Yu Zhang

Bio: Yu Zhang is an academic researcher from Fudan University. The author has contributed to research in topics: Condensed matter physics & Materials science. The author has an hindex of 2, co-authored 3 publications receiving 10 citations.

Temperature Dependence of Spin-Orbit Torques in Nearly Compensated Tb21Co79 Films by a Topological Insulator Sb2Te3.

Abstract: Topological insulators (TIs) with spin-momentum-locked metallic surface states can exert giant spin-orbit torques, offering great potential in energy-efficient magnetic memory devices. In this work, temperature (T)-dependent SOT efficiencies are investigated in Sb2Te3/Ta/TbCo heterostructures with perpendicular magnetic anisotropy. The spin Hall angle θSH is around 0.16 at room temperature (RT), which is much higher than that of the control sample without TI. Moreover, as T decreases from RT down to 10 K, θSH exhibits a conspicuous 5-fold enhancement. Detailed analysis indicates that the θSH enhancement at reduced temperatures mainly results from the improved spin-polarized surface states, as evidenced from the continuously increased ratio of surface-to-bulk conduction. The θSH difference between 20 and 10 nm Sb2Te3 gradually shrinks with the increase of T, which is due to the increase of bulk state contribution. Our findings provide a deep insight into the spin transport mechanisms and robust charge-spin conversion in TIs.

Significant and Nonmonotonic Dynamic Magnetic Damping in Asymmetric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Co</mml:mi></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Fe</mm

Abstract: Static and dynamic magnetic properties of $\mathrm{Co}$-$\mathrm{Fe}$(10 nm)/$\mathrm{Ru}$$({t}_{\mathrm{Ru}}=0\ensuremath{-}3\phantom{\rule{0.25em}{0ex}}\mathrm{nm})$/$\mathrm{Co}$-$\mathrm{Fe}$(5 nm) asymmetric trilayers are systematically investigated. The interlayer exchange coupling (IEC) strengths of bilinear $({J}_{1})$, biquadratic $({J}_{2})$, and their equivalent $({J}_{\mathrm{eff}})$ terms are determined; they show a weak nonmonotonic behavior with ${t}_{\mathrm{Ru}}$. Interestingly, the magnetic remanence ratio \ensuremath{\eta} of hard axis to easy axis has two distinct peaks; this can reasonably be interpreted by taking the strong ${J}_{2}$ term into account. Various pump-laser fluences are utilized to modulate the static IEC during the time-resolved magneto-optical Kerr effect measurements, by which individual magnetization precessions of the two $\mathrm{Co}$-$\mathrm{Fe}$ layers are achieved and the effects of dynamic IEC through mutual spin currents are highlighted. With the increase in ${t}_{\mathrm{Ru}}$, the magnetic damping factors of both layers display the same nonmonotonic behavior, which has been mainly ascribed to the spin pumping damping ${\ensuremath{\alpha}}_{\mathrm{SP}}$ associated with the dynamic IEC. Moreover, it is found that the variation trend of damping difference $\mathrm{\ensuremath{\Delta}}{\ensuremath{\alpha}}_{\mathrm{SP}}$ between the two $\mathrm{Co}$-$\mathrm{Fe}$ layers is similar to that of ${J}_{\mathrm{eff}}$, revealing that the dynamic IEC effect is actually dominated by the static IEC and thereupon a theoretical formula is proposed to describe the correlation between $\mathrm{\ensuremath{\Delta}}{\ensuremath{\alpha}}_{\mathrm{SP}}$ and ${J}_{\mathrm{eff}}$. These results suggest the feasibility of efficient control of spin pumping damping through controlled IEC, which has great significance for microwave spintronic devices based on asymmetric trilayer structures.

Significant Reorientation Transition of Magnetic Damping Anisotropy in Co2FeAl Heusler Alloy Films at Low Temperatures.

Abstract: The temperature (T) dependences of magnetization dynamics, especially for magnetic damping anisotropy, have been systematically investigated in well-ordered Co2FeAl films with a biaxial anisotropy. It is found that the damping anisotropy factor Q, defined as the fractional difference of damping between the hard and easy axes, changes from 0.35 to -0.09 as T decreases from 300 to 80 K, performing a distinctive reorientation transition at T ∼ 200 K. Through the thickness-dependent damping measurement results, the damping anisotropy reorientation is verified to originate from the competitions between the intrinsic anisotropic distribution of bulk spin orbit coupling and the interfacial two-magnon scattering. The former governs the effective damping at high temperatures, while the latter with an opposite fourfold symmetry gradually plays a dominant role at reduced temperatures, leading to the transition of the Q value from positive to negative. The clear clarification of damping anisotropy variation as well as the underlying mechanism in this study would be of great importance for designing key spintronic devices with optimized dynamic magnetic properties.

▲Current-induced effective field vector in Ta│CoFeB│MgO

Abstract: Current-induced effective magnetic fields can provide efficient ways of electrically manipulating the magnetization of ultrathin magnetic heterostructures. Two effects, known as the Rashba spin orbit field and the spin Hall spin torque, have been reported to be responsible for the generation of the effective field. However, a quantitative understanding of the effective field, including its direction with respect to the current flow, is lacking. Here we describe vector measurements of the current-induced effective field in Ta|CoFeB|MgO heterostructrures. The effective field exhibits a significant dependence on the Ta and CoFeB layer thicknesses. In particular, a 1 nm thickness variation of the Ta layer can change the magnitude of the effective field by nearly two orders of magnitude. Moreover, its sign changes when the Ta layer thickness is reduced, indicating that there are two competing effects contributing to it. Our results illustrate that the presence of atomically thin metals can profoundly change the landscape for controlling magnetic moments in magnetic heterostructures electrically.

Current-driven domain wall motion along high perpendicular anisotropy multilayers: The role of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction

Abstract: The current-induced domain wall motion along a thin cobalt ferromagnetic strip sandwiched in a multilayer (Pt/Co/AlO) is theoretically studied with emphasis on the roles of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction. The results point out that these ingredients, originated from the spin-orbit coupling, are consistent with recent experimental observations in three different scenarios. With the aim of clarifying which is the most plausible the influence of in-plane longitudinal and transversal fields is evaluated.

Spin-transfer torque generated by a topological insulator

Abstract: Magnetic devices are a leading contender for the implementation of memory and logic technologies that are non-volatile, that can scale to high density and high speed, and that do not wear out. However, widespread application of magnetic memory and logic devices will require the development of efficient mechanisms for reorienting their magnetization using the least possible current and power. There has been considerable recent progress in this effort; in particular, it has been discovered that spin–orbit interactions in heavy-metal/ferromagnet bilayers can produce strong current-driven torques on the magnetic layer, via the spin Hall effect in the heavy metal or the Rashba–Edelstein effect in the ferromagnet. In the search for materials to provide even more efficient spin–orbit-induced torques, some proposals have suggested topological insulators, which possess a surface state in which the effects of spin–orbit coupling are maximal in the sense that an electron’s spin orientation is fixed relative to its propagation direction. Here we report experiments showing that charge current flowing in-plane in a thin film of the topological insulator bismuth selenide (Bi2Se3) at room temperature can indeed exert a strong spin-transfer torque on an adjacent ferromagnetic permalloy (Ni81Fe19) thin film, with a direction consistent with that expected from the topological surface state. We find that the strength of the torque per unit charge current density in Bi2Se3 is greater than for any source of spin-transfer torque measured so far, even for non-ideal topological insulator films in which the surface states coexist with bulk conduction. Our data suggest that topological insulators could enable very efficient electrical manipulation of magnetic materials at room temperature, for memory and logic applications.

Superconducting Long-Range Proximity Effect through the Atomically Flat Interface of a Bi2Te3 Topological Insulator.

Abstract: We report on structural and electronic properties of superconducting nanohybrids made of Pb grown in the ultrahigh vacuum on the atomically clean surface of single crystals of topological Bi2Te3. In situ scanning tunneling microscopy and spectroscopy demonstrated that the resulting network is composed of Pb-nanoislands dispersed on the surface and linked together by an amorphous atomic layer of Pb, which wets Bi2Te3. As a result, the superconducting state of the system is characterized by a thickness-dependent superconducting gap of Pb-islands and by a very unusual position-independent proximity gap between them. Furthermore, the data analysis and DFT calculations demonstrate that the Pb-wetting layer leads to significant modifications of both topological and trivial electronic states of Bi2Te3, which are responsible for the observed long-range proximity effect.

Magnetoelastic interactions and magnetic damping in Co2Fe0.4Mn0.6Si and Co2FeGa0.5Ge0.5 Heusler alloys thin films for spintronic applications

Abstract: Co2Fe0.4Mn0.6Si (CFMS) and Co2FeGa0.5Ge0.5 (CFGG) Heusler alloys are among the most promising thin film materials for spintronic devices due to a high spin polarization, low magnetic damping and giant/tunneling magnetoresistance ratios. Despite numerous investigations of Heusler alloys magnetic properties performed up to now, magnetoelastic effects in these materials remain not fully understood; due to quite rare studies of correlations between magnetoelastic and other magnetic properties, such as magnetic dissipation or magnetic anisotropy. In this research we have investigated epitaxial CFMS and CFGG Heusler alloys thin films of thickness in the range of 15–50 nm. We have determined the magnetoelastic tensor components and magnetic damping parameters as a function of the magnetic layer thickness. Magnetic damping measurements revealed the existence of non-Gilbert dissipation related contributions, including two-magnon scattering and spin pumping phenomena. Magnetoelastic constant B11 values and the effective magnetic damping parameter αeff values were found to be in the range of − 6 to 30 × 106 erg/cm3 and between 1 and 12 × 10–3, respectively. The values of saturation magnetostriction λS for CFMS Heusler alloy thin films were also obtained using the strain modulated ferromagnetic resonance technique. The correlation between αeff and B11, depending on magnetic layer thickness was determined based on the performed investigations of the above mentioned magnetic properties.