Seungkyo Lee

Diameter-engineered SnO2 nanowires over contact-printed gold nanodots using size-controlled carbon nanopost array stamps.

A novel and effective methodology to control the diameters of semiconductor nanowires is reported through a versatile contact-printing method for obtaining size-controlled nanocatalysts by size-tunable carbon-based nanometer stamps. Vertically aligned carbon nanopost arrays, derived from nanoporous alumina templates, are used as the nanoscale stamps for printing of catalyst nanoparticles. The diameter of the carbon nanopost can be engineered by adjusting the pore dimension of the templates. Over the contact-printed Au nanodots in a uniform size distribution, semiconductor SnO2 nanowires are grown via a vapor-liquid-solid growth mechanism. Consequently, a direct dimension correspondence is achieved between the carbon nanopost stamp, the printed Au catalyst, and the finally obtained SnO2 nanowires. A model example of the diameter-dependent electrical properties of the semiconductor nanowires is successfully demonstrated in this work by applying three diameter-controlled SnO2 nanowires to nanowire field effect transistors.

Current-induced domain wall nucleation and its pinning characteristics at a notch in a spin-valve nanowire

The characteristics of domain wall (DW) pinning at a notch in a spin-valve nanowire were investigated when a DW was created by a current, flowing into a spin-valve nanowire. It was found that DW pinning at a notch is quite sensitive to the magnitude of the current and its polarity. The current-polarity dependence of DW pinning is likely due to the spin structure in the core of the DW, which is determined by an Oersted field from the current in a Cu layer. This indicates that the control of DW pinning at a notch in a nanowire can be achieved by a current acting on its own, which is an important advantage of this method, compared with field-induced DW control.

Sensitivity enhancement of a giant magnetoresistance alternating spin-valve sensor for high-field applications

We introduce a CoFe/Tb multilayer film as a sensing layer of alternating giant magnetoresistance (GMR) spin-valve sensors for use in a high-field measurements. The CoFe/Tb sensing layer has lower in-plane anisotropy than a single CoFe sensing layer and allows the alternating GMR sensor to show a high sensitivity, ∼0.4 MR[%]/kOe, when the sensing layer structure is [CoFe(1.92 nm)/Tb(1.22 nm)] × 2. This sensitivity is about four times larger than previously reported values. In addition, it was found that the working range of the sensor could be easily tuned by varying the CoFe thickness in the reference layer. Therefore, this study is encouraging not only for GMR alternating spin-valve sensor applications, but also for the development of tunneling magnetoresistance based alternating sensor applications with considerably higher sensitivities.

Spin-valve sensor with an out-of-plane magnetic anisotropy: For small field sensing applications

This study investigates a spin-valve sensor, which consists of ferromagnetic layers with both an out-of-plane magnetic anisotropy (NiFe/Tb/NiFe layers) and an in-plane magnetic anisotropy (CoFe/IrMn layers). The out-of-plane magnetic anisotropy was able to be tuned by varying the thickness (tTb) of the Tb layer and applying an in-plane magnetic field during film deposition. In addition, the field sensitivity of the spin-valve sensor was also found to be a function of the degree of out-of-plane magnetic anisotropy. As a result, a sensor with tTb=3 nm showed a linear and reversible magnetoresistance (MR) response to an applied in-plane magnetic field with a higher sensitivity of 0.012%/Oe by one order of magnitude than that (∼0.000 75%/Oe) of a sensor with tTb=4 nm. This suggests that the spin-valve sensor can be optimized by changing the Tb thickness so that the magnetic properties of the sensing layer can meet the requirements of a small field sensing application, such as a biosensor.

Inverse giant magnetoresistance in CoFe/Tb multilayer with perpendicular magnetic anisotropy

We detected inverse giant magnetoresistance (GMR) in a multilayer of Ta (4 nm)/[Tb (1.6 nm)/CoFe (1.2 nm)]5/Cu (3 nm)/[CoFe (1.2 nm)/Tb (0.6 nm)]5/Ta (4 nm); both the bottom [Tb (1.6 nm)/CoFe (1.2 nm)]5 and top [CoFe (1.2 nm)/Tb (0.6 nm)]5 layers revealed a perpendicular magnetic anisotropy. Furthermore, depending on the Tb layer thickness, we confirmed the magnetization of the bottom CoFe layer to be either parallel or antiparallel to the applied field. Hence, the GMR behavior could be controlled by tuning the perpendicular magnetic anisotropy, i.e., it was switchable from inverse to normal GMR or from normal to inverse. Changes in GMR occurred at a compensation composition of CoFe and Tb for which no magnetization was observed due to antiferromagnetic cancellation of the Tb and CoFe moments.

Manipulating magnetic moment in a magnetic domain wall under transverse magnetic fields near Walker threshold

Current-induced domain wall (DW) motion under transverse magnetic fields was investigated through micromagnetic simulation using the Landau–Lifshitz–Gilbert equation containing adiabatic and nonadiabatic spin torque terms. It was found that the transverse field aligned antiparallel to the magnetic moment of the DW promotes a nucleation of an antivortex core, which causes a temporal Walker breakdown and then causes the magnetic moment of the DW to be aligned parallel to the transverse field. On the other hand, the transverse field aligned parallel to the magnetic moment of the DW induces the nucleation of an antivortex core at an edge of a nanowire to be delayed, resulting in the increase in Walker threshold current. The effect of transverse field on current-induced DW motion should be considered carefully for the spintronic applications that utilize DW motion.

Quantitative detection of magnetic particles in a chromatographic membrane by a giant magnetoresistance sensor

We used streptavidin-coated magnetic particles (Dynabeads® M-280) for an immunochromatographic test, instead of colloidal gold particles, which was widely used in a conventional technique. The concentration of magnetic particles in a membrane was quantitatively analyzed by using a giant magnetoresistance (GMR) sensor with a sensitivity of 0.17%/Oe. As a specific zone with a localized concentration of the magnetic particles passed through a GMR sensor, it was observed that the magnitude of sensing signals is proportional to the density of magnetic particles. Therefore, this result suggests that a GMR sensor, which was studied in this paper, can be used for the quantitative detection with a high sensitivity of specific analytes in the immunochromatographic assays, when the analytes were coated on magnetic particles.

Control of the Degree of Out-of-Plane Anisotropy of CoFe/Tb Multilayer by Applying an External Inplane Field During Film Deposition

We investigated the out-of-plane anisotropy of Co50Fe50/Tb multilayer film with changing the thickness of Tb layer and applying an inplane filed of 200 Oe during film deposition. A strong out-of-plane anisotropy was found only for the multilayer with small thickness of Tb layer (tTb = 1 nm) and was suppressed significantly by the application of an inplane magnetic field of 200 Oe during deposition, resulting in an inplane anisotropy. It was also found that the application of the magnetic field caused the enhanced intermixing between CoFe and Tb layers to form Co-Tb and Fe-Tb alloys at the interface of the layers. While the origin of the out-of-plane anisotropy and the change of the out-of-plane to inplane anisotropy with the application of inplane field of 200 Oe are not understood completely, it seems that there is a strong correlation between the out-of-plane anisotropy and the formation of the alloys and the stress at the interface.

Origin of asymmetry of tunneling conductance in CoFeB/MgO/CoFeB tunnel junction

We investigated the top and bottom interfaces of a CoFeB∕MgO∕CoFeB tunnel junction using transmission electron microscope (TEM) and x-ray photoemission spectroscopy (XPS) in order to understand the origin of the asymmetry of dI∕dV in terms of bias polarity. It was found, from a TEM image, that there is no clear cut at the top interface, while the bottom interface has relatively clean boundary. Furthermore, XPS data show that more hydroxides were formed at the top interface than at the bottom interface. These indicate that the hydroxides would hinder the epitaxial crystallinity at the interface in CoFeB∕MgO∕CoFeB tunnel junctions. Therefore, it is most likely that the asymmetry of dI∕dV is caused by the disappearance of minority Bloch state, which is closely correlated with the existence of hydroxides at the top interface of a CoFeB∕MgO∕CoFeB tunnel junction.

Reduction of a coercive field in the bilayers of CoGdTb/NiFe with perpendicular magnetic anisotropy

We have investigated the magnetic properties of CoGdTb∕NiFe bilayer with the variation of NiFe thickness (tNiFe). It was found that the composition of CoGdTb layer can be controlled by adjusting the Ar working pressure of the sputtering system. We used a CoGdTb∕NiFe bilayer with the Co concentration of 82.2at.%, which was deposited at an Ar working pressure of 3mTorr, to investigate the bilayer coercivity. The deposition of NiFe (tNiFe=1.5nm) on the CoGdTb layer caused the increase of the coercive field. It was found from the in-plane and out-of-plane hysteresis loops that the NiFe moments were aligned out of plane due to the strong coupling between NiFe and CoGdTb, leading to the increase of the coercive field. With the increase of the NiFe thickness (tNiFe=5nm), the coercive field of CoGdTb decreased rapidly, which was likely to be caused by the in-plane component of NiFe moments. With further increase of the NiFe thickness (tNiFe=10 and 15nm), no more reduction of the coercive field was observed althou...