Generation Spintronic Devices Research Articles

All-spin logic operations: Memory device and reconfigurable computing

Exploiting spin degree of freedom of electron a new proposal is given to characterize spin-based logical operations using a quantum interferometer that can be utilized as a programmable spin logic device (PSLD). The ON and OFF states of both inputs and outputs are described by spin state only, circumventing spin-to-charge conversion at every stage as often used in conventional devices with the inclusion of extra hardware that can eventually diminish the efficiency. All possible logic functions can be engineered from a single device without redesigning the circuit which certainly offers the opportunities of designing new generation spintronic devices. Moreover, we also discuss the utilization of the present model as a memory device and suitable computing operations with proposed experimental setups.

Formation of ferromagnetic/ferrimagnetic epitaxial interfaces: Stability and magnetic properties

We have studied the formation of epitaxial ferromagnetic/ferrimagnetic MnGa(0 0 1)/Mn4N(0 0 1) heterostructures by using spin polarized first principles calculations. In particular, we have analyzed the stability of the surface and interface as function of the N content. Since the Mn4N(0 0 1) has two stable surface terminations with a (1 × 1) periodicity, we have used them as substrates to grow MnGa. The interface formation energy formalism has been applied to describe the stability of all proposed models. Our calculations show that Mn at the interface is energetically favorable. Moreover, stability of the MnGa surface has a direct nitrogen dependence. Specifically, without extra N, the Ga-terminated surface is stable in the whole range of chemical potential for both substrates. Once N is added to the surface, the MnN-terminated surface becomes stable in both substrates for different ranges of chemical potential. Although MnGa and Mn4N are not antiferromagnetic materials, they form a local antiferromagnetic-like arrangement at the interface. This thermodynamically stable system could have potential applications in the construction of new generation spintronic devices.

3d-4f coupling and multiferroicity in frustrated Cairo Pentagonal oxide DyMn2O5

In solid state science, multifunctional materials and especially multiferroics have attracted a great deal of attention, as they open the possibility for next generation spintronic and data storage devices. Interestingly, while many of them host coexisting 3d and 4f elements, the role of the coupling between these two magnetic entities has remained elusive. By means of single crystal neutron diffraction and inelastic neutron scattering experiments we shed light on this issue in the particular case of the multiferroic oxide DyMn2O5. This compound undergoes a first order magnetic transition from a high temperature incommensurate phase to a low temperature commensurate one. Our investigation reveals that although these two phases have very different magnetic structures, the spin excitations are quite similar indicating a fragile low temperature ground state with respect to the high temperature one. Such a rare scenario is argued to be a manifestation of the competition between the exchange interaction and 4f magnetic anisotropy present in the system. It is concluded that the magnetic structure, hence the ferroelectricity, can be finely tuned depending on the anisotropy of the rare earth.

Quantum size effect on charges and phonons ultrafast dynamics in atomically controlled nanolayers of topological insulators Bi2Te3

Heralded as one of the key elements for next generation spintronics devices, topological insulators (TIs) are now step by step envisioned as nanodevices like charge-to-spin current conversion or as Dirac fermions based nanometer Schottky diode for example. However, reduced to few nanometers, TIs layers exhibit a profound modification of the electronic structure and the consequence of this quantum size effect on the fundamental carriers and phonons ultrafast dynamics has been poorly investigated so far. Here, thanks to a complete study of a set of high quality molecular beam epitaxy grown nanolayers, we report the existence of a critical thickness of around ~6 nm, below which a spectacular reduction of the carrier relaxation time by a factor of ten is found in comparison to bulk Bi2 Te3 In addition, we also evidence an A1g optical phonon mode softening together with the appearance of a thickness dependence of the photoinduced coherent acoustic phonons signals. This drastic evolution of the carriers and phonons dynamics might be due an important electron-phonon coupling evolution due to the quantum confinement. These properties have to be taken into account for future TIs-based spintronic devices.

Exploring the potential of remote plasma sputtering for the production of L10 ordered FePt thin films

Lowering the temperature at which the desirable L10 phase forms in FePt thin films is a key requirement in the development of next generation high-density data storage media and spintronic devices. Remote plasma sputtering offers a higher degree of control over the sputtering parameters, allowing the properties of films to be tailored, and potentially can affect the ordering kinetics of the L10 phase of FePt. Here, we report a comprehensive study of FePt thin films deposited under a range of temperatures and sputtering conditions. X-ray diffraction and magnetometry investigations show that whilst FePt thin films ordered in the L10 phase with high perpendicular anisotropy can be produced using this technique, there is no significant reduction in the required ordering temperature compared with films produced using conventional DC sputtering. Optimally ordered L10 FePt films were fabricated when the film was deposited at a substrate temperature of 200 °C, followed by post annealing at 750 °C.

Strong Modulation of Spin Currents in Bilayer Graphene by Static and Fluctuating Proximity Exchange Fields.

Two-dimensional materials provide a unique platform to explore the full potential of magnetic proximity-driven phenomena, which can be further used for applications in next-generation spintronic devices. Of particular interest is to understand and control spin currents in graphene by the magnetic exchange field of a nearby ferromagnetic material in graphene-ferromagnetic-insulator (FMI) heterostructures. Here, we present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to FMI magnetization. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. Additionally, we discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the nonlocal spin signals in graphene, which is due to spin relaxation by thermally induced transverse fluctuations of the FMI magnetization.

Physicochemical analysis of Bi2Te3 – (Fe, Eu) – Bi2Te3 junctions grown by molecular beam epitaxy method

Topological insulators (TI) are a class of materials gaining in importance due to their unique spin/electronic properties, which may allow for the generation of quasiparticles and electronic states which are not accessible in classical condensed-matter systems. Not surprisingly, TI are considered as promising materials for multiple applications in next generation electronic or spintronic devices, as well as for applications in energy conversion, such as thermo-electrics. In this study, we examined the practical challenges associated with the formation of a well-defined junction between a model 3D topological insulator, Bi2Te3, and a metal, Fe or Eu, from which spin injection could potentially be realized. The properties of multilayer systems grown by molecular beam epitaxy (MBE), with Fe or Eu thin films sandwiched between two Bi2Te3 layers, were studied in-situ using electron diffraction and photoelectron spectroscopy. Their magnetic properties were measured using a SQUID magnetometer, while the in-depth chemical structure was assessed using secondary ion mass spectroscopy. An examination of impact of Bi2Te3 structure on chemical stability of the junction area has been realized. For Fe, we found that despite room temperature growth, a reaction between the Fe film and Bi2Te3 takes place, leading to the formation of FeTe and also the precipitation of metallic Bi. For the Eu tri-layer, a reaction also occurs, but the Te chemical state remains intact.

Interfacial perpendicular magnetic anisotropy and magnetic properties of Pd/Co2FeAl0.5Si0.5/MgO structured films

A series of Co2FeAl0.5Si0.5 (CFAS) films sandwiched by Pd and MgO layers are sputtered and subsequently annealed at various temperatures. Perpendicular magnetic anisotropy is shown to build in the structures with CFAS thickness ranging from 3 to 4.25nm after annealing. The thickness of a magnetic dead layer and the saturation magnetization are shown to increase with the annealing temperature. Perpendicular magnetic anisotropy is determined to be of 1–2×106erg/cm3. The plot of thickness dependent anisotropy indicates that the perpendicular anisotropy is contributed by the interfacial anisotropy, which is estimated to be 1.03erg/cm2 for the structures annealed at 300°C. The positive contributions to the PMA made by the interfaces of CFAS/MgO and Pd/CFAS are also identified. Our results provide some useful information on the design and integration of CFAS into the next generation of spintronic devices.

Disentanglement of bulk and interfacial spin Hall effect in ferromagnet/normal metal interface

Spin Hall effect in PdPt alloys in contact with ferromagnetic ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ alloys has been studied by spin torque ferromagnetic resonance technique. The spin torque spin Hall angle (ST-SHA) proves to be contributed by the interfacial and bulk SHAs. The bulk SHA is dominated by the skew scattering and reaches a maximal value for the largest randomization of Pt and Pd atoms at the intermediate alloy composition. In particular, the interfacial SHA becomes prominent for Pt-rich alloys. This phenomenon indicates the enhanced Rashba spin-orbit coupling at the interface, as Pt has a stronger spin-orbit coupling and larger $z$-potential gradient compared to Pd. The present work highlights the interfacial SHA and provides a pathway to improve the functionality and performance of the next generation spintronic devices.

Direct Observation of Interfacial Dzyaloshinskii-Moriya Interaction from Asymmetric Spin-wave Propagation in W/CoFeB/SiO2 Heterostructures Down to Sub-nanometer CoFeB Thickness.

Interfacial Dzyaloshinskii-Moriya interaction (IDMI) is important for its roles in stabilizing the skyrmionic lattice as well as soliton-like domain wall motion leading towards new generation spintronic devices. However, achievement and detection of IDMI is often hindered by various spurious effects. Here, we demonstrate the occurrence of IDMI originating primarily from W/CoFeB interface in technologically important W/CoFeB/SiO2 heterostructures using Brillouin light scattering technique. Due to the presence of IDMI, we observe asymmetry in the peak frequency and linewidth of the spin-wave spectra in the Damon-Eshbach (DE) geometry at finite k wave-vectors. The DMI constant is found to scale as the inverse of CoFeB thickness, over the whole studied thickness range, confirming the presence of IDMI in our system without any extrinsic effects. Importantly, the W/CoFeB interface shows no degradation down to sub-nanometer CoFeB thickness, which would be useful for devices that aim to use pronounced interface effects.

Spin Orbit Coupling Controlled Spin Pumping and Spin Hall Magnetoresistance Effects

Effective spin mixing conductance (ESMC) across the nonmagnetic metal (NM)/ferromagnet interface, spin Hall conductivity (SHC), and spin diffusion length (SDL) in the NM layer govern the functionality and performance of pure spin current devices. It is shown that all three parameters can be tuned significantly via the spin orbit coupling (SOC) strength of the NM layer by virtue of the unique Pd1‐xPtx/Y3Fe5O12 system. Surprisingly, the ESMC is observed to increase significantly with x changing from 0 to 1.0, due to the enhanced local density of states for Pt‐rich alloys. The SHC in PdPt alloys turns out to be dominated by the skew scattering term. In particular, the skew scattering parameter has for the first time been rigorously demonstrated to increase with increasing SOC strength. Meanwhile, the SDL is found to decrease when Pd atoms are replaced by heavier Pt atoms, validating the SOC induced spin flip scattering model in polyvalent PdPt alloys. A thorough grasp of the dependence of these parameters on the SOC strength in the present work can help to develop a better theoretical graph on the physics of SOC and spin orbit torque switching, relevant to next generation spintronic devices.

Exotic quantum spin Hall effect and anisotropic spin splitting in carbon based TMC6 (TM = Mo, W) kagome monolayers

Two dimensional topological insulators with high feasibility and room-temperature band gaps are desirable at present. Here, kagome monolayers TMC6 (TM = Mo, W) are systematically investigated by density functional theory and molecular dynamics simulations. TMC6 lattice with one TM layer sandwiched between two trigonal carbon layers is proved to be stable. We identify that band inversion occurs in TMC6 with opened gaps up to 226 meV, making the system suitable for room temperature applications. By analyzing orbital resolved band structures and partial charge densities, we determine that the band inversion is mainly contributed by TM-d orbitals after introducing spin orbit coupling. The nontrivial topological feature is confirmed by direct calculation of Z2 invariant (Z2 = 1) indicating that TMC6 is quantum spin Hall (QSH) insulator. Distinct from previous QSH insulators, there is spin splitting along Γ→K direction (perpendicular to the mirror plane), while there is no splitting along Γ→M direction (parallel to the mirror plane). Further investigation unravels that the anisotropic spin splitting could be attributed to the in-plane structural inversion asymmetry. TMC6 kagome monolayers with nontrivial topology and anisotropic spin splitting could open up a way for new generation spintronic and electronic devices.

Achieving room temperature ferromagnetism in ZnS nanoparticles via Eu3+ doping

This paper reports the structural as well as the magnetic properties of ZnS:Eu3+ (0, 1, 2, 3, and 4at%) nanoparticles prepared by a hydrothermal method. The compositional, structural and magnetic properties of ZnS:Eu3+ nanoparticles were investigated by energy dispersive X-ray spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry (VSM) measurements. XRD studies confirmed that all the samples had a cubic structure with good crystallinity. TEM showed that the particles were polycrystalline with a mean size of 5–8nm. XPS revealed the oxidation state of the Eu in the ZnS lattice to be +3. All the Eu3+ -doped ZnS nanoparticles exhibited room temperature ferromagnetic behavior as a function of doping concentration. The observed RTFM may be interpreted by the contribution of the magnetic moments from the unpaired 4f electrons of Eu3+ in the ZnS:Eu3+ nanoparticles. The interesting magnetic properties of ZnS:Eu3+ nanoparticles may be further explored for new generation spintronic devices.

Electric field controlled spin interference in a system with Rashba spin-orbit coupling

There have been intense research efforts over the last years focused on understanding the Rashba spin-orbit coupling effect from the perspective of possible spintronics applications. An important component of this line of research is aimed at control and manipulation of electron’s spin degrees of freedom in semiconductor quantum dot devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction in a two-dimensional electron system embedded in a host semiconducting material that lacks inversion-symmetry. This way, the Rashba spin-orbit coupling effect may potentially lead to fabrication of a new generation of spintronic devices where control of spin, thus magnetic properties, is achieved via an electric field and not a magnetic field. In this work we investigate theoretically the electron’s spin interference and accumulation process in a Rashba spin-orbit coupled system consisting of a pair of two-dimensional semiconductor quantum dots connected to each other via two conducting semi-circular channels. The strength of the confinement energy on the quantum dots is tuned by gate potentials that allow “leakage” of electrons from one dot to another. While going through the conducting channels, the electrons are spin-orbit coupled to a microscopically generated electric field applied perpendicular to the two-dimensional system. We show that interference of spin wave functions of electrons travelling through the two channels gives rise to interference/conductance patterns that lead to the observation of the geometric Berry’s phase. Achieving a predictable and measurable observation of Berry’s phase allows one to control the spin dynamics of the electrons. It is demonstrated that this system allows use of a microscopically generated electric field to control Berry’s phase, thus, enables one to tune the spin-dependent interference pattern and spintronic properties with no need for injection of spin-polarized electrons.

Atomic-Scale Engineering of Abrupt Interface for Direct Spin Contact of Ferromagnetic Semiconductor with Silicon.

Control and manipulation of the spin of conduction electrons in industrial semiconductors such as silicon are suggested as an operating principle for a new generation of spintronic devices. Coherent injection of spin-polarized carriers into Si is a key to this novel technology. It is contingent on our ability to engineer flawless interfaces of Si with a spin injector to prevent spin-flip scattering. The unique properties of the ferromagnetic semiconductor EuO make it a prospective spin injector into silicon. Recent advances in the epitaxial integration of EuO with Si bring the manufacturing of a direct spin contact within reach. Here we employ transmission electron microscopy to study the interface EuO/Si with atomic-scale resolution. We report techniques for interface control on a submonolayer scale through surface reconstruction. Thus we prevent formation of alien phases and imperfections detrimental to spin injection. This development opens a new avenue for semiconductor spintronics.

Comparative study of Fe doped ZnO based diluted and condensed magnetic semiconductors in wurtzite and zinc-blende structures by first-principles calculations

Magnetic semiconductors with simultaneous semiconducting and magnetic characteristics are significant for applications in next generation spintronic devices. However, efficiency of these materials strongly relies on the selection of the proper host and dopant/alloying materials. In this work, we explore magnetic semiconductors based on the most appropriate materials namely ZnO doped with Fe in wurtzite (w) and zinc-blende (zb) structures. For comprehensive analysis, Fe has been doped into ZnO for several Fe concentrations such as 6.25%, 12.5%, 18.5% and 25%. Investigations are achieved using density functional theory (DFT) based full potential linearized augmented plane wave plus local orbital FP-L(APW+lo) method. The exchange correlation energy has been determined using Perdew et al. proposed generalized gradient approximations (GGA) with additional Hubbard (U) parameter as well. Our results show that; in w-structure, Fe:ZnO favors antiferromagnetism (AFM) at ground state, whereas in zb structure, ferromagnetism (FM) is dominated at 6.25% and 12.5% dopant concentration. However, for 18.75% and 25% dopant concentration, AFM interactions are dominated over FM, possibly is caused by the occurrence of anti-ferromagnetic secondary phases. Moreover, effect of mismatching ionic radii of Fe and Zn atoms, and formation of secondary phases is noticed on lattice parameters of Fe:ZnO with Fe concentration. The electronic and magnetic properties of Fe:ZnO endorse them suitable for applications in spin based electronic devices.

Electric field control of spin splitting in III–V semiconductor quantum dots without magnetic field

We provide an alternative means of electric field control for spin manipulation in the absence of magnetic fields by transporting quantum dots adiabatically in the plane of two-dimensional electron gas. We show that the spin splitting energy of moving quantum dots is possible due to the presence of quasi-Hamiltonian that might be implemented to make the next generation spintronic devices of post CMOS technology. Such spin splitting energy is highly dependent on the material properties of semiconductor. It turns out that this energy is in the range of meV and can be further enhanced with increasing pulse frequency. In particular, we show that quantum oscillations in phonon mediated spin-flip behaviors can be observed. We also confirm that no oscillations in spin-flip behaviors can be observed for the pure Rashba or pure Dresselhaus cases.

New generation of two-dimensional spintronic systems realized by coupling of Rashba and Dirac fermions.

Intriguing phenomena and novel physics predicted for two-dimensional (2D) systems formed by electrons in Dirac or Rashba states motivate an active search for new materials or combinations of the already revealed ones. Being very promising ingredients in themselves, interplaying Dirac and Rashba systems can provide a base for next generation of spintronics devices, to a considerable extent, by mixing their striking properties or by improving technically significant characteristics of each other. Here, we demonstrate that in BiTeI@PbSb2Te4 composed of a BiTeI trilayer on top of the topological insulator (TI) PbSb2Te4 weakly- and strongly-coupled Dirac-Rashba hybrid systems are realized. The coupling strength depends on both interface hexagonal stacking and trilayer-stacking order. The weakly-coupled system can serve as a prototype to examine, e.g., plasmonic excitations, frictional drag, spin-polarized transport, and charge-spin separation effect in multilayer helical metals. In the strongly-coupled regime, within ~100 meV energy interval of the bulk TI projected bandgap a helical state substituting for the TI surface state appears. This new state is characterized by a larger momentum, similar velocity, and strong localization within BiTeI. We anticipate that our findings pave the way for designing a new type of spintronics devices based on Rashba-Dirac coupled systems.

Magnetic reversal in Mn5Ge3 thin films: an extensive study

We present a comprehensive study of magnetization reversal process in thin films of . For this investigation, we have studied the magnetic anisotropy of layers as a function of the film thickness using VSM and SQUID magnetometers. The samples grown by molecular beam epitaxy exhibit a reorientational transition of the easy axis of magnetization from in-plane to out-of-plane as the film thickness increases. We provide evidence that above a critical thickness estimated as 20 nm, the magnetic structure is most probably constituted of stripes with out-of-plane magnetization pointing alternately up and down. We have analyzed our results using different phenomenological models and all the calculations converge towards values for magnetocrystalline anisotropy constant and saturation magnetization that are in excellent agreement with the reported values for bulk . This study has also led to the first estimation in of the exchange constant, the surface energy of domain walls as well as their width. These parameters are essential for determining whether this material can be used in the next generation of spintronic devices.

Tunnel magnetoresistance in thermally robust Mo/CoFeB/MgO tunnel junction with perpendicular magnetic anisotropy

We report on tunnel magnetoresistance and electric-field effect in the Mo buffered and capped CoFeB/MgO magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy. A large tunnel magnetoresistance of 120% is achieved. Furthermore, this structure shows greatly improved thermal stability and stronger electric-field-induced modulation effect in comparison with the Ta/CoFeB/MgO-based MTJs. These results suggest that the Mo-based MTJs are more desirable for next generation spintronic devices.