Mechanism of spin-orbit torques in platinum oxide systems

The advent of Big Data, Machine Learning (ML) and 5G has placed a greater emphasis on certain key metrics of memory technology such as power consumption, non-volatility, speed, size, and endurance. Magnetic Random-Access Memories (MRAM) such as Spin Transfer Torque MRAM (STT-MRAM) and Spin-Orbit Torque MRAM (SOT-MRAM) have emerged as key contenders in this market, targeted towards replacing the current Complementary Metal-Oxide-Semiconductor (CMOS) based Static RAMs (SRAM) and Dynamic RAMs (DRAM). SOT-MRAMs rely on spins generated by applying a charge current through a metal with a high Spin-Orbit Coupling (SOC) to switch the magnetic memory bit. These Heavy Metals (HM) being resistive, lead to Ohmic losses during the write process. A vast body of work, both academic and industrial, has been dedicated to finding ways to minimize these losses and thereby enhance the energy efficiency. Moreover, the current that is injected into the device during the write process is controlled by a CMOS switching transistor. The size of this transistor increases with the switching current. Hence, a reduction in this current can also lead to a gain in the bit density of the memory.Various approaches have been adopted to achieve this target by means of enhancing the generation of spins per unit applied current. The earliest approaches involved using transition metals with high SOC, metallic alloys and resistive structural phase of the metal. More recent works have focused on interfacial engineering via ultra-thin insertion layers and spin-sink capping materials. One of the current focus is on using oxidation as a means of enhancing the SOTs. Different groups have studied the effect of oxidation of the HM, the Ferro-Magnet (FM) as well as the capping layer consisting of lighter metals such as copper. Although majority of these works report an increase of SOTs in general, the results and conclusions are not consistent. Differing trends of SOT increase have been reported which in turn have been attributed to varied physical phenomenon. In this work, we study the SOTs generated by oxidizing the platinum layer in a Ta/Cu/Co/Pt multilayer stack.We quantify the SOTs in this system using second harmonic torque measurements and do indeed observe an increase in torques. This is verified with spin-pumping measurements, observing an increase in the damping. In order to determine the origin of this increase, we built an oxidation model of the system based on electrical, magnetic and material characterizations and ab-initio Density Functional Theory (DFT) calculations. This led us to the conclusion that unlike previous works, which exclusively explained the findings based on a completely oxidized HM model, in practice the oxygen near the FM/HM interface gets pumped into the FM layer. This not only oxidizes the FM, but it also leaves the HM metallic near the interface. This model was further supported by measurements and calculations of the symmetric and anti-symmetric exchange, which were found to have a linear relationship. Accounting for these observations, once the quantified SOTs are corrected, we see no observable increase in torques. This leads us to conclude that although on a system level there is an increase of SOTs with platinum oxidation, there is no intrinsic contribution of platinum-oxide on the enhancement of torques. This finding has broad consequences in the design of SOT-MRAM, affecting endurance, power consumption and Tunneling Magneto-Resistance (TMR).