Site-specific growth of oriented ZnO nanocrystal arrays.

We report on the growth of ZnO nanocrystals having a hexagonal, prismatic shape, sized 700 nm × 600 nm, on bare indium tin oxide (ITO) substrates. The growth is induced by a low ion flux and involves a low-temperature electrodeposition technique. Further, vertically aligned periodic nanocrystal (NC) growth is engineered at predefined positions on polymer-coated ITO substrates patterned with ordered pores. The vertical alignment of ZnO NCs along the c-axis is achieved via ion-by-ion nucleation-controlled growth for patterned pores of size ≈600 nm; however, many-coupled branched NCs with hexagonal shape are formed when a patterned pore size of ≈200 nm is used. X-ray diffraction data is in agreement with the observed morphology. A mechanism is proposed to interpret the observed site-specific oriented/branched growth that is correlated to the pore size. As ordered NC arrays have the potential to generate new collective properties different from single NCs, our first demonstration of a cost effective and facile fabrication process opens up new possibilities for devices with versatile functionalities.

Influence of the thickness of an antiferromagnetic IrMn layer on the static and dynamic magnetization of weakly coupled CoFeB/IrMn/CoFeB trilayers.

The static and dynamic magnetization response of the CoFeB/IrMn/CoFeB trilayer system with varying thickness of the antiferromagnetic (AF) IrMn layer is investigated using magnetization hysteresis (M-H) and ferromagnetic resonance (FMR) measurements. The study shows that the two CoFeB layers are coupled via a long-range dynamic exchange effect through the IrMn layer up to a thickness of 6 nm. It is found that with the increase in IrMn layer thickness a nearly linear enhancement of the effective magnetic damping constant occurs, which is associated with the simultaneous influence of spin pumping and interlayer exchange coupling effects. An extrinsic contribution to the linewidth originating from the two-magnon scattering is also discussed. The AF-induced interfacial damping parameter is derived by studying the evolution of damping with inverse CoFeB thickness. The static magnetic measurements also reveal the interlayer exchange coupling across the IrMn layer both at room temperature and low temperature. The asymmetric hysteresis loop and training effect observed at low temperature is related to the presence of a metastable AF domain state. We show that both the static and dynamic magnetic properties of trilayer films can be adjusted over a wide range by changing the thickness of the IrMn spacer layer.

Capping Layer (CL) Induced Antidamping in CL/Py/ß-W System (CL: Al, ß-Ta, Cu, ß-W).

For achieving ultrafast switching speed and minimizing dissipation losses, the spin-based data storage device requires a control on effective damping (αeff) of nanomagnetic bits. Incorporation of interfacial antidamping spin orbit torque (SOT) in spintronic devices therefore has high prospects for enhancing their performance efficiency. Clear evidence of such an interfacial antidamping is found in Al capped Py(15 nm)/ß-W(tW)/Si (Py = Ni81Fe19 and tW = thickness of ß-W), which is in contrast to the increase of αeff (i.e., damping) usually associated with spin pumping as seen in Py(15 nm)/ß-W(tW)/Si system. Because of spin pumping, the interfacial spin mixing conductance (g↑↓) at Py/ß-W interface and spin diffusion length (λSD) of ß-W are found to be 1.63(±0.02) × 1018 m-2 (1.44(±0.02) × 1018 m-2) and 1.42(±0.19) nm (1.00(±0.10) nm) for Py(15 nm)/ß-W(tW)/Si (ß-W(tW)/Py(15 nm)/Si) bilayer systems. Other different nonmagnetic capping layers (CL), namely, ß-W(2 nm), Cu(2 nm), and ß-Ta(2,3,4 nm) were also grown over the same Py(15 nm)/ß-W(tW). However, antidamping is seen only in ß-Ta(2,3 nm)/Py(15 nm)/ß-W(tW)/Si. This decrease in αeff is attributed to the interfacial Rashba like SOT generated by nonequilibrium spin accumulation subsequent to the spin pumping. Contrary to this, when interlayer positions of Py(15 nm) and ß-W(tW) is interchanged irrespective of the fixed top nonmagnetic layer, an increase of αeff is observed, which is ascribed to spin pumping from Py to ß-W layer.

Anomalous anti-damping in sputtered ß-Ta/Py bilayer system.

Anomalous decrease in effective damping parameter αeff in sputtered Ni81Fe19 (Py) thin films in contact with a very thin ß-Ta layer without necessitating the flow of DC-current is observed. This reduction in αeff, which is also referred to as anti-damping effect, is found to be critically dependent on the thickness of ß-Ta layer; αeff being highest, i.e., 0.0093 ± 0.0003 for bare Ni81Fe19(18 nm)/SiO2/Si compared to the smallest value of 0.0077 ± 0.0001 for ß-Ta(6 nm)/Py(18 nm)/SiO2/Si. This anomalous anti-damping effect is understood in terms of interfacial Rashba effect associated with the formation of a thin protective Ta2O5 barrier layer and also the spin pumping induced non-equilibrium diffusive spin-accumulation effect in ß-Ta layer near the Ta/Py interface which induces additional spin orbit torque (SOT) on the moments in Py leading to reduction in αeff. The fitting of αeff (tTa) revealed an anomalous negative interfacial spin mixing conductance, g(↑↓) = -1.13 ± .05 × 10(18) m(-2) and spin diffusion length, λSD = 2.47 ± 0.47 nm. The increase in αeff observed above tTa = 6 nm is attributed to the weakening of SOT at higher tTa. The study highlights the potential of employing ß-Ta based nanostructures in developing low power spintronic devices having tunable as well as low value of α.