Epitaxial Alloy Films Research Articles

Ultrafast signatures of magnetic inhomogeneity in Pd1−xFex (x ≤ 0.08) epitaxial thin films

A series of Pd1−xFex alloy epitaxial films (x = 0, 0.038, 0.062, and 0.080), a material promising for superconducting spintronics, was prepared and studied with ultrafast optical and magneto-optical laser spectroscopy in a wide temperature range of 4–300 K. It was found that the transition to the ferromagnetic state causes a qualitative change of both the reflectivity and the magneto-optical Kerr effect transients. A nanoscale magnetic inhomogeneity of the ferromagnet/paramagnet type inherent in the palladium-rich Pd1−xFex alloys reveals itself through the occurrence of a relatively slow, 10–25 ps, photoinduced demagnetization component following a subpicosecond one; the former vanishes at low temperatures only in the x = 0.080 sample. We argue that the 10 ps timescale demagnetization originates most probably from the diffusive transport of d electrons under the condition of nanoscale magnetic inhomogeneities. The low-temperature fraction of the residual paramagnetic phase can be deduced from the magnitude of the slow reflectivity relaxation component. It is estimated as ≈30% for x = 0.038 and ≈15% for x = 0.062 films. The minimal iron content ensuring the magnetic homogeneity of the ferromagnetic state in the Pd1−xFex alloy at low temperatures is about 7–8 atom %.

Nb-doped ZrxSn1−xO2: Experimental and first-principles study

Ultra-wide bandgap semiconductors with exceptional advantages have potential use in ultrahigh power, ultrahigh frequency devices, and other applications. In this paper, a series of high-quality Nb-doped ZrxSn1−xO2 (Nb:ZrxSn1−xO2) alloy epitaxial films were prepared on c-plane sapphire substrates by pulsed laser deposition. A greater proportion of Zr successfully widened the optical bandgap of SnO2 up to 4.70 from 4.28 eV. Interestingly, although Nb is a common n-type dopant for SnO2, the conductivity of Nb:ZrxSn1−xO2 decreased with increasing Zr content. The greater activation energy Ea of the films with more Zr contents was determined by variable resistance measurements and rationalized by the first-principles calculations. The higher Zr content leads to a lower conductivity in the films. This is because the electronegativity of Zr is smaller than that of Sn and Nb, making it easier for O to attract electrons from Zr and Nb donating less electrons with increasing Zr content. It leads to more electrons filling the Nb 4d orbital and brings the donor level further down from the conduction band minimum. However, Nb:ZrxSn1−xO2 with a low Zr content of x = 0.1 has good electrical conductivity, with a carrier density of 5.426 × 1020 cm−3 and a resistivity of 7.89 × 10−3 Ω cm, and simultaneously a broadened bandgap of 4.4 eV. Therefore, Nb can act as an effective n-type dopant for ZrxSn1−xO2 with proper Zr content, making Nb-doped ZrxSn1−xO2 promising for developing ultraviolet-transparent conductive electrodes.

Structural and magnetic properties of CoIrMnAl equiatomic quaternary Heusler alloy epitaxial films designed using first-principles calculations

• CoIrMnAl was computationally designed as one of the advanced materials for spintronics. • We obtained B2 ordered CoIrMnAl thin films with the lattice constant similar to that predicted in fully ordered alloy. • Magnetization at 10 K and Curie temperature measured were about 70% of the predicted values in fully ordered alloys. • Experimental data were well explained with ferrimagnetism due to Co-Mn swap disorders predicted from ab-initio calculation. MgO-barrier magnetic tunnel junctions with half-metallic Heusler alloy electrodes have attracted considerable attention for spintronics applications. However, there remain a couple of issues related to materials that should be resolved before practical use. Recently, quarterly equiatomic Heusler alloys have attracted attention as advanced Heusler alloys. CoIrMnZ (Z = Al, Si, Ga, and Ge) half-metallic Heusler alloys were designed and predicted to have moderate Curie temperatures and to be lattice-matched with the MgO barrier, which is advantageous compared to traditional Co 2 Heusler alloys T. T [T. Roy et al., J. Magn. Magn. Mater. 498 (2020) 166092]. Here, we experimentally investigated the structure and magnetic properties of thin films composed of one of these alloys, CoIrMnAl, fabricated by sputtering deposition. We successfully obtained films with the B 2 chemical ordering, even without a post-annealing process. The lattice constants for the films annealed at 500–600 ∘ C were approximately equal to the predicted values. The magnetization at 10 K was close to 500 kA/m, and the Curie temperature was approximately 400 K, which were approximately 70% of the values predicted for the fully ordered structure. The magnetic properties observed in the B 2-ordered films were well explained by ferrimagnetism that appeared in the B 2-ordered CoIrMnAl with full-swap disorders of Co-Ir and Mn-Al and almost full-swap disorder of Co-Mn, which was predicted from the first-principles calculations.

RuVO2 alloy epitaxial films: Lowered insulator–metal transition temperature and retained modulation capacity

Alloying VO2 by Ru incorporation (RuxV1−xO2) should decrease the insulator–metal-transition (IMT) temperature due to the unique semi-metallic properties of RuO2. We deposit high-quality RuxV1−xO2 thin films by pulsed laser deposition on (0001) sapphire substrates. We investigate the structural, electrical, and optical properties of the RuxV1−xO2 alloy films using x-ray diffraction, x-ray photoelectron spectroscopy, UV-Vis–NIR spectrophotometry, and four-point-probe resistivity measurements. Our results confirm that Ru alloying of VO2 reduces effectively the IMT temperature while retaining the IMT characteristics of the material.

Synthesis of metastable B2-type Fe–Sn alloy epitaxial films and study of their magnetic properties

The metastable phases of Fe1−xSnx alloys were stabilized by their epitaxial growth on MgO(100). The Fe1−xSnx alloys with a body-centered-cubic-based structure were realized in the range of 0 ≤ x ≤ 0.4. For x = 0.25, the B2 structure was observed by transmission electron microscopy and X-ray diffraction analysis, although the hexagonal D019 structure was a thermodynamically stable phase. With respect to magnetic properties, all the compositions show similar hysteresis curves except for x = 0.25. According to ab initio calculations, D03-Fe3Sn and B2-Fe3Sn have band structures similar to that of Fe, which results in a large tunnel magnetoresistance by coherent tunneling.

Magnetic damping constant in Co-based full heusler alloy epitaxial films

Co-based full-Heusler alloys, such as Co2MnSi and Co2MnGe, are expected to be used as half-metallic ferromagnetic material, which has complete spin polarization. They are the most promising materials for realizing half-metallicity at room temperature due to their high Curie temperature. The optimization of the magnetic damping constant of ferromagnetic materials is extremely important for achieving high-speed magnetization switching and reducing critical current density for spin torque transfer switching. We have systematically investigated the magnetic damping constant in Co-based full Heusler alloy epitaxial films. We found that the Gilbert damping constant seems to be roughly proportional to the total density of states at the Fermi level (EF) by first principle calculation. A very small magnetic damping constant of 0.003 in the Co2Fe0.4Mn0.6Si epitaxial film was demonstrated. The small magnetic damping constant in Co2FexMn1−xSi films with x < 0.6 can be attributed to the half-metallicity of Heusler alloys. Co-based full Heusler alloys with both half-metallicity and small magnetic damping will be very useful for future applications based on spintronic devices.

Effect of Composition on the Ordered Phase Formation in Co-Pt Thin Film Deposited on MgO(111) Single-Crystal Substrate

Co x Pt 100-x (at. %) alloy epitaxial films with the close-packed plane parallel to the substrate surface are prepared on MgO(111) substrates heated at 300 °C by varying the Co content of x from 0 to 100. The stacking sequence of close-packed plane (the volume ratios of fcc and hcp crystals, V fcc and V hcp ) and the order degree are quantitatively determined. The films with Pt-rich and nearly equiatomic compositions primarily consist of fcc crystals (V fcc >0.9). With increasing the x value beyond 56, the V hcp value drastically increases. The films with Co-rich compositions consist of mixtures of hcp (V hcp ≈0.8) and fcc (V fcc ≈ 0.2) crystals. The films with 31 ≤x≤ 86 involve metastable ordered phases. The order degree of S L11 ,Bh is kept constant around 0.25 in a compositional range of 31 × 75. From the viewpoints of volume ratios of fcc and hcp crystals and order degree, the crystal structure is determined to vary A1 A1 + SFs (SFs: stacking faults) L1 1 + SFs ⇒L1 1 + Bh (+D0 19 ) ⇒A3 + SFs with increasing the x value. The films including ordered phases show strong perpendicular magnetic anisotropies. It is shown that the uniaxial magnetocrystalline anisotropy energy of Co x Pt 100-x film can be kept >10 7 erg/cm 3 while controlling the saturation magnetization from 600 to 1150 emu/cm 3 .

Characterization of metastable crystal structure for Co-Pt alloy thin film by x-ray diffraction

Co50Pt50 and Co75Pt25 (at. %) alloy epitaxial films with the close-packed plane parallel to the substrate surface are prepared on MgO(111) substrates with and without Ti(0001) underlayer at 300 °C. The structural properties are investigated by using a combination of out-of-plane, in-plane, and pole-figure x-ray diffractions. The crystal structure is determined by taking into account the order degree and the atomic stacking sequence of close-packed plane. Formation of a metastable ordered fcc-based L11-CoPt phase is recognized for the Co50Pt50 film deposited on MgO(111) substrate, whereas a metastable ordered hcp-based Bh-CoPt phase is involved in the Co50Pt50 film deposited on Ti(0001) underlayer in addition to an L11-CoPt phase. The Co75Pt25 film deposited on MgO(111) consists of a mixture of ordered hcp-based phases of Bh-CoPt and D019-Co3Pt. An accurate order degree is calculated by comparing the out-of-plane superlattice and fundamental reflection data. A simplified method for estimating the order degree, where there are only two kinds of parameter: intensity ratio of superlattice to fundamental reflection and out-of-plane lattice spacing, is also proposed. The order degree estimated by the simplified method agrees in a small error less than a few percentages with the accurate value.

Fe50Pt50–xPdxalloy thin films withL10structure epitaxially grown on MgO(001) substrates

Fe50 Pt50–x Pdx (at. %, x = 0, 12.5, 25, 37.5, 50) alloy epitaxial films are prepared on MgO(001) substrates at temperatures ranging from room temperature to 600 °C by radio-frequency magnetron sputtering. The effects of substrate temperature and Pd/Pt composition on the order degree, c -axis distribution, and magnetic property are investigated. With increasing the substrate temperature, L 10 ordering is enhanced for all the compositions. The order degree of film deposited at 600 °C decreases from 0.37 to 0.23 as the x value increases from 0 to 12.5 at. %. With further increasing the x value, the order degree increases up to 0.69. The Fe50 Pt50–x Pdx films with x ≥ 25 consist of only an L 10 (001) single-crystal with the c -axis perpendicular to the substrate surface, whereas the films with x 10 (100) variant with the c -axis lying in the film plane in addition to L 10 (001) variant. The c -axis distribution is influenced by the Pd/Pt composition. The magnetic anisotropy is reflecting the c -axis distribution and the order degree.

FePd, FePt, and CoPt alloy epitaxial thin films with flat surface grown on MgO(111) substrate

FePd, FePt, and CoPt alloy epitaxial films of 40 nm thickness are prepared on MgO(111) singlecrystal substrates by employing two different methods. One is a one-step method consisting of hightemperature deposition at 600 °C and the other is a two-step method consisting of low-temperature deposition at 200 °C followed by annealing at 600 °C. Although the preparation method is different, similar final crystal structures are realized for all the film materials. FePd and FePt alloy films grow on the substrates with six L 10 (111) variants, whose c -axes are about 35° canted from the substrate surface and rotated around the film normal by 60° each other. The order degrees of FePd and FePt films prepared by one- and two-step methods, (S one-step , S two-step ), are estimated to be (0.25, 0.33) and (0.08, 0.15), respectively. On the contrary, L 10 ordered phase formation is not recognized for CoPt alloy films. The films prepared by one-step method have rough surfaces surrounded by side facets, whereas the films prepared by two-step method have very flat surfaces with the arithmetical mean roughness of 0.3 nm. The two-step method is useful for preparation of L 10 ordered films with flat surface.

Microstructure Map for Self-Organized Phase Separation during Film Deposition

Drastically different two-phase microstructures have been reported for alloy epitaxial films, including self-organized nanoscale concentration modulations of vertical and lateral stripes. To understand the disparity of these microstructures, we study their formation mechanisms via spinodal decomposition during film deposition with the aid of computer simulations. Based on the simulation results, a microstructure map is established that describes relationships among the morphology of self-organized two-phase microstructure, initial alloy composition, and deposition rate relative to the phase separation kinetics in the film. Depending on the deposition rate relative to the kinetics of spinodal decomposition in the film, both laterally and vertically modulated microstructures could be obtained.

Magnetic patterning perpendicular anisotropy FePd alloy films by masked ion irradiation

The nanopatterning of magnetic films by ion implantation is reported. Highly L10-ordered Fe47Pd53 epitaxial alloy films on a MgO(001) substrate were covered by a monolayer of silica spheres in a Langmuir film balance. Using this sphere layer as an implantation mask, the samples were irradiated by Ne+ or Fe+ ions with energies of 35 keV and 100 keV, respectively. After the silica mask was removed, the samples were characterized via conversion electron Mössbauer spectroscopy, longitudinal and polar magneto-optical Kerr effect, and atomic force and magnetic force microscopy. We find that the magnetic stripe domains observed in the nonirradiated sample were converted into a regular 2D magnetic pattern of hcp character upon 1 × 1015/cm2 35 keV neon or 1 × 1014/cm2 100 keV iron irradiation, with the direction of magnetization remaining out of plane in the nodes of the hcp lattice and relaxed into the film plane in the inter-node region, resulting in an overall in-plane magnetic softening of the film.

Magnetooptical properties of iron based Heusler alloy epitaxial films on Ge(111)

We have examined the LMOKE of some Heusler alloy films Fe4Si, Fe3Si, Fe2CoSi, Fe2MnSi (21 at and 9 at.%Mn), Co2FeSi epitaxially grown on Ge(111) and found that Fe3Si, Fe4Si and Fe2CoSi films have larger Kerr rotation and smaller coercive fields than the amorphous Fe film and that they may be promising materials for magnetophotonic applications.

Gilbert magnetic damping constant of epitaxially grown Co-based Heusler alloy thin films

The magnetic damping constant in a series of Co2MnAlxSi1−x and Co2FexMn1−xSi Heusler alloy epitaxial films were systematically investigated by using ferromagnetic resonance technique. The determined magnetic damping constant is roughly proportional to the density of states at the Fermi energy of the first principle calculation. The result is consistent with the theoretical prediction when taking spin-orbit interaction into account. The small Gilbert damping constant for the fabricated films other than the Co2FexMn1−xSi film with x&amp;gt;0.6 can be originated in the half-metallic electronic structure of Heusler alloys.

Adaptive Modulations of Martensites

Modulated phases occur in numerous functional materials like giant ferroelectrics and magnetic shape-memory alloys. To understand the origin of these phases, we employ and generalize the concept of adaptive martensite. As a starting point, we investigate the coexistence of austenite, adaptive 14M phase, and tetragonal martensite in Ni-Mn-Ga magnetic shape-memory alloy epitaxial films. We show that the modulated martensite can be constructed from nanotwinned variants of the tetragonal martensite phase. By combining the concept of adaptive martensite with branching of twin variants, we can explain key features of modulated phases from a microscopic view. This includes metastability, the sequence of 6M-10M-14M-NM intermartensitic transitions, and the magnetocrystalline anisotropy.

Epitaxial ferromagnetic Mn5Ge3 on Ge(111)

Ferromagnetic Mn5Ge3 thin films were grown on Ge(111) with solid-phase epitaxy. The epitaxial relationship between the alloy film and substrate is Mn5Ge3(001)//Ge(111) with [100]Mn5Ge3//[11̄0]Ge. The alloy films exhibit metallic conductivity and strong ferromagnetism up to the Curie temperature, TC=296 K. These epitaxial alloy films are promising candidates for germanium-based spintronics.

"Devil's staircases" in bulk-immiscible ultrathin alloy films.

Ground-state phase diagrams of ultrathin epitaxial alloy films are studied within the framework of a discrete lattice-model Hamiltonian incorporating competing elastic and chemical interactions. For bulk-immiscible alloy systems an infinite number of commensurate, long-period stripe-superstructure ground states are obtained as a function of chemical potential. The average periodicity of these stripe superstructures is found to be a nonmonotonic function of alloy composition, in contrast to the predictions of continuum theories for two-dimensional systems with competing interactions.

Interrelation of morphology, structure, and magnetism in Fe xCo 1− x/Cu(1 0 0) epitaxial alloy films

Structure, morphology, and magnetism of epitaxial Fe x Co 1− x alloy ultrathin films grown on Cu(1 0 0) in ultrahigh vacuum were investigated by a multi-technique approach over the whole composition range up to thicknesses of nine atomic monolayers (ML). Combining the results of the different techniques it is found that the films grow at room temperature in a distorted FCC structure with random chemical order. The amount and sign of the distortion depend on thickness and composition. Below ≈60 to 70% Fe content the alloy films are smooth, and exhibit two different vertical interlayer distances. At higher Fe concentrations several superstructures are observed, which are attributed to regular structural rearrangements. The structural relaxation above 4 ML thickness known from pure Fe films is not observed in the FeCo alloy films. Instead a gradual structural change towards a BCC(1 1 0) structure with increasing thickness occurs at higher Fe concentrations, and is still not complete in films as thick as 9 ML. The interrelation between morphology and structure in Fe x Co 1− x /Cu(1 0 0) is discussed in terms of composition-dependent lattice parameters and strain. An expansion of the films in the plane leads to a stabilization of the FCC structure, whereas a compression leads to corrugated films with a tendency towards structural transformations. The absence of the relaxed FCC structure found in pure Fe films is explained by Co atoms acting as defects in the FCC structure of the alloy films either in a geometrical sense and/or because of their different magnetic moment with respect to the Fe atoms in both ferromagnetic and antiferromagnetic configurations of FCC Fe.

Growth and properties of Zn 1− xCd xS films on GaAs by low-pressure MOVPE

Cubic Zn 1− x Cd x S ternary alloy epitaxial films have been grown on (100) GaAs by low-pressure metalorganic vapor phase epitaxy (MOVPE) using dimethyl zinc, dimethyl cadmium and hydrogen sulfide as reactants. The solid composition is shown to be easily controlled over the whole range of composition, although the films tend to be Cd-rich compared with the case of atmospheric pressure MOVPE. It has been found that the solid composition slightly decreases with growth temperature due to the effect of the decrease in the sticking coefficient of cadmium with temperature. The photoluminescence spectra at 18 K of the ternary films are dominated by fairly strong free-to-acceptor emissions. Furthermore, it is shown that low-resistivity ternary films can be grown by iodine doping.

Structure and Oxidation of Thin Gold-Nickel Alloy Films

The structure and oxidation of thin alloy films of composition Au-15 at.% Ni have been investigated by means of x-ray and electron diffraction, and the oxide growth rate determined by measuring changes in the electrical resistivity of the films during oxidation. The films are prepared by sputtering and, even at low substrate temperatures, comprise single-phase homogeneous structures. Oxidation in the temperature range 250°–310°C shows a two-stage logarithmic growth rate (or resistivity change) with time similar to that observed for Ni, Cu, and Fe. This behavior is consistent with the formation of oxide only at the alloy film surface, and can be interpreted qualitatively in terms of the electron-transfer model proposed by Uhlig et al., for the oxidation of pure Ni. Epitaxial alloy films with (001) and (111) orientation are produced by sputtering onto the cleaved surfaces of NaCl and mica at about 400°C. On annealing these films at and above 500°C, pre-precipitation effects are observed in the electron-diffraction patterns. These are associated with the formation, in succession, of the superlattice structures, NiAu3 and NiAu. True precipitation of Ni-rich alloy occurs at temperatures above 600°C. Oxidation of epitaxial films of the disordered alloy at temperatures below 250°C results in the growth of poorly crystallized, non-oriented NiO. At 280°C both non-oriented and also three differently oriented oxide products, are formed, the NiO orientations corresponding to those reported by Collins and Heavens for oxide growth on Ni films. At 400°C only oxide orientations parallel to the metal film are detected.