Domain wall dynamics in antiferromagnetically-coupled double-lattice systems

In ferromagnetic materials, the rich dynamics of magnetic domain walls (DWs) under magnetic field or current have been successfully described using the well-known q-{\phi} analytical model. We demonstrate here that this simple unidimensional model holds for multiple-sublattice materials such as ferrimagnetic alloys or synthetic antiferromagnets (SAF) by using effective parameters, and is in excellent agreement with double-lattice micromagnetic simulations. We obtain analytical laws for the DW velocity and internal precession angle as a function of net magnetisation for different driving forces (magnetic field, spin transfer and spin-orbit torques) and different propagation regimes in ferrimagnetic alloys and SAFs. The model predicts that several distinctive dynamical features occur near or at the magnetic and the angular compensation points when the net magnetization or the net angular momentum of the system vanishes, and we discuss the experimental observations that have been reported for some of them. Using a higher degree-of-freedom analytical model that accounts for inter-sublattice distortions, we give analytical expressions for these distortions that agree with the micromagnetic simulations. This model shows that the DW velocity and precession rate are independent of the strength of the inter-sublattice exchange coupling, and justifies the use of the simpler effective parameters model

Skyrmion flow in periodically modulated channels

Magnetic skyrmions, topologically stabilized chiral magnetic textures with particle-like properties have so far primarily been studied statically. Here, we experimentally investigate the dynamics of skyrmion ensembles in metallic thin film conduits where they behave as quasi-particle fluids. By exploiting our access to the full trajectories of all fluid particles by means of time-resolved magneto-optical Kerr microscopy, we demonstrate that boundary conditions of skyrmion fluids can be tuned by modulation of the channel geometry. We observe as a function of channel width deviations from classical flow profiles even into the no- or partial-slip regime. Unlike conventional colloids, the skyrmion Hall effect can also introduce transversal flow-asymmetries and even local motion of single skyrmions against the driving force which we explore with particle-based simulations, demonstrating the unique properties of skyrmion liquid flow that uniquely deviates from previously known behavior of other quasi-particles

Neuromorphic weighted sum with magnetic skyrmions

Integrating magnetic skyrmion properties into neuromorphic computing promises advancements in hardware efficiency and computational power. However, a scalable implementation of the weighted sum of neuron signals, a core operation in neural networks, has yet to be demonstrated. In this study, we exploit the non-volatile and particle-like characteristics of magnetic skyrmions, akin to synaptic vesicles and neurotransmitters, to perform this weighted sum operation in a compact, biologically-inspired manner. To this aim, skyrmions are electrically generated in numbers proportional to the input with an efficiency given by a non-volatile weight. These chiral particles are then directed using localized current injections to a location where their presence is quantified through non-perturbative electrical measurements. Our experimental demonstration, currently with two inputs, can be scaled to accommodate multiple inputs and outputs using a crossbar array design, potentially nearing the energy efficiency observed in biological systems.Comment: 12 pages, 5 figure

Driving skyrmions in flow regime in synthetic ferrimagnets

Despite significant advances in the last decade regarding the room temperature stabilization of skyrmions or their current induced dynamics, the impact of local material inhomogeneities still remains an important issue that impedes to reach the regime of steady state motion of these spin textures. Here, we study the spin-torque driven motion of skyrmions in synthetic ferrimagnetic multilayers with the aim of achieving high mobility and reduced skyrmion Hall effect. We consider Pt|Co|Tb multilayers of various thicknesses with antiferromagnetic coupling between the Co and Tb magnetization. The increase of Tb thickness in the multilayers allows to reduce the total magnetic moment and increases the spin-orbit torques allowing to reach velocities up to 400 m.s-1 for skyrmions with diameters of about 160 nm. We demonstrate that due to reduced skyrmion Hall effect, combined with the edge repulsion of the magnetic track making the skyrmions moving along the track without any transverse deflection. Further, by comparing the field-induced domain wall motion and current-induced skyrmion motion, we demonstrate that the skyrmions at the largest current densities present all the characteristics of a dynamical flow regime.Comment: 14 pages, 4 figure

Current-driven domain wall dynamics in coupled ferromagnetic structures

The manipulation of the magnetic moments in ferromagnetic (FM) layers via various spin torques has enabled the spintronics research community to encode the digital data in low-power, non-volatile memory and logic devices e.g. spin-transfer torque magnetic random-access memory (STT-MRAM). In this thesis, current-induced coupled domain wall (DW) dynamics are investigated in both in-plane and perpendicular magnetic anisotropy materials via experimental and micromagnetic simulations. The effect of DW stray field on DW dynamics in a neighbouring NiFe nanowire with in-plane magnetic anisotropy is studied using micromagnetic simulations. The DWs in the multi-nanowire systems are driven by passing spin-polarized currents to one of the nanowire. The phenomenon is made possible due to the magnetostatic coupling between the DWs. The coupling strength between the DWs in two nanowires strongly depends on the chirality of the DWs and the inter-wire spacing. This coupling-induced DW motion further shifts the Walker breakdown limit towards higher current densities. Current-induced magnetization manipulation in antiferromagnetically coupled thin films with perpendicular magnetic anisotropy is investigated experimentally. FM/Ru/FM synthetic antiferromagnetic (SAF) structures are sandwiched between heavy metals to generate the spin-currents via spin Hall effect (SHE) phenomenon. A qualitative method is proposed to determine the spin-orbit torque (SOT) effective fields. The SOT fields in the SAF structures are found to be a vector sum of the individual SOT fields of the two FM layers. The SOT fields were oriented in antiparallel direction to each other in the two FM layers and efficiently modulated by tuning the net areal magnetization of the SAF structure. Furthermore, the current-induced magnetization switching in the SAF structures was achieved with the assistance of RKKY coupling. The DWs in the SAF structures are driven by nanosecond long current pulses. The RKKY exchange torque on the SOT-driven antiferromagnetically coupled Néel DWs can move the Néel DWs with a velocity of ~ 300 m/s at a current density of 1.04×1012 A/m2. The interplay between the SOT and RKKY coupling and its effect on the DWs dynamics are explained via the micromagnetic simulations. In addition, the SAF magnetic memory devices are irradiated by high energy proton beams to investigate the effect of the radiation on the data. In our investigation, the SAF magnetic memory devices are found to be radiation hard and the DW memory devices are still able to perform continuous data writing and reading even after being irradiated by several high energy proton beams. The devices are annealed at different temperatures over a range of time spans in atmosphere and the thermal stability factor (Δ) is calculated for the data retention. The SAF DW memory devices are found to have Δ ≈ 33 at an elevated temperature of T =190' C.​Doctor of Philosophy (SPMS

Observation of ice-rule violation and monopole dynamics via edge nucleation of domain walls in artificial spin ice lattice

In a patterned Co honeycomb spin ice structure, we show that violation in the ice-rule or magnetic monopoles, can be observed during a magnetization reversal process in 430 Oe≤H≤760 Oe magnetic field (H) range. The monopoles are shown to originate from the nucleation of domain walls at the edges, and they hop towards the other edge via the propagation of magnetic domain walls. The paths that the domain walls traveled or the Dirac strings, are shown to increase in length with magnetic fields increment and no random flipping of the bars are observed in the structure.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore

Efficient in-line skyrmion injection method for synthetic antiferromagnetic systems

Although it has been proposed that antiferromagnetically-coupled skyrmions can be driven at extremely high speeds, such skyrmions are near impossible to inject with current methods. In this paper, we propose the use of DMI-induced edge magnetization tilting to perform in-line skyrmion injection in a synthetic antiferromagnetic branched nanostructure. The proposed method circumvents the skyrmion topological protection and lowers the required current density. By allowing additional domain walls (DWs) to form on the branch, the threshold injection current density was further reduced by 59%. The increased efficiency was attributed to inter-DW repulsion and DW compression. The former acts as a multiplier to the effective field experienced by the pinned DW while the latter allows DWs to accumulate enough energy for depinning. The branch geometry also enables skyrmions to be shifted and deleted with the use of only three terminals, thus acting as a highly scalable skyrmion memory block.NRF (Natl Research Foundation, S’pore)Published versio

Modulation of spin-orbit torque efficiency by thickness control of heavy metal layers in Co/Pt multilayers

We investigate and quantify spin-orbit torque (SOT) strength by current induced effective in-plane magnetic fields and spin Hall angle (SHA) using AC harmonic Hall voltage measurements techniques on Ta/Pt/Co/Pt/Co/Ta thin film structures. The proposed Co/Pt thin film double stack gives property enhancement on thermal stability and perpendicular magnetization anisotropy strength over the single stack Pt/Co/Ta. In the proposed Co/Pt double stack we observed that increasing the Ta capping thickness to three times enhances the SHA in similar order, consistent with larger spin injection efficiency. Doubling the Pt spacer layer thickness reduces the SHA by nearly 1.4 times, due to partial cancellation of SOT by bottom layer Pt, negating the increase from the top Co/Pt interface. The in-plane current threshold for magnetization switching is lower with the increase of the SHA.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore

A 126 μW readout circuit in 65nm CMOS with successive approximation based thresholding for domain wall magnet based random number generator

We present a novel readout circuit for a ferromagnetic Hall cross-bar based random number generator. The random orientation of magnetic domains are result of anomalous Hall-effect. These ferromagnetic Hall cross-bar structures can be integrated with the read out circuit to form a plug and play random number generator. The system can resolve up to 15-20 μV Hall-voltages from Hall probe. Application of current densities around 10 12 A/m 2 through the Ferromagnetic Hall cross-bar produces random Hall-voltage on the output terminals. To amplify the weak Hall-voltages (10-100 μV) in the presence of DC offsets, a modulation scheme is used to up-convert the signal and a band-pass amplifier is used to amplify the modulated signal. The bandpass amplifier circuit, motivated by neural recording amplifier is designed in 65nm CMOS and consumes 126 μW of power from a 1.2 V supply. Further, we present a successive approximation algorithm and its embedded implementation to set the desired threshold for digitizing the amplified Hall-voltage in presence of signal drift. Experimental results show that the resulting system can tolerate drifts in voltage up to 440 μV.Accepted versio