Spin Hall effect, an electric generation of spin current, allows for efficient control of magnetization. Recent theory revealed that orbital Hall effect creates orbital current, which can be much larger than spin-Hall-induced spin current. However, orbital current cannot directly exert a torque on a ferromagnet, requiring a conversion process from orbital current to spin current. Here, we report two effective methods of the conversion through spin-orbit coupling engineering, which allows us to unambiguously demonstrate orbital-current-induced spin torque, or orbital Hall torque. We find that orbital Hall torque is greatly enhanced by introducing either a rare-earth ferromagnet Gd or a Pt interfacial layer with strong spin-orbit coupling in Cr/ferromagnet structures, indicating that the orbital current generated in Cr is efficiently converted into spin current in the Gd or Pt layer. Our results offer a pathway to utilize the orbital current to further enhance the magnetization switching efficiency in spin-orbit-torque-based spintronic devices.
in-plane (perpendicular) magnetic anisotropy. The spin current generated from the FM/Ti interface has a spin polarization along the m × y direction, where m is the magnetization direction of the bottom FM. For an FM with m aligned in the x-direction, a spin current with zspin polarization is generated. [22,23] This enables field-free SOT switching of the perpendicular magnetization of the top CoFeB layer.In this work, we investigate the SOTs in FM/Ta/CoFeB trilayers, in which the spin current generated by SHE in Ta can be combined with the interface-generated spin currents. Using various Ta thickness, we perform measurements of SOTinduced effective fields and of magnetization switching with various Ta thicknesses. We observe two interesting points for a sample with a thin Ta layer; first, the sign of the SOT is determined by the bottom FM layer, and is positive for NiFe and negative for CoFeB. Second, SOT-induced switching is achieved without an in-plane magnetic field. These results demonstrate that the interface-generated spin current of the FM/Ta bilayer governs the SOT of a sample with a thin Ta layer. On the other hand, as Ta thickness increases, the sign of the SOT becomes negative irrespective of the bottom FM and which is determined by the Ta, which has a negative spin Hall angle. This demonstrates that the SOT in the trilayer structure is composed of two contributions: SHE in HM and interface-generated spin current of the FM/HM bilayer, suggesting that the proper selection of an FM/HM combination and of the material thickness can enhance the SOT efficiency and induce field-free SOT switching. Results and Discussion Spin-Orbit Torques in FM/Ta/CoFeB TrilayersIn order to investigate SOT in FM/Ta/CoFeB trilayer structures, we employ Ta (5 nm)/FM (4 nm)/Ta (t Ta )/CoFeB (1.0 nm)/ MgO (3.2 nm) structures, where Ta thickness (t Ta ) ranges from 1.0 to 6.0 nm (Figure 1a). The bottom FM is in-plane magnetized CoFeB or NiFe. As the samples share identical material structures, except for the bottom FM, we refer to the trilayer CoFeB or NiFe samples according to the bottom FM layers. We first check the magnetic anisotropy of each layer by Spin-orbit torques (SOTs) in ferromagnet (FM)/Ta/CoFeB trilayers are investigated as a function of Ta thickness. When the Ta is thinner than 1.5 nm, the sign of the SOT exerting on the top perpendicularly magnetized CoFeB depends on the bottom FM layer; it is positive for NiFe and negative for CoFeB. As the Ta thickness increases, the sign becomes negative irrespective of the bottom FM, indicating that SOTs are dominated by Ta, which has a negative spin Hall angle. SOT-induced switching without an in-plane magnetic field is observed in the thickness ranges where the bottom FM or FM/Ta interface-generated SOT is dominant. The results herein demonstrate that proper design of an FM/heavy metal combination and the material thickness can lead to an enhancement of the SOT efficiency and allow for field-free SOT switching.
Physical unclonable function (PUFs) utilize inherent random physical variations of solid‐state devices and are a core ingredient of hardware security primitives. PUFs promise more robust information security than that provided by the conventional software‐based approaches. While silicon‐ and memristor‐based PUFs are advancing, their reliability and scalability require further improvements. These are currently limited by output fluctuations and associated additional peripherals. Here, highly reliable spintronic PUFs that exploit field‐free spin–orbit‐torque switching in IrMn/CoFeB/Ta/CoFeB structures are demonstrated. It is shown that the stochastic switching polarity of the perpendicular magnetization of the top CoFeB can be achieved by manipulating the exchange bias directions of the bottom IrMn/CoFeB. This serves as an entropy source for the spintronic PUF, which is characterized by high entropy, uniqueness, reconfigurability, and digital output. Furthermore, the device ensures a zero bit‐error‐rate under repetitive operations and robustness against external magnetic fields, and offers scalable and energy‐efficient device implementations.
The electrical control of antiferromagnetic moments is a key technological goal of antiferromagnet-based spintronics, which promises favourable device characteristics such as ultrafast operation and high-density integration as compared to conventional ferromagnet-based devices. To date, the manipulation of antiferromagnetic moments by electric current has been demonstrated in epitaxial antiferromagnets with broken inversion symmetry or antiferromagnets interfaced with a heavy metal, in which spin-orbit torque (SOT) drives the antiferromagnetic domain wall. Here, we report current-induced manipulation of the exchange bias in IrMn/NiFe bilayers without a heavy metal. We show that the direction of the exchange bias is gradually modulated up to ±22 degrees by an in-plane current, which is independent of the NiFe thickness. This suggests that spin currents arising in the IrMn layer exert SOTs on uncompensated antiferromagnetic moments at the interface which then rotate the antiferromagnetic moments. Furthermore, the memristive features are preserved in sub-micron devices, facilitating nanoscale multi-level antiferromagnetic spintronic devices.
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