Ding-Shuo Wang

Nonvolatile electric-field modulation of magnetic anisotropy in perpendicularly magnetized L10-FePt/(001)[Pb(Mg1/3Nb2/3)]0.7-(PbTiO3)0.3 heterostructures

We grow perpendicular L10-FePt films epitaxially on (001)[Pb(Mg1/3Nb2/3)]0.7-(PbTiO3)0.3 ferroelectric substrates. Due to the magnetostriction effect, the out-of-plane coercivity (Hc⊥) of the L10-FePt varies with applied electric fields, showing an asymmetric butterfly-like loop. The Hc⊥ at the zero-electric-field state (Hc⊥,0) shows a nonvolatile change, depending on the direction of the poling electric field. The magnitude of nonvolatile magnetic anisotropy change, induced by the ferroelectric field effect, can be comparable to the anisotropy change induced by pure electric fields. The nonvolatile magnetic anisotropy change is inversely proportional to the FePt thickness and can be eliminated by inserting a metallic intermediate layer.

Engineering spin-orbit torque in Co/Pt multilayers with perpendicular magnetic anisotropy

To address thermal stability issues for spintronic devices with a reduced size, we investigate spin-orbit torque in Co/Pt multilayers with strong perpendicular magnetic anisotropy. Note that the spin-orbit torque arises from the global imbalance of the spin currents from the top and bottom interfaces for each Co layer. By inserting Ta or Cu layers to strengthen the top-down asymmetry, the spin-orbit torque efficiency can be greatly modified without compromised perpendicular magnetic anisotropy. Above all, the efficiency builds up as the number of layers increases, realizing robust thermal stability and high spin-orbit-torque efficiency simultaneously in the multilayers structure.

High thermal stability and low Gilbert damping constant of CoFeB/MgO bilayer with perpendicular magnetic anisotropy by Al capping and rapid thermal annealing

We demonstrate that the magnetic anisotropy of the CoFeB/MgO bilayer can be manipulated by adding an aluminum capping layer. After rapid thermal annealing, we can achieve large perpendicular magnetic anisotropy of CoFeB with a high thermal stability factor (Δ = 72) while the Gilbert damping constant can be reduced down to only 0.011 simultaneously. The boron and residual oxygen in the bulk CoFeB layer are properly absorbed by the Al capping layer during annealing, leading to the enhanced exchange stiffness and reduced damping. The interfacial Fe-O bonding can be optimized by tuning annealing temperature and thickness of Al, resulting in enhanced perpendicular anisotropy.

Initialization‐Free Multilevel States Driven by Spin–Orbit Torque Switching

By engineering multidomain formation in Co/Pt multilayers, it is demonstrated how multilevel storage can be achieved by spin-orbit torque switching. It is rather remarkable that, by modulating the writing pulse conditions, the final magnetization states can be controlled, independent of the initial configurations. The initialization-free multilevel memory advances the spin-orbit-torque magnetic random access memory to higher storage density for practical applications.

Thermal dewetting with a chemically heterogeneous nano-template for self-assembled L10 FePt nanoparticle arrays

A design for the fabrication of metallic nanoparticles is presented by thermal dewetting with a chemically heterogeneous nano-template. For the template, we fabricate a nanostructured polystyrene-b-polydimethylsiloxane (PS-b-PDMS) film on a Si|SiO2 substrate, followed by a thermal annealing and reactive ion etching (RIE) process. This gives a template composed of an ordered hexagonal array of SiOC hemispheres emerging in the polystyrene matrix. After the deposition of a FePt film on this template, we utilize the rapid thermal annealing (RTA) process, which provides in-plane stress, to achieve thermal dewetting and structural ordering of FePt simultaneously. Since the template is composed of different composition surfaces with periodically varied morphologies, it offers more tuning knobs to manipulate the nanostructures. We show that both the decrease in the area of the PS matrix and the increase in the strain energy relaxation transfer the dewetted pattern from the randomly distributed nanoparticles into a hexagonal periodic array of L10 FePt nanoparticles. Transmission electron microscopy with the in situ heating stage reveals the evolution of the dewetting process, and confirms that the positions of nanoparticles are aligned with those of the SiOC hemispheres. The nanoparticles formed by this template-dewetting show an average diameter and center-to-center distance of 19.30 ± 2.09 nm and 39.85 ± 4.80 nm, respectively. The hexagonal array of FePt nanoparticles reveals a large coercivity of 1.5 T, much larger than the nanoparticles fabricated by top-down approaches. This approach offers an efficient pathway toward self-assembled nanostructures in a wide range of material systems.