Future applications of spin-orbit torque will require new mechanisms to improve the efficiency for switching nanoscale magnetic tunnel junctions (MTJs), while also controlling the magnetic dynamics to achieve fast, nanosecond scale performance with low write error rates. Here we demonstrate a strategy to simultaneously enhance the interfacial magnetic anisotropy energy and suppress interfacial spin memory loss by introducing sub-atomic and monatomic layers of Hf at the top and bottom interfaces of the ferromagnetic free layer of an in-plane magnetized three-terminal MTJ device. When combined with a beta-W spin Hall channel that generates spin-orbit torque, the cumulative effect is a switching current density of 5.4 x 10 6 A/cm 2 , more than a factor of 3 lower than demonstrated in any other spin-orbit-torque magnetic memory device at room temperature, and highly reliable switching with current pulses only 2 ns long. *buhrman@cornell.edu.

We report measurements of the thickness and temperature (T) dependencies of current-induced spin-orbit torques, especially the field-like (FL) component, in various heavy metal (HM)/normal metal (NM) spacer/ferromagnet (FM)/Oxide (MgO and HfO x /MgO) heterostructures. The FL torque in these samples originates from spin current generated by the spin Hall effect (SHE) in the HM. For a FM layer sufficiently thin that a substantial portion of this spin current can reach the FM/Oxide interface, T-dependent spin scattering there can yield a strong FL torque that is, in some cases, opposite in sign to that exerted at the NM/FM interface. DL e DL s FM H J eM t     and / / (8 ) FL e FL s FM H J eM t     , where M s is the saturation magnetization of the FM layer, t FM is its thickness, is Planck's constant and e is the electron charge [10,11]. The RE effect is generally expected, at least within the context of a Boltzmann equation or drift-diffusion analysis [10], to exert a larger FL than DL torque, while in the SHE case absorption of the transverse polarized component of J s exerts a larger DL  and reflection with some spin rotation can result in a smaller FL  . Studies of SOT excitation of nanomagnets and domain wall motion in HM/FM heterostructures have generally shown that these processes can be well explained by a DL torque due to the SHE of the HM, with an interfacial Dzyaloshinskii-Moriya interaction also important in the case of domain wall displacement [12-14]. It is therefore quite puzzling that heterostructures made of the same materials can, when the FM is thin and magnetized out of plane, exhibit FL DL   [12,15]. This is true even though experiments in which a NM layer of variable thickness (with minimal SHE) is inserted between the HM and FM, confirm the nonlocal nature of FL  in those experiments [16-18]. The origins of FL  are therefore under active debate --there are reports showing that the magnitude and even the sign of FL  can greatly depend on the thickness of the FM [17,19], the type of FM [20], the type of HM [16,21], the direction of the magnetization in FM [22,23] and temperature [15,23]. Here we report measurements of SOTs in various in-plane magnetized (IPM) and perpendicularly magnetized (PM) HM/NM/FM/Oxide (MgO and HfO x /MgO) heterostructures, using Ta, Pt, and W for the HM. We have examined FL  as a function of NM and FM thickness, t NM and t FM , and as a function of temperature T from 300 K to 5 K. The spin torque efficiencies, DL  and FL  , are measured by spin torque ferromagnetic resonance (ST-FMR) for IPM samples [3,24] and DL H  and FL H  by the harmonic response (HR) method [19,25,26] for PM samples. The t NM dependent measurements reveal that the FL torques observed in all of our samples are due to spin current that originates in the HM via the SHE. By varying t FM for PM cases we find that the FL torque in samples with very thin FM layers can be strong, and differ significantly between FM/MgO and FM/HfO x /MgO interfaces. Whereas the DL  is invariably only ...

Increasing dampinglike spin-orbit torque (SOT) is both of fundamental importance for enabling new research into spintronics phenomena and also technologically urgent for advancing low-power spin-torque memory, logic, and oscillator devices. Here, we demonstrate that enhancing interfacial scattering by inserting ultra-thin layers within a spin Hall metals with intrinsic or side-jump mechanisms can significantly enhance the spin Hall ratio. The dampinglike SOT was enhanced by a factor of 2 via sub-monolayer Hf insertion, as evidenced by both harmonic response measurements and currentinduced switching of in-plane magnetized magnetic memory devices with the record low critical switching current of ~73 μA (switching current density ≈ 3.6×10 6 A/cm 2 ). This work demonstrates a very effective strategy for maximizing dampinglike SOT for low-power spin-torque devices.

Effectively manipulating magnetism in ferromagnet (FM) thin film nanostructures with an inplane current has become feasible since the determination of a "giant" spin Hall effect (SHE) in certain heavy metal (HM)/FM system. Recently, both theoretical and experimental reports indicate that the non-collinear and collinear metallic antiferromagnet (AF) materials can have both a large anomalous Hall effect (AHE) and a strong SHE. Here we report a systematic study of the SHE in PtMn with several PtMn/FM systems. By using interface engineering to reduce the "spin memory loss" we obtain, in the best instance, a spin torque efficiency , where T int is the effective interface spin transparency. This is more than twice the previously reported spin torque efficiency for PtMn. We also find that the apparent spin diffusion length in PtMn is surprisingly long, PtMn s 2.3nm   . 2 SHE in different heavy metal (HM)/ferromagnet (FM) systems 1-4 can be characterized by the spin Hall ratio (angle)where J s is the transverse spin current density generated in the HM and J e is the applied longitudinal electrical current density. Recently a new class of heavy metal (HM) alloys, the non-collinear antiferromagnet (AF), Mn 3 Ir 5-7 and Cu-Au-I type AF, X 50 Mn 50 (X=Fe, Pd, Ir, and Pt) [8][9][10][11] have been reported to exhibit SHE as spin current sources, with an internal PtMn 0.125for PtMn 10 , opening up a new area in the rapidly advancing field of "antiferromagnet spintronics" [12][13][14][15][16][17] . To date research on the SHE from AFs has utilized the implicit assumption that there is no interfacial spin flip scattering or "spin memory loss" (SML) 18 when the spin current traverses the interface to apply a torque to the FM. However the existence of a large SML at some Pt/FM interfaces, together with the negative enthalpy of formation of Mn with both Fe and Ni 19 that can promote interface intermixing, raises the question whether there may also be a significant SML at PtMn/FM interfaces, which would mean that the internal q SH PtMn within PtMn could actually be much higher than previously reported.We performed a systematic study of the SHE in several PtMn/FM systems employing spin-torque ferromagnetic resonance (ST-FMR) 20 on in-plane magnetized (IPM) FM layers and the harmonic response technique (HR) 21,22 on FM layers with perpendicular magnetic anisotropy (PMA). We also studied samples where a thin (0.25 nm -0.8 nm) Hf layer is inserted between the PtMn and the FM to suppress strong SML at the interface 23 . We find DL  to vary significantly with both the deposition order for a given PtMn/FM system and between the different FM systems, but to be relatively consistent between IPM and PMA samples with the same constituents. We also obtained robust current-induced switching in these PMA samples demonstrating the potential for utilizing PtMn in perpendicular magnetic tunneling junction (p-MTJ) and three-terminal device applications.

As shown in Figure 1b-d, the SOT-MTJ devices were lithographically patterned from sputter-deposited multilayer stacks consisting of Si/SiO 2 /Ta 1/Au 0.25 Pt 0.75 5/Hf 0.5/ Fe 0.6 Co 0.2 B 0.2Many key electronic technologies (e.g., large-scale computing, machine learning, and superconducting electronics) require new memories that are at the same time fast, reliable, energy-efficient, and of low-impedance, which has remained a challenge. Nonvolatile magnetoresistive random access memories (MRAMs) driven by spin-orbit torques (SOTs) have promise to be faster and more energy-efficient than conventional semiconductor and spin-transfer-torque magnetic memories. It is reported that the spin Hall effect of low-resistivity Au 0.25 Pt 0.75 thin films enables ultrafast antidamping-torque switching of SOT-MRAM devices for current pulse widths as short as 200 ps. If combined with industrial-quality lithography and already-demonstrated interfacial engineering, an optimized MRAM cell based on Au 0.25 Pt 0.75 can have energy-efficient, ultrafast, and reliable switching, for example, a write energy of <1 fJ (<50 fJ) for write error rate of 50% (<10 −5 ) for 1 ns pulses. The antidamping torque switching of the Au 0.25 Pt 0.75 devices is ten times faster than expected from a rigid macrospin model, most likely because of the fast micromagnetics due to the enhanced nonuniformity within the free layer. The feasibility of Au 0.25 Pt 0.75 -based SOT-MRAMs as a candidate for ultrafast, reliable, energy-efficient, low-impedance, and unlimited-endurance memory is demonstrated.