Jangyup Son

Nanoscale patterning of complex magnetic nanostructures by reduction with low-energy protons

Techniques that can produce patterns with nanoscale details on surfaces have a central role in the development of new electronic, optical and magnetic devices and systems. High-energy ion irradiation can produce nanoscale patterns on ferromagnetic films by destroying the structure of layers or interfaces, but this approach can damage the film and introduce unwanted defects. Moreover, ferromagnetic nanostructures that have been patterned by ion irradiation often interfere with unpatterned regions through exchange interactions, which results in a loss of control over magnetization switching. Here, we demonstrate that low-energy proton irradiation can pattern an array of 100-nm-wide single ferromagnetic domains by reducing [Co(3)O(4)/Pd](10) (a paramagnetic oxide) to produce [Co/Pd](10) (a ferromagnetic metal). Moreover, there are no exchange interactions in the final superlattice, and the ions have a minimal impact on the overall structure, so the interfaces between alternate layers of cobalt (which are 0.6 nm thick) and palladium (1.0 nm) remain intact. This allows the reduced [Co/Pd](10) superlattice to produce a perpendicular magnetic anisotropy that is stronger than that observed in the metallic [Co/Pd](10) superlattices we prepared for reference. We also demonstrate that our non-destructive approach can reduce CoFe(2)O(4) to metallic CoFe.

Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor

Graphene is currently at the forefront of cutting-edge science and technology due to exceptional electronic, optical, mechanical, and thermal properties. However, the absence of a sizeable band gap in graphene has been a major obstacle for application. To open and control a band gap in functionalized graphene, several gapping strategies have been developed. In particular, hydrogen plasma treatment has triggered a great scientific interest, because it has been known to be an efficient way to modify the surface of single-layered graphene and to apply for standard wafer-scale fabrication. Here we show a monolayer chemical-vapour-deposited graphene hydrogenated by indirect hydrogen plasma without structural defect and we demonstrate that a band gap can be tuned as wide as 3.9 eV by varying hydrogen coverage. We also show a hydrogenated graphene field-effect transistor, showing that on/off ratio changes over three orders of magnitude at room temperature.

Blue and red luminescence from Si ion-irradiated SiO2/Si/SiO2 layers

Abstract Photoluminescence (PL) from the Si ion-irradiated SiO2/Si/SiO2 layers on Si substrate at room temperature has been studied to elucidate the origins of the blue and red luminescence. A luminescence band around 450 nm was observed from as-irradiated sample, which was found to be originated from the diamagnetic defect known as B2 band generated by Si ion irradiation. The intensity of this band increases with the increase of annealing temperatures up to a critical temperature. After annealing at 1100°C, the defect-related PL peaks around 450 and 600 nm disappear and a new PL peak appears around 700 nm. This luminescence band is attributed to ∼5 nm-sized Si nanocrystals formed along the Si layer between SiO2 layers.

Dependence of exchange coupling direction on cooling-field strength

We studied the dependence of exchange coupling on cooling-field strength in an exchange-biased spin valve with a synthetic antiferromagnetic layer by experiment and theory. Our theory calculates magnetic anisotropy energies in each magnetic layer composing the spin valve during the field-cooling process, finds the minimum state of total energy, and explains how the magnetizations in the layers interact with one another during field-cooling under various cooling-field strengths. Calculations based on the theory well match results of the experimental measurements. Our observation shows that one has to carefully choose the cooling-field strength optimal for designing exchange-biased spin devices having a synthetic antiferromagnetic layer; otherwise the exchange coupling direction can significantly deviate from the cooling-field direction, which impairs performance.

Photoluminescence from Si ion irradiated SiO2/Si/SiO2 films with elevated substrate temperature

Abstract Photoluminescence (PL) from the Si ion irradiated SiO2/Si/SiO2 layers on Si substrate at room temperature and elevated substrate temperatures has been studied to elucidate the luminescence origins. The irradiation of Si ions into SiO2/Si/SiO2 layers instead of SiO2 films was performed to improve the PL intensity by increasing the number of proper-sized Si nanocrystals. Before annealing at high temperature, a luminescence band around 450 nm is observed. This luminescence band was found to originate from the diamagnetic defect known as B2 band generated by Si ion irradiation. The intensity of this band increases when ion irradiation is carried out at high substrate temperature. After annealing at high temperature, the PL peaks originating from the B2 band disappear and a new PL peak appears around 700 nm. This luminescence band is associated with ∼5-nm sized Si nanocrystals. Also it can be found that the PL peak intensity around 700 nm is significantly increased with the high substrate temperature during ion irradiation. Therefore, it is concluded that ion irradiation into SiO2/Si/SiO2 layers is more effective than ion implantation into SiO2 films to obtain an intensive PL peak originating from Si nanocrystals.

Effects of Si-dose on defect-related photoluminescence in Si-implanted SiO2 layers

Si ions were implanted into 100-nm-thick SiO2 layer thermally grown on crystalline Si at an energy of 55 keV with various doses ranging from 1×1014 to 1×1017 Si/cm2 at room temperature. Si ions go through the interface between SiO2 layer and Si substrate generating defects in SiO2 layer and Si substrate as well. Defect-related phenomena were characterized by photoluminescence (PL) and electron spin resonance (ESR) measurements. The PL experiment shows that there exists a dose window for a maximum intensity of luminescence related to radiative defects, while the ESR exhibits that nonradiative defects change from E′ centers to Pb centers as the dose increases. It is considered that the intensity is controlled by the density ratio of radiative to nonradiative defects induced by ion implantation.

Angular dependence of the exchange bias direction and giant magnetoresistance on different cooling-field strengths

We studied the effect of different cooling-field strengths on the exchange bias by measuring the angular-dependent sheet resistance and the giant magnetoresistance of exchange-biased spin valves using a PtMn antiferromagnetic and a synthetic antiferromagnetic layer. When we annealed the spin valve at a cooling-field of 100 Oe, the exchange bias was antiparallel to the cooling-field. As we increased the cooling-field to 4000 Oe, the exchange bias direction gradually rotated and it ended up parallel to the cooling-field direction. The giant magnetoresistance also changed with the cooling-field strength. In the cooling-field range between 100 and 4000 Oe, the magnetoresistance ratios measured along the cooling-field direction were significantly reduced. However, the magnetoresistance ratios measured along the exchange bias direction increased, although still remaining smaller than those of the spin valve annealed at 100 or 4000 Oe. On the other hand, the exchange bias strength did not change significantly wi...

Ion beam induced atomic transport in bilayer systems

Abstract Atomic transport in ion beam mixed Co/Pt and Pd/Au bilayer systems have been studied from the shifts of maker layers in Rutherford backscattering spectroscopy. Thin layers (1 nm) of marker (Pd for Co/Pt and Ni for Pd/Au) were embedded as markers at each interfaces. 80 keV Ar + was used to irradiate the marker samples at the temperature range between 90 and 600 K. The Co/Pt system shows isotropic atomic transport ( J Co / J Pt ∼1.1) at low temperatures and anisotropic atomic transport ( J Co / J Pt ∼5.0) at high temperatures. Meanwhile, the Pd/Au system shows near isotropic atomic transport ( J Pd / J Au ∼1.2) at all temperatures examined. These results were discussed in terms of the activation energies for the normal impurity diffusion, cohesive energy difference, and the vacancy migration energy. Atomic transport in thermal spike regime is closely related with the activation energy for normal impurity diffusion. In radiation enhanced diffusion regime, the cohesive energy and/or the vacancy migration energy plays a dominant role for the atomic transport.

Side-illuminated tip-enhanced Raman study of edge phonon in graphene at the electrical breakdown limit

Nanoscale integration of graphene into a circuit requires a stable performance under high current density. However, the effects of the current density that approach the electronic breakdown limit of graphene are not well understood. We explored the effects of a high current density, close to the electronic breakdown limit of 10 A/cm (∼3.0 × 108 A/cm2), on graphene, using tip-enhanced Raman scattering. The results showed that the high current density induces Raman bands at 1456 and 1530 cm−1, which were assigned to edge-phonon modes originating from zigzag and armchair edges. This led us to conclude that C–C bonds are cleaved due to the high current density, leaving edge structures behind, which were detected through the observation of localized phonons.

A Study on the Sensitivity of a Spin Valve with Conetic-Based Free Layers

An exchange-biased spin valve with Conetic-based free layers of Co90Fe10, Co90Fe10/Conetic and Conetic was investigated. The spin valve with the Co90Fe10 free layer showed the highest giant magnetoresistance (GMR) ratio of 4% but showed the lowest normalized sensitivity of 0.02 Oe-1. The GMR ratio of 3% and the normalized sensitivity of 0.07 Oe-1 were obtained for the spin valve with the Co90Fe10/Conetic free layer after annealing. The spin valve having the Conetic free layer showed softer magnetic properties and well-defined smaller anisotropy than the other spin valves. Though the spin valve showed the lowest GMR of 0.4% after annealing, it showed the highest normalized sensitivity of 0.14 Oe-1. Our study shows that further improvement in MR response of spin valves with Conetic-based free layers can make a spin valve sensor promising for detecting extremely low fields.