Growth Of Fe Films Research Articles

Harnessing ion beam erosion engineering for controlled self-assembly and tunable magnetic anisotropy in epitaxial films

The engineering of the surface morphology and the structure of the thin film is one of the essential technological assets for regulating the physical properties and functionalities of thin film-based devices. This study presents an easy and handy approach to tailor the surface structure of epitaxial thin films utilizing low-energy ion beam. Here, we investigate the evolution of the surface structure and magnetic anisotropy (MA) in epitaxial Fe/MgO (001) model systems subjected to multiple cycles of ion beam erosion (IBE) after thin film growth. The growth of Fe film occurs in the form of three–dimensional islands and exhibits intrinsic biaxial MA. Following a few cycles of IBE, an induced uniaxial magnetic anisotropy leads to a split in the hysteresis loop, and the film displays almost uniaxial magnetic switching behavior. More distinctly, we present a clear and conclusive evidence of (2 × 2) reconstruction of the Fe surface due to the atomic rearrangement by IBE. Furthermore, 57Fe isotope sensitive nuclear resonance scattering measurement provides insight into the depth-resolved magnetic information due to the modified surface topography. We also demonstrate that thermal annealing can reversibly tune the surface reconstruction and induced UMA. The feasibility of the IBE technique by adequately selecting IBE parameters for surface structure modification has been highlighted apart from conventional tailoring of the morphology for the tuning of UMA and introduces a new dimension to our understanding of self-assembled surface morphology evolution by IBE.

Evidence of Delta Phase of Fe in MBE-Grown Thin Epitaxial Films on GaAs

Fe/GaAs is an important system for the study of spin injection behavior that can vary with the nature and interfaces of Fe films. Here, we investigate the effect of interfacial strain on the microstructure, interfaces and phase-formation behavior in epitaxially grown Fe films. To vary the strain, we have characterized Fe films of various thicknesses ranging from 10 to 1000 nm which were grown using molecular beam epitaxy on GaAs (011) and AlGaAs (001) substrates. High resolution X-ray diffraction studies revealed that films with higher thicknesses exhibited an equilibrium α-Fe phase, while the films with less than 10 nm thicknesses indicated the presence of δ-Fe. Transmission electron microscopy revealed the interface for 10-nm-thick films had strain lobes with no interfacial phase formation for films deposited at room temperature. At a higher deposition temperature of 175 °C, similar strain lobes were observed for a 10-nm-thick film. Extended annealing at 200 °C transformed the metastable δ-Fe phase to an equilibrium α-Fe. However, at higher temperature, the interface contained an intermixing layer of (FeAl)GaAs. We demonstrate that the interfacial strain plays a major role in stabilizing the metastable δ-Fe on GaAs.

In situ investigation of growth of Fe film on MgO using MOKE

Growth of Fe film on MgO is investigated using in situ MOKE for different surface conditions of MgO (001). Interfacial magnetic dead layer of Fe film on MgO is significantly affected by the presence of contaminations at surface of MgO. No magnetic dead layer is observed at the Fe/MgO interface, when the substrate was etched followed bzy annealing. However, a magnetic dead layer of 2 nm is observed at Fe/MgO interface when MgO substrate was just annealed. In order to observe the role of stoichiometry on interfacial properties, a near stoichiometric MgO film was prepared using e-beam evaporation. From x-ray photoelectron spectroscopy (XPS) measurements the stoichiometry of MgO film comes out to be slight O-deficient (Mg55O45). A high thick magnetic dead layer of 3.4 nm has been observed at Fe/MgO film interface, which may be attributed to weak off stoichiometric composition of MgO film.

Buffer layer dependence of magnetic anisotropy in Fe films grown GaAs substrate

The effect of buffer layer on the magnetic anisotropy of Fe film has been investigated by using the planar Hall effect measurement. The Fe films were grown on four different buffer layers, such as GaAs, ZnSe, ZnTe, and Ge, by molecular beam epitaxy. The field and angle dependent planar Hall effect measurements revealed the presence of two types of magnetic anisotropies, i.e., cubic crystalline anisotropy along the 〈100〉 directions and uniaxial anisotropy along the 〈110〉 crystallographic directions, in all four Fe films investigated in the study. However, the relative strength of the two types of anisotropies varies significantly depending on the type of buffer layer. Specially, in the Fe film grown on the Ge buffer layer, the uniaxial anisotropy almost disappeared and the entire magnetic anisotropy was dominated by cubic anisotropy with a four-fold symmetry. This suggests that the growth of Fe film on the Ge surface is different from its growth on other surfaces such as GaAs, ZnSe, and ZnTe buffers, where the asymmetric reconstruction process of anions (i.e., As, Se, or Te) is present at the surface.

Structure Related Magnetic Dead Layer for Ultrathin Fe/Ir(111) Films

Magnetic properties and surface structure of ultrathin Fe/Ir(111) films have been investigated using the surface magneto-optic Kerr effect and low-energy electron diffraction. Layer-by-layer growth of Fe/Ir(111) is observed for the first three monolayers at room temperature. For Fe thinner than three monolayers, pseudomorphic growth of Fe films is observed. The layer distance is close to that of fcc(111) Fe. For Fe thicker than three monolayers, the surface structure can be identified to be related to the bcc(110) arrangement of Fe atoms in Kurdjumov-Sachs orientation. As the Fe thickness increases, the linear increase of the Kerr intensity is observed. The Kerr intensity comes from the bcc-Fe and a thin magnetic dead layer is observed at the interface.

Growth evolution and superparamagnetism of ultrathin Fe films grown on GaN(0001) surfaces

AbstractWe have investigated the growth and magnetic properties of Fe films on wurtzite GaN(0001) in the ultrathin regime at room temperature (RT). As found from the coverage‐dependent reflection high‐energy electron diffraction patterns, the Fe film growth proceeds in the three‐dimensional Volmer–Weber (VM) mode at RT. Coexistence of ferromagnetic (FM) and superparamagnetic (SPM) phases has been observed at a Fe thickness of about 5 ML. The zero‐field‐cooling (ZFC) and field‐cooling (FC) magnetization measurements under a constant magnetic field with various temperatures have suggested a Fe blocking temperature (TB) of 120 K.

Reflection high-energy electron diffraction ϕ scans for in situ monitoring the heteroepitaxial growth of Fe on GaN(0001) by molecular beam epitaxy

The heteroepitaxial growth of Fe films on GaN(0001) by molecular beam epitaxy is monitored in situ by reflection high-energy electron diffraction ϕ scans. The complex epitaxial orientation-relationship between Fe and GaN can be visualized by these ϕ scans already at an Fe coverage of two monolayers. By comparing the data to ex situ x-ray ϕ scans, we show that these measurements even allow a quantitative determination of the in-plane orientation-distribution of the heteroepitaxial film. The orientation-distribution is minimized for a growth temperature of 350 °C.

Fe films grown on GaAs(110) in a two-step process: Improved structural and magnetic properties

Fe films of up to 10 ML thickness were grown on cleaved GaAs(110) in a two-step process that associates low-temperature deposition at 130K with a subsequent annealing to room temperature. Low-energy electron diffraction, scanning tunneling microscopy, and in situ magneto-optical Kerr effect were combined to study these films. The observed magnetic and structural properties are distinctly different from the characteristics of conventionally grown Fe films on GaAs. We found no indication of interface compound formation. The applied two-step growth process is a promising technique to further increase spin-injection and detection efficiencies of Fe∕GaAs hybrid structures.

Kinetic stabilization of Fe film on GaAs(1 0 0): An in situ X-ray reflectivity study

We study the growth of Fe films on GaAs(1 0 0) at a low temperature, 140 K, by in situ X-ray reflectivity (XRR) using synchrotron radiation. The XRR curves are well modeled by a single Fe layer on GaAs both at the growth temperature and after annealed at the room temperature. We found that the surface became progressively rougher during the growth with the growth exponent, β S = 0.43 ± 0.14. The observed β S is attributed to the restricted interlayer diffusion at the low growth temperature. The change of the interface width during growth was minimal. When the Fe film was annealed to room temperature, the surface smoothed, keeping the interface width almost unchanged. The confinement of the interface derives from that the diffusion of Ga and As proceeds via the inefficient bulk diffusion, and the overlying Fe film is kinetically stabilized.

Influence of Ag Underlayer on the Microstructure and the Magnetic Properties of Fe Thin Films

Fe thin films were deposited directly and via Ag underlayer on glass and MgO(100) substrates by MBE. Polycrystal Fe films grew on the glass substrate while single crystal films grew on the MgO(100) substrate. Fe film growth followed the Volmer-Weber mode for both cases. The Fe film structure was influenced by the surface roughness of Ag underlayer at the early stage of film growth. The relationships between the Fe thin film morphology and the magnetic properties are discussed.

Growth and magnetic properties of Fe films epitaxially grown on Pd/Cu(001) by pulsed laser deposition

We have grown Fe films on an epitaxial Pd monolayer on Cu(001) single crystals at room temperature, both Fe and Pd by pulsed laser deposition. The presence of the Pd interlayer influences growth, structure, and magnetism of the Fe films. Up to the thickness of 1.6 ML the Fe films show an out-of-plane easy axis of magnetization which changes to in-plane above. A linear dependence of the longitudinal Kerr rotation on the Fe thickness is obtained despite existing structural transformations. A strong increase of the Curie temperature TC is observed for the Fe films on Pd/Cu(001) compared to the Fe films grown directly on Cu(001). A correlation of the magnetic properties with possible structural changes and the role of Pd monolayer is discussed.

Formation of a body-centered-cubic Fe-based alloy at the Fe∕GaAs(001) interface

The room temperature epitaxial growth of Fe films on the As-rich GaAs(001)-(2×4) surface is studied using x-ray photoelectron spectroscopy as well as reflection high-energy electron diffraction and photoelectron diffraction. Interdiffusion mechanisms take place between Fe and GaAs during the deposition of the first 4 ML (0.7nm) Fe. The authors find that an Fe-based substitutional alloy with a body-centered-cubic structure confined on several atomic planes and containing 30% of foreign species (Ga and As atoms) sits at the Fe∕GaAs(001) interface. This intermixed layer is then buried by an almost pure Fe layer.

Thickness dependence of X-ray absorption and photoemission in Fe thin films on Si(0 0 1)

To investigate the initial growth of Fe films on Si(0 0 1) and the Fe/Si interface, Fe films at various thicknesses have been systematically studied by soft X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). The Fe L edge XAS spectrum shows a strong thickness dependence with broader line-width for thinner films. Detailed analysis of the Fe absorption signal as a function of the thickness shows that the broad linewidth of Fe L edge XAS spectra is mostly contributed by the first Fe layer at the Fe/Si interface. In contrast to XAS, Fe 2p photoemission spectra for these films are identical. However, valence band photoemission also shows a strong thickness dependence. Comparing the valence band photoemission spectra of the thin Fe/Si(0 0 1) films with that of pure Si and the thickest Fe film, the difference spectra at all thicknesses show almost identical shape indicating the same origin: the Fe/Si interface. Thus, it is mainly the first Fe layer at Fe/Si layer that is reactive with the Si substrate changing its electronic structure.

Structural and magnetic anisotropy properties in epitaxial Fe films on Al/sub 0.48/In/sub 0.52/As(001)

The structural and magnetic properties of epitaxial Fe(001) ultrathin films (0.9-17 nm) deposited by molecular beam epitaxy at room temperature on Al/sub 0.48/In/sub 0.52/As(001) layers have been studied by magneto-optical Kerr effect and X-ray diffraction (XRD) experiments. Fe grows in a body centered cubic (bcc) structure with epitaxial relationship Fe(001)<100>//Al/sub 0.48/In/sub 0.52/As(001)<100>. XRD measurements demonstrate a pseudomorphous growth of Fe films up to 4.4 nm and then the layers progressively relax with the Fe thickness. All Fe films are ferromagnetic at room temperature and show an in-plane magnetization with a fourfold anisotropy (with <100> as easy axes) superimposed to an uniaxial anisotropy (with [110] as easy axis). The uniaxial contribution finds its origin essentially in the interface anisotropy, as it is the case for the Fe/GaAs(001) system. We found that in the pseudomorphous range of the Fe films, the volume fourfold anisotropy constant is 60% larger than the bulk Fe value. We attribute this increase to the in-plane biaxial tensile strain in the films through magneto-elastic coupling effects.

Epitaxial growth of bcc Fe films on Al(1 0 0) surfaces using Ti as an interface stabilizer

An approach is described to promote epitaxial growth of thin metal films on single-crystal metal substrates by stabilizing the interface with an extremely thin metallic interlayer. A single atomic layer of a metal is deposited at the interface, Ti on Al(1 0 0) in this case, prior to the growth of the metal film of interest to produce an epitaxial interface in a system that is otherwise characterized by interdiffusion and disorder. The stabilized interface reduces interdiffusion and serves as a template for ordered film growth. Using Rutherford backscattering and channeling techniques along with low-energy electron diffraction and low-energy He + scattering, it is demonstrated that an atomically thin layer of Ti metal deposited at the Fe–Al interface, a system well known for considerable intermixing at room temperature, reduces interdiffusion and promotes the epitaxial growth of Fe films on the Al(1 0 0) surface. The decrease in ion scattering yield for Al atoms, Fe–Fe shadowing and long-range order of the surface suggest that the epitaxial growth of Fe is greatly improved when the Ti interlayer is introduced prior to Fe deposition. Off-normal ion channeling experiments provide clear evidence for the bcc structure of Fe on the Ti/Al(1 0 0) template with the measured average (1 0 0) interplanar distance of 1.44 Å in the Fe overlayer.

Magnetic anisotropies and dispersion relation of epitaxial Fe/InAs(001) films

Magneto-transport effects investigated for the application in some hybrid structures need also a careful magnetic characterization. In our studies the angular dependence of the uniform mode excited in ferromagnetic resonance was measured at various frequencies for epitaxially grown Fe films on a semiconducting InAs(001) substrate. The results enable a precise determination of the magnetic anisotropies in this system. A large out-of-plane twofold anisotropy (K 2⊥=2.8×10 6 erg/ cm 3) and an in-plane fourfold anisotropy (K 4∥=0.51×10 6 erg/ cm 3) were found. The latter appeared to be modulated by a small uniaxial term with the easy axis rotated by 45° with respect to the [100] direction, the easy axis of the fourfold anisotropy. Based on multi-frequency measurements the dispersion relation has been verified, i.e. the resonance field versus the microwave frequency has been plotted and compared with calculations.

Ion scattering studies of the formation and thermal stability of the Ti–Al interface

Promoting the ordered growth of component thin films with chemically abrupt interfaces for magnetic/non-magnetic metal/metal systems, such as Fe/Al, is an important problem in magneto-resistive device technology. The focus of the present investigation is the Ti–Al interface, a stable template for promoting epitaxial growth of Fe films. The present work was performed to characterize the formation and thermal stability of the Ti–Al interface and thereby provide a better understanding of the physical mechanisms underlying the success of the Fe–Ti–Al interlayer concept, and how it might be extended to other metal–metal systems. We investigated interface formation for thin Ti films deposited on Al(1 0 0) surfaces at room temperature using low-energy ion scattering spectroscopy and Rutherford backscattering. The results indicate that Ti does not grow in a simple layer-by-layer fashion for initial deposition on Al(1 0 0), but are instead more consistent with some combination of island growth and/or alloy formation at the interface. The Ti films are stable up to temperatures on the order of 673 K, at which point Ti diffuses into the Al lattice to occupy substitutional sites.

Using MeV ion backscattering/channeling and MC simulations to characterize the composition and structure of buried metal–metal interfaces

The technique of MeV ion backscattering and channeling has been used to study the interface structure and stoichiometry for thin transition metal films deposited on Al single crystal surfaces. Analysis of the backscattering data is based on the concepts of shadowing and the well-known theory of Rutherford scattering. In some cases we have used X-ray photoemission spectroscopy (XPS), low-energy (keV) He + ion scattering spectroscopy, and Monte Carlo computer simulations of interface evolution to obtain additional information about the structure of buried interfaces. We first describe the growth of epitaxial fcc Ti films on Al single crystal surfaces, and alloy formation at the epitaxial Ag–Al interface. We next describe measurements for alloy formation at the Ni–Al(1 1 0) interface and present Monte Carlo simulations of the interface evolution. Finally, we describe an approach to stabilize metal–metal epitaxial interfaces using an extremely thin metallic interlayer. Specifically, we show that a single ML of Ti metal deposited at the Fe–Al interface, a system well known for considerable intermixing at room temperature, prevents interdiffusion and enables the epitaxial growth of Fe films on the Al(1 0 0) surface. The resulting structure is observed to be stable for temperatures up to about 200 °C.

Thermal stability of thin Ti films on Al single crystal surfaces

Chemical roughness and alloy formation at metallic interfaces can significantly degrade the performance of multilayer thin film magnetic device structures. We have investigated the use of Ti interlayers, one or two atoms thick, to stabilize the interface for ordered growth of Fe films on Al(100), a system characterized by considerable interdiffusion at room temperature. The practicality of the interlayer concept is strongly coupled to the stability of the interlayer at elevated temperatures. In this investigation we have characterized the structure of thin Ti layers on Al single crystal surfaces as a function of temperature using Rutherford backscattering and channeling and low-energy ion scattering. The Ti layers are shown to be stable up to temperatures of about 675 K, at which point diffusion of Ti into the Al lattice occurs. Channeling measurements show that the Ti atoms sit on Al lattice sites as substitutional impurities. The stability of the Ti film appears to increase with the packing density of the Al surface, being slightly more stable for the close-packed Al(111) surface, and diffusing into the more open Al(110) surface at a lower temperature.

Temperature-induced structure instability and magnetism of Fe/Cu(100)

The structure and magnetism of epitaxially grown Fe films on Cu(100) at 300 K have been investigated in situ using high energy electron diffraction, low energy electron diffraction, scanning tunneling microscopy, and the magneto-optical Kerr effect. Based on a careful thickness calibration, the three well-known regions with distinct magnetic properties are determined accurately for Fe films on Cu(100). We claim that the traditional region II should be further divided into two parts II(1) and II(2), where the former remains region II in the traditional sense but the latter is a transition region between regions II and III. A theoretically predicted temperature driven Martensitic phase transition from face-centered-cubic (fcc) Fe to body-centered-cubic Fe is indeed realized in the transition region II(2).