Maxwell Mann

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.

Current-induced switching in a magnetic insulator

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.

Spin-orbit torques in Ta/TbxCo100-x ferrimagnetic alloy films with bulk perpendicular magnetic anisotropy

We quantified the bulk perpendicular magnetic anisotropy (PMA) and spin-orbit torques (SOTs) in bilayer Ta/TbxCo100-x ferrimagnetic alloy films with varying Tb concentration. The coercivity increases dramatically with increasing TbxCo100-x thickness and is enhanced by the presence of a Ta underlayer. The Ta underlayer simultaneously serves as a source of SOT due to the spin Hall effect, which we show provides an efficient means to manipulate the magnetization in bulk PMA materials. It is further shown that the sign of the anomalous Hall voltage is different for rare-earth (RE) and transition-metal (TM) dominated alloy compositions, whereas the sign of the SOT effective field remains the same, suggesting that the former is related to the TM sublattice magnetization whereas the latter is related to the net magnetization. Our results suggest that Ta/TbxCo100-x is a potential candidate for spin-orbitronic device applications and give insight into spin transport and SOTs in rare-earth/transition-metal alloys.

Effect of rare earth metal on the spin-orbit torque in magnetic heterostructures

We report the effect of the rare earth metal Gd on current-induced spin-orbit torques (SOTs) in perpendicularly magnetized Pt/Co/Gd heterostructures, characterized using harmonic measurements and spin-torque ferromagnetic resonance (ST-FMR). By varying the Gd metal layer thickness from 0 nm to 8 nm, harmonic measurements reveal a significant enhancement of the effective fields generated from the Slonczewski-like and field-like torques. ST-FMR measurements confirm an enhanced effective spin Hall angle and show a corresponding increase in the magnetic damping constant with increasing Gd thickness. These results suggest that Gd plays an active role in generating SOTs in these heterostructures. Our finding may lead to spin-orbitronics device application such as non-volatile magnetic random access memory, based on rare earth metals.

Magneto-ionic effect in CoFeB thin films with in-plane and perpendicular-to-plane magnetic anisotropy

The magneto-ionic effect is a promising method to control the magnetic properties electrically. Charged mobile oxygen ions can easily be driven by an electric field to modify the magnetic anisotropy of a ferromagnetic layer in contact with an ionic conductor in a solid-state device. In this paper, we report on the room temperature magneto-ionic modulation of the magnetic anisotropy of ultrathin CoFeB films in contact with a GdOx layer, as probed by polar micro-Magneto Optical Kerr Effect during the application of a voltage across patterned capacitors. Both Pt/CoFeB/GdOx films with perpendicular magnetic anisotropy and Ta/CoFeB/GdOx films with uniaxial in-plane magnetic anisotropy in the as-grown state exhibit a sizable dependence of the magnetic anisotropy on the voltage (amplitude, polarity, and time) applied across the oxide. In Pt/CoFeB/GdOx multilayers, it is possible to reorient the magnetic anisotropy from perpendicular-to-plane to in-plane, with a variation of the magnetic anisotropy energy greater...

A multi-state memory device based on the unidirectional spin Hall magnetoresistance

We report on a memory device concept based on the recently discovered unidirectional spin Hall magnetoresistance (USMR), which can store multiple bits of information in a single ferromagnetic heterostructure. We show that the USMR with possible contribution of Joule heating-driven magnetothermal effects in ferromagnet/normal metal/ferromagnet (FM/NM/FM) trilayers gives rise to four different 2nd harmonic resistance levels corresponding to four magnetization states ( ⇉, ⇄, ⇆, ⇇) in which the system can be found. Combined with the possibility of controlling the individual FMs by spin-orbit torques, we propose that it is possible to build an all-electrical lateral two-terminal multi-bit-per-cell memory device.

Reduction of in-plane field required for spin-orbit torque magnetization reversal by insertion of Au spacer in Pt/Au/Co/Ni/Co/Ta

Spin-orbit torques and current-induced switching are studied in perpendicularly magnetized Pt/Au/(Co/Ni/Co) films as a function of Au insertion layer thickness tAu. By simultaneously varying the ferromagnet layer thickness, a parametric series of samples with nearly constant anisotropy were prepared. On this series, spin orbit torques were characterized by harmonic voltage and hysteresis loop shift measurements, and current-induced switching was examined as a function of the in-plane bias field. Little variation is seen for tAu 0.5 nm, a series of well-correlated effects appear. Both the loop shift efficiency and the Slonczewski-like spin-orbit torque effective field double, while the in-plane field required to saturate the loop shift efficiency decreases by a factor of ∼10. Correspondingly, the current and in-plane field required for spin-orbit torque switching are reduced by about 90%. These results suggest that a thin Au insertion layer reduces the Dzyaloshinskii-Moriya inte...

Fast current-driven domain walls and small skyrmions in a compensated ferrimagnet

Spintronics is a research field that aims to understand and control spins on the nanoscale and should enable next-generation data storage and manipulation. One technological and scientific key challenge is to stabilize small spin textures and to move them efficiently with high velocities. For a long time, research focused on ferromagnetic materials, but ferromagnets show fundamental limits for speed and size. Here, we circumvent these limits using compensated ferrimagnets. Using ferrimagnetic Pt/Gd44Co56/TaOx films with a sizeable Dzyaloshinskii–Moriya interaction, we realize a current-driven domain wall motion with a speed of 1.3 km s–1 near the angular momentum compensation temperature (TA) and room-temperature-stable skyrmions with minimum diameters close to 10 nm near the magnetic compensation temperature (TM). Both the size and dynamics of the ferrimagnet are in excellent agreement with a simplified effective ferromagnet theory. Our work shows that high-speed, high-density spintronics devices based on current-driven spin textures can be realized using materials in which TA and TM are close together.Ferrimagnetic Gd44Co56 near the compensation temperature enables domain wall motion with a speed of 1.3 km s–1 and room temperature skyrmions with diameters close to 10 nm.

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.