Magnetic skyrmions are particle-like magnetization configurations which can be found in materials with broken inversion symmetry. Their topological nature allows them to circumvent around random ...pinning sites or impurities as they move within the magnetic layer, which makes them interesting as information carriers in memory devices. However, when the skyrmion is driven by a current, a Magnus force is generated which leads to the skyrmion moving away from the direction of the conduction electron flow. The deflection poses a serious problem to the realization of skyrmion-based devices, as it leads to skyrmion annihilation at the film edges. Here, we show that it is possible to guide the movement of the skyrmion and prevent it from annihilating by surrounding and compressing the skyrmion with strong local potential barriers. The compressed skyrmion receives higher contribution from the spin transfer torque, which results in the significant increase of the skyrmion speed.
Aim
To investigate whether static magnetic fields (SMFs) have a positive effect on the migration and dentinogenesis of dental pulp stem cells (DPSCs) to promote reparative dentine formation.
...Methodology
In vitro scratch assays and a traumatic pulp exposure model were performed to evaluate the effect of 0.4‐Tesla (T) SMF on DPSC migration. The cytoskeletons of the DPSCs were identified by fluorescence immunostaining and compared with those of a sham‐exposed group. Dentinogenic evaluation was performed by analysing the expressions of DMP‐1 and DSPP marker genes using a quantitative real‐time polymerase chain reaction (qRT‐PCR) process. Furthermore, the formation of calcified deposits was examined by staining the dentinogenic DPSCs with Alizarin Red S dye. Finally, the role played by the p38 MAPK signalling pathway in the migration and dentinogenesis of DPSCs under 0.4‐T SMF was investigated by incorporating p38 inhibitor (SB203580) into the in vitro DPSC experiments. The Student's t‐test and the Kruskal–Wallis test followed by Dunn's post hoc test with a significance level of P < 0.05 were used for statistical analysis.
Results
The scratch assay results revealed that the application of 0.4‐T SMF enhanced DPSCs migration towards the scratch wound (P < 0.05). The cytoskeletons of the SMF‐treated DPSCs were found to be aligned perpendicular to the scratch wound. After 20 days of culture, the SMF‐treated group had a greater number of out‐grown cells than the sham‐exposed group (nonmagnetized control). For the SMF‐treated group, the DMP‐1 (P < 0.05) and DSPP genes (P < 0.05), analysed by qRT‐PCR, exhibited a higher expression. The distribution of calcified nodules was also found to be denser in the SMF‐treated group when stained with Alizarin Red S dye (P < 0.05). Given the incorporation of p38 inhibitor SB203580 into the DPSCs, cell migration and dentinogenesis were suppressed. No difference was found between the SMF‐treated and sham‐exposed cells (P > 0.05).
Conclusion
0.4‐T SMF enhanced DPSC migration and dentinogenesis through the activation of the p38 MAPK‐related pathway.
A linear array of periodically spaced and individually controllable skyrmions is introduced as a magnonic crystal. It is numerically demonstrated that skyrmion nucleation and annihilation can be ...accurately controlled by a nanosecond spin polarized current pulse through a nanocontact. Arranged in a periodic array, such nanocontacts allow the creation of a skyrmion lattice that causes a periodic modulation of the waveguide's magnetization, which can be dynamically controlled by changing either the strength of an applied external magnetic field or the density of the injected spin current through the nanocontacts. The skyrmion diameter is highly dependent on both the applied field and the injected current. This implies tunability of the lowest band gap as the skyrmion diameter directly affects the strength of the pinning potential. The calculated magnonic spectra thus exhibit tunable allowed frequency bands and forbidden frequency bandgaps analogous to that of conventional magnonic crystals where, in contrast, the periodicity is structurally induced and static. In the dynamic magnetic crystal studied here, it is possible to dynamically turn on and off the artificial periodic structure, which allows switching between full rejection and full transmission of spin waves in the waveguide. These findings should stimulate further research activities on multiple functionalities offered by magnonic crystals based on periodic skyrmion lattices.
An all-magnetic logic scheme has the advantages of being non-volatile and energy efficient over the conventional transistor based logic devices. In this work, we present a reconfigurable magnetic ...logic device which is capable of performing all basic logic operations in a single device. The device exploits the deterministic trajectory of domain wall (DW) in ferromagnetic asymmetric branch structure for obtaining different output combinations. The programmability of the device is achieved by using a current-controlled magnetic gate, which generates a local Oersted field. The field generated at the magnetic gate influences the trajectory of the DW within the structure by exploiting its inherent transverse charge distribution. DW transformation from vortex to transverse configuration close to the output branch plays a pivotal role in governing the DW chirality and hence the output. By simply switching the current direction through the magnetic gate, two universal logic gate functionalities can be obtained in this device. Using magnetic force microscopy imaging and magnetoresistance measurements, all basic logic functionalities are demonstrated.
•Enhancement of spin Hall conductivity for Cu40W60|Fe by 120 % compared to α-W|Fe.•The ratio of spin Hall angle to damping was found to be reduced by a factor four.•Cu40W60|Fe is a promising ...candidate for energy efficient spintronic applications.
We report on the enhancement of the spin Hall conductivity in tungsten by alloying with copper, measured by using the spin-torque ferromagnetic resonance technique. The alloying leads to an increase in spin-dependent scattering events and results in an enhancement of the contributing extrinsic spin Hall effects. The measured damping property shows a slight increase with higher tungsten concentration, due to spin current losses from the ferromagnetic layer into the tungsten-copper alloy. At a tungsten concentration of 60%, the spin Hall conductivity reaches a maximum of 3.68±0.68×105Ω-1m-1, corresponding to an enhancement of 120% compared to the pure tungsten sample. At the same concentration, the ratio of the spin Hall angle to the damping of the ferromagnetic layer, which offers a quick estimation for the critical switching current density, is found to be four times smaller as compared to pure tungsten.
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.
•Multi-state behaviour due to geometrically induced inhomogeneous current density.•Temperature-sensitive multi-state write and read.•Reduced spin-orbit torque efficiency at elevated temperature.
We ...demonstrate spin-orbit torque (SOT) driven multi-state magnetization switching in Co/Pt Hall crosses in the presence of varying externally applied in-plane (IP) bias fields from room temperature (RT = 295 K) to 360 K. In-situ Kerr imaging at various resistance states reveal the evolution of up and down magnetic domain expansion due to current-induced SOT switching. The control of magnetization states in the Hall cross is attributed to the inhomogeneous current-density and current-induced effective out-of-plane field due to the device geometry. The critical switching current density scales inversely with device temperature, and the SOT-driven change in Hall resistance varies across device temperatures and IP fields. Subsequently, the current-induced SOT efficiency, χsat, and the Dzyaloshinskii-Moriya interaction effective field, HDMI, at RT and 360 K are determined using the chiral domain wall model and current-induced loop-shift method. The χsat and HDMI values are found to decrease by ~26% and ~15%, respectively, with increasing device temperature. These results demonstrate the thermal sensitivity of current-induced SOT magnetization switching in multi-state devices.
Abstract
High temperature studies of spin Hall effect have often been neglected despite its profound significance in real-world devices. In this work, high temperature spin torque ferromagnetic ...resonance measurement was performed to evaluate the effects of temperature on the Gilbert damping and spin Hall efficiency of Pt
x
Cu
1−
x
. When the temperature was varied from 300 K to 407 K, the Gilbert damping was relatively stable with a change of 4% at composition
x
= 66%. Alloying Pt and Cu improved the spin Hall efficiency of Pt
75
Cu
25
/Co/Ta by 29% to a value of 0.31 ± 0.03 at 407 K. However, the critical switching current density is dependent on the ratio between the Gilbert damping and spin Hall efficiency and the smallest value was observed when
x
= 47%. It was found that at this concentration, the spin transparency was at its highest at 0.85 ± 0.09 hence indicating the importance of interfacial transparency for energy efficient devices at elevated temperature.
Skyrmions are the prime candidate as the information carrier of tomorrow’s data storage devices. But they face the risk of annihilation on encountering a boundary. We show how skyrmions can avoid ...this undesired outcome through management of their anisotropy energies. Specifically we derive an edge potential and a contact interaction term which only operate close to the edge. We verify the accuracy of these two terms by modelling a realistic skyrmion system and comparing it with micromagnetic simulations. We not only observe and explain the speeding up of skyrmions near the edge (which had been expected) but also observe and explain a peculiar asymmetry in the motion (where it speeds up more on one edge compared to the other). We devise a means to stabilize a skyrmion against Magnus force (without the need for a transverse current) by modifying the damping parameters. Finally, we note a link between these damping parameters and the anisotropy energy. Our results will be of value in the design of skyrmion-based devices and would give fresh impetus to the study of magnetic anisotropy.
Skyrmions are the prime candidate as information carrier of tomorrow’s ever ubiquitous data storage devices. But they face the risk of annihilation on encountering a boundary. We propose a model for the skyrmion-edge dynamics by deriving an edge potential and a contact interaction, which are incorporated into the standard Thiele equation for the current-induced motion of skyrmions. We compare this model with micromagnetic simulations which leads us to observe and explain the speeding up of skyrmions near the edge. We also observe and explain a peculiar asymmetry in the motion (where skyrmions speed up more on one edge compared to the other) and discover the important role played by the damping constants of the Thiele equation. We devised a means to stabilize a skyrmion against Magnus force (without the need for a transverse current) by modifying the damping parameters. A connection with the anisotropy energy is noted.
We report on the conduction mechanisms of novel Ru/MgO/Cu and Ru/MgO/Ta resistive switching memory (RSM) devices. Current-voltage (I-V) measurements revealed Schottky emission (SE) as the dominant ...conduction mechanism in the high resistance state (HRS), which was validated by varying temperatures and transmission electron microscopy (TEM) results. Retention of more than 10 years at 85 °C was obtained for both Ru/MgO/Ta and Ru/MgO/Cu RSM devices. In addition, annealing processes greatly improved the consistency of HRS and LRS switching paths from cycle to cycle, exhibiting an average ON/OFF ratio of 10
. Further TEM studies also highlighted the difference in crystallinity between different materials in Ru/MgO/Cu RSM devices, confirming Cu filament identification which was found to be 10 nm in width.
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