Epitaxial Fe3Si Research Articles

Electric field-controlled deterministic magnetization reversal in nanomagnet Fe3Si/PMN-PT multiferroic heterostructures

Pure electric field-controlled 180° magnetization switching plays a vital role in low-power magnetoelectric memory devices. Using micromagnetic simulation, we engineered a square-shaped epitaxial Fe3Si nanomagnet on a PMN-PT piezoelectric substrate to make the magnetic easy axis slightly deviate 18° from the piezostrain axis, aiming to break the symmetry of the magnetization distribution and achieve deterministic magnetization reversal paths. Under the coaction of a magnetic field and an electric field, the simulated magnetic hysteresis loops and magnetic domain patterns reveal a fourfold to twofold magnetic anisotropy transition and magnetization reversal paths. Stimulated by pure electric field-induced piezoelectric strain, deterministic 180° magnetization reversals are accomplished by the two successive clockwise 90° switching process. The results help to comprehend electrically regulated deterministic magnetization reversal and pave an avenue for designing multistate spintronics devices.

Soft magnetism in single phase Fe3Si thin films deposited on SrTiO3(001) by pulsed laser deposition

The development of devices relying on spin phenomena requires of an ideal spin polarized electron source. This can be achieved by taking advantage of half-metallic full Heusler alloy thin films. However, their implementation requires a controlled growth of stoichiometric films with large activation volumes. In this work, we report on the growth of epitaxial Fe3Si ultra-thin films by pulsed laser deposition on SrTiO3(001) substrates, analyzing the effect of deposition temperature in the structural, morphological and magnetic properties of the deposited films. We conclude that optimal compromise between phase purity and interface quality is obtained at 200 ºC, obtaining the best magnetic response under this condition.

Effect of substrate on superconducting and magnetic properties for the epitaxial FeSe0.5Te0.5 thin films

High quality FeSe0.5Te0.5 (FST) epitaxial thin films were successfully prepared on SrTiO3 (STO), MgO and LaAlO3 substrates with a TiO2 buffer layer by pulsed laser deposition method, respectively. We present the results of a collaborative study on the superconducting and magnetic properties of the three types films, such as the critical temperature T c, the anisotropy, and the critical current density J c. A stronger matching between film and substrate plays an important role in enhancing the superconducting and magnetic properties. FST film on STO substrate shows the high superconducting transition temperature T c about 14.6 K and the largest critical current density J c above 106 A cm−2 at 4 K in self-field. In addition, the high-angle annular dark field imaging implies the interface sharpness at FST/TiO2 heterostructure, contributing to a deep understanding of bonding for the iron-based superconductors.

Direct-ARPES and STM Investigation of FeSe Thin Film Growth by Nd:YAG Laser

Research on ultrathin quantum materials requires full control of the growth and surface quality of the specimens in order to perform experiments on their atomic structure and electron states leading to ultimate analysis of their intrinsic properties. We report results on epitaxial FeSe thin films grown by pulsed laser deposition (PLD) on CaF2 (001) substrates as obtained by exploiting the advantages of an all-in-situ ultra-high vacuum (UHV) laboratory allowing for direct high-resolution surface analysis by scanning tunnelling microscopy (STM), synchrotron radiation X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) on fresh surfaces. FeSe PLD growth protocols were fine-tuned by optimizing target-to-substrate distance d and ablation frequency, atomically flat terraces with unit-cell step heights are obtained, overcoming the spiral morphology often observed by others. In-situ ARPES with linearly polarized horizontal and vertical radiation shows hole-like and electron-like pockets at the Γ and M points of the Fermi surface, consistent with previous observations on cleaved single crystal surfaces. The control achieved in growing quantum materials with volatile elements such as Se by in-situ PLD makes it possible to address the fine analysis of the surfaces by in-situ ARPES and XPS. The study opens wide avenues for the PLD based heterostructures as work-bench for the understanding of proximity-driven effects and for the development of prospective devices based on combinations of quantum materials.

LaO as a candidate substrate for realizing superconductivity in an FeSe epitaxial film

The significantly enhanced superconducting transition temperature ($T_c$) of an FeSe monolayer on SrTiO$_3$(001) substrate has attracted extensive attention in recent years. Here, based on first-principles electronic structure calculations, we propose another candidate substrate LaO(001) for the epitaxial growth of FeSe monolayer to realize superconductivity. Our calculations show that for the optimal adsorption structure of FeSe monolayer on LaO(001), the stripe antiferromagnetic state and the dimer antiferromagnetic state are almost energetically degenerate, indicating the existence of strong magnetic fluctuation that is beneficial to the appearance of superconductivity. According to the Bader charge analysis, the calculated electron doping from the LaO substrate to the FeSe monolayer is about 0.18 electrons per Fe atom, even larger than that in case of FeSe/SrTiO$_3$(001). Since LaO was also reported to be a superconductor with $T_c$ ~ 5 K, it may have a superconducting proximity effect on the epitaxial FeSe film and vice versa. These results suggest that LaO would be an interesting substrate to study the interface-related superconductivity.

Observing the magnetization reversal processes and anisotropic effective damping of epitaxial FeSi/MgO (001)

Epitaxial FeSi film on MgO (001) substrate was fabricated via a radio frequency magnetron sputtering technology. The epitaxial relationship of FeSi(001)/[110]//MgO(001)/[100] was characterized by crystal structure measurements and confirmed by in-plane biaxial magnetic anisotropy through vibrating sample magnetometer (VSM). By measuring Kerr magnetic hysteresis loops and recording the real-time magnetic domain images through surface magneto-optic Kerr effect (MOKE), two successive 90° domain wall displacement along easy axis and two discontinuous 90° domain wall displacement along hard axis were directly observed. Meanwhile, the difference of magnetic hysteresis loops obtained by VSM and MOKE devices were discussed. Furthermore, from the results of magnetic field sweeping ferromagnetic resonance measurements, it was found that FeSi film possesses an anisotropic effective damping constant of 0.0042 for easy axis direction and 0.0053 for hard axis direction, which was ascribed to the crystallographic defects induced two magnon scattering contributions.

Anisotropic field dependence of the electronic transport in superconducting FeSe thin films

Electronic transport measurements were carried out on c-axis oriented epitaxial FeSe thin films in static magnetic fields up to 14 T parallel to the crystallographic c-axis and parallel to the ab-plane. The temperature dependence of the upper critical fields and was determined from the field dependence of the resistivity. While was orbitally limited, the larger was paramagnetically limited. The anisotropy was decreasing with decreasing temperature and revealed a pseudoisotropic behaviour towards zero temperature. This temperature dependence is indicative of the multiband character of FeSe. From measurements of the temperature- and field-dependent critical current densities the effective depinning or irreversibility field was obtained, and its angular dependence yielded the effective mass anisotropy . The temperature dependence of , which was different from that of , can be considered as another signature of multiband FeSe.

Stoichiometry and defect superstructures in epitaxial FeSe films on SrTiO3

Cryogenic scanning tunneling microscopy is employed to investigate the stoichiometry and defects of epitaxial FeSe thin films on ${\mathrm{SrTiO}}_{3}$(001) substrates under various postgrowth annealing conditions. Low-temperature annealing with an excess supply of Se leads to the formation of Fe vacancies and superstructures, accompanied by a superconductivity (metal)-to-insulator transition in FeSe films. By contrast, high-temperature annealing could eliminate the Fe vacancies and superstructures, and thus recover the high-temperature superconducting phase of monolayer FeSe films. We also observe multilayer FeSe during low-temperature annealing, which is revealed to link with Fe vacancy formation and adatom migration. Our results document the very special roles of film stoichiometry and help unravel several controversies in the properties of monolayer FeSe films.

Enhanced superconducting properties of FeSe0.8Te0.2 thin films grown by pulsed laser deposition

Despite FeSe1-xTex bulk samples with low Te content are difficult to have a single phase due to phase separation, the superconducting epitaxial FeSe0.8Te0.2 thin films with high phase purity and excellent crystallinity are fabricated on CaF2 (00l) substrates by pulsed laser deposition. The superconducting properties are studied by complimentary experimental approaches: structural investigation, elemental analysis, and transport measurements. Prepared films with thickness of 150 nm are c-axis oriented with no evidence of impurity phases. In addition, the critical transition temperature Tc is pushed up to 24 K and the upper critical fields are inferred to be improved compared to the most extensively investigated FeSe0.5Te0.5 films. Furthermore, the transport critical current density Jc reaches values of 5.05 × 105 A cm−2 in self-field at 4.2 K and shows a small anisotropy value of 2.34 at 9 T. Pinning analysis confirms the presence of ab-correlated pinning centers.

Modulation of magnetization dynamics in an epitaxial Heusler ferromagnet due to pure spin current in a laterally configured structure

Using a laterally configured structure, we demonstrate modulation of the magnetization dynamics of an epitaxial Heusler ferromagnet, Fe3Si, by nonlocally injecting pure spin currents. From the ferromagnetic resonance signals, the effective Gilbert damping constant () of the epitaxial Fe3Si nanomagnet is estimated to be ∼0.003 at room temperature. By using the lateral and nonlocal spin injection, the value of the epitaxial Fe3Si nanomagnet can be modulated from 0.0034 to 0.0024. This study will open a way for control of the magnetization dynamics of high-performance ferromagnetic Heusler alloys due to pure spin currents even in laterally configured structures.

Effect of chemical ordering on optical properties of Fe3Si epitaxial films

Optical characteristics (electron energy loss function, optical conductivity σ, permittivity ε, refractive index n, extinction coefficient k, and absorption coefficient α) of a 30 nm thick epitaxial Fe3Si iron silicide films grown at different silicon substrate temperature (26, 100, 200, 300 ˚C) were determined within E = 0.74–6.46 eV photon energy range using spectroscopic ellipsometry technique. The experimental data are compared to the optical characteristics calculated in the framework of the density functional theory using the GGA–PBE approximation. Variations of the optical characteristics spectra are discussed from the point of view of chemical ordering of DO3 type crystal structure. It is asserted that the electron energy-loss function, optical conductivity and extinction coefficient of the Fe3Si iron silicide films undergo noticeable changes in different spectral ranges over the whole spectrum between 0.74 and 6.46 eV due to variation in the chemical order. Information on the effect of chemical ordering on the optical properties obtained here allows one to carry out quick qualitative analysis of Fe3Si film crystal quality during the synthesis procedures by ellipsometry method in situ.

Thermally activated flux flow in superconducting epitaxial FeSe0.6Te0.4 thin film

The thermally activated flux flow effect has been studied in epitaxial FeSe0.6Te0.4 thin film grown by a PLD method through the electrical resistivity measurement under various magnetic fields for B//c and B//ab. The results showed that the thermally activated flux flow effect is well described by the nonlinear temperature-dependent activation energy. The evaluated apparent activation energy U0(B) is one order larger than the reported results and showed the double-linearity in both magnetic field directions. Furthermore, the FeSe0.6Te0.4 thin film shows the anisotropy of 5.6 near Tc and 2D-like superconducting behavior in thermally activated flux flow region. In addition, the vortex glass transition and the temperature dependence of the high critical fields were determined.

Iron silicide-based ferromagnetic metal/semiconductor nanostructures

Ferromagnetic single-crystal epitaxial Fe3Si films and polycrystalline Fe5Si3 films are obtained on Si substrates by molecular-beam epitaxy with in situ control of the structure, optical, and magnetic properties. The results of the structural, magnetic, and optical measurements are discussed. The experimental data are compared to the results of the microscopic calculation of the spin-polarized structure, the permittivity, and the optical conductivity spectra.

Angular-dependent vortex pinning mechanism and magneto-optical characterizations of FeSe0.5Te0.5 thin films grown on CaF2 substrates

Magneto-optical (MO) characterizations and the angular-dependent critical current density (Jc(Θ)) of epitaxial FeSe0.5Te0.5 (FST) thin films grown on CaF2 single-crystalline substrates were performed. The MO images show typical rooftop patterns in the remanent state from which a large, homogeneous, and almost isotropic self-field Jc over 2 × 106 A cm−2 at 8 K was obtained. The vortex pinning mechanism is investigated measuring the magnetic field and angular-dependent critical current density Jc. The FST films exhibit small anisotropy of Jc in the whole applied magnetic field range below 15 K. The Dew-Hughes model and angular scaling analyses prove that pointlike normal cores, which are distributed randomly in the FST film, dominate the pinning in the FST films on CaF2 substrates.

Electrical field control of non-volatile 90° magnetization switching in epitaxial FeSi films on (001) 0.7[Pb(Mg1/3Nb2/3)O3]-0.3[PbTi0.3O3

The epitaxial FeSi thin films on (001) 0.7[Pb(Mg1/3Nb2/3)O3]-0.3[PbTi0.3O3] (PMN-0.3PT) substrates were fabricated by radio frequency magnetron sputtering. The observed asymmetric strain-electric field curve suggests a tensile strain switching between two perpendicular directions in PMN-0.3PT with the variation of polarity of electric fields. A simple theoretical simulation of the free energy landscape shows that the stable magnetization easy axis (MEA) of FeSi with the inherent cubic magnetocrystalline anisotropy depends on the strength and direction of an extra strain induced uniaxial anisotropy. A reversible and non-volatile 90° switching of MEA by ±6 kV/cm pulses was confirmed by the angular dependence of remanent magnetization and Kerr hysteresis loops in Ta/FeSi/PMN-0.3PT/Pt heterostructures.

High temperature and full-in-plane-direction workable high-frequency soft magnetic epitaxial FeSi thin films on MgO(0 0 1)

The epitaxial FeSi(0 0 1)[1 1 0]//MgO(0 0 1)[1 0 0] films were fabricated by sputtering and post annealing at 800 °C. A four-fold symmetric angular dependence of remanence ratios and coercivities of FeSi films were observed and well fitted by theoretical models considering the cubic anisotropy. The experimental ferromagnetic resonance frequency (fr) of epitaxial FeSi films reaches to 8.0 GHz, which is in agreement with the theoretical value derived from Landau–Lifshitz–Gilbert equation at room temperature. Moreover, the resonance phenomenon can be observed in any in-plane directions in contrast with the absence of resonance phenomenon in some specific directions for in-plane uniaxial soft magnetic Fe2Co films. Although the saturation magnetization, cubic anisotropy constant and fr all decrease with increasing temperature, fr still can keep as high as 3.2 GHz at 800 K, indicating that the epitaxial FeSi films with high Curie temperature have potential application in full angle workable microwave devices at relatively high temperature.

Preparation and characterization of Bi2Se3(0001) and of epitaxial FeSe nanocrystals on Bi2Se3(0001)

Procedures to prepare clean Bi2Se3(0001) surfaces from bulk samples and epitaxial FeSe nanocrystals on Bi2Se3(0001) are reported. Bi2Se3(0001) substrates are prepared by in vacuo cleavage of bulk samples, followed by ion bombardment and annealing cycles. FeSe is prepared by Fe deposition onto Bi2Se3 at 303K, followed by annealing at T≈623K. We use low-energy electron diffraction, surface X-ray diffraction, photoemission spectroscopy, scanning tunneling microscopy and spectroscopy, and stress measurements to elucidate the correlation between structural and electronic properties of the pristine Bi2Se3(0001) and FeSe covered surfaces. Our analysis reveals the formation of epitaxial FeSe nanocrystals with a thickness of three unit cells (1.5nm). Electron diffraction experiments indicate an anisotropic epitaxial strain in FeSe. A negligible strain close to 0.0% and a tensile strain of +2.1% are observed along the in-plane 011¯0 and 211¯0 Bi2Se3 directions, respectively. The out-of-plane strain is +4.2%. The role of this strain state for the reported high-TC superconductivity in bulk FeSe is discussed.

High performance FeSe0.5Te0.5 thin films grown at low temperature by pulsed laser deposition

We report on fully epitaxial FeSe0.5Te0.5 (FST) thin films of high quality grown on CaF2 (00l) substrate at a low temperature of 300 °C by pulsed laser deposition. The transport Jc of thin films is up to 1.36 MA cm−2 in self-field and 0.97 MA cm−2 in 9 T at 4.2 K, indicating very weak field dependence. A near isotropy of Jc (γ = as low as 1.09 at 9 T is achieved in the FST thin films. Moreover, there is no clear amorphous interfacial layer between the film and the substrate, probably due to low temperature and low laser repetition rate, while the thickness of the reaction layer is approximate 5 nm in many other works. The transmission electron microscopy evidence shows that some lattices with lateral size <5 nm × 20 nm seem to be disturbed. These location defects are thought to be responsible for the nearly isotropic behavior of the superconductivity.

Grain boundary junctions of FeSe0.5Te0.5 thin films on SrTiO3 bi-crystal substrates

Grain boundary junctions were fabricated in the epitaxial FeSe0.5Te0.5 thin films on [001] tilt SrTiO3 bi-crystal substrates with a CeO2 buffer layer. Critical current densities across the junctions with different mis-orientation angles of 4°, 7°, 15°, and 24° were measured at magnetic fields up to 30 T. It was found that the 4° and 7° junctions carry critical current densities comparable to that of the intra-grain film while those of the 15° and 24° junctions were suppressed drastically. A critical mis-orientation angle of around 9° was identified that separates the strong coupling region from the weak link region. We found that the critical current densities across the grain boundary with a 24° mis-orientation angle are modulated by the magnetic field, indicating a Josephson Effect. This junction is estimated to be in the intermediate-size regime with an effective transverse junction width L ∼ 2.6–2.8 μm and a Josephson penetration depth λJ ∼ 1.2 μm.

Optical characteristics of an epitaxial Fe3Si/Si(111) iron silicide film

The dispersion of the relative permittivity ɛ of a 27-nm-thick epitaxial Fe3Si iron silicide film has been measured within the E = 1.16–4.96 eV energy range using the spectroscopic ellipsometry technique. The experimental data are compared to the relative permittivity calculated in the framework of the density functional theory using the GGA-PBE approximation. For Fe3Si, the electronic structure and the electronic density of states (DOS) are calculated. The analysis of the frequencies corresponding to the transitions between the DOS peaks demonstrates qualitative agreement with the measured absorption peaks. The analysis of the single wavelength laser ellipsometry data obtained in the course of the film growth demonstrates that a continuous layer of Fe3Si iron silicide film is formed if the film thickness achieves 5 nm.