickness of underlayer. The influence of the underlayer on the surface morphology and micro‐magnetic structure of films were investigated by AFM and MFM. It was suggested that the modification of the structure of Fe65Co35 films by using an underlayer is an important way to improve their soft magnetic properties. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Effect of Fe Underlayer on the Magnetic Properties of Fe<tex>$_3$</tex>Pt Thin Films

The role of Fe underlayer on the magnetic properties of Fe/sub 3/Pt has been studied. Thin films of Fe/sub 3/Pt are grown epitaxially onto Al/sub 2/O/sub 3/ (00 /spl middot/ 1) substrate by electron beam evaporation. These samples grown at around 400/spl deg/C show orientation and exhibit large 6-fold basal plane magnetic anisotropy (10/sup 7/ erg/cc) which could be enhanced by introducing Fe underlayer. Based on perpendicular hysteresis curves measured by magneto-optical polar Kerr effect and the anomaly of the perpendicular torque curves, it is interpreted that a large perpendicular magnetic anisotropy in Fe/sub 3/Pt thin films is generated by Fe underlayer.

Structure and magnetic properties of RF sputtered Fe–N films

Fe–N films with 20 nm thick Fe underlayers were deposited on Si(100) substrates by RF magnetron sputtering, and their structure and magnetic properties were investigated. It was found that the optimum nitrogen partial pressure for α″ phase formation is about 2 × 10−4 Torr, and the saturation magnetization measured at room temperature of the Fe–N films deposited at this pressure is as high as 2179 emu cc−1, which is much higher than that of pure iron. Film thickness dependence of the saturation magnetization shows that it initially increases with the increase in film thickness and reaches a maximum of ∼2179 emu cc−1 in the thickness range of 300–570 nm, and decreases with a further increase in film thickness.