Material and Thickness Investigation in Ferromagnet/Ta/CoFeB Trilayers for Enhancement of Spin–Orbit Torque and Field‐Free Switching

Abstract

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.

Current‐Induced Spin–Orbit Torques for Spintronic Applications

“…Various approaches have been suggested to achieve field-free SOT switching of perpendicular magnetization; one utilizes an internal effective magnetic field generated within the SOT device such as an exchange-biased field from antiferromagnet (AFM), [105][106][107] or an interlayer or stray fields from a remote FM layer in the device. [108][109][110] Another approach is to generate spin current with an out-of-plane component of spin polarization using FM/NM/FM trilayers [111][112][113][114] or competing spin currents in NM/FM multilayers, [115] or material systems with lateral inversion asymmetry. [61,[116][117][118][119] In this Progress Report, we present recent progress in SOT research and discuss the advantages and challenges of SOT-based spintronic devices.…”

“…[108][109][110]149] The other approach is to generate spin current and an associated SOT that includes an out-of-plane (z-) component, which has been demonstrated in laterally asymmetric structures, [61,[116][117][118][119] or in magnetic trilayers where the interface-generated spin current has a z-component of spin polarization. [111][112][113][114][115]…”

“…Furthermore, one recent study of SOT in a magnetic trilayer of FM/Ta/CoFeB shows the interface-generated spin current in FM/Ta is combined with the SHE in Ta. [113] This leads to an enhancement of the SOT efficiency and simultaneously successful field-free switching. The interface-generated spin current has been also demonstrated in structures consisting of PML/Cu/NiFe, [114] where PML (representing the perpendicular magnetic multilayer of [Co 90 Fe 10 /Ni] 8 /Co 90 Fe 10 ) and NiFe are used as a spin current source and analyzer, respectively.…”

Giant charge-to-spin conversion in ferromagnet via spin-orbit coupling

“…However, these materials require single-crystal structures epitaxially grown on special substrates and are therefore incompatible with device applications. Apart from such symmetry-reduced materials, FMs (such as Co, Fe, and Ni) have a great potential to lift the geometrical restriction through time-reversal asymmetry due to the presence of magnetization [15][16][17][18][19][20][21] . From a symmetry point of view, the spin orientation of the out-of-plane flowing spin current in FM can be expressed as follows (Fig.…”

“…Therefore, SOT with σ MD is of great interest, because it enables switching of perpendicular magnetization. However, there have been very few reports on the generation of σ MD [15][16][17][18] and the efficiency of spin-current generation, characterized by spincurrent conductivity, has been low in FMs. To overcome this issue and achieve high spin-current conductivity, it is necessary to clarify the physical origin of the charge-to-spin conversion in FM, for instance, whether the interfacial contribution or bulk contribution is dominant.…”