arXiv subjects

Chi-Feng Pai

Publications and source records attributed to Chi-Feng Pai.

34 records · Page 2Linked to original sources

arXiv2018

The current-induced spin-orbit torque and field-free switching from Mo-based magnetic heterostructures

Magnetic heterostructure Mo/CoFeB/MgO has strong perpendicular magnetic anisotropy and thermal stability. Through current-induced hysteresis loop shift measurements, we show that the dampinglike spin-orbit torque (SOT) efficiency of Mo/CoFeB/MgO heterostructure is $ξ_{DL}\approx -0.003\pm 0.001$ and fairly independent of the annealing temperature from 300$^\circ$C to 400$^\circ$C. Though $|ξ_{DL}|$ is small while compare to those from Ta or W-based heterostructures, reversible current-induced SOT switching of a thermally-stable Mo/CoFeB/MgO heterostruture can still be achieved. Furthermore, we observe field-free current-induced switching from a Mo/CoFeB/MgO structure with the Mo layer being wedge-deposited. Our results indicate that even for a weak spin-orbit interaction 4d transition metal such as Mo, it is still possible to generate sufficient spin current for conventional SOT switching and to realize field-free current-induced switching by structural engineering.

cond-mat.mtrl-sci↗

arXiv2018

Spin-orbit torque magnetometry by wide-field magneto-optical Kerr effect

Magneto-optical Kerr effect (MOKE) is an efficient approach to probe surface magnetization in thin film samples. Here we present a wide-field MOKE technique that adopts a K$ö$hler illumination scheme to characterize the current-induced damping-like spin-orbit torque (DL-SOT) in micronsized and unpatterned magnetic heterostructures with perpendicular magnetic anisotropy. Through a current-induced hysteresis loop shift analysis, we quantify the DL-SOT efficiency of a Ta-based heterostructure with bar-shaped geometry, Hall-cross geometry, and unpatterned geometry to be $|ξ_{DL}|\approx$ 0.08. The proposed wide-field MOKE approach therefore provides an instant and direct characterization of DL-SOT, without the need of any further interpretation on electrical signals.

cond-mat.mtrl-sci↗

arXiv2017

A Comparative Study on Spin-Orbit Torque Efficiencies from W/ferromagnetic and W/ferrimagnetic Heterostructures

It has been shown that W in its resistive form possesses the largest spin-Hall ratio among all heavy transition metals, which makes it a good candidate for generating efficient dampinglike spin-orbit torque (DL-SOT) acting upon adjacent ferromagnetic or ferrimagnetic (FM) layer. Here we provide a systematic study on the spin transport properties of W/FM magnetic heterostructures with the FM layer being ferromagnetic Co$_{20}$Fe$_{60}$B$_{20}$ or ferrimagnetic Co$_{63}$Tb$_{37}$ with perpendicular magnetic anisotropy. The DL-SOT efficiency $|ξ_{DL}|$, which is characterized by a current-induced hysteresis loop shift method, is found to be correlated to the microstructure of W buffer layer in both W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ systems. Maximum values of $|ξ_{DL}|\approx 0.144$ and $|ξ_{DL}|\approx 0.116$ are achieved when the W layer is partially amorphous in the W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ heterostructures, respectively. Our results suggest that the spin Hall effect from resistive phase of W can be utilized to effectively control both ferromagnetic and ferrimagnetic layers through a DL-SOT mechanism.

cond-mat.mtrl-sci↗

arXiv2017

Tunable Spin-Orbit Torques in Cu-Ta Binary Alloy Heterostructures

The spin Hall effect (SHE) is found to be strong in heavy transition metals (HM), such as Ta and W, in their amorphous and/or high resistivity form. In this work, we show that by employing a Cu-Ta binary alloy as buffer layer in an amorphous Cu$_{100-x}$Ta$_{x}$-based magnetic heterostructure with perpendicular magnetic anisotropy (PMA), the SHE-induced damping-like spin-orbit torque (DL-SOT) efficiency $|ξ_{DL}|$ can be linearly tuned by adjusting the buffer layer resistivity. Current-induced SOT switching can also be achieved in these Cu$_{100-x}$Ta$_{x}$-based magnetic heterostructures, and we find the switching behavior better explained by a SOT-assisted domain wall propagation picture. Through systematic studies on Cu$_{100-x}$Ta$_{x}$-based samples with various compositions, we determine the lower bound of spin Hall conductivity $|σ_{SH}|\approx2.02\times10^{4}[\hbar/2e]Ω^{-1}\cdot\operatorname{m}^{-1}$ in the Ta-rich regime. Based on the idea of resistivity tuning, we further demonstrate that $|ξ_{DL}|$ can be enhanced from 0.087 for pure Ta to 0.152 by employing a resistive TaN buffer layer.

cond-mat.mtrl-sci↗

arXiv2017

The Spin-Orbit Torque from a Magnetic Heterostructure with High-Entropy Alloy

High-entropy alloy (HEA) is a family of metallic materials with nearly equal partitions of five or more metals, which might possess mechanical and transport properties that are different from conventional binary or tertiary alloys. In this work, we demonstrate current-induced spin-orbit torque (SOT) magnetization switching in a Ta-Nb-Hf-Zr-Ti HEA-based magnetic heterostructure with perpendicular magnetic anisotropy (PMA). The maximum damping-like SOT efficiency from this particular HEA-based magnetic heterostructure is further determined to be $|ζ^{\operatorname{HEA}}_{DL}|\approx0.033$ by hysteresis loop shift measurements, while that for the Ta control sample is $|ζ^{\operatorname{Ta}}_{DL}|\approx0.04$. Our results indicate that HEA-based magnetic heterostructures can serve as a new group of potential candidates for SOT device applications.

cond-mat.mtrl-sci↗

arXiv2016

On the origin of field-like spin-orbit torques in heavy metal-ferromagnet-oxide thin film heterostructures

We report measurements of the thickness and temperature (T) dependencies of current-induced spin-orbit torques, especially the field-like (FL) component, in various heavy metal (HM)/normal metal (NM) spacer/ferromagnet (FM)/Oxide (MgO and HfOx/MgO) heterostructures. The FL torque in these samples originates from spin current generated by the spin Hall effect (SHE) in the HM. For a FM layer sufficiently thin that a substantial portion of this spin current can reach the FM/Oxide interface, T-dependent spin scattering there can yield a strong FL torque that is, in some cases, opposite in sign to that exerted at the NM/FM interface.

cond-mat.mtrl-sci↗

arXiv2016

Determination of the Spin-Hall-Effect-Induced and the Wedged-Structure-Induced Spin Torque Efficiencies in Heterostructures with Perpendicular Magnetic Anisotropy

We report that by measuring current-induced hysteresis loop shift versus in-plane bias magnetic field, the spin Hall effect (SHE) contribution of the current-induced effective field per current density, $χ_{SHE}$, can be estimated for Pt and Ta-based magnetic heterostructures with perpendicular magnetic anisotropy (PMA). We apply this technique to a Pt-based sample with its ferromagnetic (FM) layer being wedged-deposited and discover an extra effective field contribution, $χ_{Wedged}$, due to the asymmetric nature of the deposited FM layer. We confirm the correlation between $χ_{Wedged}$ and the asymmetric depinning process in FM layer during magnetization switching by magneto-optical Kerr (MOKE) microscopy. These results indicate the possibility of engineering deterministic spin-orbit torque (SOT) switching by controlling the symmetry of domain expansion through the materials growth process.

cond-mat.mtrl-sci↗

arXiv2016

Current Control of Magnetic Anisotropy via Stress in a Ferromagnetic Metal Waveguide

We demonstrate that in-plane charge current can effectively control the spin precession resonance in an Al2O3/CoFeB/Ta heterostructure. Brillouin Light Scattering (BLS) was used to detect the ferromagnetic resonance field under microwave excitation of spin waves at fixed frequencies. The current control of spin precession resonance originates from modification of the in-plane uniaxial magnetic anisotropy field H_k, which changes symmetrically with respect to the current direction. Numerical simulation suggests that the anisotropic stress introduced by Joule heating plays an important role in controlling H_k. These results provide new insights into current manipulation of magnetic properties and have broad implications for spintronic devices.

cond-mat.mtrl-sci↗

arXiv2015

Enhancement of the Anti-Damping Spin Torque Efficacy of Platinum by Interface Modification

We report a strong enhancement of the efficacy of the spin Hall effect (SHE) of Pt for exerting anti-damping spin torque on an adjacent ferromagnetic layer by the insertion of $\approx$ 0.5 nm layer of Hf between a Pt film and a thin, < 2 nm, Fe$_{60}$Co$_{20}$B$_{20}$ ferromagnetic layer. This enhancement is quantified by measurement of the switching current density when the ferromagnetic layer is the free electrode in a magnetic tunnel junction. The results are explained as the suppression of spin pumping through a substantial decrease in the effective spin-mixing conductance of the interface, but without a concomitant reduction of the ferromagnet\' s absorption of the SHE generated spin current.

cond-mat.mtrl-sci↗

arXiv2014

Dependence of the Efficiency of Spin Hall Torque on the Transparency of Pt-Ferromagnetic Layer Interfaces

We report that spin current transport across Pt-ferromagnet (FM) interfaces is strongly dependent on the type and the thickness of the FM layer and on post-deposition processing protocols. By employing both harmonic voltage measurements and spin-torque ferromagnetic resonance measurements, we find that the efficiency of the Pt spin Hall effect in exerting a damping-like spin torque on the FM ranges from < 0.05 to > 0.10 under different interfacial conditions. We also show that the temperature dependence of the spin torque efficiencies for both the damping-like torque and field-like torque is dependent upon the details of the Pt-FM interface. The "internal" spin Hall angle of the Pt thin films used in this study, after taking the interfacial spin transmission factor into account, is estimated to be ~ 0.20. This suggests that a careful engineering of Pt-FM interfaces can improve the spin-Hall-torque efficiency of Pt-based spintronic devices.

cond-mat.mes-hall↗

arXiv2014

Control of Propagating Spin Waves via Spin Transfer Torque in a Metallic Bilayer Waveguide

We investigate the effect of a direct current on propagating spin waves in a CoFeB/Ta bilayer structure. Using the micro-Brillouin light scattering technique, we observe that the spin wave amplitude may be attenuated or amplified depending on the direction of the current and the applied magnetic field. Our work suggests an effective approach for electrically controlling the propagation of spin waves in a magnetic waveguide and may be useful in a number of applications such as phase locked nano-oscillators and hybrid information processing devices.

cond-mat.mtrl-sci↗

arXiv2014

Enhancement of Perpendicular Magnetic Anisotropy and Transmission of Spin-Hall-Effect-Induced Spin Currents by a Hf Spacer Layer in W/Hf/CoFeB/MgO Layer

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.

cond-mat.mtrl-sci↗

arXiv2012

Gate voltage modulation of spin-Hall-torque-driven magnetic switching

Two promising strategies for achieving efficient control of magnetization in future magnetic memory and non-volatile spin logic devices are spin transfer torque from spin polarized currents and voltage-controlled magnetic anisotropy (VCMA). Spin transfer torque is in widespread development as the write mechanism for next-generation magnetic memory, while VCMA offers the potential of even better energy performance due to smaller Ohmic losses. Here we introduce a 3-terminal magnetic tunnel junction (MTJ) device that combines both of these mechanisms to achieve new functionality: gate-voltage-modulated spin torque switching. This gating makes possible both more energy-efficient switching and also improved architectures for memory and logic applications, including a simple approach for making magnetic memories with a maximum-density cross-point geometry that does not require a control transistor for every MTJ.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic oscillations driven by the spin Hall effect in 3-terminal magnetic tunnel junction devices

We show that 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.

cond-mat.mtrl-sci↗

arXiv2012

Spin transfer torque devices utilizing the giant spin Hall effect of tungsten

We report a giant spin Hall effect (SHE) 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 |θ|=0.30\pm0.02, 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 |θ|=0.33\pm0.06. 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.

cond-mat.mtrl-sci↗

arXiv2012

Spin torque switching with the giant spin Hall effect of tantalum

We report a giant spin Hall effect (SHE) in β-Ta that generates spin currents intense enough to induce efficient spin-transfer-torque switching of ferromagnets, thereby providing a new approach for controlling magnetic devices that can be superior to existing technologies. We quantify this SHE by three independent methods and demonstrate spin-torque (ST) switching of both out-of-plane and in-plane magnetized layers. We implement a three-terminal device that utilizes current passing through a low impedance Ta-ferromagnet bilayer to effect switching of a nanomagnet, with a higher-impedance magnetic tunnel junction for read-out. The efficiency and reliability of this device, together with its simplicity of fabrication, suggest that this three-terminal SHE-ST design can eliminate the main obstacles currently impeding the development of magnetic memory and non-volatile spin logic technologies.

cond-mat.mtrl-sci↗