Asymmetric Magnetic Tunnel Junctions Research Articles

Efficient domain wall motion in asymmetric magnetic tunnel junctions with vertical current flow

In this paper, we study the domain wall motion induced by vertical current flow in asymmetric magnetic tunnel junctions. The domain wall motion in the free layer is mainly dictated by the current-induced field-like torque acting on it. We show that as we increase the MTJ asymmetry, by considering dissimilar ferromagnetic contacts, a linear-in-voltage field-like torque behavior is accompanied by an enhancement in the domain wall displacement efficiency and a higher degree of bidirectional propagation. Our analysis is based on a combination of a quantum transport model and magnetization dynamics as described by the Landau–Lifshitz–Gilbert equation, along with comparison to the intrinsic characteristics of a benchmark in-plane current injection domain wall device.

Current-induced torques in magnetic Weyl semimetal tunnel junctions.

We study the current-induced torques in asymmetric magnetic tunnel junctions containing a conventional ferromagnet and a magnetic Weyl semimetal contact. The Weyl semimetal hosts chiral bulk states and topologically protected Fermi arc surface states which were found to govern the voltage behavior and efficiency of current-induced torques. We report how bulk chirality dictates the sign of the non-equilibrium torques acting on the ferromagnet and discuss the existence of large field-like torques acting on the magnetic Weyl semimetal which exceeds the theoretical maximum of conventional magnetic tunnel junctions. The latter are derived from the Fermi arc spin texture and display a counter-intuitive dependence on the Weyl nodes separation. Our results shed light on the new physics of multilayered spintronic devices comprising of magnetic Weyl semimetals, which might open doors for new energy efficient spintronic devices.

Bidirectional switching assisted by interlayer exchange coupling in asymmetric magnetic tunnel junctions.

We study the combined effects of spin transfer torque, voltage modulation of interlayer exchange coupling and magnetic anisotropy on the switching behavior of perpendicular magnetic tunnel junctions (p-MTJs). In asymmetric p-MTJs, a linear-in-voltage dependence of interlayer exchange coupling enables the effective perpendicular anisotropy barrier to be lowered for both voltage polarities. This mechanism is shown to reduce the critical switching current and effective activation energy. Finally, we analyze the possibility of having switching via interlayer exchange coupling only.

Shot noise and tunneling magnetoresistance in disordered MgO-based double-barrier magnetic tunnel junctions: First-principles study

We report first-principles study of shot noise and tunneling magnetoresistance in Fe/MgO/Fe- /MgO/Fe double-barrier magnetic tunnel junctions (MTJs). We mainly investigate the effects of disordered interfacial oxygen vacancies and barrier asymmetry on the spin-dependent tunneling. It is found that interchannel scattering induced by interfacial oxygen vacancies can substantially enhance the tunneling conductance of the antiparallel magnetic configurations, and results in the dramatic decrease of tunneling magnetoresistance. Moreover, we find the interfacial disorder scattering favors the sub-Poissonian tunneling process. As a result, Fano factors of symmetric MTJs maintain at around 0.5, or are suppressed, while Fano factors of asymmetric MTJs can all be significantly suppressed, illustrating the important correlations in tunneling induced by interfacial disorders. Interchannel scattering induced by interfacial oxygen vacancies can effectively couple the electron to high-transmission channels, enhancing the transmission and reducing the shot noise. In comparison with interfacial disorder, middle-layer disordered Fe vacancies present limited modulation on the Fano factor. Increasing the asymmetry of barriers can quickly decrease high-transmission channels, and make the tunneling process Poissonian in double-barrier MTJs.

Symmetric and Asymmetric Magnetic Tunnel Junctions with Embedded Nanoparticles: Effects of Size Distribution and Temperature on Tunneling Magnetoresistance and Spin Transfer Torque

The problem of the ballistic electron tunneling is considered in magnetic tunnel junction with embedded non-magnetic nanoparticles (NP-MTJ), which creates additional conducting middle layer. The strong temperature impact was found in the system with averaged NP diameter dav < 1.8 nm. Temperature simulation is consistent with experimental observations showing the transition between dip and classical dome-like tunneling magnetoresistance (TMR) voltage behaviors. The low temperature approach also predicts step-like TMR and quantized in-plane spin transfer torque (STT) effects. The robust asymmetric STT respond is found due to voltage sign inversion in NP-MTJs with barrier asymmetry. Furthermore, it is shown how size distribution of NPs as well as quantization rules modify the spin-current filtering properties of the nanoparticles in ballistic regime. Different quantization rules for the transverse component of the wave vector are considered to overpass the dimensional threshold (dav ≈ 1.8 nm) between quantum well and bulk-assisted states of the middle layer.

비대칭 자기터널접합에서의 수직 스핀 전달 토크: 물질 변수에 대한 의존성

스핀전달토크는 나노구조에서 자성상태를 제어하는데 유용한 수단이다. 자기터널접합에서 스핀전달토크는 자성물질층의 자화가 이루는 평면에 평행한 성분과 수직인 성분으로 나눌 수 있다. 이중 평행한 성분의 스핀전달토크의 성질은 상당히 잘 알려져 있으나, 수직인 성분의 스핀전달토크의 성질에 대해서는 여전히 이견이 많다. 비대칭 자기터널접합에서의 최근 실험에서, 수직전달토크의 전압 의존성이 전압의 이차항 성분뿐만 아니라 일차항 성분도 가짐을 보고하였다. 하지만 물질 변수에 대한 의존성은 여전히 잘 알려지지 않았다. 이 논문에서는 비대칭 자기터널접합에서의 스핀전달토크의 전압의존성을, 강자성층의 스핀 갈라짐 에너지와 일함수의 차이, 그리고 페르미 에너지를 변화시켜 가면서 체계적인 조사를 하였다. Spin-transfer torque is a useful tool to control the magnetic state in nanostructures. In magnetic tunnel junctions, the spin-transfer torque has two components, the in-plane spin torque and the perpendicular spin torque. While properties of the in-plane spin-transfer torque are relatively well understood, properties of the perpendicular spin-transfer torque still remain controversial. A recent experiment demonstrated that in asymmetric magnetic tunnel junctions, the bias voltage dependence of the perpendicular spin-transfer torque contains both linear and quadratic terms in the bias. However it still remains unexplored how the bias voltage dependence changes as a function of material parameters. In this paper, we systematically investigate the perpendicular spin-transfer torque in asymmetric magnetic tunnel junction by varying spin splitting energy, work function difference, and Fermi energy of the ferromagnetic metal leads.

Bias-voltage dependence of perpendicular spin-transfer torque in asymmetric MgO-based magnetic tunnel junctions

Spin-transfer torque allows the magnetization of nanopillar devices to be switched electrically. Incorporating asymmetries into the design of such a device generates a linear out-of-plane torque component that could help prevent the unwanted spontaneous reversal of the nanopillar’s magnetization. Spin-transfer torque1,2 (STT) allows the electrical control of magnetic states in nanostructures3,4,5. The STT in magnetic tunnel junctions (MTJs) is of particular importance owing to its potential for device applications6,7. It has been demonstrated8,9,10,11 that the MTJ has a sizable perpendicular STT ( , field-like torque), which substantially affects STT-driven magnetization dynamics. In contrast to symmetric MTJs where the bias dependence of is quadratic8,9,10,12,13, it is theoretically predicted that the symmetry breaking of the system causes an extra linear bias dependence11. Here, we report experimental results that are consistent with the predicted linear bias dependence in asymmetric MTJs. The linear contribution is quite significant and its sign changes from positive to negative as the asymmetry is modified. This result opens a way to design the bias dependence of the field-like term, which is useful for device applications by allowing, in particular, the suppression of the abnormal switching-back phenomena.

Controlling the Nonequilibrium Interlayer Exchange Coupling in Asymmetric Magnetic Tunnel Junctions

We predict an oscillatory bias behavior of the fieldlike spin torque, T(perpendicular), in magnetic tunnel junctions, which can be selectively controlled via the asymmetry in band filling between the ferromagnetic leads. This can lead to a linear or quadratic low-bias behavior, including tuning the bias-induced reversal of T(perpendicular). These findings reconcile the apparently contradictory experimental results recently reported in the literature. The underlying mechanism for the nonequilibrium interlayer exchange coupling (IEC) of noncollinear configurations is the interplay of four independent IEC for the majority- and minority-spin bands of the leads solely in the ferromagnetic configuration.