S.D. Kim

Involvement of nitric oxide-induced NADPH oxidase in adventitious root growth and antioxidant defense in Panax ginseng

Nitric oxide (NO) affects the growth and development of plants and also affects plant responses to various stresses. Because NO induces root differentiation, we examined whether or not it is involved in increased ROS generation. Treatments with sodium nitroprusside (SNP), an NO donor, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO), a specific NO scavenger, and Nω-nitro-l-arginine methyl ester hydrochloride (l-NAME), an NO synthase (NOS) inhibitor, revealed that NO is involved in the adventitious root growth of mountain ginseng. Supply of an NO donor, SNP, activates NADPH oxidase activity, resulting in increased generation of O2·−, which subsequently induces growth of adventitious roots. Moreover, treatment with diphenyliodonium chloride (DPI), an NADPH oxidase inhibitor, individually or with SNP, inhibited root growth, NADPH oxidase activity, and O2·− anion generation. Supply of the NO donor, SNP, did not induce any notable isoforms of enzymes; it did, however, increase the activity of pre-existing bands of NADPH oxidase, superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, and glutathione reductase. Enhanced activity of antioxidant enzymes induced by SNP supply seems to be responsible for a low level of H2O2 in the adventitious roots of mountain ginseng. It was therefore concluded that NO-induced generation of O2·− by NADPH oxidase seems to have a role in adventitious root growth of mountain ginseng. The possible mechanism of NO involvement in O2·− generation through NADPH oxidase and subsequent root growth is discussed.

Transfer ratio of the spin-valve transistor

We describe the factors that control the transfer ratio of the spin-valve transistor. An increase in transfer ratio is obtained by a systematic variation of the height of emitter and collector Schottky barrier, and of the nonmagnetic metals. Next, we found that in some cases, a thicker base leads to a higher transfer ratio. Finally, the thickness of the magnetic layers in the Ni 80 Fe 20 /Au/Co spin-valve base can be optimized for a maximum absolute change of collector current. An overall increase by a factor of 24 was achieved, without loss of the magnetocurrent.

The spin-valve transistor: Fabrication, characterization and physics

An overview is given of the fabrication, basic properties, and physics of the spin-valve transistor. We describe the layout of this three-terminal ferromagnet/semiconductor hybrid device, as well as the operating principle. Fabrication technologies are discussed, including vacuum metal bonding. We characterize properties of the device relevant for possible applications in magneto-electronics, such as relative magnetic response, output current, and noise behavior. Furthermore, we illustrate the unique possibilities of the spin-valve transistor for fundamental studies of the physics of hot-electron spin transport in magnetic thin film structures.

300% magnetocurrent in a room temperature operating spin-valve transistor

Here we present the realization of a room temperature operating spin-valve transistor with huge magnetocurrent (MC=300%) at low fields. This spin-valve transistor employs hot-electron transport across a Ni81Fe19/Au/Co spin valve. Hot electrons are injected into the spin valve across a Si–Pt Schottky barrier. After traversing the spin valve, these hot electrons are collected using a second Schottky barrier (Si–Au), which provides energy and momentum selection. The collector current is found to be extremely sensitive to the spin-dependent scattering of hot electrons in the spin valve, and therefore on the applied magnetic field. We also illustrate the role of the collector diode characteristics in determining the magnetocurrent under collector bias.

Hot-electron transport through Ni80Fe20 in a spin-valve transistor

Hot-electron transport in Ni80Fe20 thin films was studied using a spin-valve transistor. By varying the NiFe thickness from 10 to 100 A we obtain an attenuation length of 43 A for majority-spin hot electrons at 0.9 eV above the Fermi level. Based on such relatively long bulk attentuation lengths, one would expect a current transfer ratio that is much larger than the measured value. We propose that the discrepancy can be accounted for by considering interfacial scattering. Increasing the growth quality should thus provide a means to improve the current transfer ratio.

Noise properties of the spin-valve transistor

Noise measurements have been performed on a spin-valve transistor. This transistor consists of a Pt/NiFe/Au/Co/Au multilayer sandwiched between two semiconductors. For comparison, we also studied metal base transistors with a Pt/Au or Pt/NiFe/Au base. All samples show full shot noise in the collector current. The inclusion of a spin-valve in the base layer decreases the absolute value of the collector current and with it the noise level but it does not change the nature of the noise in this device. Similarly, the collector current, and therefore, the noise changes as a function of magnetic field for the spin-valve transistor, but no additional noise of magnetic origin is observed.

Fabrication technology for miniaturization of the spin-valve transistor

A new fabrication technology that allows miniaturization of the spin-valve transistor is presented. The spin-valve transistor consists of a spin-valve base (Pt 2 nm/NiFe 3 nm/Au 3.5 nm/Co 3 nm/Au 4 nm) sandwiched between a Si emitter and collector. With the use of a silicon-on-insulator wafer and vacuum metal bonding, spin-valve transistors down to a few tens of micron size are realized through conventional photolithography and etching processes. These spin-valve transistors show 275% magnetocurrent at 87 K and 170% at room temperature in small magnetic fields.

Temperature dependence of magnetocurrent of hot electrons in a spin-valve transistor

Spin-dependent transport of hot electrons across a spin valve has been studied as function of temperature using a spin-valve transistor with a soft Ni80Fe20/Au/Co spin-valve base. Spin-dependent scattering makes the collector current highly sensitive to small magnetic fields that change the magnetic state of the base. The magnetocurrent approaches 400% at 100K but decays to about 240% at room temperature. The reduction is attributed to mixing of the two spin channels due to spin-flip scattering of hot electrons by thermal spin waves.

A Highly Sensitive Spin-Valve Transistor

In this paper we present a spin-valve transistor made with a silicon on insulator wafer as emitter and a double sided polished Si wafer as collector. Using vacuum metal bonding we obtain a three terminal device in which a spin-valve layer is sandwiched between two Si wafers. We measure a 217% change in the collector current with magnetic field using a spin valve that shows only 0.5% resistance change in a current in plane measurement.