Chi-Feng Pai
Cornell University
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Featured researches published by Chi-Feng Pai.
Science | 2012
Luqiao Liu; Chi-Feng Pai; Yingxia Li; Hsin-wei Tseng; D. C. Ralph; R. A. Buhrman
Giant Spin Hall One of the primary challenges in the field of spin-electronics, which exploits the electrons spin rather than its charge, is to create strong currents of electrons with polarized spins. One way to do this is to use a ferromagnet as a polarizer, a principle used in magnetic tunnel junctions; however, these devices suffer from reliability problems. An alternative is the spin Hall effect, where running a charge current through a material generates a spin current in the transverse direction, but the efficiency of this process tends to be small. Liu et al. (p. 555) now show that the spin Hall effect in Tantalum in its high-resistance β phase generates spin currents strong enough to induce switching of the magnetization of an adjacent ferromagnet; at the same time, Ta does not cause energy dissipation in the ferromagnet. These properties allowed efficient and reliable operation of a prototype three-terminal device. Tantalum is found to generate strong spin currents that can induce switching of ferromagnets efficiently and reliably. Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We report a giant spin Hall effect (SHE) in β-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main obstacles to the development of magnetic memory and nonvolatile spin logic technologies.
Applied Physics Letters | 2012
Chi-Feng Pai; Luqiao Liu; Yingxia Li; Hsin-wei Tseng; D. C. Ralph; R. A. Buhrman
We report a giant spin Hall effect in β-W thin films. Using spin torque induced ferromagnetic resonance with a β-W/CoFeB bilayer microstrip, we determine the spin Hall angle to be |θSHβ-W|=0.30±0.02, large enough for an in-plane current to efficiently reverse the orientation of an in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction adjacent to a thin β-W layer. From switching data obtained with such 3-terminal devices, we independently determine |θSHβ-W|=0.33±0.06. We also report variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable (α and β) phase composition.
Physical Review Letters | 2012
Luqiao Liu; Chi-Feng Pai; D. C. Ralph; R. A. Buhrman
We show that a direct current in a tantalum microstrip can induce steady-state magnetic oscillations in an adjacent nanomagnet through spin torque from the spin Hall effect (SHE). The oscillations are detected electrically via a magnetic tunnel junction (MTJ) contacting the nanomagnet. The oscillation frequency can be controlled using the MTJ bias to tune the magnetic anisotropy. In this 3-terminal device, the SHE torque and the MTJ bias therefore provide independent controls of the oscillation amplitude and frequency, enabling new approaches for developing tunable spin torque nano-oscillators.
Applied Physics Letters | 2014
Chi-Feng Pai; Minh-Hai Nguyen; Carina Belvin; Luis Henrique Vilela-Leão; D. C. Ralph; R. A. Buhrman
We report that strong perpendicular magnetic anisotropy of the ferromagnetic layer in a W/CoFeB/MgO multilayer structure can be established by inserting a Hf layer as thin as 0.25 nm between the W and CoFeB layers. The Hf spacer also allows transmission of spin currents generated by an in-plane charge current in the W layer to apply strong spin torque on the CoFeB, thereby enabling current-driven magnetic switching. The antidamping-like and field-like components of the spin torque exerted on a 1 nm CoFeB layer are of comparable magnitudes in this geometry. Both components originate from the spin Hall effect in the underlying W layer.
Physical Review B | 2014
OukJae Lee; Luqiao Liu; Chi-Feng Pai; Yingxia Li; Hsin-wei Tseng; P. G. Gowtham; Junbo Park; D. C. Ralph; R. A. Buhrman
We study deterministic magnetic reversal of a perpendicularly magnetized Co layer in a Co/MgO/Ta nano-square driven by spin Hall torque from an in-plane current flowing in an underlying Pt layer. The rate-limiting step of the switching process is domain-wall (DW) depinning by spin Hall torque via a thermally-assisted mechanism that eventually produces full reversal by domain expansion. An in-plane applied magnetic field collinear with the current is required, with the necessary field scale set by the need to overcome DW chirality imposed by the Dzyaloshinskii-Moriya interaction. Once Joule heating is taken into account the switching current density is quantitatively consistent with a spin Hall angle {\theta}
Nature Materials | 2017
Can Onur Avci; Andy Quindeau; Chi-Feng Pai; Maxwell Mann; Lucas Caretta; Astera S. Tang; Mehmet C. Onbasli; Caroline A. Ross; Geoffrey S. D. Beach
_{SH}
Physical Review B | 2016
Yongxi Ou; Chi-Feng Pai; Shengjie Shi; D. C. Ralph; R. A. Buhrman
Physical Review B | 2014
Kyongmo An; Daniel R. Birt; Chi-Feng Pai; Kevin Olsson; D. C. Ralph; R. A. Buhrman; Xiaoqin Li
{\approx}
Applied Physics Letters | 2016
Kohei Ueda; Maxwell Mann; Chi-Feng Pai; Aik-Jun Tan; Geoffrey S. D. Beach
0.07 for 4 nm of Pt.
Applied Physics Letters | 2016
Kohei Ueda; Chi-Feng Pai; Aik Jun Tan; Maxwell Mann; Geoffrey S. D. Beach
The spin Hall effect in heavy metals converts charge current into pure spin current, which can be injected into an adjacent ferromagnet to exert a torque. This spin-orbit torque (SOT) has been widely used to manipulate the magnetization in metallic ferromagnets. In the case of magnetic insulators (MIs), although charge currents cannot flow, spin currents can propagate, but current-induced control of the magnetization in a MI has so far remained elusive. Here we demonstrate spin-current-induced switching of a perpendicularly magnetized thulium iron garnet film driven by charge current in a Pt overlayer. We estimate a relatively large spin-mixing conductance and damping-like SOT through spin Hall magnetoresistance and harmonic Hall measurements, respectively, indicating considerable spin transparency at the Pt/MI interface. We show that spin currents injected across this interface lead to deterministic magnetization reversal at low current densities, paving the road towards ultralow-dissipation spintronic devices based on MIs.