arXiv subjects

Jing Shi

Publications and source records attributed to Jing Shi.

At least 127 records · Page 7Linked to original sources

arXiv2017

Direct observation of magnon-phonon coupling in yttrium iron garnet

The magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic transition temperature of $\sim$560 K has been widely used in microwave and spintronic devices. Anomalous features in the spin Seeback effect (SSE) voltages have been observed in Pt/YIG and attributed to the magnon-phonon coupling. Here we use inelastic neutron scattering to map out low-energy spin waves and acoustic phonons of YIG at 100 K as a function of increasing magnetic field. By comparing the zero and 9.1 T data, we find that instead of splitting and opening up gaps at the spin wave and acoustic phonon dispersion intersecting points, magnon-phonon coupling in YIG enhances the hybridized scattering intensity. These results are different from expectations of conventional spin-lattice coupling, calling for new paradigms to understand the scattering process of magnon-phonon interactions and the resulting magnon-polarons.

cond-mat.str-el↗

arXiv2017

Scaling of Dzyaloshinskii Moriya interaction at heavy metal and ferromagnetic metal interfaces

The Dzyaloshinskii Moriya Interaction (DMI) at the heavy metal (HM) and ferromagnetic metal (FM) interface has been recognized as a key ingredient in spintronic applications. Here we investigate the chemical trend of DMI on the 5d band filling (5d^3~5d^10) of the HM element in HM/CoFeB/MgO multilayer thin films. DMI is quantitatively evaluated by measuring asymmetric spin wave dispersion using Brillouin light scattering. Sign reversal and 20 times modification of the DMI coefficient D have been measured as the 5d HM element is varied. The chemical trend can be qualitatively understood by considering the 5d and 3d bands alignment at the HM/FM interface and the subsequent orbital hybridization around the Fermi level. Furthermore, a positive correlation is observed between DMI and spin mixing conductance at the HM/FM interfaces. Our results provide new insights into the interfacial DMI for designing future spintronic devices.

cond-mat.mtrl-sci↗

arXiv2017

Strong electron-hole symmetric Rashba spin-orbit coupling in graphene/monolayer transition metal dichalcogenide heterostructures

Despite its extremely weak intrinsic spin-orbit coupling (SOC), graphene has been shown to acquire considerable SOC by proximity coupling with exfoliated transition metal dichalcogenides (TMDs). Here we demonstrate strong induced Rashba SOC in graphene that is proximity coupled to a monolayer TMD film, MoS2 or WSe2, grown by chemical vapor deposition with drastically different Fermi level positions. Graphene/TMD heterostructures are fabricated with a pickup-transfer technique utilizing hexagonal boron nitride, which serves as a flat template to promote intimate contact and therefore a strong interfacial interaction between TMD and graphene as evidenced by quenching of the TMD photoluminescence. We observe strong induced graphene SOC that manifests itself in a pronounced weak anti-localization (WAL) effect in the graphene magnetoconductance. The spin relaxation rate extracted from the WAL analysis varies linearly with the momentum scattering time and is independent of the carrier type. This indicates a dominantly Dyakonov-Perel spin relaxation mechanism caused by the induced Rashba SOC. Our analysis yields a Rashba SOC energy of ~1.5 meV in graphene/WSe2 and ~0.9 meV in graphene/MoS2, respectively. The nearly electron-hole symmetric nature of the induced Rashba SOC provides a clue to possible underlying SOC mechanisms.

cond-mat.mtrl-sci↗

arXiv2017

Above 400 K Robust Perpendicular Ferromagnetic Phase in a Topological Insulator

The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TI) exhibits many fascinating physical properties for potential applications in nano-electronics and spintronics. However, in transition-metal doped TI, the only experimentally demonstrated QAHE system to date, the effect is lost at practically relevant temperatures. This constraint is imposed by the relatively low Curie temperature (Tc) and inherent spin disorder associated with the random magnetic dopants. Here we demonstrate drastically enhanced Tc by exchange coupling TI to Tm3Fe5O12, a high-Tc magnetic insulator with perpendicular magnetic anisotropy. Signatures that the TI surface states acquire robust ferromagnetism are revealed by distinct squared anomalous Hall hysteresis loops at 400 K. Point-contact Andreev reflection spectroscopy confirms that the TI surface is indeed spin-polarized. The greatly enhanced Tc, absence of spin disorder, and perpendicular anisotropy are all essential to the occurrence of the QAHE at high temperatures.

cond-mat.mtrl-sci↗

arXiv2017

Deficiency of the Bulk Spin Hall Effect Model for Spin-Orbit Torques in Magnetic Insulator/Heavy Metal Heterostructures

Electrical currents in a magnetic insulator/heavy metal heterostructure can induce two simultaneous effects, namely, spin Hall magnetoresistance (SMR) on the heavy metal side and spin-orbit torques (SOTs) on the magnetic insulator side. Within the framework of the pure spin current model based on the bulk spin Hall effect (SHE), the ratio of the spin Hall-induced anomalous Hall effect (SH-AHE) to SMR should be equal to the ratio of the field-like torque (FLT) to damping-like torque (DLT). We perform a quantitative study of SMR, SH-AHE, and SOTs in a series of thulium iron garnet/platinum or Tm3Fe5O12/Pt heterostructures with different Tm3Fe5O12 thicknesses, where Tm3Fe5O12 is a ferrimagnetic insulator with perpendicular magnetic anisotropy. We find the ratio between measured effective fields of FLT and DLT is at least 2 times larger than the ratio of the SH-AHE to SMR. In addition, the bulk SHE model grossly underestimates the spin torque efficiency of FLT. Our results reveal deficiencies of the bulk SHE model and also address the importance of interfacial effects such as the Rashba and magnetic proximity effects in magnetic insulator/heavy metal heterostructures.

cond-mat.mtrl-sci↗

arXiv2017

Observation of Inverse Edelstein Effect in Rashba-Split 2DEG between SrTiO3 and LaAlO3 at Room Temperature

The Rashba physics has been intensively studied in the field of spin orbitronics, for the purpose of searching novel physical properties and the FM magnetization switching for technology applications. Here, we report the observation of the inverse Edelstein effect up to room temperature in the Rashba-split 2DEG between two insulating oxides SrTiO3 and LaAlO3 with the LaAlO3 layer thickness from 3 to 40 unit cells (UC). We further demonstrate that the spin voltage could be dramatically manipulated by electric field effect for the 2DEG between SrTiO3 and 3 UC LaAlO3. These results demonstrate that the Rashba-split 2DEG at the complex oxide interface can be used for room temperature efficient charge-and-spin conversion for the generation and detection of spin current.

cond-mat.mes-hall↗

arXiv2016

Spin Injection and Inverse Edelstein Effect in the Surface States of Topological Kondo Insulator SmB6

There has been considerable interest in exploiting the spin degrees of freedom of electrons for potential information storage and computing technologies. Topological insulators (TI), a class of quantum materials, have special gapless edge/surface states, where the spin polarization of the Dirac fermions is locked to the momentum direction. This spin-momentum locking property gives rise to very interesting spin-dependent physical phenomena such as the Edelstein and inverse Edelstein effects. However, the spin injection in pure surface states of TI is very challenging because of the coexistence of the highly conducting bulk states. Here, we experimentally demonstrate the spin injection and observe the inverse Edelstein effect in the surface states of a topological Kondo insulator, SmB6. At low temperatures when only surface carriers are present, a clear spin signal is observed. Furthermore, the magnetic field angle dependence of the spin signal is consistent with spin-momentum locking property of surface states of SmB6.

cond-mat.mes-hall↗

arXiv2016

Hierarchical Memory Networks for Answer Selection on Unknown Words

Recently, end-to-end memory networks have shown promising results on Question Answering task, which encode the past facts into an explicit memory and perform reasoning ability by making multiple computational steps on the memory. However, memory networks conduct the reasoning on sentence-level memory to output coarse semantic vectors and do not further take any attention mechanism to focus on words, which may lead to the model lose some detail information, especially when the answers are rare or unknown words. In this paper, we propose a novel Hierarchical Memory Networks, dubbed HMN. First, we encode the past facts into sentence-level memory and word-level memory respectively. Then, (k)-max pooling is exploited following reasoning module on the sentence-level memory to sample the (k) most relevant sentences to a question and feed these sentences into attention mechanism on the word-level memory to focus the words in the selected sentences. Finally, the prediction is jointly learned over the outputs of the sentence-level reasoning module and the word-level attention mechanism. The experimental results demonstrate that our approach successfully conducts answer selection on unknown words and achieves a better performance than memory networks.

cs.IR↗

arXiv2016

Anomalous Hall hysteresis in Tm3Fe5O12/Pt with strain-induced perpendicular magnetic anisotropy

We demonstrate robust interface strain-induced perpendicular magnetic anisotropy in atomically flat ferrimagnetic insulator Tm3Fe5O12 films grown with pulsed laser deposition on substituted-Gd3Ga5O12 substrate which maximizes the tensile strain at the interface. In bilayers consisting of Pt and TIG, we observe large squared Hall hysteresis loops over a wide range of thicknesses of Pt at room temperature. When a thin Cu layer is inserted between Pt and TIG, the Hall hysteresis magnitude decays but stays finite as the thickness of Cu increases up to 5 nm. However, if the Cu layer is placed atop Pt instead, the Hall hysteresis magnitude is consistently larger than when the Cu layer with the same thickness is inserted in between for all Cu thicknesses. These results suggest that both the proximity-induced ferromagnetism and spin current contribute to the anomalous Hall effect.

cond-mat.mtrl-sci↗

arXiv2016

Tunable spin-orbit coupling and symmetry-protected edge states in graphene/WS$_2$

We demonstrate clear weak anti-localization (WAL) effect arising from induced Rashba spin-orbit coupling (SOC) in WS$_2$-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single-layer graphene flake, WAL in WS$_2$-covered graphene occurs over a wide range of carrier densities on both electron and hole sides. At high carrier densities, we estimate the Rashba SOC relaxation rate to be $\sim 0.2 \rm{ps}^{-1}$ and show that it can be tuned by transverse electric fields. In addition to the Rashba SOC, we also predict the existence of a `valley-Zeeman' SOC from first-principles calculations. The interplay between these two SOC's can open a non-topological but interesting gap in graphene; in particular, zigzag boundaries host four sub-gap edge states protected by time-reversal and crystalline symmetries. The graphene/WS$_2$ system provides a possible platform for these novel edge states.

cond-mat.mes-hall↗

arXiv2016

An integrated approach to doped thin films with strain tunable magnetic anisotropy: Powder synthesis, target preparation and pulsed laser deposition of Bi:YIG

We present a synthesis/processing method for fabricating ferrimagnetic insulator (Bi-doped yttrium iron garnet) thin films with tunable magnetic anisotropy. Since the desired magnetic properties rely on controllable thickness and successful doping, we pay attention to the entire synthesis/processing procedure (nanopowder synthesis, nanocrystalline target preparation and pulsed laser deposition (PLD)). Atomically flat films were deposited by PLD on (111)-orientated yttrium aluminum garnet. We show a significant enhancement of perpendicular anisotropy in the films, caused by strain-induced anisotropy. In addition, the perpendicular anisotropy is tunable by decreasing the film thickness and overwhelms the shape anisotropy at a critical thickness of 3.5 nm.

cond-mat.mtrl-sci↗

arXiv2016

Crystal Structure Manipulation of the Exchange Bias in an Antiferromagnetic Film

Exchange bias is one of the most extensively studied phenomena in magnetism, since it exerts a unidirectional anisotropy to a ferromagnet (FM) when coupled to an antiferromagnet (AFM) and the control of the exchange bias is therefore very important for technological applications, such as magnetic random access memory and giant magnetoresistance sensors. In this letter, we report the crystal structure manipulation of the exchange bias in epitaxial hcp Cr2O3 films. By epitaxially growing twined (10-10) oriented Cr2O3 thin films, of which the c axis and spins of the Cr atoms lie in the film plane, we demonstrate that the exchange bias between Cr2O3 and an adjacent permalloy layer is tuned to in-plane from out-of-plane that has been observed in (0001) oriented Cr2O3 films. This is owing to the collinear exchange coupling between the spins of the Cr atoms and the adjacent FM layer. Such a highly anisotropic exchange bias phenomenon is not possible in polycrystalline films.

cond-mat.mtrl-sci↗

arXiv2016

Platinum/Yttrium Iron Garnet Inverted Structures for Spin Current Transport

30-80 nm thick yttrium iron garnet (YIG) films are grown by pulsed laser deposition on a 5 nm thick sputtered Pt atop gadolinium gallium garnet substrate (GGG) (110). Upon post-growth rapid thermal annealing, single crystal YIG(110) emerges as if it were epitaxially grown on GGG(110) despite the presence of the intermediate Pt film. The YIG surface shows atomic steps with the root-mean-square roughness of 0.12 nm on flat terraces. Both Pt/YIG and GGG/Pt interfaces are atomically sharp. The resulting YIG(110) films show clear in-plane uniaxial magnetic anisotropy with a well-defined easy axis along <001> and a peak-to-peak ferromagnetic resonance linewidth of 7.5 Oe at 9.32 GHz, similar to YIG epitaxilly grown on GGG. Both spin Hall magnetoresistance and longitudinal spin Seebeck effects in the inverted bilayers indicate excellent Pt/YIG interface quality.

cond-mat.mtrl-sci↗

arXiv2016

Enhanced spin Seebeck effect signal due to spin-momentum locked topological surface states

Spin-momentum locking in protected surface states enables efficient electrical detection of magnon decay at a magnetic-insulator/topological-insulator heterojunction. Here we demonstrate this property using the spin Seebeck effect, i.e. measuring the transverse thermoelectric response to a temperature gradient across a thin film of yttrium iron garnet, an insulating ferrimagnet, and forming a heterojunction with (BixSb1-x)2Te3, a topological insulator. The non-equilibrium magnon population established at the interface can decay in part by interactions of magnons with electrons near the Fermi energy of the topological insulator. When this decay channel is made active by tuning (BixSb1-x)2Te3 to a bulk insulator, a large electromotive force emerges in the direction perpendicular to the in-plane magnetization of yttrium iron garnet. The enhanced, tunable spin Seebeck effect which occurs when the Fermi level lies in the bulk gap offers unique advantages over the usual spin Seebeck effect in metals and therefore opens up exciting possibilities in spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Experimental Investigation of Temperature-Dependent Gilbert Damping in Permalloy Thin Films

The Gilbert damping of ferromagnetic materials is arguably the most important but least understood phenomenological parameter that dictates real-time magnetization dynamics. Understanding the physical origin of the Gilbert damping is highly relevant to developing future fast switching spintronics devices such as magnetic sensors and magnetic random access memory. Here, we report an experimental study of temperature-dependent Gilbert damping in permalloy (Py) thin films of varying thicknesses by ferromagnetic resonance. From the thickness dependence, two independent contributions to the Gilbert damping are identified, namely bulk damping and surface damping. Of particular interest, bulk damping decreases monotonically as the temperature decreases, while surface damping shows an enhancement peak at the temperature of ~50 K. These results provide an important insight to the physical origin of the Gilbert damping in ultrathin magnetic films.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic anisotropy of the single-crystalline ferromagnetic insulator Cr2Ge2Te6

Cr2Ge2Te6 (CGT), a layered ferromagnetic insulator, has attracted a great deal of interest recently owing to its potential for integration with Dirac materials to realize the quantum anomalous Hall effect (QAHE) and to develop novel spintronics devices. Here, we study the uniaxial magnetic anisotropy energy of single-crystalline CGT and determine that the magnetic easy axis is directed along the c-axis in its ferromagnetic phase. In addition, CGT is an insulator below the Curie temperature. These properties make CGT a potentially promising candidate substrate for integration with topological insulators for the realization of the high-temperature QAHE.

cond-mat.mtrl-sci↗

arXiv2016

Observation of magnon-mediated current drag in Pt/yttrium iron garnet/Pt(Ta) trilayers

Pure spin current, a flow of spin angular momentum without flow of any companying net charge, is generated in two common ways. One makes use of the spin Hall effect in normal metals (NM) with strong spin-orbit coupling, such as Pt or Ta. The other utilizes the collective motion of magnetic moments or spin waves with the quasi-particle excitations called magnons. A popular material for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we demonstrate in NM/MI/NM trilayers that these two types of spin currents are interconvertible across the interfaces, predicated as the magnon-mediated current drag phenomenon. The transmitted signal scales linearly with the driving current without a threshold and follows the power-law with n ranging from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can serve as a scalable pure spin current valve device which is an essential ingredient in spintronics.

cond-mat.mes-hall↗

arXiv2015

Multiferroicity and Magnetoelectric Coupling in TbMnO3 Thin Films

In this work, we report the growth and functional characterizations of multiferroic TbMnO3 thin films grown on Nb-doped SrTiO3 (001) substrates using pulsed laser deposition. By performing detailed magnetic and ferroelectric properties measurements, we demonstrate that the multiferroicity of spin origin known in the bulk crystals can be successfully transferred to TbMnO3 thin films. Meanwhile, anomalous magnetic transition and unusual magnetoelectric coupling related to Tb moments are observed, suggesting a modified magnetic configuration of Tb in the films as compared to the bulk counterpart. In addition, it is found that the magnetoelectric coupling enabled by Tb moments can even be seen far above the Tb spin ordering temperature, which provides a larger temperature range for the magnetoelectric control involving Tb moments.

cond-mat.mtrl-sci↗