Magnetization switching by spin–orbit torque in an antiferromagnet–ferromagnet bilayer system

Abstract

Spin-orbit torque (SOT)-Electrical manipulation of magnetization allows us to realize low-power and high-performance solid-state devices. Recent studies have revealed that an in-plane electric current applied to a heterostructure with large spin-orbit interaction and structural inversion asymmetry gives rise to a torque, so-called SOT, which in turn induces a magnetization switching [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] . The SOT-induced switching is useful for three-terminal spintronics device that is capable of high-speed and reliable operation 19 . The origin of SOT is considered to arise from the bulk spin Hall effect (SHE) in the NM and/or the heterostructure interface(s). According to the scenario attributed to the SHE, the in-plane current flowing in the NM generates a pure spin current in the orthogonal direction that is manifested in a Slonczewski-like torque and a field-like torque acting upon the magnetization of FM 5,8,9 , where the ratio of the spin current to the charge current is represented by the spin Hall angle. Among the two magnetic configurations presented so far, the perpendicular one has an advantage in terms of fast switching 11,16 and scalability; however, the requirement of a static magnetic field being applied collinear with the current to achieve bipolar switching poses a technological challenge. While recent studies have shown that the need for the field can be eliminated by introducing a lateral structural asymmetry 14,15,17 , the present work addresses this issue by employing a new material system. 23,24 . These works promise the existence of a direct SHE in AFM that eventually generates SOT. Note that the AFM can exert an internal effective field on the adjacent FM through the exchange-bias [25][26][27] , which has been a core technology for the read heads of the hard-disk drives. Therefore, by replacing the NM by the AFM, external-field-free switching of perpendicular magnetization may be possible in the AFM/FM bilayer systems through the SOT arising from the direct SHE in the AFM.Here we study the SOT switching in an AFM/FM bilayer system. We develop stacks consisting of an in-plane exchange-biased ferromagnetic Co/Ni multilayer with a perpendicular easy axis on top of an antiferromagnetic PtMn (Fig. 1a). Evidence of direct SHE and resulting SOT-induced switching under zero magnetic fields are obtained. Furthermore, we find that, in the AFM/FM system, the portion of reversed magnetization can be controlled continuously by adjusting the magnitude of the applied current; in other words, the system has the potential to serve as a memristor [28][29][30][31] , which is useful for neuromorphic computing 32 . samples. For t PtMn = 6.0 nm (non-biased device), the diagram shows fourfold symmetry, that is, the variation of H C depends on neither the switching direction nor the sign of I CH ; on the other hand, for t PtMn = 8.0 nm (exchange-biased device), it shows twofold symmetry, that is, the sign of I CH specifies the preferable switching direction. We ...

Spin-Polarized Current in Noncollinear Antiferromagnets

“…This has been predicted theoretically [32][33][34] and experimentally observed [32,[35][36][37]. It was also recently demonstrated that both collinear [38] and non-collinear AFMs [33,[39][40][41][42] can generate a large SOT on FMs in AFM/FM heterostructures. While the origin of such a torque is not clear [41] it is known that in heavy metal/FM heterostructures, the SHE plays an important role [43,44].…”

Antiferromagnetic spintronics

“…b Data for Fe 50 Rh 50 are for the specific antiferromagnetic-to-ferromagnetic transition. Fukami et al, 2016;Lau et al, 2016;Oh et al, 2016) (see Sec. III.D.4)].…”

Antiferromagnetic spintronics

“…38 It has been experimentally demonstrated that antiferromagnets with strong spin-orbit coupling can also act as efficient SHE spin injectors. Measurements showing electrical reorientation of a ferromagnet by the SHE induced in an adjacent antiferromagnet 59,60 highlight that antiferromagnetic spintronics aims not only at replacing ferromagnets but also at assisting ferromagnets in their performance in spintronic devices. We also note that, according to recent theory predictions, 61 even insulating antiferromagnets can act as efficient spin-current sources via the antiferromagnetic resonance spin pumping.…”

“…In previous sections we already mentioned examples of metal antiferromagnets which have so far driven much of the research in antiferromagnetic spintronics. Alloys of Ir and Mn are a prime example of metal antiferromagnets gradually progressing from favorable passive exchange-bias materials to active electrodes in TAMR devices, 8,16,17,19 to SHE injectors of spin current controlling adjacent ferromagnets, 59,60 or to sensitive spin detectors.…”