Magnetic Silicide Research Articles

Large Magnetoresistance and Perfect Spin-Injection Efficiency in Two-Dimensional Strained VSi2N4 -Based Room-Temperature Magnetic-Tunnel-Junction Devices

Two-dimensional (2D) materials provide a promising platform for exploring spintronic devices. However, the low Curie temperature of most 2D magnetic materials limits their development and application. Based on density-functional theory and the nonequilibrium-Green's-function formalism, we present a systematic simulation study of room-temperature strained ${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$-based magnetic tunnel junctions (MTJs), using a high-accuracy Heyd-Scuseria-Ernzerhof (HSE) functional. In contrast to Perdew-Burke-Ernzerhof results, a spin-conductance match is observed in $\mathrm{Ag}/{\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$-${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}/\mathrm{Ag}$ MTJs at the HSE06 level. Thus, a 1200% tunnel-magnetoresistance (TMR) ratio and a perfect spin-injection efficiency are theoretically predicted in $\mathrm{Ag}/{\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$-${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}/\mathrm{Ag}$ MTJs. Interestingly, the coexistence of a Weyl semimetal and a half-metal state is found in 1.965% tensile-biaxial-strained monolayer ${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$. In addition, a slight compressive strain leads to a boost of the TMR ratio by 2 orders of magnitude, up to ${10}^{5}\mathrm{%}$ in strained ${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$-based MTJs. Other MTJs based on ${\mathrm{VSi}}_{2}{\mathrm{P}}_{4}$, ${\mathrm{VSi}}_{2}{\mathrm{As}}_{4}$, and ${\mathrm{Nb}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ are investigated. Our results show that these spin-polarized magnetic silicide compounds, with Curie temperatures close to room temperature, are promising candidates for next-generation spintronic devices.

Low-Temperature Growth of Graphene on a Semiconductor

The industrial realization of graphene has so far been limited by challenges related to the quality, reproducibility, and high process temperatures required to manufacture graphene on suitable substrates. We demonstrate that epitaxial graphene can be grown on transition metal treated 6H-SiC(0001) surfaces, with an onset of graphitization starting around $450-500^\circ\text{C}$. From the chemical reaction between SiC and thin films of Fe or Ru, $\text{sp}^{3}$ carbon is liberated from the SiC crystal and converted to $\text{sp}^{2}$ carbon at the surface. The quality of the graphene is demonstrated using angle-resolved photoemission spectroscopy and low-energy electron diffraction. Furthermore, the orientation and placement of the graphene layers relative to the SiC substrate is verified using angle-resolved absorption spectroscopy and energy-dependent photoelectron spectroscopy, respectively. With subsequent thermal treatments to higher temperatures, a steerable diffusion of the metal layers into the bulk SiC is achieved. The result is graphene supported on magnetic silicide or optionally, directly on semiconductor, at temperatures ideal for further large-scale processing into graphene based device structures.

Microstructural, magnetic and magneto-transport properties of NiO thin film deposited on Si (100) substrates

NiO thin film on p- and n-type Si (100) substrates were fabricated by electron beam evaporation technique and studied by structural, morphological, magnetic and magneto-electronic transport characterization techniques. The room temperature interfacial chemistry across the interfaces is found to affect the morphology, transport and magnetic behaviour of the structures. Structural study has shown the presence of magnetic nickel silicide phases along with some metallic phases as a result of interfacial intermixing. Isolated nano-granular features of magnetic grains along with the presence of clustered grains are observed. Strong magnetic signal strength is observed for NiO/nSi structure as compared to the interface on pSi substrate which suggests the formation of more magnetic silicide phases resulting in a higher magnetization value with increased coercivity. Magnetization characteristics of the NiO thin films on Si substrates have shown a nearly superparamagnetic behaviour with weak ferromagnetic contribution as compared to bulk antiferromagnetic nature of NiO. Electronic transport study measured across NiO/nSi interface have shown the enhancement in conductivity up to 2–3 orders of magnitude than for NiO film on pSi substrate with significant magnetic field sensitivity. Magnetic field induced enhanced current has been observed for NiO/nSi structure which is related to the magnetic field response of magnetic grains (of nickel silicide phases) or isolated magnetic clusters favouring the paths for spin alignment or spin-dependent scattering resulting in a negative magnetoresistance of ~50 %.

Effect of swift heavy ion irradiation on magnetic, surface morphology and electronic transport across CoFe/n-Si interfacial structures

In this study, swift heavy ion induced modifications on magnetic, morphological and electronic transport properties of CoFe/n-Si bilayers was investigated. Structural investigations have revealed the interfacial intermixing across the interface upon irradiation to result in the formation of magnetic silicide phases with enhanced crystallite size as compared to unirradiated structure. On irradiation, surface topography (from atomic force microscopy) has revealed the columnar arrangement of grains with increased value of rms surface roughness which in turn also affects the magnetic behaviour. Magnetization measurements have shown the enhancement in saturation magnetization and coercivity value with increased magnetic signal strength after irradiation. Current–voltage measurement across the irradiated CoFe/n-Si interface has shown the enhancement in current data by two orders of magnitude as compared to unirradiated interface. The observed significant changes in magnetic and transport properties for the irradiated interface has been explained on the basis of disorder/defect creation and interfacial chemistry modifications in the structure due to swift heavy ions.

Magnetic resonance in ion‐beam synthesized Fe3Si films (computer simulation)

AbstractHigh dose Fe+ ion implantation into Si assisted by an external magnetic field parallel to silicon surface results in the formation of thin granular film with pronounced uniaxial magnetic anisotropy in the film plane. It was suggested that the anisotropy is caused by the growth of elongated clusters of magnetic silicide Fe3Si. In the present work, the features of magnetic resonance spectra for two‐dimensional array of elongated clusters are numerically studied. Absorption spectra reveal anisotropy when observed in the magnetic field lying in the film plane. In magnetic field perpendicular to the film the dipole‐dipole interaction between the clusters results in a bimodal resonance signal at low level of the film filling. The dipolar field distribution over the (400x400) lattice for several values of the lattice filling is computer simulated. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The formation of magnetic silicide Fe3Si clusters during ion implantation

A simple two-dimensional model of the formation of magnetic silicide Fe3Si clusters during high-dose Fe ion implantation into silicon has been proposed and the cluster growth process has been computer simulated. The model takes into account the interaction between the cluster magnetization and magnetic moments of Fe atoms random walking in the implanted layer. If the clusters are formed in the presence of the external magnetic field parallel to the implanted layer, the model predicts the elongation of the growing cluster in the field direction. It has been proposed that the cluster elongation results in the uniaxial magnetic anisotropy in the plane of the implanted layer, which is observed in iron silicide films ion-beam synthesized in the external magnetic field.

Irradiation Induced Modifications in Magnetic Property of Mn/n-Si Structure

Mn films of ~50 nm were deposited on n-Si (100) substrates by electron beam evaporation technique. The Mn/n-Si structures were irradiated from swift heavy ions with a fluence of 1×1013 ions/cm2. The irradiated and unirradiated structures have been characterized from atomic force microscopy, X-ray diffractometry and vibrating sample magnetometer facilities. It has been found that surfacial/interfacial granular magnetic silicide phases (of MnxSiy) are formed before and after the irradiation with irradiation induced modifications of surface morphology and magnetic property. The surface roughness has been found to decrease on the irradiation from the atomic force microscopy data. From the X-ray diffraction data, it has been found that after the irradiation MnSi is formed in addition to Mn5Si2 as compared to unirradiated ones. The magnetization characteristics show that the magnetic parameters such as coercivity, saturation magnetization, remanance and squareness have decreased on the irradiation for in-plane orientation whereas coercivity and remanance is increased after the irradiation for out of plane orientation. The increase of remanance shows the presence of strong exchange coupling in the structure after the irradiation.

Nano granular magnetic phase by interfacial intermixing in [formula omitted]/Si bilayer structure

The bilayer structures of Fe 50Ni 50 films on silicon substrate have been studied before and after the swift (∼100 MeV) Fe 7+ ion irradiation. The AFM (atomic force microscopy) study of the structures shows the formation of clear ellipsoidal grains of nano dimensions. XRD (X-ray diffraction) data of the structures show the formation of magnetic silicide phases of Fe 5Si 3, Ni 3Si 2 and Ni 2Si. Electronic flow across the interface has been found to significantly increase by two orders of magnitude on the irradiation. Moreover the electronic flow shows a low magnetic field (of ∼500 G) sensitivity for the irradiated structures and not for the unirradiated ones. The M – H (magnetization versus magnetic field) characteristics show the typical behaviour of nano magnetic grains which are antiferromagnetically coupled in a paramagnetic phase. The observed results have been understood in the realm of interfacial intermixing (which has been further tailored by the irradiation) to result the nano granular phases of magnetic silicides (of ∼50 nm). The intermixing seems to make the interface intimate enough to result in a significant increase of current across interface of the bilayer and its sensitivity for an in plane low magnetic field.

Growth and magnetic properties of the sandwich structure Fe/magnetic silicide/Si(100) obtained from in situ optic and magneto-optic data

It is demonstrated by the Surface Magneto-Optic Kerr Effect and Differential Reflectance Spectroscopy methods that structures free of magnetically dead layers can be created by the deposition of iron at room temperature onto a prefabricated magnetic silicide layer. The magnetic silicide can be formed by the deposition of iron at 70 ∘C onto a layer of amorphous silicon prefabricated on Si(100). Both in the silicide and the iron film, magnetism onsets after the iron amount deposited reaches some critical value. The spontaneous magnetization vector in the iron film changes its direction twice during the film growth. Sufficiently thick iron films persist being ferromagnetic in air for years.

Giant magnetoresistance (GMR) in swift heavy ion irradiated Fe films on c-silicon (Fe/c-Si)

Fe/c-Si devices have been irradiated with 100 MeV swift heavy ions of Fe7+ at a dose of 1014 ions cm−2. The devices have been studied using XRD and SEM. Electronic transport across the interface of the devices (i.e. in current perpendicular to the plane (CPP) of the device) has been measured from room temperature to liquid N2 temperature. The CPP current has also been studied in a magnetic field (up to 10 kG which has been applied along the plane of the device). Unirradiated devices do not show any effect of the magnetic field whereas large magnetoresistance (MR) up to 2400% giant magnetoresistance (GMR) has been observed for the irradiated devices. An M–H study of the irradiated devices shows a behaviour of coupled magnetic nanograins. The results have been understood by considering the formation of a nanogranular magnetic silicide phase (of Fe5Si3) due to intermixing at the interface (as evidenced from XRD and SEM features). The electronic and magnetotransport characteristics of the irradiated devices show that the interface becomes intimate enough (due to the irradiation induced strong intermixing) to result in a tunnel transmission of carriers. A tunnelling barrier seems to form (for the irradiated ones) between Fe5Si3 magnetic nanocrystals separated by a nanometre scaled silicon tunnelling barrier. The observed very large (strong) GMR could be due to the spin dependent interface scattering in the presence of the strong AF coupling across the tunnelling barrier.

Effect of MgO, Y 2O 3, and Fe 2O 3 on silicothermal synthesis and sintering of X-sialon. An XRD, multinuclear MAS NMR and 57Fe Mössbauer study

The separate effects of 10 wt.% MgO, Y 2O 3 and Fe 2O 3 on the silicothermal formation and sintering of X-sialon were investigated by XRD, multinuclear MAS NMR and Mössbauer spectroscopy. The effects of the three oxides on densification follow a similar trend to samples containing 1 wt.% oxide (Y>Mg>Fe), but at the higher additive concentration, the maximum densities are higher and are achieved at lower temperature (1400°C). 10 wt.% MgO and Y 2O 3 facilitate low-temperature mullite formation and its conversion to X-sialon by forming increased amounts of liquid phases. The 25Mg NMR spectra of the MgO-containing samples suggest that their liquid phase is a N-containing Mg silicate glass. These samples also form MgAl 2O 4, depleting the amount of Al available to form X-sialon. The Y 2O 3-containing samples form liquid of a similar composition to N-melilite, depleting the available Si, with the excess alumina appearing as corundum. Under silicothermal conditions, Fe 2O 3 reacts with the Si to form a mixture of magnetic silicide and aluminide, suppressing X-sialon formation by depleting the Si but enhancing the formation and stability of mullite by Fe-for Al substitution in the mullite lattice. Some of the magnetic phases may occur as very fine superparamagnetic particles possibly located in an X-sialon matrix.