We report the experimental demonstration of long-range orbital torques generated by Nb and Ru and detected by spin-torque ferromagnetic resonance (ST-FMR) measurements in Nb/Ni and Ru/Ni bilayers. We identify the orbital torques from the sign-reversal and a strong enhancement in the damping-like torque observed in Nb (Ru)/Ni bilayers as compared to Nb (Ru)/FeCoB bilayers as theoretically predicted1. The long-range nature of orbital transport in the ferromagnet was revealed by varying the thickness of Ni in Nb (Ru)/Ni bilayers which is markedly different compared to the regular spin absorption in the ferromagnet that takes place in the first few layers confirming the recent prediction2. Finally, we show that the external injection of orbital current in a heavy metal such as Pt can convert the orbital current into a spin-current leading to higher switching efficiency and the enhanced spin-current generation that could both be potentially attractive for the application3,4. 
**References** 1. Go, D. & Lee, H.-W. Orbital torque: Torque generation by orbital current injection. Phys. Rev. Res. 2, 013177 (2020).
2. Go, D. et al. Long-Range Orbital Magnetoelectric Torque in Ferromagnets. arXiv 2106.07928, 1–6 (2021).
3. Ding, S. et al. Harnessing Orbital-to-Spin Conversion of Interfacial Orbital Currents for Efficient Spin-Orbit Torques. Phys. Rev. Lett. 125, 177201 (2020).
4. Go, D., Jo, D., Lee, H.-W., Kläui, M. & Mokrousov, Y. Orbitronics: Orbital currents in solids. EPL (Europhysics Lett. 135, 37001 (2021).
5. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/337689b4cc2bcaa0dfb3521d12ccce04.png)

Detection of long range orbital torques

Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research [1]. It arises from the dependence of the resistance of a magnetically ordered solid on the relative angle between the applied electric field and the magnetization vector independent of their polarity. The origins of the AMR are usually ascribed to extrinsic effects (i.e. spin-dependent electron scattering), whereas intrinsic (i.e. scattering-independent) contributions are neglected [2]. Here, we perform a broadband measurement of the AMR of prototypical ferromagnets such as Co, Ni or NiFe alloys in a frequency range from DC to 28 THz [3]. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni and NiFe alloys is of predominantly extrinsic nature. However, the Co thin film exhibits a sizeable intrinsic, frequency-independent AMR contribution, which amounts to more than 2/3 of the DC AMR contrast of 1%. These features are attributed to the lower symmetry of the hexagonal structure of Co crystallites which allows for larger anisotropies in their bandstructure. The new insight into the nature of AMR is not only relevant for the fundamental understanding of the magnetotransport but might also advance the ultrafast probing techniques in spintronic devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/b939fa383f615a86b5ac200134a15b4d.jpg)

Fig. 1. THz detection of the AMR. The incident THz pulse applies transient electric field in the direction of polarization, in which, as a consequence of the AMR, the electrical resistance and the transmission of the THz pulse depend on the angle of the magnetization. Variations in transmitted amplitude allow us to calculate the AMR contrast.


![](https://assets.underline.io/uploads/markdown_image/1/image/584dca932356943c776e64e9fc76bbd6.jpg)

Fig. 2. Spectral composition of the AMR in different metals. (a) The absolute value of the AMR contrast in permalloy versus frequency is well described within the Drude formalism (curve) assuming only magnetization-dependent electron scattering rates. (b) The same measurement for a cobalt sample shows a scalable frequency-independent offset which witnesses the intrinsic contribution to the AMR.

Broadband terahertz spectroscopy of anisotropic magnetoresistance reveals intrinsic contributions

Spin-orbit torque (SOT) induced magnetic switching has attracted an extensive interest due to its potential high efficiency in terms of switching time and power consumption [1]. Chiral domain wall (DW) motion driven by SOT in nanowires in the presence of Dzyaloshinskii–Moriya Interaction (DMI) at interfaces between HM and FM layers has shown a great potential for applications to racetrack memory devices [2]. One potential advantage of DW driven switching is that multi-level states can be obtained by controlling the position of the DW across a nanostructure such as wires or dots. As such, domain wall Memristors and the possibility to use DWs motion as synapses [3] have been recently demonstrated. Here, we study SOT driven multi-level states in Hall bars based on W-CoFeB-MgO structures by reducing locally the perpendicular magnetic anisotropy through He+ ion irradiation across a mask [4]. The structure of the investigated samples is W (4 nm)/Co20Fe60B20 (0.6 nm)/MgO (2 nm)/Ta (3 nm). The films were then patterned into Hall bars consisting in 20 μm wide and 140 μm long wires including 2 Hall crosses with 5 μm wide pads. As shown in previous studies [6], light He+ ion irradiation induced interface intermixing can be used to finely tune Ms, interface anisotropy and DMI in CoFeB-MgO structures. One of the Hall cross was then subsequently irradiated through a mask by He+ ions with an energy of 15 keV at a fluence of 1×1019 ions/m2 to reduce locally the perpendicular anisotropy Keff. Anomalous Hall Effect measurements combined with Kerr microscopy indicate that the SOT switching process is dominated by domain wall nucleation in the irradiated region followed by rapid domain propagation to switch the irradiated area (figure 1). The reduction of Keff by 20% under ion irradiation is accompanied by a large reduction of the switching current from 4.07 to 1.68 MA/cm2 under an in-plane 50 mT field; The switching current is determined by the threshold current to induce DW nucleation as checked by Kerr microscopy. As Keff is lowered in the irradiated Hall bar, the energy barrier to induce DW nucleation by SOT is also reduced. Due to the strong pinning of the DW at the transition between the irradiated and the non-irradiated region as evidenced by Kerr microscopy, we observe an intermediate Hall resistance state between the 2 saturated states (Figure 2). The possibility to increase the number of intermediate states by designing different value of perpendicular anisotropy through He+ ion irradiation paves the way towards efficient neuromorphic and memristor devices where the nucleation, motion and pinning can be tailored.

**References:**

[1] M. Cubukcu, et al, IEEE Transactions on Magnetics 54, 9300204 (2018) [2] K. S. Ryu et al, Nat. Nanotechnol. 8, 527 (2013) [3] S. Lequeux, et al, Scientific Reports 6, 31510 (2016) [4] X.Zhao, Appl. Phys. Lett 116, 242401 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/15951bba9072bcb7b7faba4b2ff3dd61.jpg)
 
Figure 1. Kerr images showing the SOT induced magnetization switching process of the Hall bar for an increasing current under a 50 mT in-plane magnetic field. The white dashed lines indicate the longitudinal wire and the transverse Hall voltage pads. The gray box corresponds to the irradiated area. The yellow arrow in (iv) shows the direction of domain wall motion.

![](https://assets.underline.io/uploads/markdown_image/1/image/50371af00eff66ca4a5d33d727cefb07.jpg)
Figure 2. SOT switching curve for an irradiated Hall cross measured under 50 mT. States 1, 2, 3 correspond to down magnetization, intermediate state and up magnetization respectively

 Spin orbit torque driven multi-level switching in He+ irradiated W-CoFeB-MgO Hall bars with perpendicular anisotropy

EOF: Spin-Orbitronics II

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Detection of long range orbital torques

Broadband terahertz spectroscopy of anisotropic magnetoresistance reveals intrinsic contributions

Spin orbit torque driven multi-level switching in He+ irradiated W-CoFeB-MgO Hall bars with perpendicular anisotropy

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