(a) Schematic diagram; (b) Experimental confirmation that the MTT effect is real. (a) A spin current in Pt transforms into a magnon current within NiO, which then transfers spin angle momenta to Y3 Fe5 O12 (YIG). (b) Hall resistance of anomalous as a function applied current under different applied fields. Under opposite fields, magnetization switching loops can be observed with opposing switching directions. Image by IOP
Spin torque is a convenient electric method to control magnetizations. This can be either spin-polarized current, or pure spin current via the spin Hall effect. These are called spin orbit torque (SOT) and spin transfer torque (STT). These tools were used to develop the second generation STT-MRAM, Magnetic Random-Access Memory (Magnetic Random-Access Memory), the third generation STT-MRAM, with in-plane magnet anisotropy, and the fourth generation SOT-MRAM, among other spintronic devices. Perpendicular STT/MRAM chips are also demonstrated to be suitable for large-scale applications.
The transmission of spin angular momentum over long distances by collective excitations in spin-ordered or spin waves, or magnons, has been demonstrated only by the adjoining coupling between local spins. This is a Joule free version of microelectronics. Magnons can be used as a medium for processing, transmitting and storing spin information.
It is still a frontier question in magnonics, spintronics, and magnons if magnons can transfer torque and not just the spin current. This question will determine whether it is possible to develop pure magnonic ways to control magnetizations. While the theory of magneton transfer torque (MTT), has been studied extensively, it is difficult to confirm its existence in experimental data.
Dr. Guo Chenyang, Associate Prof. WAN Caihua and Prof. HAN Xiufeng etc. The M02 group of the Institute of Physics of Chinese Academy of Sciences has recently created an insulating magnetic homostructure Y3 Fe 5 O12 /NiO/Pt, in which the MTT effect was unambiguously confirmed through the current-driven magnetic switching phenomenon.
This structure contains platinum, a heavy metal that has strong spin orbit coupling. It is responsible for producing pure spinning current by applying a current through the structure. NiO, an antiferromagnetic magnet, is used to convert electronic spinning current to magnonic (magnon) spin current. As a magnon sink, the insulating Y3 Fe5 O 12 (YIG), with PMA, is used. Once the MTT is present, the magnetization can be tilted by or switched by magnon current.
YIG or NiO, as insulators, can prevent any electronic current from entering them. This can eliminate any influence from the electronic spin current. Perpendicular YIG can not be deterministically turned by an Oersted field in the plane, which can completely rule out the possibility of the Oersted mechanism. Only the magnon current from the antiferromagnetic NickelO can exert torque on YIG, and cause its magnetization switching.
This observation clearly confirms the MTT effect. The characteristics of SOT-induced magneticization switching indicate that if the applied field is reversed, then the SOT switching direction should be reversed. This can also be an indicator for the MTT effect.
This work clearly establishes the physical scenario for the magneton-transfer torque effect. It also shows that the MTT can be used to control magnetization in magnetic insulators. This could help advance the development and use of pure magnon memories and logic devices.
Physical Review B published the related work entitled "Switching Perpendicular Magnetization of a Magnetic Insulator by Magnon Transfer Torque."
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C. Y. Guo et. al., Switching perpendicular magnetization in a magnetic insulator using magnon torque, Physical Review B (2021). Information from the Journal: Physical Review B C. Y. Guo et. al., Switching perpendicular magneticization of a magnet insulator using magnon torque, (2021). DOI: 10.1103/PhysRevB.104.094412
