K. S. Yoon

Performance of Co/Al2O3/NiFe magnetic tunnel junctions prepared by a two-step rf plasma oxidation method

A two-step rf plasma oxidation technique of an insulating layer has been performed to enhance electrical and structural properties of magnetic tunnel junction (MTJ) devices. Comparison was made by analyzing properties of the MTJ oxidized by conventional rf and two-step rf plasma oxidation methods. Experimentally observed results give improved surface imaging and sufficient oxygen contents of the insulating layer under the two-step oxidation method. In addition, electrical breakdown voltage and magnetoresistance of the MTJ were increased from 0.7 to 1.8 V and from 4.5% to 6.8%, respectively, correlated with improved structural information.

Ferromagnetic quantum dots formed by external laser irradiation

We present an efficient method for the formation of cobalt ferromagnetic quantum dots (FMQDs) by exposing a Nd: YAG laser (wavelength=355 nm) on ferromagnetic thin films grown on Si substrates. Primary emphasis is placed on the simple and direct fabrication of the FMQDs without performing any micro- or nanopattering process. Scanned laser irradiation of low power causes localized segregation of ultrathin as-deposited films by the laser-induced heat, resulting in the fabrication of FMQDs. Observations by scanning electron microscope and atomic force microscope clearly confirm highly uniform controllable nanoscale FMQDs in our case. The average size and density of the FMQDs were about 10 nm and 2×1011/cm2, respectively. Magnetic characteristics of the FMQDs were also measured by a superconducting quantum interference device at 5 K.

Efficient characterization of ultrathin AlOx insulating barriers in magnetic tunnel junctions fabricated by masked rf plasma oxidation technique

AlOx insulating barriers in magnetic tunnel junctions were prepared by a masked rf plasma oxidation process to reduce direct ion-bombardment effect on the barrier. In our method, the root-mean-square value of 2.1 A in the barrier and the magnetic resistance ratio up to 32% were observed. In addition, the surface plasmon resonance spectroscopy (SPRS) measurement was performed to efficiently determine optimum oxidation time, dielectric properties, and oxidation states inside thin barrier. The SPRS results revealed that the dielectric value and thickness of the optimum barrier were found to be 1.3576+i1.4488 and 16.3 A, respectively, with a thickness confirmation by a high resolving transmission electron microscope measurement.

Enhanced performance of CoFe/AlOx/CoFe magnetic tunnel junctions prepared by an off-axis rf remote plasma oxidation method

A new off-axis rf remote oxidation technique for the insulating barrier was carried out to enhance performance of CoFe/AlOx/CoFe magnetic tunneling junctions. The rf remote plasma method was designed to reduce self-bias voltage effect on the barrier during the rf oxidation process and to increase atomic oxygen concentration at high power operation, compared with a conventional rf plasma oxidation method. In addition, the off-axis geometry was chosen to give rise to high uniformity of the insulating layer. Experimentally observed root mean square of the barrier was decreased from 5 A to 1.5 A in our method. Electrical breakdown voltage and magnetoresistance of our MTJ devices increased from 0.8 V up to 1.4 V and from 20.2% up to 33.6%, respectively, correlated with the improved structural information of the barrier.

Surface plasmon resonance measurement of AlOx insulating barriers in magnetic tunnel junctions formed by remote rf plasma oxidation process

A surface plasmon resonance spectroscopy (SPRS) measurement was carried out to determine dielectric properties, oxidation states, and optimum thickness of AlOx insulating barrier in magnetic tunnel junctions. The insulating barriers were prepared as a function of oxidation time by using a remote rf plasma oxidation process. The experimental results revealed that the dielectric value and optimum thickness of the AlOx insulating barrier in our junctions were found to be 1.79+i1.77 and 11.26 A, respectively. For comparison, the magnetic tunneling junction was also fabricated using the same oxidation condition. The best magneto-resistance ratio of about 34% was observed at an optimum oxidation time of 70 s, as expected by the SPRS measurement.

Spin-Polarized Transport Phenomena in Double Magnetic Tunnel Junctions Caused by Ferromagnetic CoFe Nanoparticles

We present some unusual spin-polarized transport phenomena in asymmetric double barrier magnetic tunneling junctions (ADBMTJs) with CoFe/AlOx/ferromagnetic nanoparticle (FM-NPs)/AlOx/Ta structures. The conductance curves and the magnetoresistance ratio clearly oscillate with applied bias voltage, indicating the presence of Coulomb blockade effects due to isolated ferromagnetic nanoparticles in the parallel configuration in the ADBMTJ. The oscillation period is about 1.5 mV at 2 K.

Vertical Transport Control of Electrical Charge Carriers in Insulator/Oxide Semiconductor Hetero-structure

The technology for electrical current passing through an insulator thin-film between two electrodes is newly getting spotlights for substantial potentials toward advanced functional devices including a diode and a resistive switching device. However, depending on an electrode-limited conduction mechanisms of the conventional devices, a narrow processing window for a thickness of the insulator thin-film and an inability to control a magnitude and direction of the currents are challenges to overcome. Herein, we report a new approach to enable electrical charge carriers to pass stably through a relatively-thick insulator layer and to control a magnitude and polarity of the currents by applying an oxide semiconductor electrode in a metal/insulator/metal structure. We reveal that the electrical conduction in our devices follows a space charge-limited conduction mechanism which mainly depends on the charge carriers injected from contacts. Therefore, characteristics of the current including a current value and a rectification ratio of input signal are precisely controlled by electrical properties of the oxide semiconductor electrode. The unique current characteristics in metal/insulator/oxide semiconductor structures give extendable inspirations in electronic materials science, even a prominent solution for various technology areas of electronics.

In-plane direct current probing for spin orbit torque-driven effective fields in perpendicularly magnetized heavy metal/ferromagnet/oxide frames

Electrical manipulation of magnetization states has been the subject of intense focus as it is a long-standing goal in the emerging field of spintronics. In particular, torque generated by an in-plane current with a strong spin-orbit interaction shows promise for control of the adjacent ferromagnetic state in heavy-metal/ferromagnet/oxide frames. Thus, the ability to unlock precise spin orbit torque-driven effective fields represents one of the key approaches in this work. Here, we address an in-plane direct current measurement approach as a generic alternative tool to identify spin orbit torque-driven effective fields in a full polar angle range without adopting the commonly used harmonic analyses. Our experimental results exhibited a strongly polar angular dependency of the spin orbit torque-driven effective fields observed from Ta or W/CoFeM/MgO frames.

A new rf plasma oxidation method for the insulating AlOx barrier in magnetic tunneling junctions

Abstract An rf remote plasma oxidation technique to form an insulating barrier was carried out to enhance properties of CoFe/AlO x /CoFe magnetic tunneling junctions. The rf remote plasma method was found to reduce self-bias effect on the barrier during the rf oxidation process and to increase atomic oxygen concentration in a plasma state. Experimentally observed rms roughness of the barrier in our magnetic tunnel junction was decreased from 5 to 1.5 A. In addition, electrical breakdown voltage and magnetoresistance of our magnetic tunnel junction devices were increased from 0.8 V up to 1.2 V and from 7% up to 30%, respectively.