Spin torques in ferri- and antiferromagnetic systems

In application such as magnetic random access memory (MRAM), spin-orbit torques (SOTs) have advantages over STTs due to their higher efficiency and the ability to switch a MTJ without passing a large current through the tunnel barrier. Usually the spin Hall angle (SHA) of adjacent materials determines the direction of σ (spin polarization of the spin current) and thus the switching direction for a given H. Usually the adjacent materials can be classified into two basic types with a positive SHA, such as Pt, and a negative SHA, such as Ta and W. Therefore, the spin polarization and the switching directions in Pt/ferromagnet (Pt/FM) and Ta/FM structures are opposite.

We demonstrated that the synthetic antiferromagnets (SAFs) can also be efficiently switched like a single ferromagnet. Surprisingly, the switching scheme of SAFs does not obey the usual switching rule shown. It is shown that the SOT switching direction of SAFs can be reversed depending on the strength of applied in-plane fields even with the same sign of SHA. These results indicate that the switching of SAFs can be achieved without any direction changes of the applied in-plane field and current, contrary to the switching of a single ferromagnet in which the direction of either current or in-plane field has to be reversed. To explain the anomalous magnetization switching (AMS) behaviors, we then propose a new SOT switching mechanism directly arising from the asymmetric domain expansion/contraction due to the field-modulated chiral DW motion. The new switching mechanism suggests that the SOT switching direction is only determined by the in-plane field modulated relative velocity between ↑↓ and ↓↑ domains (V_RD), regardless of the initially nucleated domains through the macrospin model, and thus does not directly depend on the sign of SHA.

The SOT switching with a (a) positive or (b) negative SHA. (c) SOT induced anomalous switching observed in this work, in which m can be switched to both up and down states under the SOT with the same sign. (d) Top-view of a Hall bar structure showing the configurations of electrical measurements and coordinate system. (e) Magnetic properties of the Pt/SAF structure characterized by AHE (black) and VSM (blue).

The current driven magnetization switching under (a) ±1 kOe and (b) ±5 kOe in-plane magnetic fields. (c) The magnetization switching induced by ±21 mA current pulses as a function of in-plane field. The red and black arrows indicate the corresponding switching sequence of red and black curves, respectively. (d) M_R/M_S ratio (black) and |H_t| (blue) as a function of t_Ru. |H_t| = 0 indicates no AMS observed.

We have also demonstrated the voltage control of the HM/FM interface in a Pt/Co/GdO_x structure. By combining independent control of the Co/GdO_x interfaces, we have reached an unprecedentedly effective manipulation of PMA in HM/FM/oxide structures, greatly expanding the controllable magnetic states to those that cannot be achieved by controlling only the FM/oxide interface in previous studies. Moreover, both the PMA field (H_k) and M_s can be controlled independently and the magnetism can be configured to any possible state, in sharp contrast to previous studies where M_s and H_k were always controlled together when only the FM/oxide interface was modulated. The efficiency of SOTs is also controlled by voltage and the critical SOT switching current (I_c) is reduced by about one order of magnitude. These results highlight the unexplored voltage control of metallic HM/FM interfaces in electrical control of magnetism, which may be extended to control other interfacial effects.

STEM EELS results at different EF-controlled states. (a) Top-view and typical STEM image of a Pt/Co/GdO_x sample. (b) The perpendicular field dependent R_H for samples c, as-deposited state, d, after V_G = -5 V application, e, fully oxidized state, f, after V_G = -5 V and then V_G = +5 V applications. (d-f) The corresponding Co and oxygen distribution from EELS line scan along z direction at the Pt/Co interface

SOT switching under V_G control. (a) Typical current induced SOT switching under in-plane fields. (b) The SOT switching curves under V_G control. (c, d) The extracted H_k (c) and corresponding I_c (d) as a function of R_H at each magnetic state controlled by V_G. The arrows indicate the time evolution of control process. (e) I_c is replotted as a function of R_H∙H_k. The solid lines are the liner fitting results.

Related publications:

“Anomalous spin-orbit torque switching in synthetic antiferromagnets”, Phys. Rev. B. 95, 104434 (2017)
"Electrical Control of Metallic Heavy-Metal-Ferromagnet Interfacial States", Phys. Rev. Appl., 8, 034003 (2017)

Voltage controlled antiferromagnetism in tunnel junctions

We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB magnetic tunnel junctions that is related to the interfacial Fe(Co)O_x formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly-magnetized CoFeB allows sensitive detection of the exchange bias. It is found that the exchange bias field can be isothermally controlled by magnetic fields at low temperatures. More importantly, the exchange bias can also be effectively manipulated by the electric field applied to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy in this system. (a), Representative TMR curves measured under different bias voltages at 30 K. (b), The voltage dependence of magnetization switching fields H_C1 and H_C2 at RT, as a result of the VCMA effect. (c), The voltage dependence of H_C1 and H_C2 at 30K, as a result of both the VCMA and voltage-controlled exchange bias effects. (d), Voltage dependence of the exchange bias field at 30K. Experimental data are fitted using equation (1) assuming a linear voltage dependence for K_AF.

Hot-electron-induced Magnonic switching with AF barriers

Magnons in anitiferromagnets can deliver orders of magnitude larger spin torques than conduction electrons
Self-induced temperature gradient by relaxation of tunneling electrons reaches 1-2K/nm
Very efficient switching in MTJ with AF barrier can be potentially achieved
Calculation has been done by our collaborators ( Shufeng Zhang group)

Related publications:

"Amplification of spin-transfer torque in magnetic tunnel junctions with an antiferromagnetic barrier", Physical Review B 99 (10), 104417 (2019)
"Voltage-Controlled Antiferromagnetism in Magnetic Tunnel Junctions", Physical Review Letters 124 (18), 187701 (2020)

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