Anomalous anti-damping in sputtered β-Ta/Py bilayer system

Is Semiconducting Transition-Metal Dichalcogenide Suitable for Spin Pumping?

Spin pumping has been reported on interfaces formed with ferromagnetic metals and layered transition-metal dichalcogenides (TMDs), as signified by enhanced Gilbert damping parameters extracted from magnetodynamics measurements. However, whether the observed damping enhancement purely arises from the pumping effect has remained debatable, given that possible extrinsic disturbances on the interfaces cannot be excluded in most of the experiments. Here, we explore an atomically clean interface formed with CoFeB and atomically thin MoSe2, achieved by an all in situ growth strategy based on molecular beam epitaxy. Taking advantage of ferromagnetic resonance analysis, we find that the Gilbert damping of the CoFeB/MoSe2 interface closely resembles that of CoFeB/SiO2, suggesting the absence of spin pumping. With similar findings demonstrated on a few more representative interfaces, this work clarifies the unsuitability of semiconducting TMDs for spin pumping and suggests that the observed damping enhancement in the previous reports may be predominantly attributed to extrinsic contributions during the experimental process.

Observation of the out-of-plane orbital antidamping-like torque

The out-of-plane antidamping-like orbital torque fosters great hope for high-efficiency spintronic devices. Here we report experimentally the observation of out-of-plane antidamping-like torque that could be generated by z-polarized orbital current in ferromagnetic-metal/oxidized Cu (CuOx) bilayers, which is presented unambiguously by the magnetic field angle dependence of the spin-torque ferromagnetic resonance signal. The CuOx thickness dependence of the orbital torque ratios highlights that the interfacial effect would be responsible for the generation of orbital current. Besides that, the CuOx thickness dependence of the damping parameter further proves the observation of antidamping-like torque. This result contributes to enriching the orbital-related theory of the generation mechanism of the orbital torque.

Heavy metal deposition temperature tuned spin pumping efficiency control in permalloy/tantalum bilayers

Spin pumping is a key property for spintronic application that can be realized in heavy metal/ferromagnet bilayers. Here we demonstrate the possibility of improving spin pumping in permalloy (Py)/tantalum (Ta) bilayers through control of Ta heavy metal deposition temperature. Through a combination of structural and ferromagnetic resonance based magnetization dynamics study, we reveal the role of Ta deposition temperature in improving spin mixing conductance which is a key parameter for spin pumping across the Py/Ta interface. The results show that by depositing Ta above room temperature, a high spin mixing conductance of 7.7 ×10¹⁸m^-2is obtained withα-Ta layer. The results present an understanding of the correlation between heavy metal deposition temperature and interface structure improvement and consequent control of spin pumping in Py/Ta bilayers.

Simultaneous observation of anti-damping and the inverse spin Hall effect in the La0.67Sr0.33MnO3/Pt bilayer system

Manganites have shown potential in spintronics because they exhibit high spin polarization. Here, by ferromagnetic resonance we have studied the damping properties of La0.67Sr0.33MnO3/Pt bilayers which are prepared by oxide molecular beam epitaxy. The damping coefficient (α) of a La0.67Sr0.33MnO3 (LSMO) single layer is found to be 0.0104. However the LSMO/Pt bilayers exhibit a decrease in α with an increase in Pt thickness. This decrease in the value of α is probably due to high anti-damping like torque. Furthermore, we have investigated the angle dependent inverse spin Hall effect (ISHE) to quantify the spin pumping voltage from other spin rectification effects such as the anomalous Hall effect and anisotropic magnetoresistance. We have observed a high spin pumping voltage (∼20 μV). The results indicate that both anti-damping and spin pumping phenomena occur simultaneously.

Roles of Joule heating and spin-orbit torques in the direct current induced magnetization reversal

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmO_x and Pt/Co/AlO_x structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (J_c) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained J_c for Pt/Co/SmO_x and Pt/Co/AlO_x structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and J_c also immensely relies on the depinning field.

Current-Induced Domain Wall Motion and Tilting in Perpendicularly Magnetized Racetracks

The influence of C insertion on Dzyaloshinskii-Moriya interaction (DMI) as well as current-induced domain wall (DW) motion (CIDWM) and tilting in Pt/Co/Ta racetracks is investigated via a magneto-optical Kerr microscope. The similar DMI strength for Pt/Co/Ta and Pt/Co/C/Ta samples reveals that DMI mainly comes from the Pt/Co interface. Fast DW velocity around tens of m/s with current density around several MA/cm² is observed in Pt/Co/Ta. However, it needs double times larger current density to reach the same magnitude in Pt/Co/C/Ta, indicating DW velocity is related to the spin-orbit torque efficiency and pinning potential barrier. Moreover, in CIDWM, DW velocity is around 10³ times larger than that in field-induced DW motion (FIDWM) with current-generated effective field keeping the same magnitude as applied magnetic field, revealing that the current-generated Joule heating has an influence on DW motion. Interestingly, current-induced DW tilting phenomenon is observed, while this phenomenon is absent in FIDWM, demonstrating that the current-generated Oersted field may also play an essential role in DW tilting. These findings could provide some designing prospects to drive DW motion in SOT-based racetrack memories.

Magnetization switching through domain wall motion in Pt/Co/Cr racetracks with the assistance of the accompanying Joule heating effect

Heavy metal/ferromagnetic layers with perpendicular magnetic anisotropy (PMA) have potential applications for high-density information storage in racetrack memories and nonvolatile magnetic random access memories.

Capping Layer (CL) Induced Antidamping in CL/Py/β-W System (CL: Al, β-Ta, Cu, β-W)

For achieving ultrafast switching speed and minimizing dissipation losses, the spin-based data storage device requires a control on effective damping (α_eff) of nanomagnetic bits. Incorporation of interfacial antidamping spin orbit torque (SOT) in spintronic devices therefore has high prospects for enhancing their performance efficiency. Clear evidence of such an interfacial antidamping is found in Al capped Py(15 nm)/β-W(t_W)/Si (Py = Ni₈₁Fe₁₉ and t_W = thickness of β-W), which is in contrast to the increase of α_eff (i.e., damping) usually associated with spin pumping as seen in Py(15 nm)/β-W(t_W)/Si system. Because of spin pumping, the interfacial spin mixing conductance (g^↑↓) at Py/β-W interface and spin diffusion length (λ_SD) of β-W are found to be 1.63(±0.02) × 10¹⁸ m^-2 (1.44(±0.02) × 10¹⁸ m^-2) and 1.42(±0.19) nm (1.00(±0.10) nm) for Py(15 nm)/β-W(t_W)/Si (β-W(t_W)/Py(15 nm)/Si) bilayer systems. Other different nonmagnetic capping layers (CL), namely, β-W(2 nm), Cu(2 nm), and β-Ta(2,3,4 nm) were also grown over the same Py(15 nm)/β-W(t_W). However, antidamping is seen only in β-Ta(2,3 nm)/Py(15 nm)/β-W(t_W)/Si. This decrease in α_eff is attributed to the interfacial Rashba like SOT generated by nonequilibrium spin accumulation subsequent to the spin pumping. Contrary to this, when interlayer positions of Py(15 nm) and β-W(t_W) is interchanged irrespective of the fixed top nonmagnetic layer, an increase of α_eff is observed, which is ascribed to spin pumping from Py to β-W layer.

Influence of intermixing at the Ta/CoFeB interface on spin Hall angle in Ta/CoFeB/MgO heterostructures

When a current is passed through a non-magnetic metal with strong spin-orbit coupling, an orthogonal spin current is generated. This spin current can be used to switch the magnetization of an adjacent ferromagnetic layer or drive its magnetization into continuous precession. The interface, which is not necessarily sharp, and the crystallographic structure of the nonmagnetic metal can both affect the strength of current-induced spin-orbit torques. Here, we investigate the effects of interface intermixing and film microstructure on spin-orbit torques in perpendicularly magnetized Ta/Co40Fe40B20/MgO trilayers with different Ta layer thickness (5 nm, 10 nm, 15 nm), greater than the spin diffusion length. Effective spin-orbit torques are determined from harmonic Hall voltage measurements performed at temperatures ranging from 20 K to 300 K. We account for the temperature dependence of damping-like and field-like torques by including an additional contribution from the Ta/CoFeB interface in the spin diffusion model. Using this approach, the temperature variations of the spin Hall angle in the Ta underlayer and at the Ta/CoFeB interface are determined separately. Our results indicate an almost temperature-independent spin Hall angle of [Formula: see text] in Ta and a strongly temperature-dependent [Formula: see text] for the intermixed Ta/CoFeB interface.

Two magnon scattering and anti-damping behavior in a two-dimensional epitaxial TiN/Py(tPy)/β-Ta(tTa) system

Anti-damping in two-magnon scattering free two-dimensional epitaxial Si(400)/TiN(200) (8 nm)/Py(200) (12 nm)/Ta(200) (6 nm) system.

Growth of Co2FeAl Heusler alloy thin films on Si(100) having very small Gilbert damping by Ion beam sputtering

The influence of growth temperature T_s (300–773 K) on the structural phase ordering, static and dynamic magnetization behaviour has been investigated in ion beam sputtered full Heusler alloy Co₂FeAl (CFA) thin films on industrially important Si(100) substrate. The B2 type magnetic ordering is established in these films based on the clear observation of the (200) diffraction peak. These ion beam sputtered CFA films possess very small surface roughness of the order of subatomic dimensions (<3 Å) as determined from the fitting of XRR spectra and also by AFM imaging. This is supported by the occurrence of distinct Kiessig fringes spanning over the whole scanning range (~4°) in the x-ray reflectivity (XRR) spectra. The Gilbert damping constant α and effective magnetization 4πM_eff are found to vary from 0.0053 ± 0.0002 to 0.0015 ± 0.0001 and 13.45 ± 00.03 kG to 14.03 ± 0.04 kG, respectively. These Co₂FeAl films possess saturation magnetization ranging from 4.82 ± 0.09 to 5.22 ± 0.10 μ_B/f.u. consistent with the bulk L2₁-type ordering. A record low α-value of 0.0015 is obtained for Co₂FeAl films deposited on Si substrate at T_s ~ 573 K.