Controlling the spin-torque efficiency with ferroelectric barriers

Abstract

KAUST RepositoryNonequilibrium spin-dependent transport in magnetic tunnel junctions comprising a ferroelectric barrier is theoretically investigated. The exact solutions of the free electron Schrödinger equation for electron tunneling in the presence of interfacial screening are obtained by combining Bessel and Airy functions. We demonstrate that the spin transfer torque efficiency, and more generally the bias dependence of tunneling magneto-and electroresistance, can be controlled by switching the ferroelectric polarization of the barrier. In particular, the critical voltage at which the in-plane torque changes sign can be strongly enhanced or reduced depending on the direction of the ferroelectric polarization of the barrier. This effect provides a supplementary way to electrically control the current-driven dynamic states of the magnetization and related magnetic noise in spin transfer devices. The electrical control of magnetization in thin films is currently attracting intensive efforts due to its major potential for applications [1]. Current-and voltage-induced magnetization dynamics have been observed in a wide variety of magnetic heterostructures such as metallic and semiconducting spin valves and domain walls [2][3][4][5]. Of most technological interest is the manipulation of magnetization in magnetic tunnel junctions (MTJs)-trilayers comprising a tunnel barrier embedded between two ferromagnets-by means of either gate voltages or spin-polarized currents. In the first case, a large voltage pulse is applied across the barrier and charge reordering at the interface modifies the interfacial magnetic anisotropy [5]. In the second case, a spin-polarized current tunnels through the barrier and transfers its spin angular momentum to the local magnetization of the free magnetic layer [2][3][4]. This last phenomenon, known as spin transfer torque [6], enables the design of promising components such as on-chip tunable microwave generators [7], magnetic memory cells [8], race-track memories [9], and so on.The most successful candidates to date that combine low critical switching current densities with large tunneling magnetoresistance ratios (TMR) are MTJs based on MgO barriers and transition metal electrodes [10] (i.e., Fe, Co, Ni, and their compounds). Significant progress has been achieved towards understanding the complex microscopic nature of the junction's interfaces [11,12] and establishing the characteristics of spin transfer torque [13,14]. Despite undeniable successes [15,16], the actual exploitation of spin transfer torque in devices is facing major hurdles, among which the necessary reduction of its critical switching current as well as the control of current-driven magnetic instabilities. Serious efforts are being made towards improving the device performances, and solutions such as engineering the junction structural asymmetries [17] or designing the metallic electrodes stacking [18] have been proposed to enhance the device * aurelien.manchon@kaust.edu.sa operation through modifying the bias depende...

Enhancement of the spin transfer torque efficiency in magnetic STM junctions

“…In our 3D-WKB model, the bias-and angle-asymmetry are related to the integrated electronic structures in the bias window, where the transport quantities involving the magnetic PDOS (m S or m T ) can have a nonmonotonic behavior and can even change sign depending on the bias voltage, see Eq. (11). Such a behavior is reported for the spin-polarized part of the charge current in Ref.…”

“…5(e,f) are comparable in size with the in-plane STT efficiencies and they show complex z-and φ-dependences resulting from a complex interplay of orbital-dependent transmission and electronic structure effects, see Eq. (11).…”

“…Tight-binding models in combination with Keldysh nonequilibrium Green's function formalism have been extensively used [7][8][9]. Very recently, the effects of magnetic insulating [10] and ferroelectric [11] barriers on the STT have been investigated by the tight-binding method. A scattering theory of STT has been proposed by Xiao et al [12].…”

Theory of high-resolution tunneling spin transport on a magnetic skyrmion

“…So far, theoretical research of spin transport partly focused on the calculation of the tunneling spin transfer torque (STT) in symmetric or asymmetric planar MTJs. The theoretical approaches cover free-electron models [2][3][4], a transfer Hamiltonian method [5], tightbinding-based transport models [6][7][8][9][10], and scattering methods [11], partly combined with first-principles techniques [12,13]. The first direct measurement of the outof-plane and in-plane components of the STT vector in a planar MTJ was provided by Sankey et al [14].…”

Analytical description of ballistic spin currents and torques in magnetic tunnel junctions

“…( 2) for symmetric MTJs yielding quadratic bias voltage behavior 12,13,[17][18][19] as confirmed later experimentally 22,23 . The roles of inelastic scattering, structural asymmetries and material compositions have also been theoretically investigated [24][25][26][27][28][29][30][31] resulting in the forms displayed in Eqs. ( 1)- (2) with some of them supported by a number of recent experiments [32][33][34][35][36][37] .…”