Youngman Jang

Low Energy Magnetic Domain Wall Logic in Short, Narrow, Ferromagnetic Wires

We present circuit simulation results of an implementation of universal logic that operates at low switching energy. Information is stored in the position of a single domain wall in a thin, short ferromagnetic wire. The gate is switched by current-driven domain wall motion, and information is read out using a magnetic tunnel junction. The inputs and outputs of the device are currents controlled by voltage clocks, making it compatible with CMOS. Using devices that operate at 100-1 mV, we simulate a shift register circuit and a full-adder circuit. The simulations show that the magnetic logic gates can operate at lower switching energy than CMOS electronics.

Magnetic field sensing scheme using CoFeB∕MgO∕CoFeB tunneling junction with superparamagnetic CoFeB layer

The authors investigated the tunneling magnetoresistance (TMR) of CoFeB∕MgO∕CoFeB tunnel junctions by varying the thickness (tCoFeB) of the top CoFeB layer. Linear and hysteresis-free switching was observed in junctions with tCoFeB⩽10A, while normal tunneling behavior occurred for tCoFeB>10A. The field sensitivity and the sensing field range were found to be controlled by varying the thickness of the sensing layer. This finding means that the magnetic tunneling junction (MTJ) provides a scheme for magnetic field sensing, which has a simple sensor design and low power consumption. The magnetic properties of the sensing layer with tCoFeB⩽10A were found to show the characteristics of superparamagnetism. Although the detailed mechanism of TMR in MTJs with a superparamagnetic layer is not fully understood at present, this phenomenon is observed repeatedly. Therefore, this sensing scheme would be an alternative method for overcoming the problems in magnetic sensors with a crossed magnetization pattern.

Variation in the properties of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing

Variation in the quality of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing was investigated using x-ray photoemission spectroscopy. The formation of B oxide and the reduction of Fe oxide at the bottom interface after thermal annealing near Ta=300°C were found to enhance the tunneling magnetoresistance ratio significantly. At the same time, an asymmetry of the conductance (dV∕dI) in the bias polarity and a local minimum of conductance in a positive bias state were measured which were attributed to the presence of a minority state at the bottom interface. The authors believe that the existence of the Bloch state was also responsible for the failure of the application of the Brinkman-Dynes-Rowell or Simmons models to the CoFeB∕MgO∕CoFeB junction.

Formation and structure of 360 and 540 degree domain walls in thin magnetic stripes

360°, 540°, and other complex transverse domain walls have been created in narrow cobalt wires connected to injection pads by cycling a magnetic field perpendicular to the wire length. The composite walls, formed by impingement of 180° transverse walls of alternating chirality, are stable over a wide field range. The structure of the walls observed at remanence by scanning electron microscopy with polarization analysis and by magnetic force microscopy is in good quantitative agreement with the prediction of micromagnetic simulations.

Work Function Modification of Indium–Tin Oxide by Surface Plasma Treatments Using Different Gases

We report on the effects of surface treatment with N2, O2, and N2O plasmas on the work function of indium–tin oxide (ITO). UV photoelectron spectroscopy (UPS) showed that the work function on the ITO surface treated with N2O plasma increased more than that on the samples treated with N2 or O2 plasma. X-ray photoelectron spectroscopy (XPS) showed that the intensity of the O–O bonding peak at 532.3 eV markedly increased owing to the adsorption of O- ions on the ITO surface from breaking bonds in N2O gas by the plasma. The dipole layer formed by O- ions on the ITO surface increases the work function of ITO. Accordingly, N2O plasma treatment leads to a reduction of the potential barrier between the Fermi level of ITO and the highest occupied molecular orbital (HOMO) level of an organic layer when ITO is used as an anode for organic light-emitting devices (OLEDs) and related devices. Therefore, N2O plasma treatment enhances the hole-injection properties from the ITO thin film to the organic layer.

Current-driven domain wall motion in heterostructured ferromagnetic nanowires

Micromagnetic modeling shows that the placement of non-magnetic conductive pads on a ferromagnetic wire affects the current-induced velocity of a domain wall (DW) in the wire and can act as a DW chirality filter. The pads shunt the current, causing a non-uniform spin current distribution inside the ferromagnetic wire and an Oersted field transverse to the wire. This suppresses Walker breakdown allowing higher current densities to be imposed before breakdown occurs. The transverse Oersted field pins the DW under some regimes of current density and pad geometry, selectively allowing transmission of DWs of only one chirality.

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.

Effect of Fe–O distance on magnetocrystalline anisotropy energy at the Fe/MgO(001) interface

We report first-principles calculations on the magnetocrystalline anisotropy energy (MAE) of an Fe monolayer sandwiched by MgO. We found that by increasing the interlayer distance between Fe and O by about 8% from its equilibrium value, the perpendicular interfacial magnetic anisotropy can be enhanced as high as 2.75 erg/cm2, which is three times larger than that at the equilibrium distance. The analysis of MAE based on the second-order interactions of the spin-orbit coupling shows that the energy position of the majority-spin dz2 orbital is of central importance in determining MAE. Our results suggest that increasing the Fe–O distance in the Fe/MgO system is an important material-design direction for high-performance magnetic memories.

Characteristics of domain wall pinning and depinning in a three-terminal magnetic Y-junction

The characteristics of domain wall (DW) pinning and propagation in a three-terminal magnetic Y-junction were investigated, where the junction consisted of two input and one output wires. The output switching depends strongly on the junction angle (α). Junctions with high angles of α>9.5° lead to DW pinning at the junction, whereas junctions with low angles of α<9.5° have no DW pinning effect. At the critical angle of α = 9.5°, the Y-junction showed a multimode DW propagation, which was ascribed to a moderate transverse field effect.