Single Crystalline MgO Substrates Research Articles

Field-free spin-orbit torque switching assisted by in-plane unconventional spin torque in ultrathin [Pt/Co

Electrical manipulation of magnetization without an external magnetic field is critical for the development of advanced non-volatile magnetic-memory technology that can achieve high memory density and low energy consumption. Several recent studies have revealed efficient out-of-plane spin-orbit torques (SOTs) in a variety of materials for field-free type-z SOT switching. Here, we report on the corresponding type-x configuration, showing significant in-plane unconventional spin polarizations from sputtered ultrathin [Pt/Co]N, which are either highly textured on single crystalline MgO substrates or randomly textured on SiO2 coated Si substrates. The unconventional spin currents generated in the low-dimensional Co films result from the strong orbital magnetic moment, which has been observed by X-ray magnetic circular dichroism (XMCD) measurement. The x-polarized spin torque efficiency reaches up to −0.083 and favors complete field-free switching of CoFeB magnetized along the in-plane charge current direction. Micromagnetic simulations additionally demonstrate its lower switching current than type-y switching, especially in narrow current pulses. Our work provides additional pathways for electrical manipulation of spintronic devices in the pursuit of high-speed, high-density, and low-energy non-volatile memory.

Controlling the Metamagnetic Phase Transition in FeRh/MnRh Superlattices and Thin-Film Fe50-xMnxRh50 Alloys.

Equiatomic and chemically ordered FeRh and MnRh compounds feature a first-order metamagnetic phase transition between antiferromagnetic and ferromagnetic order in the vicinity of room temperature, exhibiting interconnected structural, magnetic, and electronic order parameters. We show that these two alloys can be combined to form hybrid metamagnets in the form of sputter-deposited superlattices and alloys on single-crystalline MgO substrates. Despite being structurally different, the magnetic behavior of the alloys with substantial Mn content resembles that of the FeRh/MnRh superlattices in the ultrathin individual layer limit. For FeRh/MnRh superlattices, dissimilar lattice distortions of the constituent FeRh and MnRh layers at the antiferromagnetic-ferromagnetic transition cause double-step transitions during cooling, while the magnetization during the heating branch shows a smooth, continuous trend. For Fe50-xMnxRh50 alloy films, the substitution of Mn at the Fe sites introduces an effective tensile in-plane strain and magnetic frustration in the highly ordered epitaxial films, largely influencing the phase transition temperature TM (by more than 150 K). In addition, Mn acts as a surfactant, enabling the growth of continuous thin films at higher temperatures. Thus, the introduction of hybrid FeRh-MnRh systems with adjustable parameters provides a pathway for the realization of tunable spintronic devices based on magnetic phase transitions.

Structural and Magnetic Properties of Mn3Ge Grown on a Thin Polycrystalline MgO Seed Layer

D022‐Mn3Ge has been studied recently for its large perpendicular magnetic anisotropy (PMA) and low magnetization, which enables it as a potential alternative material for the scaling of magnetic tunnel junctions (MTJs) used in spin‐transfer‐torque magnetic random access memory (STT‐MRAM). Most of the research was carried out on D022‐Mn3Ge deposited by molecular beam epitaxy (MBE) on single crystalline MgO substrates. As the use of single crystalline substrates is not compatible with an industrial integration flow, we developed a polycrystalline MgO seed layer by physical vapor deposition (PVD) on Si. PMA is observed in MBE‐grown D022‐Mn3Ge with coercive field up to 4.5 Tesla, even when the PVD‐MgO seed is only 1 nm thick. It indicates that PVD‐MgO is able to serve as the seed layer for D022‐Mn3Ge integration to MTJs. However, no PMA is observed in Mn3Ge grown by PVD, which is attributed to different kinetics taking place during PVD deposition.

Impact of carbon segregant on microstructure and magnetic properties of FePt-C nanogranular films on MgO (001) substrate

Carbon is the essential segregant to achieve the nanogranular structure of FePt-based heat-assisted magnetic recording media. However, a fundamental understanding of its effect on the microstructure and magnetic properties of the medium is yet to be elucidated. Here, a systematic investigation of FePt-C nanogranular films deposited on the single-crystalline MgO substrate with various C concentrations was performed. While the averaged FePt grain size can be successfully reduced to 5.8 nm, introducing excess C significantly degrades the degree of L10 ordering, the magnetic anisotropy, and the effective total magnetic moments of FePt films. Such degradation is mainly due to the finite size effects. Besides, the possible contribution form the dissolution of C atoms in the interstitial sites of the FePt lattice for the resulting magnetic properties was also studied via the first-principles calculations. More interestingly, fully in-plane c-axis-oriented FePt grains were observed even for growth on the MgO (001) substrate with a C concentration beyond 34.6 vol.%. Their formation is attributed to the reduced epitaxial strain energy due to the suppressed contact area at the FePt/MgO interface with excess C. Such grains were then identified as the main source of the in-plane component of the medium through micromagnetic simulation.

Microscopic origin of highly enhanced current carrying capabilities of thin NdFeAs(O,F) films.

Fe-based superconductors present a large variety of compounds whose physical properties strongly depend on the crystal structure and chemical composition. Among them, the so-called 1111 compounds show the highest critical temperature Tc in the bulk form. Here we demonstrate the realization of excellent superconducting properties in NdFeAs(O1−xFx). We systematically investigated the correlation between the microstructure at the nanoscale and superconductivity in an epitaxial 22 nm NdFeAs(O1−xFx) thin film on a MgO single crystalline substrate (Tc = 44.7 K). Atomic resolution analysis of the microstructure by transmission electron microscopy and atom probe tomography identified several defects and other inhomogeneities at the nanoscale that can act as extrinsic pinning centers. X-Ray diffraction and transmission electron microscopy displayed a broad variation of the a-axis lattice parameter either due to a partially strained layer at the interface to the substrate, high local strain at dislocation arrays, mosaicity, or due to composition variation within the film. The electrical transport properties are substantially affected by intrinsic pinning and a matching field corresponding to the film thickness and associated with the Bean–Livingston surface barrier of the surfaces. The thin film showed a self-field critical current density Jc(4.2 K) of ∼7.6 MA cm−2 and a record pinning force density of Fp ≈ 1 TN m−3 near 35 T for H‖ab at 4.2 K. These investigations highlight the role of the microstructure in fine-tuning and possibly functionalizing the superconductivity of Fe-based superconductors.

Growth, microstructure and thermal transformation behaviour of epitaxial Ni-Ti films

Epitaxial films have the potential to be used as model systems for fundamental investigations on the martensitic transformation in binary NiTi. In this paper, we discuss growth of binary NiTi thin films on single crystalline MgO substrates. Sputter deposition is used to grow NiTi films. Films prepared by complementary preparation routes (with different deposition temperatures and subsequent heat treatments) are investigated by X-ray diffraction, electron microscopy, atomic force microscopy, and electrical resistivity measurements, with the aim of optimizing film properties, particularly to obtain a well defined orientation of the austenitic unit cell and smooth surfaces. Our results show that deposition at elevated temperatures and carefully controlled subsequent heat treatments allow to produce epitaxially grown and smooth NiTi films that exhibit reversible one- or two-step martensitic transformations.

Pt surface segregation in L10-FePt nano-grains

The influence of the chemical compositions of FePt films on the surface segregations of Pt was investigated by cross-sectional high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) of L10-FePt granular films grown on single crystalline MgO substrates. Ab initio atomistic simulations revealed the surface segregation of Pt is detremental for the magentocrystalline anisotropy energy of the L10-FePt grains. This detremental effect is pronounced for the grain size of less than 15nm. We found that the Pt surface segregation can be suppressed by a trace addition of Ag to the FePt film.

Transparent and conductive Ta doped BaSnO3 films epitaxially grown on MgO substrate

Transparent and conductive Ta doped BaSnO3 (BaSn1−xTaxO3, BSTO) films with x = 0–0.15 were epitaxially grown on MgO single crystalline substrates by a pulsed laser deposition method. The effects of Ta ions incorporation on the microstructure, electrical and optical properties of BSTO films were investigated. X-ray diffraction measurements indicate that the out-of-plane lattice parameters increase gradually with Ta concentration increasing, and the films are relaxed due to the large lattice mismatch between the films and substrate. Atomic force microscopy images reveal that all the films have smooth surface and low roughness. X-ray photoelectron spectra of BSTO films confirm that the Ta ions are presented in the +5 state. The lowest room-temperature resistivity and the highest carrier concentration and Hall mobility were observed in BSTO film at x = 0.07, with the values of 2.525 mΩcm, 5.024 × 1020 cm−3, and 4.921 cm2/Vs, respectively. Temperature dependent resistivity behavior shows that the BSTO films with low doping content exhibit metal-semiconductor transition at low temperature. The optical transmittances of all BSTO films are more than 80% in the visible region. The optical band gaps were found to increase from 3.52 to 4.23 eV with the increase of Ta doping content and can be attributed to the Burstein-Moss effect. Thus, the superior electrical and optical properties of Ta doped BaSnO3 films are comparable to that of La and Sb doped BaSnO3 films, and have potential applications in the optoelectronic devices.

Characterization of superconducting Fe y (Se1−x Te x ) thin films deposited on MgO substrates by sputtering

We have investigated the feasibility of RF sputtering for the deposition of superconducting Fe y (Se1−x Te x ) (Fe-11) thin films on single-crystalline MgO substrates. Phase evolution and texture development as a function of processing conditions were studied, and the optimum conditions for high-quality films are reported. The epitaxial growth of the pure Fe-11 phase occurs in a temperature range of 300–320 °C with a critical film thickness of 50 nm. C-axis alignment is obtained in almost all processing conditions, while in-plane alignment is shown to be strongly dependent on the substrate temperature and the film thickness. When the substrate temperature exceeds 325 °C, an interlayer is formed between films and substrates which precludes epitaxial growth. Using sputtering with a single 3-element target, the deposited films tend to be enriched in Fe content and the excess Fe suppresses superconductivity, but a sharp transition at 10.2 K is found for the films with an Fe/(Se + Te) ratio close to 1.

Structural and Magnetic Properties of Ni<sub>50</sub>Mn<sub>35</sub>In<sub>15</sub> Thin Films

We present the results of macroscopic measurements, X-ray diffraction and neutron reflectivity experiments on ≈ 25 nm thin films of Ni50Mn35In15 grown using Pulsed laser deposition technique on MgO single-crystalline substrate. Intrinsic magnetization of the film below Tc ≈ 290 K was confirmed. Structural measurements show the large temperature-dependent residual strain on the substrate with no indication of martensitic transition.

Interfacial exchange coupling in cubic Heusler Co2FeZ (Z = Al and Si)/tetragonal Mn3Ga bilayers

We have fabricated bilayer films of tetragonal Heusler-like D022 Mn3Ga and cubic Heusler Co2FeZ (Z = Si and Al) on (100) single-crystalline MgO substrates and investigated their structural and interfacial exchange coupling. The coupling in the Mn3Ga/Co2FeAl bilayer was either ferromagnetic or antiferromagnetic, depending on annealing temperature, whereas only antiferromagnetic exchange coupling was observed in the Mn3Ga/Co2FeSi bilayers. The effects of annealing on the structure and coupling strength in the bilayers are discussed.

Analysis of superconducting FeSe thin films deposited by a sputtering technique

Superconducting FeSe thin films have been deposited onto (001)-oriented single-crystalline MgO substrates by conventional radio-frequency magnetron co-sputtering of a FeSe target and a pure Se target. The composition of the films is within the homogeneity range of the tetragonal β phase close to stoichiometry. Films with a minimum residual resistivity and a maximum superconducting transition temperature can be prepared within a narrow substrate temperature range from 480 to 510 °C. In contrast to the equilibrium phase diagram the β phase surprisingly exists in thin films deposited at substrate temperatures as high as 680 °C. The polycrystalline tetragonal films grow with a preferred c-axis texture and a tiny admixture of misaligned grains. The temperature-dependent resistivity reveals a low-temperature linear part and a high-temperature part with a pronounced negative curvature. The origin of the non-Fermi liquid behaviour and the underlying electron scattering mechanisms are still unknown so far.

Altering the exchange bias in Co90Fe10/(Co,Fe)O bilayers by changing the antiferromagnet's magnetism via interfacial ion-beam bombardment and different single crystalline MgO substrates

In this study, we investigated the exchange bias (coupling) effects in CoFe/(Co,Fe)O bilayers by using different single crystal substrates of MgO(100) and MgO(110) and Ar ion-beam bombardment on the surface of the bottom antiferromagnet (Co,Fe)O layer before capping with ferromagnet CoFe. In the CoFe/(Co,Fe)O/MgO(110) bilayer, above the irreversibility temperature (Tirr. ∼ 170 K), there was a rapid decrease in M(T) with increasing temperature, unlike the CoFe/(Co,Fe)O/MgO(100) film that showed an increased Tirr. ∼ 300 K and no observable decrease in M(T) above Tirr. The different M vs T zero-field-cooled/field-cooled behavior of the CoFe/(Co,Fe)O bilayers on MgO(100) and MgO(110) indicated that the FM CoFe spin orientations were affected by the different substrates used via exchange coupling to the AF (Co,Fe)O layer altered by MgO.

Synthesis and luminescence properties of Eu 2+ -doped 8-coordinated SrO phosphors

Abstract Eu2+-doped SrO (SrO:Eu2+) phosphors were synthesized and their luminescence properties were investigated. Phosphors doped with different Eu2+ concentrations were synthesized by the thermal treatment of SrO and Eu powders on a single crystalline MgO substrate at 1500 °C in a reducing atmosphere of Ar and H2. X-ray diffraction measurements showed that the synthesized phosphors contained 8-coordinated SrO with an orthorhombic crystal system. The phosphors showed a strong blue emission at 456 nm, which was attributed to the Eu2+ concentration. In addition, a phosphor doped with 2 at% Eu2+ showed an internal quantum efficiency of 30%.

Dielectric oxynitride LaTiOxNy thin films deposited by reactive radio-frequency sputtering

LaTiOxNy thin films have been deposited by RF sputtering on (001) Nb-doped SrTiO3 and (001) MgO single-crystalline substrates at high temperature (TS=800°C) under different nitrogen ratios in the plasma (vol.% N2=0, 25, 71). The band gaps ranged from Eg=3.30eV for the epitaxial transparent film containing no nitrogen to Eg=2.65eV for the textured coloured film containing a moderate amount of nitrogen. Dielectric characterization in the frequency range [100Hz–1MHz], using a metal-insulator-metal structure, has shown a stable permittivity and loss tangent of the epitaxial low-nitrided LaTiOxNy film with values of ε′=135 and tanδ=1.210−2 at 100kHz (RT).

Structural and Magnetic Properties of [Fe/Ni]N Multilayers

[Fe/Ni]N multilayered structure grows epitaxially on the single crystalline MgO substrate. Due to the different directions of magnetic easy axes of Fe and Ni and the strong strain, large anisotropy dispersion is assumed. According to the layer model, the magnetization of Fe and Ni layers cannot follow each easy axis because of exchange coupling, and then the anisotropies are averaged out. The reduction of the effective anisotropy enhances with the decrease of periodic thickness. Thus, the coercivity of [Fe/Ni]N multilayers reduces with decreasing periodic thickness.

Potential of Nd:YAG pulsed laser deposition method for coated conductor production

A Nd:YAG laser is environmentally safe and economical with no poisonous or hazardous gases and no expensive gases. We prepared Y123 films by using the fourth harmonic Nd:YAG pulsed laser deposition (PLD) method and optimized the deposition conditions on MgO single crystalline substrates and IBAD-MgO substrates for Y123 coated conductor. We found that the optimal deposition conditions acquired bi-axially aligned Y123 films on both substrates with Tc∼90K and Jc>1MA/cm2 at 77K in self-field. For obtaining high Ic, we fabricated thick Y123 films on both substrates and the maximum Ic per 1cm in width reached 186A/cm-width on the IBAD-MgO substrate. Interestingly, there were no a-axis oriented grains within the films up to 1.8μm thick. This might be an especial feature of the Nd:YAG-PLD method. We believe that the Nd:YAG-PLD method is promising method for RE123 coated conductor production.

Wear-resistant magnetic thin film material based on aTi1−xFexC1−ynanocomposite alloy

In this study we report on the film growth and characterization of thin (approximately 50 nm thick) Ti-Fe-C films deposited on amorphous quartz. The experimental studies have been complemented by first principles density functional theory (DFT) calculations. Upon annealing of as-prepared films, the composition of the metastable Ti-Fe-C film changes. An iron-rich phase is first formed close to the film surface, but with increasing annealing time this phase is gradually displaced toward the film-substrate interface where its position stabilizes. Both the magnetic ordering temperature and the saturation magnetization changes significantly upon annealing. The DFT calculations show that the critical temperature and the magnetic moment both increase with increasing Fe and C-vacancy concentration. The formation of the metastable iron-rich Ti-Fe-C compound is reflected in the strong increase of the magnetic ordering temperature. Eventually, after enough annealing time ($\geq 10$ minutes), nano-crystalline $\alpha$-Fe starts to precipitate and the amount and size of these precipitates can be controlled by the annealing procedure; after 20 minutes of annealing, the experimental results indicate a nano-crystalline iron-film embedded in a wear resistant TiC compound. This conclusion is further supported by transmission electron microscopy studies on epitaxial Ti-Fe-C films deposited on single crystalline MgO substrates where, upon annealing, an iron film embedded in TiC is formed. Our results suggest that annealing of metastable Ti-Fe-C films can be used as an efficient way of creating a wear-resistant magnetic thin film material.

Epitaxial Heterostructures of KNbO3/LaNiO3/SrTiO3 and KNbO3/La1-xKxMnO3/SrTiO3 : MOCVD preparation and properties

Epitaxial conducting electrodes (60-300 nm thick) of LaNiO3, La1-xKxMnO3 and ferroelectric KNbO3 thin films were deposited by MOCVD using thd-volatile precursors on SrTiO3 and MgO single crystalline substrates. Influence of epitaxial strain on properties is discussed. Ferroelectric properties of the title heterostructures were characterized by hysteresis measurements, piezo-response force microscopy and second harmonic generation.

Electrical and structural properties of ultrathin polycrystalline and epitaxial TiN films grown by reactive dc magnetron sputtering

AbstractUltrathin TiN films were grown by reactive dc magnetron sputtering on amorphous SiO2 substrates and single-crystalline MgO substrates at 600°C. The resistance of the films was monitored in-situ during growth to determine the coalescence and continuity thicknesses. TiN films grown on SiO2 are polycrystalline and have coalescence and continuity thicknesses of 8 Å and 19 Å, respectively. TiN films grow epitaxially on the MgO substrates and the coalescence thickness is 2 Å and the thickness where the film becomes continuous cannot be resolved from the coalescence thickness. X-ray reflection measurements indicate a significantly higher density and lower roughness of the epitaxial TiN films.