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1.
Phys Rev Lett ; 124(2): 027203, 2020 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-32004033

RESUMO

Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI), which gives rise to topological spin textures, such as skyrmions. However, the DMI has not yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping for spin waves. In this work, we experimentally evidence the interfacial DMI in a 7-nm-thick YIG film by measuring the nonreciprocal spin-wave propagation in terms of frequency, amplitude, and most importantly group velocities using all electrical spin-wave spectroscopy. The velocities of propagating spin waves show chirality among three vectors, i.e., the film normal direction, applied field, and spin-wave wave vector. By measuring the asymmetric group velocities, we extract a DMI constant of 16 µJ/m^{2}, which we independently confirm by Brillouin light scattering. Thickness-dependent measurements reveal that the DMI originates from the oxide interface between the YIG and garnet substrate. The interfacial DMI discovered in the ultrathin YIG films is of key importance for functional chiral magnonics as ultralow spin-wave damping can be achieved.

2.
Sci Adv ; 5(8): eaaw3415, 2019 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-31497642

RESUMO

Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi2Se3 and an insulating ferromagnet BaFe12O19. A charge current in Bi2Se3 can switch the magnetization in BaFe12O19 up and down. When the magnetization is switched by a field, a current in Bi2Se3 can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe12O19. These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.

3.
Nat Nanotechnol ; 14(7): 691-697, 2019 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-31011219

RESUMO

Spin waves may constitute key components of low-power spintronic devices. Antiferromagnetic-type spin waves are innately high-speed, stable and dual-polarized. So far, it has remained challenging to excite and manipulate antiferromagnetic-type propagating spin waves. Here, we investigate spin waves in periodic 100-nm-wide stripe domains with alternating upward and downward magnetization in La0.67Sr0.33MnO3 thin films. In addition to ordinary low-frequency modes, a high-frequency mode around 10 GHz is observed and propagates along the stripe domains with a spin-wave dispersion different from the low-frequency mode. Based on a theoretical model that considers two oppositely oriented coupled domains, this high-frequency mode is accounted for as an effective antiferromagnetic spin-wave mode. The spin waves exhibit group velocities of 2.6 km s-1 and propagate even at zero magnetic bias field. An electric current pulse with a density of only 105 A cm-2 can controllably modify the orientation of the stripe domains, which opens up perspectives for reconfigurable magnonic devices.

4.
Phys Rev Lett ; 121(10): 107204, 2018 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-30240252

RESUMO

Exact fractals of nonlinear waves that rely on strong dispersion and nonlinearity and arise spontaneously out of magnetic media were observed for the first time. The experiments make use of a microwave to excite a spin wave in a quasi-one-dimensional magnonic crystal. When the power of the input microwave (P_{in}) is low, the output signal has a power-frequency spectrum that consists of a single peak. When P_{in} is increased to a certain level, new side modes are generated through modulational instability, resulting in a comblike frequency spectrum. With a further increase in P_{in}, each peak in the frequency comb can evolve into its own finer comb through the modulational instability. As P_{in} is increased further, one can observe yet another set of finer frequency combs. Such a frequency-domain fractal manifests itself as multiple layers of amplitude modulation in the time-domain signal.

5.
Phys Rev Lett ; 120(21): 217202, 2018 May 25.
Artigo em Inglês | MEDLINE | ID: mdl-29883138

RESUMO

We observe strong interlayer magnon-magnon coupling in an on-chip nanomagnonic device at room temperature. Ferromagnetic nanowire arrays are integrated on a 20-nm-thick yttrium iron garnet (YIG) thin film strip. Large anticrossing gaps up to 1.58 GHz are observed between the ferromagnetic resonance of the nanowires and the in-plane standing spin waves of the YIG film. Control experiments and simulations reveal that both the interlayer exchange coupling and the dynamical dipolar coupling contribute to the observed anticrossings. The coupling strength is tunable by the magnetic configuration, allowing the coherent control of magnonic devices.

6.
Phys Rev Lett ; 120(20): 207206, 2018 May 18.
Artigo em Inglês | MEDLINE | ID: mdl-29864320

RESUMO

The nontrivial feature and penetration depth of the topological surface states (TSS) in SmB_{6} were studied via spin pumping. The experiments used SmB_{6} thin films grown on the bulk magnetic insulator Y_{3}Fe_{5}O_{12} (YIG). Upon the excitation of magnetization precession in the YIG, a spin current is generated in the SmB_{6} that produces, via spin-orbit coupling, a lateral electrical voltage in the film. This spin-pumping voltage signal becomes considerably stronger as the temperature decreases from 150 to 10 K, and such an enhancement most likely originates from the spin-momentum locking of the TSS and may thereby serve as evidence for the nontrivial nature of the TSS. The voltage data also show a unique film thickness dependence that suggests a TSS depth of ∼32 nm. The spin-pumping results are supported by transport measurements and analyses using a tight binding model.

7.
Nat Commun ; 9(1): 738, 2018 02 21.
Artigo em Inglês | MEDLINE | ID: mdl-29467416

RESUMO

Recent years have witnessed a rapidly growing interest in exploring the use of spin waves for information transmission and computation toward establishing a spin-wave-based technology that is not only significantly more energy efficient than the CMOS technology, but may also cause a major departure from the von-Neumann architecture by enabling memory-in-logic and logic-in-memory architectures. A major bottleneck of advancing this technology is the excitation of spin waves with short wavelengths, which is a must because the wavelength dictates device scalability. Here, we report the discovery of an approach for the excitation of nm-wavelength spin waves. The demonstration uses ferromagnetic nanowires grown on a 20-nm-thick Y3Fe5O12 film strip. The propagation of spin waves with a wavelength down to 50 nm over a distance of 60,000 nm is measured. The measurements yield a spin-wave group velocity as high as 2600 m s-1, which is faster than both domain wall and skyrmion motions.

8.
ACS Nano ; 11(12): 12257-12265, 2017 12 26.
Artigo em Inglês | MEDLINE | ID: mdl-29182851

RESUMO

Manipulation of spin degree of freedom (DOF) of electrons is the fundamental aspect of spintronic and valleytronic devices. Two-dimensional transition metal dichalcogenides (2D TMDCs) exhibit an emerging valley pseudospin, in which spin-up (-down) electrons are distributed in a +K (-K) valley. This valley polarization gives a DOF for spintronic and valleytronic devices. Recently, magnetic exchange interactions between graphene and magnetic insulator yttrium iron garnet (YIG) have been exploited. However, the physics of 2D TMDCs with YIG have not been shown before. Here we demonstrate strong many-body effects in a heterostructure geometry comprising a MoS2 monolayer and YIG. High-order trions are directly identified by mapping absorption and photoluminescence at 12 K. The electron doping density is up to ∼1013 cm-2, resulting in a large splitting of ∼40 meV between trions and excitons. The trions exhibit a high circular polarization of ∼80% under optical pumping by circularly polarized light at ∼1.96 eV; it is confirmed experimentally that both phonon scattering and electron-hole exchange interaction contribute to the valley depolarization with temperature; importantly, a magnetoresistance (MR) behavior in the MoS2 monolayer was observed, and a giant MR ratio of ∼30% is achieved, which is 1 order of magnitude larger than the reported ratio in MoS2/CoFe2O4 heterostructures. Our experimental results confirm that the giant MR behaviors are attributed to the interfacial spin accumulation due to YIG substrates. Our work provides an insight into spin manipulation in a heterostructure of monolayer materials and magnetic substrates.

9.
Chem Commun (Camb) ; 53(84): 11572-11575, 2017 Oct 19.
Artigo em Inglês | MEDLINE | ID: mdl-28990598

RESUMO

A straightforward Fe-catalyzed method for the synthesis of pyrrolo[1,2-a]quinoxalines from 1-(2-aminophenyl)pyrroles and cyclic ethers, which includes functionalization of C(sp3)-H bonds and the construction of C-C and C-N bonds, has been developed. The features of this reaction are Fe catalysis, low-cost and readily accessible starting materials. Moreover, this procedure exhibits good functional group tolerance and a series of pyrrolo[1,2-a]quinoxaline derivatives are obtained in moderate to good yields.

10.
J Org Chem ; 82(19): 10110-10120, 2017 10 06.
Artigo em Inglês | MEDLINE | ID: mdl-28929749

RESUMO

One-pot Cu-catalyzed tandem aerobic oxidative cyclization for the synthesis of quinolines from 2-vinylanilines/2-arylanilines and 2-methylquinolines via C(sp3)-H/C(sp2)-H bond functionalization has been developed. Dioxygen as an ideal oxidant has been employed for this transformation. The substrates bearing various functional groups perform well in this process and generate the desired products in moderate to good yields.

11.
Phys Rev Lett ; 119(2): 024101, 2017 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-28753356

RESUMO

The formation and properties of envelope dispersive shock wave (DSW) excitations from repulsive nonlinear waves in a magnetic film are studied. Experiments involve the excitation of a spin wave step pulse in a low-loss magnetic Y_{3}Fe_{5}O_{12} thin film strip, in which the spin wave amplitude increases rapidly, realizing the canonical Riemann problem of shock theory. Under certain conditions, the envelope of the spin wave pulse evolves into a DSW that consists of an expanding train of nonlinear oscillations with amplitudes increasing from front to back, terminated by a black soliton. The onset of DSW self-cavitation, indicated by a point of zero power and a concomitant 180° phase jump, is observed for sufficiently large steps, indicative of the bidirectional dispersive hydrodynamic nature of the DSW. The experimental observations are interpreted with theory and simulations of the nonlinear Schrödinger equation.

12.
Sci Adv ; 3(4): e1601614, 2017 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-28435873

RESUMO

The role of damping in the spin Seebeck effect (SSE) was studied experimentally for the first time. The experiments used Y3Fe5O12 (YIG)/Pt bilayered structures where the YIG films exhibit very similar structural and static magnetic properties but very different damping. The data show that a decrease in the damping gives rise to an increase in the SSE coefficient, which is qualitatively consistent with some of the theoretical models. This response also shows quasi-linear behavior, which was not predicted explicitly by previous studies. The data also indicate that the SSE coefficient shows no notable correlations with the enhanced damping due to spin pumping, which can be understood in the frame of two existing models.

13.
Nat Commun ; 7: 12688, 2016 09 01.
Artigo em Inglês | MEDLINE | ID: mdl-27581060

RESUMO

As an in-plane charge current flows in a heavy metal film with spin-orbit coupling, it produces a torque on and thereby switches the magnetization in a neighbouring ferromagnetic metal film. Such spin-orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. Here we report the SOT-assisted switching in heavy metal/magnetic insulator systems. The experiments used a Pt/BaFe12O19 bilayer where the BaFe12O19 layer exhibits perpendicular magnetic anisotropy. As a charge current is passed through the Pt film, it produces a SOT that can control the up and down states of the remnant magnetization in the BaFe12O19 film when the film is magnetized by an in-plane magnetic field. It can reduce or increase the switching field of the BaFe12O19 film by as much as about 500 Oe when the film is switched with an out-of-plane field.

14.
Phys Rev Lett ; 117(7): 076601, 2016 Aug 12.
Artigo em Inglês | MEDLINE | ID: mdl-27563980

RESUMO

We report the observation of ferromagnetic resonance-driven spin pumping signals at room temperature in three-dimensional topological insulator thin films-Bi_{2}Se_{3} and (Bi,Sb)_{2}Te_{3}-deposited by molecular beam epitaxy on Y_{3}Fe_{5}O_{12} thin films. By systematically varying the Bi_{2}Se_{3} film thickness, we show that the spin-charge conversion efficiency, characterized by the inverse Rashba-Edelstein effect length (λ_{IREE}), increases dramatically as the film thickness is increased from two quintuple layers, saturating above six quintuple layers. This suggests a dominant role of surface states in spin and charge interconversion in topological-insulator-ferromagnet heterostructures. Our conclusion is further corroborated by studying a series of Y_{3}Fe_{5}O_{12}/(Bi,Sb)_{2}Te_{3} heterostructures. Finally, we use the ferromagnetic resonance linewidth broadening and the inverse Rashba-Edelstein signals to determine the effective interfacial spin mixing conductance and λ_{IREE}.

15.
ACS Appl Mater Interfaces ; 8(12): 8175-83, 2016 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-26967756

RESUMO

The influence of interface structure on the magnetic proximity effect (MPE) in Pt/Y3Fe5O12 (YIG) bilayered heterostructures is studied by first-principles calculations based on the density functional theory (DFT). When Pt atoms are in close proximity with Y or Fe ions at the interface, Pt-Y and Pt-Fe bonds are observed. The crystalline orientations and interface termination layers of the YIG strongly modify both the strength and the length of the Pt-Fe bonding and thereby influence the magnetic properties of the Pt. Point defects including tetrahedral Fe, octahedral Fe, and Y vacancies are introduced at the Pt/YIG interface to quantitatively evaluate the influence on the MPE from individual atoms. For the Pt(100)/YIG(100) structure, the interface tetrahedral Fe vacancies can significantly reduce or even completely diminish the magnetic moments in the Pt. In a stark contrast, the octahedral Fe vacancies slightly enhance the Pt magnetism, and the nonmagnetic Y vacancies cause little influences to the Pt magnetism. These results indicate that the strength of the MPE at the Pt/YIG interface strongly depends on the interface structure. This dependence originated from the direct exchange interaction between the Fe 3d and Pt 5d electrons via electronic state hybridization as well as the electron exchange coupling between the Pt atoms.

16.
Phys Rev Lett ; 111(10): 106601, 2013 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-25166689

RESUMO

Strong damping enhancement in nm-thick yttrium iron garnet (YIG) films due to Pt capping layers was observed. This damping is substantially larger than the expected damping due to conventional spin pumping, is accompanied by a shift in the ferromagnetic resonance field, and can be suppressed by the use of a Cu spacer in between the YIG and Pt films. The data indicate that such damping may originate from the ferromagnetic ordering in Pt atomic layers near the YIG/Pt interface and the dynamic exchange coupling between the ordered Pt spins and the spins in the YIG film.

17.
Phys Rev Lett ; 108(25): 257202, 2012 Jun 22.
Artigo em Inglês | MEDLINE | ID: mdl-23004648

RESUMO

Relaxation control in magnetic thin films via thermally induced interfacial spin transfers was demonstrated for the first time. The experiments used a trilayered structure that consisted of an yttrium iron garnet (YIG) thin film grown on a gadolinium gallium garnet substrate and capped with a nanometer-thick Pt layer. As a temperature gradient is applied across the thickness of the structure, there exists a spin angular momentum transfer across the YIG/Pt interface. This spin transfer results in a torque on YIG magnetic moments. The torque can either speed up or slow down the relaxation in the YIG film, depending on the sign of the temperature gradient with respect to the trilayered structure.

18.
Phys Rev Lett ; 107(14): 146602, 2011 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-22107222

RESUMO

Control of spin waves in a ferrite thin film via interfacial spin scattering was demonstrated. The experiments used a 4.6 µm-thick yttrium iron garnet (YIG) film strip with a 20-nm thick Pt capping layer. A dc current pulse was applied to the Pt layer and produced a spin current across the Pt thickness. As the spin current scatters off the YIG surface, it can either amplify or attenuate spin-wave pulses that travel in the YIG strip, depending on the current or field configuration. The spin scattering also affects the saturation behavior of high-power spin waves.

19.
Phys Rev Lett ; 107(11): 114102, 2011 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-22026671

RESUMO

Chaotic spin-wave solitons in magnetic film active feedback rings were observed for the first time. At some ring gain level, one observes the self-generation of a single spin-wave soliton pulse in the ring. When the pulse circulates in the ring, its amplitude varies chaotically with time. Numerical simulations based on a gain-loss nonlinear Schrödinger equation reproduce the observed responses.

20.
Phys Rev Lett ; 107(6): 066604, 2011 Aug 05.
Artigo em Inglês | MEDLINE | ID: mdl-21902353

RESUMO

Spin injection across the ferrimagnetic insulator (YIG)/normal metal (Au) interface was studied by ferromagnetic resonance. The spin mixing conductance was determined by comparing the Gilbert damping in bare YIG films with those covered by a Au/Fe/Au structure. The Fe layer in Au/Fe/Au acted as a spin sink as displayed by an increased Gilbert damping parameter α compared to that in the bare YIG. In particular, for the 9.0 nm YIG/2.0 nm Au/4.3 nm Fe/6.1 nm Au structure, the YIG and Fe films were coupled by an interlayer exchange coupling, and the exchange coupled YIG exhibited an increased Gilbert damping compared to the bare YIG. This relationship between static and dynamic coupling provides direct evidence for spin pumping. The transfer of spin momentum across the YIG interface is surprisingly efficient with the spin mixing conductance g(↑↓) ≃ 1.2 × 10(14) cm(-2).

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