Soogil Lee

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.

An Array of Ferromagnetic Nanoislands Nondestructively Patterned via a Local Phase Transformation by Low-Energy Proton Irradiation

Low-energy proton irradiation was applied to pattern an array of metallic, ferromagnetic nanoislands through the local phase transformation of an oxidic, paramagnetic phase in a complex superlattice composed of repetitions of an oxidic and metallic layer. The irradiation inflicted minimal damage on the structure, resulting in the absence of unwanted defects and side effects. This nondestructive pattern transfer was clearly confirmed by the contrast between irradiated and unirradiated regions in electrical, chemical, and magnetic images. Simulation based on the magnetic properties suggests that this low-energy proton irradiation can nondestructively pattern an array of ferromagnetic islands with 8.2 nm in diameter and 7.4 nm in spacing between islands, which means it can achieve an areal density of ∼3 Tb/in.(2) with a thermal stability of over 80 kBT. Such an array is strong enough to overcome the so-called superparamagnetism limit in magnetic recording. The attributes demonstrated here corroborate that proton irradiation can be applied to design and pattern devices on a nanometer scale not only for magnetic but also for electric and optical materials systems in all such systems in which a local phase transformation is available.

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.

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...

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.

Anisotropy dispersion in the exchange-biased pinned layer of a spin valve prepared by 550 eV hydrogen-ion irradiation

By investigating angular dependence of resistance and applying the Boltzmann distribution to the anisotropy dispersion of the magnetization in an exchange-biased pinned layer, we quantized the intrinsic anisotropy dispersion σγ of spin valves. The σγ was estimated to be 0.412° for the as-deposited spin valve and 0.183° for the ion-irradiated spin valve. This indicates that the dispersion indeed narrowed when the spin valve was field-annealed or irradiated by 550 eV hydrogen ions under a magnetic field, which is consistent with our previous attribution to the significant improvement in both exchange anisotropy and giant magnetoresistance of spin valves thus treated. Our methodology can be applied for other spin devices characterized by angular dependence of resistance to determine useful device properties such as the intrinsic anisotropy dispersion and the exchange bias of the exchange-biased reference layer.

In vivo skin reactions from pulsed-type, bipolar, alternating current radiofrequency treatment using invasive noninsulated electrodes

Bipolar, alternating current radiofrequency (RF) conduction using invasive noninsulated electrodes consecutively generates independent tissue coagulation around each electrode and then, the converged coagulation columns.

Effect of tunnel-spin polarization on spin accumulation in n-type Ge(001)/MgO/Co40Fe40B20

We found that a huge enhancement of electrical spin accumulation in n-type Ge(001) with the MgO/Co40Fe40B20 (CFB) spin-tunnel contact (STC) is realized by postannealing. The spin-resistance–area product (R s A) of this STC on n-type Ge after postannealing at 350 °C (1.97 × 106 Ωµm2) is nearly one order of magnitude larger than that of the as-deposited one (2.34 × 105 Ωµm2). The dependence of R s A on contact resistance, a scaling property, is also greatly modulated after postannealing. The epitaxial growth of CFB on an MgO(001) template and the consequent TSP improvement are responsible for such changes.

Strongly (001)-textured MgO/Co40Fe40B20 spin-tunnel contact on n-Ge(001) and its spin accumulation: Structural modification with ultrathin Mg insertion by sputtering

The sputter-deposited fcc-MgO (001)[100]/bcc-Co40Fe40B20 (001)[110] spin-tunnel contact (STC) was successfully prepared on n-Ge(001). We found that the interfacial modification by ultrathin (6 A) Mg insertion at the interface between n-Ge and MgO plays an important role in spin injection into Ge. The significantly amplified spin accumulation was observed in this STC as a result of the structural modification. The three-terminal Hanle signal of this STC was 2.7 times larger than that of the STC without Mg insertion. Our study confirms that a sputtering technique is indeed practical and useful to modify interfacial structures for the efficient injection of spins into semiconductors.