Strong Bias Effect on Voltage-Driven Torque at Epitaxial Fe-MgO Interface

Abstract

Torque can be provided to magnetization in nanomagnets directly by electric current and/or voltage. This technique enables electric current (voltage)-to-spin conversion without electromagnetic induction, and has been intensively studied for memory device applications. Among the various kinds of torque, torque induced by spin-orbit splitting has recently been found. However, quantitative understanding of bulkrelated torque and interface-related torque is still lacking because of their identical symmetry for currentin-plane devices. In this paper, we propose that a pure interface-related torque can be characterized by spintorque ferromagnetic resonance with a current-perpendicular-to-plane tunnel junction. Epitaxial Fe-MgO-V tunnel junctions are prepared to characterize the interface-related torque at Fe-MgO. We find that the current-driven torque is negligible, and a significant enhancement of the voltage-driven torque is observed when the MgO barrier thickness decreases. The maximum torque obtained is as large as 2.8 × 10 −5 J=ðVm 2 Þ, which is comparable to the voltage-controlled magnetic anisotropy of 180 fJ=Vm. The voltage-driven torque shows strong dc-bias-voltage dependence that cannot be explained by conventional voltage-controlled magnetic anisotropy. Tunnel anisotropic magnetoresistance spectroscopy suggests that the torque is correlated to an interface state at the Fe-MgO. This surface-state-sensitive electric modulation of magnetic properties provides new insight into the field of interface magnetism.

Recent Progress in the Voltage-Controlled Magnetic Anisotropy Effect and the Challenges Faced in Developing Voltage-Torque MRAM

“…In addition to ultrathin epitaxial films with large PMA [35,59,60,61,62,63,64,65,66,67], VCMA effects have been observed in various materials systems, for example, in sputter-deposited CoFeB [68,69,70,71,72,73,74,75,76,77,78,79,80,81], which is an important practical material that is used in the mass production of MTJs, and in self-assembled nano-islands [82], nanocomposite structures [83], and ultrathin layers with quantum well states [84]. The VCMA effect can also be applied for the control of domain wall motion [85,86,87] and magnetic skyrmions [88,89,90].…”

Electronic voltage control of magnetic anisotropy at room temperature in high- κ SrTiO3/Co/Pt trilayer

“…In VCMA [4], an electric field is applied across a dielectric/ferromagnet interface. Charge accumulation at this interface leads to a modulation of interfacial orbital magnetic moments [5,8] and interfacial electric quadrupoles [5,9]. Both mechanisms lead to a modification of the interfacial anisotropy energy [5].…”

“…Both mechanisms lead to a modification of the interfacial anisotropy energy [5]. VCMA is mostly obtained using MgO as a dielectric, and MRAM-compatible ferromagnetic layers such as Fe [4,[9][10][11][12], FeCo [13], Co x Fe y B [14][15][16][17], and Co [18][19][20][21][22][23]. The deposition method has a decisive impact on the VCMA effect strength.…”

Alternating current-induced interfacial spin-transfer torque

“…We show that this resonance allows us to estimate directly the interfacial exchange interaction strength from the domain wall motion. We also find that the β-term includes an unconventional contribution which is proportional to the time derivative of the current and exists even in absence of any spin relaxation processes.Introduction.-A variety of physical phenomena arises near interfaces, such as spin-dependent transports [1][2][3][4][5][6], interfacial magnetic phenomena [7][8][9][10][11][12], and chiral/topological phenomena [13][14][15][16], which have attracted attention from many years ago [17]. Among these, the spin-dependent transport has been closely related to the aspect of not only fundamental physics but device application; especially the tunneling magnetoresistance [2][3][4] impacted upon the invention of the magnetoresistive random access memory [18].The spin-dependent transports near the interfaces are important from the viewpoint of the understanding of recent developments in spintronics, such as the spin pumping effect (SPE) [19][20][21][22][23][24][25] and the spin Seebeck effect (SSE) [26,27], because the mutual dependence between the magnetization dynamics and the spin-dependent transports is the key mechanism in various spin-dependent phenomena.…”

“…in the Fourier space (α = x, y and i = y, z). From the linear response theory, we can obtain the response coefficientχ αi (q, ω) fromχ αi (q; ω) = (K αi (q; ω) −K αi (q; 0))/iω, whereK αi (q; ω) can be evaluated from the following spin-current correlation function in the Matsubara form (6) through the analytic continuation iω λ → ω + i0;K αi (q; ω) = K αi (q; ω + i0).…”