Field-free spin-orbit torque switching in interlayer exchange coupled Co/Ta/CoTb

This study investigates a T-type field-free spin-orbit torque device with an in-plane magnetic layer coupled to a perpendicular magnetic layer via a non-magnetic spacer. The device utilizes a Co/Ta/CoTb structure, in which the in-plane Co layer and the perpendicular CoTb layer are ferromagnetically (FM) coupled through the Ta spacer. ‘T-type’ refers to the magnetization arrangement in the FM/spacer/FIM structure, where the magnetization in FM is in-plane, while in FIM, it is out-of-plane. This configuration forms a T-shaped arrangement for the magnetization of the two magnetic layers. Additionally, ‘interlayer exchange coupling (IEC)’ denotes the interaction between the two magnetic layers, which is achieved by adjusting the material and thickness of the spacer. Our results show that an in-plane effective field from the IEC enables deterministic current-induced magnetization switching of the CoTb layer. The field-driven and the current-driven asymmetric domain wall motion are observed and characterized by magneto-optic Kerr effect measurements. The functionality of multistate synaptic plasticity is demonstrated by understanding the relationship between the anomalous Hall resistance and the applied current pulses, indicating the potential for the device in spintronic memory and neuromorphic computing.


Introduction
Spin-orbit torque (SOT) has emerged as an efficient method for electrically manipulating magnetization due to its low 3 These authors contributed equally to this work. * Authors to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. power consumption, fast write/read speed, and improved endurance [1,2]. In contrast to conventional spin-transfer torque (STT) that relies on spin momenta transfer between spin-polarized electrons and local magnetization, the SOT describes a charge-spin conversion process in which the injected charge current and the induced spin current are separated in a spin source material [3,4] or at the inversionsymmetry-breaking surface/interface [5,6]. The spin current can further propagate and transfer the spin momenta to the adjacent ferromagnetic (FM) layer for magnetization switching. Depending on the different magnetic anisotropy of the FM layer, SOT devices can be classified mainly into three types: X-type with in-plane magnetization along the direction of current flow, Y-type with in-plane magnetization orthogonal to the direction of current flow, and Z-type with out-of-plane magnetization perpendicular to the film plane. Although Z-type SOT devices hold great promise for constructing high-density memories, an additional magnetic field is generally required to break the symmetry for deterministic magnetization switching, thus inevitably increasing power consumption and circuit complexity. In order to realize the field-free perpendicular magnetization switching, several ingenious designs have been introduced in the SOT devices, such as a built-in effective field (exchange bias [7][8][9], exchange coupling [10][11][12], and magnetic hard mask [13]), lateral structural asymmetry (wedged structure [14], lateral composition gradient [15], and geometric engineering [16]), the combination of STT and SOT [17], ferroelectric control [18], spin current manipulation [19], orbital current contribution [20], and crystal asymmetry [21,22]. However, most approaches are not yet mature for the memory industry due to their compromise on fabrication compatibility, device density, and power consumption.
Recently, a relatively new structure of field-free SOT device has been proposed, in which an in-plane magnetic layer is coupled to a perpendicular magnetic layer via a nonmagnetic spacer, providing an in-plane field through interlayer exchange coupling (IEC) for the SOT-driven perpendicular magnetization switching [10,[23][24][25]. This type of device has drawn much attention for its simplified design, complementary metal-oxide-semiconductor-compatible fabrication, and extended functionality such as logic-in-memory [26] and neuromorphic computing [27]. However, only FM materials are used in these devices; other materials, such as compensated ferrimagnets for potentially ultrafast and energyefficient switching [28], are rarely reported. In this work, we report field-free magnetization switching in a Co/Ta/CoTb composite free layer. The deterministic switching of the top perpendicularly magnetized CoTb layer is assisted by a FM coupled exchange field as large as 14 Oe induced from the bottom in-plane magnetized Co layer. Magneto-optic Kerr effect (MOKE) images reveal the nucleation and propagation of domain walls (DWs) in the ferrimagnetic CoTb layer under the influence of the IEC field, which is consistent with the electrical transport behavior. Furthermore, field-free SOT-driven multistate synaptic plasticity is also demonstrated in this structure. Our work sheds light on using ferrimagnetic materials in conjunction with IEC-based structures in spintronic memory and neuromorphic computing.

Experimental details
Samples with multilayer stacks were deposited on thermally oxidized Si substrates via a AJA-ATC-2000 magnetron sputtering system. All film layers in the stack were deposited by confocal sputtering except for Co, which was elaborately designed to induce an in-plane uniaxial magnetic anisotropy by oblique sputtering. Figure 1(a) shows the schematic diagram of the deposited multilayer stacks. Hall-bar devices with channel widths of 20 µm were fabricated using standard photolithography and subsequent Ar + beam etching for electrical transport measurement and DW motion characterization.
During the electrical transport measurement, the positive directions of the magnetic field (+H x ), electric current (+I), and Hall voltage (+V) are along +X, +X, and +Y, respectively, as shown in figure 1(b). The current source (Keithley model 6221) was used to apply currents, a nanovoltmeter (Keithley model 2182A) to measure the Hall voltage.
The MOKE imaging measurements was utilized to study the SOT-induced DW motion, we first saturate the CoTb magnetization along the +z or −z-axis with an in-plane external magnetic field H, then we remove H and apply a pulsed dc current (50 µs pulse width) to observe the magnetization switching using MOKE microscope. The dark and grey contrast represent the magnetization along the −z and +z axis, respectively.

Results and discussion
We confirm that both films and devices show perpendicular magnetic anisotropy (PMA) by MOKE imaging and anomalous Hall effect (AHE) measurements. The resulting square hysteresis loops (as seen in figure 1(c)) indicate that the top CoTb (6 nm) layer indeed has good PMA. As shown in figure 1(d), the magnetization switching is accomplished by the nucleation and propagation of magnetic domains. The structure of the nucleated magnetic domains shows an elliptical shape, which is consistent with the in-plane uniaxial magnetic anisotropy induced during the deposition process. In figure 2, we further measured the M-H loops of the sample along the x, y, and z directions using a vibrating sample magnetometer. The CoTb layer shows good PMA and couples to the bottom Co layer in the z-direction. Measurements in the x and y directions further confirm the in-plane uniaxial magnetic anisotropy induced in the Co layer, and the easy axis is along the x direction.
Next, the effects of the IEC field on the DW motion were investigated. An in-plane field of 3000 Oe was first applied to set the Co magnetization along the −x direction, followed by a large field along the +z direction to saturate the CoTb magnetization (see the first panel of figure 3(a)) and a reversed field (H z = −600 Oe) to induce magnetization switching. The results indicate that the DW motion along the x direction is faster than that in the y direction, leading to the formation of an elliptical domain shape with its long axis in the x direction. Furthermore, a significant asymmetry of the DW motion along the +x and −x directions was observed, as shown in figure 3(a). This asymmetry was confirmed when the field polarity was reversed in figure 3(b), where the DW motion along the −x direction was faster than that in the +x direction under the +600 Oe switching field. These findings suggest that the IEC induces an in-plane effective field, and its direction depends on the Co layer's initial magnetization state (here along the −x direction). This effective IEC field will break the right-handed DW chirality stabilized by the Dzyaloshinskii-Moriya interaction [29], resulting in the asymmetric DW motion as sketched in figures 3(c) and (d).
The effects of SOT on magnetization switching were investigated using a sweeping current pulse with a width of 50 µs applied to the device. The magnetization state of the CoTb layer was detected by measuring the Hall resistance with a smaller current of 1 mA after each pulse. Figures 4(a) and (b) show the current-induced magnetization switching loop with an in-plane external field along the x direction. When the field is scanned from +500 Oe to −500 Oe, the switching polarity at zero-field is the same as that with the +500 Oe field, shown in figure 4(a). Similarly, when the external field was applied from −500 Oe to + 500 Oe, the polarity of zero-field switching was the same as that with the −500 Oe field, shown in figure 4(b). The deterministic field-free switching has been observed in both cases. However, the effective IEC field has different directions depending on the initial magnetization state of the Co layer, resulting in the opposite switching polarity in both cases. The consistent directions between the IEC field and the magnetization of the Co layer indicate that the coupling between CoTb and Co is FM. The suppression of magnetization switching at H x ≈ ±14 Oe was also observed, which corresponds to the compensation of the IEC strength. It is noted that reversing the polarity of the current stimuli typically results in opposite but symmetric DW motion. Hence an additional in-plane field is required to break the symmetry for the expansion/compression of perpendicularly magnetized domains. In figure 4(c), we use magneto-optical Kerr microscopy to characterize the SOT-induced DW motion further. The applied current pulse is 80 mA with a width of 50 µs. With the same initial state (zerofield state) as in figure 4(a), where the magnetization of Co and CoTb were along the +x and +z directions, a pulse current was subsequently applied and resulted in the DW expansion. Additionally, it is found that the DW motion along the −x direction is significantly faster than the +x direction, further supporting the broken symmetry of the system due to the presence Finally, we investigated the memristive behavior of the device. A series of pulse currents were applied to the device at zero-filed. The maximum amplitude of the pulse (I max ) ranges from 45 mA to 85 mA. In figure 5(a), a series of different Hall resistances were obtained, corresponding to different saturated states of the CoTb layer. Next, successive pulses with variable amplitudes were applied to understand the memristive behavior better. As shown in figure 5(b), the pulse amplitude directly affects the Hall resistance, indicating that the magnetization state of the CoTb layer can be modulated by changing the amplitude of the current pulse. It is also observed that the Hall resistance remains unchanged when the pulse is off, which is typical behavior of non-volatile memories. As shown in figure 5(c), fifty pulses of +55 mA and fifty pulses of −55 mA were applied repeatedly to the device, and the Hall resistance change was monitored. It is found that the different memristive behavior can be switched between positive and negative pulse regimes. Furthermore, as the applied pulse increases, the number of pulses required for the device to reach saturation decreases, as shown in figure 5(d). This result demonstrates the device's non-volatile memristive properties, enabling it to simulate the function of synapses-the junctions through which neurons transmit signals to one another. Synaptic plasticity refers to the ability of synapses to dynamically adjust their weight based on the activity of the presynaptic and postsynaptic neurons. This modulation of synaptic weights allows neural networks to learn and store information, as is observed in the human brain through changes in synaptic weights [27,30]. In this context, the device's artificial synapse is characterized by the modulation of the magnetization state in the CoTb layer, which serves as a surrogate for synaptic weight.

Conclusions
In conclusion, our study shows that the coupled CoTb and Co layers with perpendicular and in-plane anisotropies generate an in-plane effective field of ±14 Oe, enabling the deterministic SOT switching. The field-driven and the currentdriven asymmetric DW motion are observed and characterized by MOKE measurements. Furthermore, field-free SOTdriven multistate synaptic plasticity is also demonstrated in this device. Our work sheds light on using ferrimagnetic materials in conjunction with IEC-based structures in spintronic memory and neuromorphic computing.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).