Single Crystal Fe Films Research Articles

A pathway to synthesizing single-crystal Fe and FeCr films

Nuclear reactor environments provide a unique scientific and engineering challenge wherein materials must tolerate prolonged exposure to concurrent irradiation, elevated temperatures, and corrosive media. However, uncontrolled variability in material composition and structure often prohibits truly single-variable experiments that can reveal basic aspects of environmental damage. Magnetron sputtering is used here to provide a more controlled model system for these fundamental studies, yielding reproducible single-crystal Fe and FeCr thin films containing 8 and 18 at.% Cr. Electron microscopy is used to determine the systematic correlations between growth conditions and the resulting film microstructure and surface morphology. It is found that the substrate temperature and applied radio frequency (RF) bias can be tuned to obtain consistent homogeneous and single crystal films with a minimal amount of Ar impurities from the RF bias process. Epitaxial, single-crystal Fe films are obtained on MgO substrates at 500 °C with 10 Watt (W) RF bias deposition. However, when Cr is alloyed with Fe, higher substrate temperatures (600 °C) and applied RF biases (15 W) are required to achieve a similar epitaxial single-crystal FeCr film. Accelerated molecular dynamics simulations reveal that Cr impedes surface transport, explaining the need for higher temperature and bias during the growth of the Cr-bearing films.

Crystallographic and magnetostriction properties of Fe and FeB-alloy thin films formed on MgO(100) single-crystal substrates

Fe(100)bcc single-crystal film, Fe-B amorphous film, and Fe-B film consisting of a mixture of epitaxial bcc(100) crystal and amorphous are prepared on MgO(100) single-crystal substrates. The influence of crystallographic property on the magnetostriction behavior under rotating magnetic fields is investigated. The output waveform of magnetostriction is sinusoidal for the amorphous film, whereas that of single-crystal film shows a triangle shape. 90° magnetic domain walls are observed for the single-crystal Fe film and the film shows a four-fold symmetry in in-plane magnetic anisotropy. The observation of triangle waveforms is related to the domain wall motion in magnetically unsaturated Fe(100)bcc film under rotating magnetic fields. A distortion from triangle wave is observed for the Fe-B film consisting of a mixture of bcc-crystal and amorphous. The magnetostriction behavior is influenced by the magnetization structure.

Magnetic anisotropy and magnetization reversal of ultrathin iron films with in-plane magnetization on Si(111) substrates

The magnetic anisotropy and magnetization reversal of single crystal Fe films with thickness of 45 monolayer (ML) grown on Si(111) have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM). Owing to the significant modification of the energy surface in remanent state by slight misorientation from (111) plane and a uniaxial magnetic anisotropy, the azimuthal angular dependence of in-plane resonance field shows a six-fold symmetry with a weak uniaxial contribution, while the remanence of hysteresis loops displays a two-fold one. The competition between the first and second magnetocrystalline anisotropies may result in the switching of in-plane easy axis of the system. Combining the FMR and VSM measurements, the magnetization reversal mechanism has also been determined.

A broadband ferromagnetic resonance spectrometer to measure thin films up to 70 GHz

We report the development of a broadband ferromagnetic resonance (FMR) system operating in the frequency range from 10 MHz to 70 GHz using a closed-cycle He refrigeration system for measurements of thin films and micron/nano structures. The system is capable of carrying out measurements in frequency and field domain. Using two coplanar waveguides, it is capable of simultaneously measuring two samples in the out of plane and in plane FMR geometries. The system operates in the temperature range of 27-350 K and is sensitive to less than one atomic monolayer of a single crystal Fe film.

The effect of interelement dipole coupling in patterned ultrathin single crystal Fe square arrays

The correlation between the magnetic properties and the interelement separation in patterned arrays of ultrathin single crystal Fe films of 12 monolayers (ML) grown on GaAs(100) has been studied. The critical condition to form single domain remanent states in the square elements was found to be 10 μm in size and 20 μm for the interelement separation. The coercivity was also found to increase with the increasing interelement separation in the patterned arrays. These results are attributed to the competition between the large in-plane uniaxial anisotropy, the demagnetizing field, and interelement dipole coupling as determined semiqualitatively by the ferromagnetic resonance measurements.

An interface study of crystalline Fe/Ge multilayers grown by molecular beam epitaxy

Fe/Ge multilayers were grown on single crystal Ge(001) substrates by molecular beam epitaxy. The structural, electronic and magnetic properties of Fe/Ge have been studied. The analysis shows that Fe grows in a layer-by-layer epitaxial growth mode on Ge(001) substrates at 150°C and no intermixing has been observed. Growth of a crystalline Ge film at 150°C on a single crystal Fe film has been observed. At this temperature Ge films grow by means of the island growth mode according to reflection of high energy electron diffraction patterns. Fe layers of 36nm thickness, deposited at 150°C on Ge(001) substrates, show two magnetization reversal values indicating the growth of Fe in two different crystal orientations. 36nm thick Fe and Ge layers grown at 150°C in Ge/Fe/Ge/Fe/Ge(001) sequence shows ferromagnetic behavior, however, the same structure grown at 200°C shows paramagnetic behavior.

Single crystal Fe elements patterned by one-step selective chemical wet etching

A technique has been developed to pattern single crystal ultrathin Fe films by selective chemical wet etching of the Au capping layer and then simultaneous oxidization of the ferromagnetic Fe layer underneath. The focused magneto-optical Kerr effect and ferromagnetic resonance measurements demonstrate that the intrinsic magnetic anisotropy has not been changed in the patterned elements, showing that the chemical bonding at the metal–semiconductor interface remains the same. Further x-ray energy dispersive spectroscopy measurements show that this selective wet-etching technique is suitable for the patterning of thin Fe films with thicknesses less than around 25 ML.

A study on ferromagnetic resonance linewidth of single crystalline ultrathin Fe film grown on GaAs substrate

We report ferromagnetic resonance studies on linewidth of single crystal ultrathin Fe films on GaAs(100). For different orientations of the magnetic field in the film plane, anisotropic linewidth has been observed. An expression for anisotropy of linewidth with isotropic damping was derived and also an expression for anisotropic damping constant was proposed. With these two expressions the data of the angular dependence of linewidth of a 5.2 ML (monolayer) film were fitted well. For the thicker film of 25 ML, the fitting of experimental angular dependence of linewidth could be improved if an inhomogeneous broadening is further considered.

Magnetic anisotropy engineering in square magnetic elements

Square magnetic elements with side in the 100–500 nm range have been fabricated using the focused ion beam (FIB) milling technique from a 10 nm thick, single-crystal Fe film, epitaxially grown on MgO(0 0 1). Thanks to the good crystal quality of the film, magnetic elements with well-defined magnetocrystalline anisotropy have been prepared, while the fine control of the size and shape of the magnets allows for the effective engineering of the anisotropic behavior of the magnetostatic energy that determines the so-called configurational anisotropy. Micromagnetic calculations and experiments show that the angular dependence of the transverse susceptibility has a strong dependence on the material parameters as well as on the static applied field. This allows the effective engineering of the total anisotropy of the magnets.

Single crystal Fe films grown on Ge (001) substrates by magnetron sputtering

Single crystal Fe films were grown on Ge (001) substrates by using dc magnetron sputtering. It was found that the microstructures and magnetic properties of Fe films on Ge substrates were strongly dependent upon the substrate temperature during the deposition process. There existed a narrow substrate temperature window of 125±25°C for achieving single crystal Fe film on Ge. Lower substrate temperature led to polycrystalline Fe films due to limited mobility of Fe atoms, while higher substrate temperatures resulted in amorphous Fe–Ge alloy due to severe interdiffusion.

Increase of magnetic damping in thin polycrystalline Fe films induced by Cu/Fe overlayers

The ferromagnetic resonance properties of thin polycrystalline layers in the sequence Cu/Fe/Cu/Fe/Cu, grown on Si wafers, were studied. Fe grown on Cu can have a very narrow ferromagnetic resonance (FMR) linewidth. Similar structures are of interest for spin transport studies and for giant magnetoresonance applications. The thinner Fe underlayer ranged from 2 to 5 nm, the intermediate Cu spacer thickness from 2 to 10 nm, and the Fe outer layer was 20 nm thick. The increased damping of the thinner Fe layer in this structure, as reflected in the FMR linewidths observed at 9.46 and 33.5 GHz, is similar to that predicted by Berger and recently observed experimentally using epitaxial single crystal Fe films. In addition to confirming the 1/(Fe layer thickness) dependence of the additional damping, the present measurements show a decrease of damping with increasing Cu spacer thickness, implying a short spin transport decay length in our Cu of about 3.5 nm. The linewidth in isolated Cu/Fe/Cu layers also increased with decreasing Fe thickness, as predicted and observed in some other cases. Magnetization data indicate negligible magnetic coupling of the Fe layers except for the smallest interlayer Cu thicknesses used. The linewidths studied increased moderately with cooling to 77 K.

X-ray diffraction and ferromagnetic resonance study of sputtered (110) GaAs/Fe epitaxial films

A series of epitaxial (110) Fe films with thicknesses from 5 to 164 nm were deposited on (110) GaAs substrates by rf magnetron sputtering. X-ray diffraction pole figures, in-plane φ-scan measurements, and ferromagnetic resonance linewidths show decreasing crystal quality with increasing thickness. In-plane and out-of-plane θ–2θ scans show a tensile stress decreasing with film thickness. Magnetic crystalline anisotropy and effective in-plane uniaxial anisotropy values were obtained from calculations of the free energy of a (110) cubic crystal and curve fitting of resonance measurements. The quality of the films thinner than 84 nm is close to the reported values of single crystal (110) Fe films deposited by molecular-beam epitaxy systems.

Evolution of magnetic anisotropy in epitaxial Fe films by ferromagnetic resonance

Single-crystal Fe films of 4 to 33 monolayers (ML) on GaAs (100) prepared by molecular-beam epitaxy were studied by ferromagnetic resonance (FMR). The evolution of both in-plane and out-of-plane anisotropies was observed. The FMR data show a predominant in-plane uniaxial magnetic anisotropy in the films with the thickness t less than 12.7 ML, with an easy axis along the 〈11_0〉 direction. An in-plane fourfold anisotropy due to cubic magnetocrystalline anisotropy starts to appear and coexists with uniaxial magnetic anisotropy when t=8.4 ML, and increases with increasing film thickness. For t=33 ML, the cubic anisotropy constant K1 reaches 28.2×104 erg/cm3, which is 60% of the value for bulk bcc Fe. A strong perpendicular anisotropy in the ultrathin Fe films was observed. For t=4 ML, the perpendicular anisotropy constant is as high as 14.6×106 erg/cm3. It decreases with increasing the thickness t and reaches about 2.1×106 erg/cm3 for 33 ML, while a small fourth-order perpendicular anisotropy appears and increases with the same trend as the cubic anisotropy.

Temperature dependence of anisotropy in exchange biased Fe/KCoF3 system

We used molecular beam epitaxy to deposit a novel ferro-/antiferromagnet (Fe/KCoF3) system on gallium terminated GaAs (100) substrates. We varied the thicknesses of single crystal Fe (001) layers from 1.05 to 3 nm. The antiferromagnetic fluoride, with a thickness of 30 nm, was deposited either in a single-crystal or a polycrystalline form, depending on the deposition conditions. KCoF3 is an antiferromagnet with a Néel temperature of 114 K. Its cubic structure almost perfectly matches the Fe film structure. The growth was monitored by reflection high energy electro diffraction. The magnetic properties of the system were studied using ferromagnetic resonance. Uniaxial and unidirectional anisotropy fields, due to exchange bias, were measured at low temperatures in the field-cooled samples and were smaller than 45 and 72 Oe, respectively. Both anisotropy fields, unidirectional and uniaxial, decreased with increasing thickness of the Fe film or with increases in temperature. The temperature dependence of the fourfold anisotropy was studied for the samples with different thicknesses of Fe films and different crystalline states of the fluoride layer. Room temperature values of the fourfold anisotropy also increased with increasing thickness of the Fe layer and were in a fairly good agreement with previously reported values for single crystal Fe films. Also, the temperature characteristics of the fourfold anisotropy for the structures with single crystal fluoride seemed to reproduce low-temperature literature data for Fe (001) films. In contrast, the structures with polycrystalline KCoF3 demonstrated a significant enhancement of the fourfold anisotropy at low temperatures. For the thinnest sample (tFe=1.05 nm) the low temperature anisotropy is more than tripled compared to room temperature values. It is believed that the surface effects can be responsible for this enhancement.

Optical property of Fe/GaAs(0 0 1) hybrid structures grown by molecular beam epitaxy

We report on the epitaxy of Fe thin films on GaAs(0 0 1) using molecular beam epitaxy with two new growth methods which optimize the interface quality for spin injection. These methods make use of low temperature deposition at −150°C and/or of an ultra-thin Al interlayer, respectively. High quality, epitaxial single crystal Fe films have been obtained using these methods. The Fe films have easy axes along [1 1 0] with square hysteresis loops and hard axes along [ 1 ̄ 1 0] . Photoluminescence of an Al 0.3Ga 0.7As/GaAs quantum well in close proximity to the Fe film is used to determine the quality of the Fe/GaAs interface.

Properties of a Fe/GaAs(001) hybrid structure grown by molecular-beam epitaxy

We report on the epitaxy of Fe thin films on GaAs(001) using molecular-beam epitaxy with two different growth methods aimed at suppressing Fe and GaAs interdiffusion. These methods make use of low-temperature deposition at −150 °C and/or of an ultrathin Al interlayer, respectively. Good-quality single-crystal Fe films were obtained. The magnetic properties of the Fe films show square hysteresis loops and clear in-plane magnetic anisotropy with well-defined easy hard axes. The photoluminescence of an Al0.3Ga0.7As/GaAs quantum well in close proximity to the Fe film is measured in order to examine the quality of the Fe/GaAs interface.

Ferromagnetic metal/semiconductor heterostructures for magneto-electronic devices

We have successfully grown single crystal Fe films on InAs(100) and InAs(graded)/GaAs(100) substrates using molecular beam epitaxy. In situ magneto-optical Kerr effect (MOKE) and ex situ alternating gradient field magnetometry (AGFM) measurements show that the films have well defined magnetic properties, and I– V measurements in the temperature range 2.5–304 K show that Fe forms an ohmic contact on InAs. This demonstrates that Fe/InAs is a very promising system for use in future magneto-electronic devices as it has both favorable magnetic and electrical properties. We also show that with careful substrate preparation and suitable growth conditions Fe/GaAs films do not exhibit a magnetically `dead' layer at the interface. A spin-polarized field effect transistor based on Fe/InAs/GaAs has been proposed, which could operate using either an external electric field or an external magnetic field.

Magnetization reversal in (0 0 1)Fe thin films studied by combining domain images and MOKE hysteresis loops

The magnetization reversal of epitaxial single-crystal Fe films has been studied by combining domain images and hysteresis loops. The reversal is quantitatively described by combining the coherent rotation model and the domain wall displacement model. The pinning energy exerted on the domain walls and the domain wall angle at the switching fields are obtained by fitting this model to experimental hysteresis loops. The field-dependent pinning energy and the domain wall angle in the reversal process, and the contributions of second-order magneto-optic effect to hysteresis loops, are revealed to be two important features of single-crystal Fe films.

Fabrication and magnetic properties of electron beam lithography patterned arrays of single crystals

The fabrication method of arrays of iron squares separated by submicrometric distances is presented. The magnetic elements are defined on a single crystal Fe(001) film by electron beam lithography followed by a ion beam etching process. The obtained Fe squares present smooth surfaces and homogeneous sizes over areas as large as 500 μm x 500 μm as analyzed by scanning electron and atomic force microscopies. The lateral size of the squares have been varied in the range 1–10 μm in order to analyze the size dependence. The magnetic properties of the single crystalline squares have been studied by magnetooptical Kerr effect. The results reveal an important change in the hysteresis loops as the square dimensions are reduced below a threshold of 3 μm.

Structure and magnetic properties of epitaxial Fe films on GaAs(1 0 0) and InAs(1 0 0)

Single crystal BCC Fe films have been stabilized on both GaAs(1 0 0)-4×6 and InAs(1 0 0)-4×2 substrates at room growth temperature. The magnetic properties of the Fe/GaAs were found to proceed via three phases determined by the growth morphology; a non-magnetic phase, a short-range-ordered superparamagnetic phase and a ferromagnetic phase above about five monolayers. A uniaxial magnetic anisotropy (UMA) was observed in ultrathin Fe/InAs(1 0 0)-4×2 films within a narrow thickness range of about 5–10ML. The easy axis is along the [0 1 1] direction rather than [0 1 ̄ 1] , the easy axis in Fe/GaAs(1 0 0)-4×6 and 4×2. As ultrathin Fe films tend to be compressed in-plane on GaAs and expanded on InAs, the strikingly different anisotropy behaviour in the Fe/GaAs(1 0 0) and the Fe/InAs(1 0 0) may indicate the importance of magneto-elastic interactions.