Seonghoon Woo

Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets

Magnetic skyrmions are topologically protected spin textures that exhibit fascinating physical behaviours and large potential in highly energy-efficient spintronic device applications. The main obstacles so far are that skyrmions have been observed in only a few exotic materials and at low temperatures, and fast current-driven motion of individual skyrmions has not yet been achieved. Here, we report the observation of stable magnetic skyrmions at room temperature in ultrathin transition metal ferromagnets with magnetic transmission soft X-ray microscopy. We demonstrate the ability to generate stable skyrmion lattices and drive trains of individual skyrmions by short current pulses along a magnetic racetrack at speeds exceeding 100 m s(-1) as required for applications. Our findings provide experimental evidence of recent predictions and open the door to room-temperature skyrmion spintronics in robust thin-film heterostructures.

Enhanced spin-orbit torques in Pt/Co/Ta heterostructures

Spin-orbit torques (SOTs) are studied in perpendicularly magnetized ultrathin Co films sandwiched between two heavy metals, Pt and Ta. A significant enhancement of the Slonczewski-like torque is achieved by placing dissimilar metals with opposite spin Hall angles on opposite sides of the ferromagnet. SOTs were characterized through harmonic measurements and the contribution by the Ta overlayer was isolated by systematically varying its thickness. An effective spin Hall angle of up to 34% is observed, along with a sizable field-like torque that increases with increasing Ta layer thickness. Current-induced switching measurements reveal a corresponding increase in switching efficiency, suggesting that by engineering both interfaces in trilayer structures, the SOTs can be significantly improved.

Chiral magnetization textures stabilized by the Dzyaloshinskii-Moriya interaction during spin-orbit torque switching

We study the effect of the Dzyaloshinskii-Moriya interaction (DMI) on current-induced magnetic switching of a perpendicularly magnetized heavy-metal/ferromagnet/oxide trilayer both experimentally and through micromagnetic simulations. We report the generation of stable helical magnetization stripes for a sufficiently large DMI strength in the switching region, giving rise to intermediate states in the magnetization and confirming the essential role of the DMI on switching processes. We compare the simulation and experimental results to a macrospin model, showing the need for a micromagnetic approach. The influence of the temperature on the switching is also discussed.

Large voltage-induced modification of spin-orbit torques in Pt/Co/GdOx

We report on large modifications of current-induced spin-orbit torques in a gated Pt/Co/Gd-oxide microstrip due to voltage-driven O2− migration. The Slonczewski-like and field-like torques are quantified using a low-frequency harmonic technique based on the polar magneto-optical Kerr effect. Voltage-induced oxidation of Co enhances the Slonczewski-like torque by as much as an order of magnitude and simultaneously reduces the anisotropy energy barrier by a factor of ∼5. Such magneto-ionic tuning of interfacial spin-orbit effects may significantly enhance the efficiency of magnetization switching and provide additional degrees of freedom in spintronic devices.

Control of propagating spin-wave attenuation by the spin-Hall effect

The spin-Hall effect induced modification of the attenuation of propagating exchange-mode spin waves (SWs) is studied micromagnetically and analytically in heavy-metal/ferromagnet bilayers. Micromagnetic simulations of spin-wave propagation in Pt/NiFe show that at a relatively low current density of ∼ 6 × 1011 A/m2, Gilbert damping is exactly balanced by the spin-Hall torque and long-distance SW transmission is possible. An analytical model is developed to explain the micromagnetic results and relate the current density to the characteristic attenuation length. The results suggest that the spin Hall effect can be used as an effective means to control the attenuation length of propagating spin waves in nanostructures.

Characterization of spin-orbit torques in Pt/Co/Ta structures

Current-induced torques in heavy-metal/ferromagnet/oxide stacks have been of significant recent interest for highly efficient magnetization switching and domain wall motion. These spin-orbit torques (SOTs) arise through the spin-hall effect (SHE) and Rashba effects at the heavy-metal/ ferromagnet interface. In such structures, the oxide layer plays the role of breaking the inversion symmetry of the structure, but typically does not actively contribute to the SOT. Here we examine SOTs in ultrathin Co films sandwiched between two spin Hall metals whose spin Hall angles are of opposite sign. In this case, the Slonczewski-like torques generated at the top in bottom interface work in concert to enhance the total SOT. We examine Pt/Co/Ta stacks, where effective spin Hall angles have been reported in the ranges θTa=-0.12~0.15 and θPt= +0.04~0.08 in torque measurements. A series of Pt(3nm)/Co(0.9nm)/Ta(t) tri-layer structure capped by 1.5nm of TaOx were prepared by sputter deposition, where the thickness t of the Ta metal top layer varied from t=0.5nm to t=4nm. These films all exhibited perpendicular magnetic anisotropy in the as-deposited state. As shown in the figure 1, SOTs were measured using the harmonic Hall voltage measurement scheme[7-8], in which the variation of the first and second harmonics of the anomalous Hall voltage with in-plane fields are used to quantify the longitudinal and transverse induced effective fields generated, respectively, by the Slonczewski-like and field-like SOTs. Figure 1(a) and (b) shows the experimental geometry for the torque measurements, and the first and second harmonics of the anomalous Hall voltage, Vω and Vω, are then measured while sweeping either a longitudinal field HL or transverse field H T to yield HSL, and H FL, respectively. Figure 1(g) shows the measured effective fields, HFL and HSL, depending on the thickness tTa of the Ta top metal layer (left axis). The Sloncewski-like torque increases substantially up to 190 Oe per 1011 A/m2, along with a sizable field-like torque that exceeds 120 Oe per 1011 A/m2 with increasing Ta layer thickness. The effective spin Hall angle computed from the HSL is shown referenced to the right-hand axis of Fig 1(g). An effective spin Hall angle of up to 34% is observed, exceeding the record value of 0.30 for W. X-ray photoelectron spectroscopy (XPS) sputter-depth profiling was performed to extract the depth-dependent material compositions. Based on the XPS profiling results, we speculate that the presence of Ta within the Co layer and the compositionally-graded Co/Ta interface may increase asymmetric spin scattering within the Co layer and/or enhance the spin injection efficiency from the Ta to Co due to the diffuse nature of the interface. Finally, we characterized current-induced switching and extracted a measure of the switching efficiency to compare with the effective fields obtained from harmonic SOT measurements. Figure 2(a, b) shows exemplary switching phase diagrams for tTa=0.5nm and tTa=4nm in which the mean normalized Mz after current pulse injection was determined for each pair (Hx, jpulse) from 10 measurement cycles. As shown in Figure 2(c), the switching efficiency increased significantly with the addition of a metallic Ta overlayer, by about a factor of 2 over the range of tTa examined, implying that the large enhancement in the Slonczewski-like torque significantly increased the current-induced switching efficiency. These results point to significant opportunities to engineer the interfaces of ultrathin transition ferromagnets to enhance SOTs for spintronic device applications.