Feilong Luo

Highly Efficient Domain Walls Injection in Perpendicular Magnetic Anisotropy Nanowire

Electrical injection of magnetic domain walls in perpendicular magnetic anisotropy nanowire is crucial for data bit writing in domain wall-based magnetic memory and logic devices. Conventionally, the current pulse required to nucleate a domain wall is approximately ~1012 A/m2. Here, we demonstrate an energy efficient structure to inject domain walls. Under an applied electric potential, our proposed Π-shaped stripline generates a highly concentrated current distribution. This creates a highly localized magnetic field that quickly initiates the nucleation of a magnetic domain. The formation and motion of the resulting domain walls can then be electrically detected by means of Ta Hall bars across the nanowire. Our measurements show that the Π-shaped stripline can deterministically write a magnetic data bit in 15 ns even with a relatively low current density of 5.34 × 1011 A/m2. Micromagnetic simulations reveal the evolution of the domain nucleation – first, by the formation of a pair of magnetic bubbles, then followed by their rapid expansion into a single domain. Finally, we also demonstrate experimentally that our injection geometry can perform bit writing using only about 30% of the electrical energy as compared to a conventional injection line.

Deterministic Spin-Orbit Torque Induced Magnetization Reversal In Pt/[Co/Ni] n /Co/Ta Multilayer Hall Bars

Spin-orbit torque (SOT) induced by electric current has attracted extensive attention as an efficient method of controlling the magnetization in nanomagnetic structures. SOT-induced magnetization reversal is usually achieved with the aid of an in-plane bias magnetic field. In this paper, we show that by selecting a film stack with weak out-of-plane magnetic anisotropy, field-free SOT-induced switching can be achieved in micron sized multilayers. Using direct current, deterministic bipolar magnetization reversal is obtained in Pt/[Co/Ni]2/Co/Ta structures. Kerr imaging reveals that the SOT-induced magnetization switching process is completed via the nucleation of reverse domain and propagation of domain wall in the system.

Quantitative characterization of spin-orbit torques in Pt/Co/Pt/Co/Ta/BTO heterostructures due to the magnetization azimuthal angle dependence

Substantial understanding of spin-orbit interactions in heavy-metal (HM)/ferromagnet (FM)heterostructures is crucial in developing spin-orbit torque (SOT) spintronics devices utilizing spin Hall and Rashba effects. Though the study of SOT effective field dependence on the out-of-plane magnetization angle has been relatively extensive, the understanding of in-plane magnetization angle dependence remains unknown. Here, we analytically propose a method to compute the SOT effective fields as a function of the in-plane magnetization angle using the harmonic Hall technique in perpendicular magnetic anisotropy (PMA) structures. Two different samples with PMA, a Pt/Co/Pt/Co/Ta/BaTi O3 (BTO) test sample and a Pt/Co/Pt/Co/Ta reference sample, are studied using the derived formula. Our measurements reveal that only the dampinglike field of the test sample with a BTO capping layer exhibits an in-plane magnetization angle dependence, while no angular dependence is found in the reference sample. The presence of the BTO layer in the test sample, which gives rise to a Rashba effect at the interface, is ascribed as the source of the angular dependence of the dampinglike field.

Characterizing Angular Dependence of Spin-Orbit Torque Effective Fields in Pt/(Co/Ni) 2 /Co/IrMn Structure

We investigate the interplay between the spin-orbit torque (SOT)-induced effective fields and the azimuth angle of the magnetization vector with respect to the applied current. A method to quantify the ratio of the planar Hall effect to the anomalous Hall effect in perpendicular magnetic anisotropy (PMA) structures by using the low field harmonic Hall voltage measurement technique is devised. The validity of the ratio is confirmed by measuring the PMA effective field. In addition, a technique to characterize the SOT effective fields as a function of the magnetization vector azimuth angle with respect to the current direction is proposed and experimentally validated. The experimental results are in quantitative agreement with our derivations. Our measurement reveals that the field-like effective SOT fields are minima when the azimuth angle of magnetization with respect to the current is at 45°.

Characterizing the spin orbit torque field-like term in in-plane magnetic system using transverse field

In this work, we present an efficient method for characterizing the spin orbit torque field-like term in an in-plane magnetized system using the harmonic measurement technique. This method does not require a priori knowledge of the planar and anomalous hall resistances and is insensitive to non-uniformity in magnetization, as opposed to the conventional harmonic technique. We theoretically and experimentally demonstrate that the field-like term in the Ta/Co/Pt film stack with in-plane magnetic anisotropy can be obtained by an in-plane transverse field sweep as expected, and magnetization non-uniformity is prevented by the application of fixed magnetic field. The experimental results are in agreement with the analytical calculations.

Spin Hall effect induced SOT fields in perpendicular magnetized Co/Ni multilayer systems

Recently, Spin-Orbit torque (SOT) arising from the ferromagnetic heterostructures consisting of an ultra-thin ferromagnet (FM) sandwiched between a heavy metal (HM) and an Oxide layer has attracted extensive attention in the low power magnetic memory and logic devices.

Observation of large planar Hall effect on the current induced spin-orbit effective fields in Ta/MgO/CoFeB/Ta

Current induced spin-orbit effective magnetic fields in metal/ferromagnet/ oxide trilayer provide an alternative switching mechanism that is more efficient than the conventional spin torque transfer.

Spin-orbit torque induced magnetization anisotropy modulation in Pt/(Co/Ni)4/Co/IrMn heterostructure

In this work, we show that domain wall (DW) dynamics within a system provide an alternative platform to characterizing spin-orbit torque (SOT) effective fields. In perpendicularly magnetized wires with a Pt/(Co/Ni)4/Co/IrMn stack structure, differential Kerr imaging shows that the magnetization switching process is via the nucleation of the embryo state followed by domain wall propagation. By probing the current induced DW motion in the presence of in-plane field, the SOT effective fields are obtained using the harmonic Hall voltage scheme. The effective anisotropy field of the structure decreases by 12% due to the SOT effective fields, as the in-plane current in the wire is increased.

Quaternary memory based on magnetic skyrmion

Magnetic skyrmions are nanoscale spin textures that are highly stable due to their protected topo-logical charge [1-2]. Skyrmions also experience almost no intrinsic pinning, allowing them to be easily manipulated with low current densities. These positive attributes allows the skyrmion to a very attractive candidate for next generation memory devices [2-4]. In this work, we demonsrate by micromagnetic simulations [5] that a skyrmion can be excited to move in a gyrotropic fashion with the application of a spin-polarized current perpendicular to the nanodisk. In a patterned nanodisk (see Fig. 1), the gyrotropic motion can be exploited to create a four-state memory device. Due to repulsive edge interactions, the skyrmion is forced to reside in one of the 4 quadrants of the device. The four possible positions of the skyrmion represents the four states of our device. To alter the state, a perpendicular current can be applied to drive the skyrmion into either of its neighboring quadrant, depending on the current density and pulse duration. Fig. 2 shows the relative position of the skyrmion against the pulse duration of the applied current density (the material parameters are chosen similar to Ref. 4). Starting from position 0 marked in Fig. 1, we showed that it is possible to move the skyrmion into different quadrants by varying the pulse duration. A marginal increase in applied current density was able to produce a significant decrease in pulse duration required for shifting the devices state. Figure 1(b) shows the trajectory of skyrmion core with current density J = 4E10 A/m2 and pulse duration t = 4.5 ns, which reveal that skyrmion moves away from position 0 and finally relaxes at position 2. Our proposed device has the advantage of having a low threshold current-density for state-switching and very small size (as small as about 50 nm). The solution provided in this work will facilitate the creation of high density skyrmion-based devices.