Ki-Su Lee

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.

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.

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.

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.

Stress polarity dependence of breakdown characteristics in magnetic tunnel junctions

Time-dependent dielectric breakdown (TDDB) measurements under constant voltage stress with positive and negative bias polarities are carried out for magnetic tunnel junctions (MTJs) with different oxidation status (under-, optimal, and overoxidation). We found that there is significant polarity dependence in the TDDB and speculated that the polarity dependence is due to both intrinsic and extrinsic origins. Optimally oxidized MTJs with positive bias on the top electrode show shorter times to breakdown (tBD’s) and lower barrier height than with negative bias, indicating that asymmetric band structure, in part, causes the polarity dependence. On the other hand, under- and overoxidized MTJs show much shorter tBD’s than optimally oxidized one and show a higher 1∕f noise power density for positive bias than for negative bias, indicating that the polarity dependence is also, in part, due to the interface states, which acts like precursors for the dielectric breakdown.

Enhancement of magnetoresistance with low interlayer coupling by insertion of a nano-oxide layer into a free magnetic layer

We studied the interlayer coupling strength (Hin) and GMR ratio of a spin-valve with the top free layer, separated by a nano-oxide layer (NOL). With the total thickness of the top free layer being fixed at 60A, the physical properties of the NOL spin-valve were studied with the thickness (tf) of the free layer under the inserted NOL and compared with those of the normal spin-valve with the same thickness as tf. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve at the optimal condition (tf=40A) after thermal annealing at T=250°C. The NOL spin-valve also shows a lower Hin than that of the optimal normal spin-valve with tf=40A, which is comparable to that of the normal spin-valve with tf=60A. This indicates that the enhancement of GMR, while keeping the Hin to be low, can be achieved by inserting a NOL into the top free layer.

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.