Haifeng Ding

Distinguishing spin pumping from spin rectification in a Pt/Py bilayer through angle dependent line shape analysis

A pure spin current driven by spin pumping is converted to a DC voltage and detected electrically in a Py/Pt bilayer sample. This DC voltage mixes with a DC voltage produced through spin rectification. The ferromagnetic resonance line shape strongly depends on the microwave magnetic h field distribution. We have systematically studied the line shapes by changing the external magnetic field orientation in plane of a Pt/Py bilayer. A method is demonstrated which allows us to calculate the microwave h field vector distribution, and distinguish spin pumping from spin rectification.

Width dependent edge distribution of graphene nanoribbons unzipped from multiwall carbon nanotubes

We present the width dependent study of edge distribution of graphene nanoribbons unzipped from multi-wall nanotubes. The partial unzipping of the carbon nanotubes yields a mixture of carbon nanotubes and nanoribbons. Comparing atomic resolution images of scanning tunneling microscopy with the lattice of graphene, the edge structures of nanoribbons are identified. Below 10 nm, the edges are closer to armchair type. Above 20 nm, the ribbons prefer to have edges close to zigzag type. In between, a more random distribution of the edges is found. The findings are of potential usages for the edge control in graphene nanoribbon based applications.

Observation of spin-orbit magnetoresistance in metallic thin films on magnetic insulators

A new type of spin-orbit magnetoresistance effect is observed in Cu/YIG with interface decorated with nanosize Pt islands. A magnetoresistance (MR) effect induced by the Rashba spin-orbit interaction was predicted, but not yet observed, in bilayers consisting of normal metal and ferromagnetic insulator. We present an experimental observation of this new type of spin-orbit MR (SOMR) effect in the Cu[Pt]/Y3Fe5O12 (YIG) bilayer structure, where the Cu/YIG interface is decorated with nanosize Pt islands. This new MR is apparently not caused by the bulk spin-orbit interaction because of the negligible spin-orbit interaction in Cu and the discontinuity of the Pt islands. This SOMR disappears when the Pt islands are absent or located away from the Cu/YIG interface; therefore, we can unambiguously ascribe it to the Rashba spin-orbit interaction at the interface enhanced by the Pt decoration. The numerical Boltzmann simulations are consistent with the experimental SOMR results in the angular dependence of magnetic field and the Cu thickness dependence. Our finding demonstrates the realization of the spin manipulation by interface engineering.

Spectroscopy of self-assembled one-dimensional atomic string: The role of step edge

Step-edge guided Gd one-dimensional atomic strings on Ag(111) surface are studied by scanning tunneling microscopy and spectroscopy at low temperature. Spectroscopy measurements show a characteristic peak, 65 meV above Fermi level in sharp contrast with tight-binding calculations of a string on a flat terrace. Good agreement can be obtained when the step edges are included in the calculations, revealing their roles on the electronic spectra of the self-assembled strings guided by step edges. The results are further confirmed by the spectroscopy study of Fe strings on a flat terrace and near step edges constructed by atomic manipulation.

Self-consistent determination of spin Hall angle and spin diffusion length in Pt and Pd: The role of the interface spin loss

Self-consistent determination of the key parameters in spin-charge conversion via spin pumping–induced inverse spin Hall effect. Spin Hall angle (θSH) and spin diffusion length (λsd) are the key parameters in describing the spin-charge conversion, which is an integral part of spintronics. Despite their importance and much effort devoted to quantifying them, significant inconsistencies in the reported values for the same given material exist. We report a self-consistent method to quantify both θSH and λsd of nonmagnetic materials by spin pumping with various ferromagnetic (FM) pumping sources. We characterize the spin-charge conversion for Pt and Pd with various FM combinations using (i) effective spin-mixing conductance, (ii) microwave photoresistance, and (iii) inverse spin Hall effect measurements and find that the pumped spin current suffers an interfacial spin loss (ISL), whose magnitude varies for different interfaces. By properly treating the ISL effect, we obtained consistent values of θSH and λsd for both Pt and Pd regardless of the ferromagnet used.

Highly Efficient Spintronic Terahertz Emitter Enabled by Metal–Dielectric Photonic Crystal

Spintronic terahertz (THz) emitter provides the advantages such as apparently broader spectrum, significantly lower cost, and more flexibility in compared with the commercial THz emitters, and thus attracts great interests recently. In past few years, efforts have been made in optimizing the material composition and structure geometry, and the conversion efficiency has been improved close to that of ZnTe crystal. One of the drawbacks of the current designs is the rather limited laser absorption - more than 50% energy is wasted and the conversion efficiency is thus limited. Here, we theoretically propose and experimentally demonstrate a novel device that fully utilizes the laser intensity and significantly improves the conversion efficiency. The device, which consists of a metal-dielectric photonic crystal structure, utilizes the interference between the multiple scattering waves to simultaneously suppress the reflection and transmission of the laser, and to reshape the laser field distributions. The experimentally detected laser absorption and THz generations show one-to-one correspondence with the theoretical calculations. We achieve the strongest THz pulse emission that presents a 1.7 times improvement compared to the currently designed spintronic emitter. This work opens a new pathway to improve the performance of spintronic THz emitter from the perspective of optics.