Elucidation of the Mechanism for Maintaining Ultrafast Domain Wall Mobility Over a Wide Temperature Range

To realize a data rate of 20 Gbps in the communication standard 5G with a racetrack memory, it is crucial to stably recognize a domain-wall (DW) velocity (vDW) of 1200 m/s when the minimum bit length is 60 nm. However, general vDW is as slow as about 100 m/s. Recent reports indeed showed that the fast DW motion occurs using an in-plane external magnetic field however, this mechanism is unsuitable because the rear-edge vDW decelerated, which contrary to the front-edge of DW velocity. Therefore, we designed magnetic wires by bringing the g values of rare-earth and transition-metals close to each other and suppressing the Joule heat generation distribution due to short pulse current, we successfully demonstrated the vDW of 1200 m/s in a wide temperature range without using an external magnetic field. Moreover, the current density (J) is low, and the DW mobility (vDW/J) is significantly improved 10-times over a wide temperature range compared to other reports.

Modelling and Measurement of Magnetically Soft Nanowire Arrays for Sensor Applications

Soft magnetic wires and microwires are currently used for the cores of magnetic sensors. Due to their low demagnetization, they contribute to the high sensitivity and the high spatial resolution of fluxgates, Giant Magnetoimpedance (GMI), and inductive sensors. The arrays of nanowires can be prepared by electrodeposition into predefined pores of a nanoporous polycarbonate membrane. While high coercivity arrays with square loops are convenient for information storage and for bistable sensors such as proximity switches, low coercivity cores are needed for linear sensors. We show that coercivity can be controlled by the geometry of the array: increasing the diameter of nanowires (20 µm in length) from 30 nm to 200 nm reduced the coercivity by a factor of 10, while the corresponding decrease in the apparent permeability was only 5-fold. Finite element simulation of nanowire arrays is important for sensor development, but it is computationally demanding. While an array of 2000 wires can be still modelled in 3D, this is impossible for real arrays containing millions of wires. We have developed an equivalent 2D model, which allows us to solve these large arrays with acceptable accuracy. Using this tool, we have shown that as a core of magnetic sensors, nanowires are efficiently employed only together with microcoils with diameter comparable to the nanowire length.

Enhanced magnetic performance of aligned wires assembled from nanoparticles: from nanoscale to macroscale

Magnetic wires in highly dense arrays, possessing unique magnetic properties, are eagerly anticipated for inexpensive and scalable fabrication technologies. This study reports a facile method to fabricate arrays of magnetic wires directly assembled from well-dispersed α ″ -Fe 16 N 2 /Al 2 O 3 and Fe 3 O 4 nanoparticles with average diameters of 45 nm and 65 nm, respectively. The magnetic arrays with a height scale of the order of 10 mm were formed on substrate surfaces, which were perpendicular to an applied magnetic field of 15 T. The applied magnetic field aligned the easy axis of the magnetic nanoparticles (MNPs) and resulted in a significant enhancement of the magnetic performance. Hysteresis curves reveal that values of magnetic coercivity and remanent magnetization in the preferred magnetization direction are both higher than that of the nanoparticles, while these values in the perpendicular direction are both lower. Enhancement in the magnetic property for arrays made from spindle-shape α ″ -Fe 16 N 2 /Al 2 O 3 nanoparticles is higher than that made from cube-like α ″ -Fe 16 N 2 /Al 2 O 3 ones, owing to the shape anisotropy of MNPs. Furthermore, the assembled highly magnetic α ″ -Fe 16 N 2 /Al 2 O 3 arrays produced a detectable magnetic field with an intensity of approximately 0.2 T. Although high-intensity external field benefits for the fabrication of magnetic arrays, the newly developed technique provides an environmentally friendly and feasible approach to fabricate magnetic wires in highly dense arrays in open environment condition.

Synthesis of Magnetic Wires from Polyol-Derived Fe-Glycolate Wires

Fe-glycolate wires with micrometer-scale lengths can be synthesized by the polyol process. Although the as-produced wires are in the paramagnetic state at room temperature, they are transformed into ferrimagnetic iron oxides and ferromagnetic metallic iron wires by reductive annealing. The shape of the wires is unchanged by reductive annealing, and it is possible to control the magnetic properties of the resulting wire-shaped ferri/ferromagnets by adjusting the annealing conditions. Consequently, the reductive annealing of polyol-derived Fe-glycolate wires is an effective material-processing route for the production of magnetic wires.