Epitaxial Fe Films Research Articles

Computer simulation of Fe epitaxial films on a Cu(100) substrate

The article addresses the problem of modeling thin epitaxial iron films on a Cu (100) substrate by calculating the surface potential. The purpose of the simulation is to determine the film structure. The model uses the paired Lennard-Jones potential to describe the interatomic interaction. We calculate the surface potential of the copper substrate using the first 10 surface atomic layers. The iron atoms in the first layer are placed in the minima of the surface potential. The surface potential of the monoatomic iron layer and the 9 copper layers underneath determine the position of the second iron layer. The position of atoms in the following layers is formed on the basis of similar calculations. Calculations show that the copper substrate stabilizes the fcc crystal lattice in the iron film at low temperatures. Bulk iron samples at low temperatures have a bcc crystal lattice. The period of the iron lattice along the substrate is equal to the period of the copper lattice. The crystal lattice period of iron perpendicular to the substrate has an intermediate value between the values for copper and for γ-ferrum. The interface effect causes additional vertical deformations in the first two layers of iron above the substrate.

Growth temperature dependent magnetic properties of Fe film on BaTiO3

We report on the growth temperature effect on magnetic coercivity (HC) and saturation magnetization (MS) of epitaxial Fe films grown on BaTiO3 (BTO). A tensile strain is observed in Fe films grown on tetragonal BTO below the growth temperature of 120 °C, whereas a compressive strain is induced in Fe films grown on cubic BTO above the growth temperature of 120 °C. Combining the lattice mismatch compressive strain and tensile thermal strain, the strongest tensile strain in Fe30 and the compressive strain in Fe400 are opposite trend to the expectation, which may be due to the FeOx interface layer with larger lattice constant. The HC grew from 14.6 to 66.6 Oe as the growth temperature increased from 30 to 400 °C, while the MS reduced from 2129 to 1244 emu/cm3.

Temperature dependence of angular-dependent magnetoresistance in epitaxial Fe(001) film

Angular-dependent magnetoresistance (ADMR) of an epitaxial Fe(001) film is investigated with current along hard and easy axes in-plane at different temperatures. The temperature dependences of ordinary magnetoresistance (OMR), anisotropic magnetoresistance (AMR), and magnetocrystalline anisotropy magnetoresistance (MMR) in ADMR were determined. As temperature decreases, negative OMR and positive AMR with a twofold symmetry show increasing and decreasing amplitudes, respectively. The amplitude of MMR with twofold and fourfold symmetries is almost invariable with both temperature and magnetic field. Meantime, when current was applied at hard and easy axes, MMR contributes an anti-phase signal to ADMR. Our findings of OMR, AMR, and MMR in the Fe(001) ADMR are of great significance to the understanding of magnetoresistance in magnetic films.

Lattice dynamics of β−FeSi2 nanorods

Here we present a combined experimental and ab initio lattice dynamics study of the semiconducting $\ensuremath{\beta}$ phase of $\mathrm{Fe}{\mathrm{Si}}_{2}$. A polycrystalline $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ film was prepared on Si(111) and single-crystalline, self-assembled $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ nanorods were grown on Si(110) by molecular beam epitaxy. Both types of nanostructures were obtained by annealing of precursor structures, an epitaxial Fe film in the case of the film and high-aspect-ratio $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ nanowires in the case of the nanorods. The morphology and crystalline structure of the samples were investigated by reflection high-energy electron diffraction, atomic force microscopy, as well as x-ray diffraction and x-ray absorption spectroscopy. The Fe-partial phonon density of states (PDOS) was obtained from nuclear inelastic scattering. The PDOS of the film was investigated in the temperature range of 296 K down to 11 K and shows an excellent agreement with the ab initio calculations. In the PDOS of the nanorods, a shift in the number of states in the main features and an additional vibrational mode at 20 meV are observed. While the first effect can fully be explained by the specific orientation of the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ unit cell on the Si(110) surface, the second effect is attributed to the formation of an $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}$ interlayer at the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Si}}_{2}/\mathrm{Si}$ interface. Furthermore, the thermoelastic properties of the film show a harmonic behavior in the investigated temperature range. For the nanorods, no significant deviation from the film is observed, except for a small decrease of the sound velocity.

Propagating backward-volume spin waves in epitaxial Fe films

The propagation characteristics of backward-volume magnetostatic spin-waves in epitaxial Fe(001) films were studied by frequency-domain and time-domain spin-wave propagation spectroscopies using a vector network analyser. Due to the combination of cubic-magnetocrystalline anisotropy and anisotropic spin-wave dispersion, the backward-volume spin-wave exhibited a complicated packet propagation. For the hard-axis propagation, the group velocity of the spin wave was greatly enhanced at low external magnetic fields and propagation occurred even under no magnetic field. By analysing within a theoretical model and micromagnetic simulations, these transmission character of the backward-volume magnetostatic spin-waves in an epitaxial iron film was well reproduced. The observed characteristics are essential information to promote two-dimensional magnonic devices utilizing cubic-anisotropic materials.

Second-order magnetoelastic effects: From the Dirac equation to the magnetic properties of ultrathin epitaxial films for magnetic thin-film applications

Abstract It has always been the endeavour of Professor Kronmüller to combine various methods in order to attain a comprehensive understanding of a physical phenomenon on all relevant scales. In this spirit, we combine the phenomenological nonlinear theory of magnetoelasticity with the relativistic density functional electron theory to describe the magnetoelastic behaviour of epitaxial films. We explain the two already performed cantilever bending beam experiments on the magnetoelasticity and underpin the assumption that second-order magnetoelastic effects are very important for epitaxial Fe films. A complete set of six cantilever experiments is introduced which allows to calculate the intrinsic first- and second-order magnetoelastic constants for cubic materials, and these constants are calculated ab initio for Fe, face-centred cubic Co, Ni, Ni3Fe and CsCl-type CoFe.

Modification of the anomalous Hall effect in Fe films by [formula omitted]-doping Bi impurities with strong spin-orbit coupling

Modification of the anomalous Hall effect (AHE) in epitaxial Fe films with δ-doping of Bi has been investigated by the multivariable scaling. We observe that the intrinsic contribution of the AHE and the side jump coming from dynamic scatterings are robust despite of the strong spin–orbit coupling of Bi impurities. However, the skew scattering and the side jump coming from static scatterings have noticeable changes due to the δ-doping of Bi.

Magnetic Damping in Epitaxial Iron Alloyed with Vanadium and Aluminum

To develop low-moment, low-damping metallic ferromagnets for power-efficient spintronic devices, it is crucial to understand how magnetic relaxation is impacted by the addition of nonmagnetic elements. Here, we compare magnetic relaxation in epitaxial Fe films alloyed with light nonmagnetic elements of V and Al. FeV alloys exhibit lower intrinsic damping compared to pure Fe, reduced by nearly a factor of 2, whereas damping in FeAl alloys increases with Al content. Our experimental and computational results indicate that reducing the density of states at the Fermi level, rather than the average atomic number, has a more significant impact in lowering damping in Fe alloyed with light elements. Moreover, FeV is confirmed to exhibit an intrinsic Gilbert damping parameter of $\simeq$0.001, among the lowest ever reported for ferromagnetic metals.

Anomalous Hall magnetoresistance in single-crystal Fe(001) films

The angular-dependent magnetoresistance (MR) in epitaxial Fe(001) films grown on MgO(001) is systematically investigated at room temperature. The resistivities with in-plane magnetic fields parallel (ρx) and transverse (ρy) to a current and a perpendicular field (ρz) show a correlation of ρz ≈ ρx > ρy for Fe film thickness (dFe) between 3 and 50 nm, which is distinctly different from the conventional anisotropic MR relation of ρx > ρy ≈ ρz. The dependence of such unusual MR on dFe is quantitatively explained by the competition between the anomalous Hall MR, intrinsic anisotropic MR, and the MR induced by the geometrical size effect in Fe films. Our results also reveal the strong four-fold symmetric terms in the measured angular-dependent MR with a linear dependence of 1/dFe.

Large anisotropy of magnetic damping in ultrathin epitaxial Fe/GaAs (0 0 1) film

The magnetization dynamics of ultrathin epitaxial Fe films on GaAs (0 0 1) with different thicknesses have been investigated by all-optical time-resolved magneto-optical Kerr effect. For the Fe film with thickness of 8 monolayers, the magnetic damping constant along orientation increases by 66% compared with that along orientation, showing the uniaxial anisotropy of magnetic damping. By the measurement of angular dependent time-resolved magneto-optical Kerr effect, the uniaxial magnetic damping is clearly correlated to the in-plane uniaxial anisotropy field of the Fe film. In addition, the anisotropy of the damping constants is found to disappear for the Fe films thicker than 15 monolayers, suggesting that the anisotropic damping originates from the interfacial effect between Fe and GaAs.

Recovering in-plane six-fold magnetic symmetry of epitaxial Fe films by N<sup>+</sup> implantation

In order to study the effect of ion implantation on the in-plane magnetic anisotropy of epitaxial magnetic films, a 3-nm Al buffer layer is epitaxially grown on an Si (111) substrate with a miscut angle, and then 25-nm Fe is grown on the buffer layer. High-resolution X-ray diffraction reveals that the epitaxial Fe film has a (111)-oriented bcc structure. The epitaxial Fe films are implanted by 10 keV N<sup>+</sup> ions with dose up to 5 × 10<sup>16</sup> ions/cm<sup>2</sup>. The change and mechanism of the in-plane magnetic anisotropy of the epitaxial Fe film are studied systematically. It is found that the in-plane magnetic anisotropy of the epitaxial Fe film is gradually changed from two-fold to six-fold symmetry with the increase of N<sup>+</sup> implantation dose. It is confirmed by transmission electron microscopy and etching experiments that ion implantation changes the surface and interface state of Fe film. This result is consistent with the result from the SRIM software simulation. The in-plane magnetic uniaxial anisotropy of epitaxial Fe film comes from atomic steps at the surface and the interface of the Fe film. These steps result from Si (111) substrate with a miscut angle. Ion implantation has effects on sputtering and atomic diffusion. The sputtering effect causes the step at the surface of the Fe film to be erased, and the diffusion of the atom leads the step at the interface of the Fe film to disappear. The in-plane uniaxial anisotropy induced by the atomic step is weakened, and the magnetocrystalline anisotropy induced by the Fe (111) plane is dominant. Therefore, the epitaxial Fe film exhibits Fe (111) plane induced six-fold magnetic symmetry after high-dose N<sup>+</sup> implantation. This work indicates that the in-plane magnetic anisotropy of Fe films epitaxially grown on Si (111) substrate with miscut angle can be modified and precisely controlled by ion implantation. This work may be of practical significance for improving the density of in-plane magnetic recording material.

Interface-induced spiral magnetic structure of epitaxial Fe films on GaAs(001)

We investigated the magnetic structure in epitaxial Fe films on GaAs(001) by taking advantage of planar Hall effect combining with static and dynamic magnetization measurements. The depth dependence of the magnetic structure was evidenced as a result of competition between in-plane interfacial uniaxial and bulk cubic magnetic anisotropies. The competing results exposed by these techniques allow us to image a spiral magnetic structure nearby the interface of the Fe/GaAs(001) system. This work provides an insight for electrical and magnetic properties in an ultrathin hybrid ferromagnet/non-magnet system.

Thickness Dependence on the Magnetism in Mo-Capped Epitaxial Fe Films

When magnetic metal films are oxidized, in many cases, their saturation magnetization values decrease. The importance of high magnetization is well-known because it is directly related to the maximum energy product. Thus, prevention of oxidation in magnetic metal films via capping is important not only for studying the magnetism in magnetic metal films but also for developing new packaging technology for such films. In this research, we successfully grow epitaxial (110) Fe films on (0001) Al2O3 substrates by using radio-frequency (RF) magnetron sputtering. We capped 10-nm-thick Mo layers on the Fe films to prevent oxidation. By varying the thickness of the films, we systematically observed the changes in both the coercivity and the saturation magnetization. Especially, when the film’s thickness was below 8.5 nm, the coercivity of the film started to decrease. We believe the drastic change in the coercivity appeared in the Fe films when the film’s thickness approached the critical domain size for a magnetic domain transition.

Structure and magnetic properties of an epitaxial Fe(110)/MgO(111)/GaN(0001) heterostructure

We present the structural and magnetic properties of fully epitaxial Fe(110)/MgO(111)/GaN(0001) tunnel barrier structures grown by molecular beam epitaxy. In-situ reflection high-energy electron diffraction and ex-situ X-ray diffraction measurements indicate epitaxial Fe(110) films on top of an epitaxial 2 nm MgO(111) tunnel barrier on GaN(0001). X-ray reflectivity measurements confirm a roughness of approximately 0.3 nm and 0.7 nm for the MgO/GaN and the Fe/MgO interfaces, respectively. Results of in-situ magneto-optical Kerr effect measurements indicate that 1 nm thick Fe film shows signs of in-plane ferromagnetism at room temperature. Vibrating sample magnetometer measurements determine the saturation magnetisation of the 5 nm thick film to be 1660 ± 100 emu/cm3 and show that this system has a predominant uniaxial anisotropy contribution despite the presence of cyclic twinned crystals. We estimate the values of effective uniaxial (KUeff) and cubic (K1eff) anisotropy constants to be 11700 ± 170 erg cm−3 and −3300 ± 700 erg cm−3 by fitting the angular dependence of the magnetising energy.

Temperature dependence of magnetic properties of ultrathin Fe thin films on GaAs substrates

The magnetic properties of epitaxial Fe films on GaAs(001) in the initial stage have been investigated by means of XMCD and SQUID. Fe with a layer of thickness of 5 ML shows a magnetic property above RT. It is also found that the ferromagnetism character is still existed above the TB (the temperature giving relaxation time comparable to the measurement time is defined as the blocking temperature) but vanishing at RT. The conclusion is that a weaker exchange interaction in the Fe thin films on GaAs (001) and the intrinsically magnetically dead layers at the interface has been excluded. The superparamagnetic character coexists with ferromagnetism above the TB at the initial growth stage.

A photoemission spectroscopy study of the initial oxidation of epitaxial fcc and bcc Fe films grown on Cu(100)

The initial oxidation of Fe is a fundamental issue due to its importance for corrosion and catalysis. In this study, we investigate the initial oxidation of epitaxial fcc and bcc Fe thin films grown on a Cu(100) substrate at room temperature. Fe thin films deposited with two different thicknesses on Cu(100) were prepared by molecular beam epitaxy in order to obtain the fcc Fe(100) and bcc Fe(110) surfaces. The structures and the chemical composition of the Fe films were evaluated by LEED and photoelectron spectroscopy, using the facilities at the TEMPO beamline of the Synchrotron SOLEIL. The Fe 2p and O 1s high-resolution spectra were collected during oxidation at a constant O2 pressure in order to build the kinetic curves, which are discussed taking into account the Fromhold-Cook theory. Our result revealed the different initial oxidation kinetics of the films and showed that bcc Fe(110) surface has a much higher reactivity with O2 than the fcc Fe(100). The Fe 2p peak analysis suggests the formation of Fe1−xO and an Fe3+-rich oxide on the bcc Fe(110) surface and Fe1−xO on the fcc Fe(100) surface.

Tailoring the magnetodynamic properties of nanomagnets using magnetocrystalline and shape anisotropies

Magnetodynamical properties of nanomagnets are affected by the demagnetizing fields created by the same nanoelements. In addition, magnetocrystalline anisotropy produces an effective field that also contributes to the spin dynamics. In this article we show how the dimensions of magnetic elements can be used to balance crystalline and shape anisotropies, and that this can be used to tailor the magnetodynamic properties. We study ferromagnetic ellipses patterned from a 10 nm thick epitaxial Fe film with dimensions ranging from 50 x 150 nm to 150 x 450 nm. The study combines ferromagnetic resonance (FMR) spectroscopy with analytical calculations and micromagnetic simulations, and proves that the dynamical properties can be effectively controlled by changing the size of the nanomagnets. We also show how edge defects in the samples influence the magnetization dynamics. Dynamical edge modes localized along the sample edges are strongly influenced by edge defects, and this needs to be taken into account in understanding the full FMR spectrum

In-plane sixfold symmetry forα-Fe(110) on GaN{0001}: Measurement of the cubic anisotropy constantK3of Fe

We investigate the magnetic anisotropy of epitaxial Fe films on GaN(0001) and $\mathrm{GaN}(000\overline{1})$. Due to the particular orientation relationship between Fe and GaN{0001}, the leading term proportional to ${K}_{1}$ does not contribute to the magnetic anisotropy, allowing us to determine ${K}_{3}$ with unprecedented accuracy. Using two experimental techniques, we obtain values for ${K}_{3}$ of $4.7--9.2\ifmmode\times\else\texttimes\fi{}{10}^{5} \mathrm{erg}/\mathrm{cm}{}^{3}$, i.e., comparable to and even larger than ${K}_{1}$.

The anomalous Hall effect in epitaxial Fe(110) films grown on GaAs(110)

The anomalous Hall effect in epitaxial Fe(110) films grown on GaAs(110) is investigated as a function of both film thickness and temperature. The Berry curvature-induced intrinsic contribution of 996Ω−1 cm−1 is determined experimentally for the first time. Together with 821Ω−1 cm−1 in Fe(111) and 1100Ω−1 cm−1 in Fe(001) obtained earlier, we show unambiguously the anisotropy of the Berry curvature contribution to the anomalous Hall effect in single-crystal Fe.

Tailoring of magnetic properties of ultrathin epitaxial Fe films by Dy doping

We report on the controlled modification of relaxation parameters and magnetic moments of epitaxial Fe thin films through Dy doping. Ferromagnetic resonance measurements show that an increase of Dy doping from 0.1% to 5% gives a tripling in Gilbert damping, and more importantly a strongly enhanced anisotropic damping that can be qualitatively understood through the slow-relaxing impurity model. X-ray magnetic circular dichroism measurements show a pronounced suppression of the orbital moment of the Fe with Dy doping, leading to an almost threefold drop in the orbital to spin moment ratio, ml/ms. Doping with Dy can therefore be used to control both dynamic and static properties of thin ferromagnetic films for improved performance in spintronics device applications, mediated through the antiferromagnetic interaction of the 4f and 3d states.