Shi-Jing Gong

Picosecond electric field pulse induced coherent magnetic switching in MgO/FePt/Pt(001)-based tunnel junctions: a multiscale study

Combined methods of first-principles calculations and Landau-Lifshitz-Gilbert (LLG) macrospin simulations are performed to investigate the coherent magnetization switching in the MgO/FePt/Pt(001)-based magnetic tunnel junctions triggered by short pulses of electric field through the control of magnetic anisotropy energy (MAE) electrically. First-principles calculations indicate that the MAE of MgO/FePt/Pt(001) film varies linearly with the change of the electric field, whereas the LLG simulations show that the change in MAE by electric field pulses could induce the in-plane magnetization reversal of the free layer by tuning the pulse parameters. We find that there exist a critical pulse width τmin to switch the in-plane magnetization, and this τmin deceases with the increasing pulse amplitude E0. Besides, the magnetization orientation cannot be switched when the pulse width exceeds a critical value τmax, and τmax increases asymptotically with E0. In addition, there exist some irregular switching areas at short pulse width due to the high precessional frequency under small initial angle. Finally, a successive magnetization switching can be achieved by a series of electric field pulses.

Concepts of ferrovalley material and anomalous valley Hall effect

Valleytronics rooted in the valley degree of freedom is of both theoretical and technological importance as it offers additional opportunities for information storage, as well as electronic, magnetic and optical switches. In analogy to ferroelectric materials with spontaneous charge polarization, or ferromagnetic materials with spontaneous spin polarization, here we introduce a new member of ferroic family, that is, a ferrovalley material with spontaneous valley polarization. Combining a two-band k·p model with first-principles calculations, we show that 2H-VSe2 monolayer, where the spin–orbit coupling coexists with the intrinsic exchange interaction of transition-metal d electrons, is such a room-temperature ferrovalley material. We further predict that such system could demonstrate many distinctive properties, for example, chirality-dependent optical band gap and, more interestingly, anomalous valley Hall effect. On account of the latter, functional devices based on ferrovalley materials, such as valley-based nonvolatile random access memory and valley filter, are contemplated for valleytronic applications.

Spintronic properties of graphene films grown on Ni(111) substrate

Graphene is believed to be a promising candidate for spintronic applications. In this study, we investigate the electronic, magnetic, and, especially, spintronic properties of graphene films grown on Ni(111) substrate using relativistic density-functional calculations. Enhanced Rashba spin-orbit coupling (SOC), with a magnitude of up to 20 meV—several orders of magnitude larger than the intrinsic SOC strength in freestanding graphene—is found at the graphene–Ni(111) interface. The hybridization between graphene’s pz states and Ni’s 3d states magnetizes the interfacial carbon atoms and induces a sizable exchange splitting in the π band of the graphene sheet. The calculated results agree well with the recently reported experimental data and provide a deep understanding of the spintronic behavior of graphene in contact with a 3d-ferromagnet.

Manipulation of magnetic anisotropy of Fe/graphene by charge injection

We propose that charge injection can be used to tune the magnetic anisotropy of transition metal monolayer adsorbed on graphene substrate. Using relativistic density-functional calculations, we calculate magnetocrystalline anisotropy energy (MAE) of freestanding Fe monolayer and Fe/graphene complex system. We find MAE of Fe atom is drastically changed, from meV/atom in freestanding Fe monolayer to μeV/atom in Fe/graphene system. The more interesting finding is, through charge injection, the suppressed MAE of Fe atoms in Fe/graphene system can be restored back, which provides an effective approach to control MAE. We expect such strategy would be beneficial to graphene based spintronic devices.

Manipulation of the large Rashba spin splitting in polar two-dimensional transition-metal dichalcogenides

Transition metal dichalcogenide (TMD) monolayers MXY (M=Mo, W, X(not equal to)Y=S, Se, Te) are two-dimensional polar semiconductors. Setting WSeTe monolayer as an example and using density functional theory calculations, we investigate the manipulation of Rashba spin orbit coupling (SOC) in the MXY monolayer. It is found that the intrinsic out-of-plane electric field due to the mirror symmetry breaking induces the large Rashba spin splitting around the Gamma point, which, however, can be easily tuned by applying the in-plane biaxial strain. Through a relatively small strain (from -2% to 2%), a large tunability (from around -50% to 50%) of Rashba SOC can be obtained due to the modified orbital overlap, which can in turn modulate the intrinsic electric field. The orbital selective external potential method further confirms the significance of the orbital overlap between W-dz2 and Se-pz in Rashba SOC. In addition, we also explore the influence of the external electric field on Rashba SOC in the WSeTe monolayer, which is less effective than strain. The large Rashba spin splitting, together with the valley spin splitting in MXY monolayers may make a special contribution to semiconductor spintronics and valleytronics.

Ferroelectric control of in-plane to out-of-plane magnetization switching at poly(vinylidene fluoride)/iron interface

Using relativistic density-functional theory calculations, we investigate magnetocrystalline anisotropy energy (MAE) of the poly(vinylidene fluoride)(PVDF)/Fe/Cu/Ag heterostructure. We find that MAE of this heterostructure can be flexibly manipulated by the ferroelectric polarization of PVDF. In particular, by carefully designing the interface structure, we demonstrate that the ferroelectric polarization reversal can switch the easy axis of the Fe layer from in-plane to out-of-plane, due to the surface/interface magnetoelectric effect. We expect such strategy would be beneficial to electric-field controlled magnetic data storage.

Spin-dependent optical response of multiferroic EuO: First-principles DFT calculations

Using first-principles density functional calculations, electronic and optical properties of ferromagnetic semiconductor EuO are investigated. In particular, we have developed a way to obtain the spin-dependent optical response of the magnetic materials, which is helpful to verify the spin-dependent band structure of EuO. Significantly different optical responses from spin-up and spin-down channels are obtained in both linear and nonlinear cases, making it possible to distinguish contributions from different spin channels in the optical absorption spectra if the spin-flip process can be neglected. In addition, the red-shift of the absorption edge from paramagnetic to ferromagnetic ordering is explained by exchange interactions. Using such a method, we have also compared the optical properties of multiferroic EuO which is induced by strong epitaxial strain. Our results show that from tensile to compressive strain, the blue-shift of the leading absorption peaks in the optical spectra, the red-shift of the optical band gap in the spin-up state can be observed, consistent to the energy difference between spin-splitting orbits. The spin-dependent nonlinear optical properties reveal that in the infrared and visible light regions, the contributions to second-harmonic generation (SHG) susceptibilities are mainly from spin-majority channels. In addition, the strain effect is also discussed. With the increase of epitaxial strain, the larger energy shift of the leading absorption peaks and the more remarkable nonlinear optical response can be obtained.

Improved multiferroic behavior in [111]-oriented BiFeO3/BiAlO3 superlattice

We report a systematic study on the structural, electronic, magnetic, and ferroelectric properties of [111]-oriented BiFeO3/BiAlO3 (BFO/BAO) superlattice using density-functional calculations. It is found that the Fe-O-Fe superexchange interactions in BFO/BAO superlattice are greatly suppressed by the inserted BAO layers, with the antiferromagnetic-ferromagnetic transition energy decreasing from around 280 meV per BFO formula unit (five atoms) to 11.6 meV per BFO/BAO formula unit (ten atoms). The tensile strain can further decrease this energy, making the magnetic transition more plausible. In addition, we find that BFO/BAO superlattice preserves the large ferroelectric polarization as well as energy gap of bulk BFO. Therefore, BAO may be a good candidate for constructing the BFO-based superlattice with improved multiferroicity.

First-principles studies of the magnetic anisotropy of the Cu/FePt/MgO system

Using first-principles density-functional theory calculations, we systematically investigate the magnetic anisotropy of the multilayer system Cu/(FePt)n/MgO, a promising spintronics structure. Particularly, we have studied the influence of the epitaxial strain, thickness of the ferromagnetic layer, and different interfaces on the magnetic anisotropy energy (MAE) of the system. It is found that the thickness of FePt has slight influence on the MAE, while the increase of the in-plane lattice constant a, or tensile strain, can significantly reduce and even change the sign of the MAE. The calculated density of states shows that the occupation number of the minority spin channel of Fe dx(2)-y(2) orbital decreases with the increase of a, which leads to the reduction of the orbital moment anisotropy of the Fe atom and therefore the decrease of MAE. We also consider the influence of the Cu/FePt and FePt/MgO interfaces on the MAE, and find that both interfaces can reduce the MAE. Especially, the effect of the Cu/FePt interface is more pronounced due to the increased occupation number of the minority spin channel of Fe dz(2) orbital.

Orbital control of Rashba spin orbit coupling in noble metal surfaces

Rashba spin orbit coupling (SOC) in noble metal surfaces is of great importance for the application of metal films in spintronic devices. By combining the density-functional theory calculations with our recently developed orbital selective external potential method, we investigate the Rashba SOC in the Shockley surface states of Au(111) and Ag(111). We find that the large Rashba SOC in the sp-character surface states of Au(111) is mainly contributed by the minor d-orbitals in the surface states. While for the sd-character surface states, although they are dominated by the d-orbitals, Rashba splitting is found to be rather small. Band structure analysis reveals that this is mainly because the sd-character surface states are well below the Fermi level and can be less influenced by the asymmetric surface potential. We demonstrate that the Rashba SOC in noble metal surfaces can be effectively manipulated by shifting the d-orbitals in the surface states, which can be physically implemented through surface decoration. Our investigation provides a deep understanding on Rashba SOC in noble metal surfaces and could be helpful to their applications in spintronic devices.