arXiv subjects

Jae Hyun Kwon

Publications and source records attributed to Jae Hyun Kwon.

15 recordsLinked to original sources

arXiv2017

Oscillatory spin-orbit torque switching induced by field-like torques

Deterministic magnetization switching using spin-orbit torque (SOT) has recently emerged as an efficient means to electrically control the magnetic state of ultrathin magnets. The SOT switching still lacks in oscillatory switching characteristics over time, therefore, it is limited to bipolar operation where a change in polarity of the applied current or field is required for bistable switching. The coherent rotation based oscillatory switching schemes cannot be applied to SOT because the SOT switching occurs through expansion of magnetic domains. Here, we experimentally achieve oscillatory switching in incoherent SOT process by controlling domain wall dynamics. We find that a large field-like component can dynamically influence the domain wall chirality which determines the direction of SOT switching. Consequently, under nanosecond current pulses, the magnetization switches alternatively between the two stable states. By utilizing this oscillatory switching behavior we demonstrate a unipolar deterministic SOT switching scheme by controlling the current pulse duration.

cond-mat.mtrl-sci↗

arXiv2017

Anomalous spin-orbit torque switching due to field-like torque-assisted domain wall reflection

Spin-orbit torques (SOT) allow the electrical control of magnetic states. Current-induced SOT switching of the perpendicular magnetization is of particular technological importance. The SOT consists of damping-like and field-like torques so that the efficient SOT switching requires to understand combined effects of the two torque-components. Previous quasi-static measurements have reported an increased switching probability with the width of current pulses, as predicted with considering the damping-like torque only. Here we report a decreased switching probability at longer pulse-widths, based on time-resolved measurements. Micromagnetic analysis reveals that this anomalous SOT switching results from domain wall reflections at sample edges. The domain wall reflection is found to strongly depend on the field-like torque and its relative sign to the damping-like torque. Our result demonstrates a key role of the field-like torque in the deterministic SOT switching and notifies the importance of sign correlation of the two torque-components, which may shed light on the SOT switching mechanism.

cond-mat.mtrl-sci↗

arXiv2016

Giant nonreciprocal emission of spin waves in Ta/Py bilayers

Spin waves are propagating disturbances in the magnetization of magnetic materials. One of their interesting properties is the nonreciprocity, exhibiting that their amplitude depends on the magnetization direction. Nonreciprocity in spin waves is of great interest in both fundamental science and applications, as it offers an extra knob to control the flow of waves for the technological fields of logics and switch applications. We show a high nonreciprocity in spin waves from Ta/Py bilayer systems with out-of-plane magnetic fields. The nonreciprocity depends on the thickness of Ta underlayer which is found to induce an interfacial anisotropy. The origin of observed high nonreciprocity is twofold; different polarities of the in-plane magnetization due to different angles of canted out-of-plane anisotropy and the spin pumping effect at the Ta/Py interface. Our findings provide an opportunity to engineer highly efficient nonreciprocal spin wave based applications such as nonreciprocal microwave devices, magnonic logic gates, and information transports.

cond-mat.mtrl-sci↗

arXiv2014

Determination of intrinsic spin Hall angle in Pt

The spin Hall angle in Pt is evaluated in Pt/NiFe bilayers by spin torque ferromagnetic resonance (ST-FMR) measurements, and is found to increase with increasing the NiFe thickness. To extract the intrinsic spin Hall angle in Pt by estimating the total spin current injected into NiFe from Pt, the NiFe thickness dependent measurements are performed and the spin diffusion in the NiFe layer is taken into account. The intrinsic spin Hall angle of Pt is determined to be 0.068 at room temperature, and is found to be almost constant in the temperature range 13 - 300 K.

cond-mat.mtrl-sci↗

arXiv2014

Investigation of the temperature-dependence of ferromagnetic resonance and spin waves in Co2FeAl0.5Si0.5

Co2FeAl0.5Si0.5 (CFAS) is a Heusler compound that is of interest for spintronics applications, due to its high spin polarization and relatively low Gilbert damping constant. In this study, the behavior of ferromagnetic resonance as a function of temperature was investigated in CFAS, yielding a decreasing trend of damping constant as the temperature was increased from 13 to 300 K. Furthermore, we studied spin waves in CFAS using both frequency domain and time domain techniques, obtaining group velocities and attenuation lengths as high as 26 km/s and 23.3 um, respectively, at room temperature.

cond-mat.mtrl-sci↗

arXiv2013

Nonreciprocity engineering in magnetostatic spin waves

Magnetostatic surface spin waves (MSSW) excited from a coplanar waveguide antenna travel in different directions with different amplitudes. This effect, called nonreciprocity of MSSW, has been investigated by micromagnetic simulations. The ratio of amplitude of two counter propagating spin waves, the nonreciprocity parameter κ, is obtained for different ferromagnetic materials, such as NiFe (Py), CoFeAl, yttrium iron garnet (YIG), and GaMnAs. A device schematic has been proposed in which κ can be tuned to a large value by varying simple geometrical parameters of the device.

cond-mat.mtrl-sci↗

arXiv2013

Spin wave nonreciprocity for logic device applications

The utilization of spin waves as eigenmodes of the magnetization dynamics for information processing and communication has been widely explored recently due to its high operational speed with low power consumption and possible applications for quantum computations. Previous proposals of spin wave Mach-Zehnder devices were based on the spin wave phase, a delicate entity which can be easily disrupted. Here, we propose a complete logic system based on the spin wave amplitude utilizing the nonreciprocal spin wave behavior excited by microstrip antennas. The experimental data reveal that the nonreciprocity of magnetostatic surface spin wave can be tuned by the bias magnetic field. Furthermore, engineering of the device structure could result in a high nonreciprocity factor for spin wave logic applications.

cond-mat.mtrl-sci↗

arXiv2013

Characterization of magnetostatic surface spin waves in magnetic thin films: evaluation for microelectronic applications

The authors have investigated the possibility of utilizing spin waves for inter- and intra-chip communications, and as logic elements using both simulations and experimental techniques. Through simulations it has been shown that the decay lengths of magnetostatic spin waves are affected most by the damping parameter, and least by the exchange stiffness constant. The damping and dispersion properties of spin waves limit the attenuation length to several tens of microns. Thus, we have ruled out the possibility of inter-chip communications via spin waves. Experimental techniques for the extraction of the dispersion relationship have also been demonstrated, along with experimental demonstrations of spin wave interference for amplitude modulation. The effectiveness of spin wave modulation through interference, along with the capability of determining the spin wave dispersion relationships electrically during manufacturing and testing phase of chip production may pave the way for using spin waves in analog computing wherein the circuitry required for performing similar functionality becomes prohibitive.

cond-mat.mtrl-sci↗

arXiv2012

Detection of domain wall eigenfrequency in infinity-shaped magnetic nanostructures

The dynamics of a magnetic infinity-shaped nanostructure has been experimentally studied by two different techniques such as the sinusoidal resonance excitation and the damped short pulse excitation to measure the eigenfrequency of domain walls. Direct observation of the magnetic domain wall nucleation has been measured in the frequency domain. Electrical measurements of the domain wall dynamics in the frequency domain reveal the existence of multi-eigenmodes for large excitation amplitudes. The time-resolved measurements show that the frequency of the damped gyration is similar to that of the frequency domain and coexistence of spin wave excitations.

cond-mat.mtrl-sci↗

arXiv2012

Spin waves interference from rising and falling edges of electrical pulses

The authors have investigated the effect of the electrical pulse width of input excitations on the generated spin waves in a NiFe strip using pulse inductive time domain measurements. The authors have shown that the spin waves resulting from the rising- and the falling-edges of input excitation pulses interfere either constructively or destructively, and have provided conditions for obtaining spin wave packets with maximum intensity at different bias conditions.

cond-mat.mtrl-sci↗

arXiv2012

Attenuation characteristics of spin pumping signal due to travelling spin waves

The authors have investigated the contribution of the surface spin waves to spin pumping. A Pt/NiFe bilayer has been used for measuring spin waves and spin pumping signals simultaneously. The theoretical framework of spin pumping resulting from ferromagnetic resonance has been extended to incorporate spin pumping due to spin waves. Equations for the effective area of spin pumping due to spin waves have been derived. The amplitude of the spin pumping signal resulting from travelling waves is shown to decrease more rapidly with precession frequency than that resulting from standing waves and show good agreement with the experimental data.

cond-mat.mtrl-sci↗

arXiv2012

Interference-mediated intensity modulation of spin waves

The modulation of propagating spin-wave amplitude in Ni81Fe19 (Py) films, resulting from constructive and destructive interference of spin wave, has been demonstrated. Spin waves were excited and detected inductively using pulse inductive time domain measurements. Two electrical impulses were used for launching two interfering Gaussian spin wave packets in Py films. The applied bias magnetic field or the separation between two pulses was used for tuning the amplitude of the resulting spin wave packets. This may thus be useful for spin wave based low-power information transfer and processing.

cond-mat.mtrl-sci↗

arXiv2011

Electrical measurement of antivortex wall eigenfrequency

The dynamics of a ferromagnetic antivortex wall has been experimentally studied in a magnetic nanostructure. Two different techniques have been used to independently measure the eigenfrequency of an antivortex wall such as the resonance excitation by sinusoidal microwave and the damped resonance excitation induced by short voltage pulses. Direct observation of antivortex wall nucleation has been measured in the frequency domain for the first time. Electrical measurements of the antivortex dynamics in frequency domain reveal the existence of multi-eigenmodes as well as nonlinear behaviors for large excitation amplitudes. The time resolved measurements of the antivortex wall show that the frequency of the damped gyration is similar to that of frequency domain and coexistence of spin wave excitations.

cond-mat.mtrl-sci↗

arXiv2011

Tunneling characteristics of graphene

Negative differential conductance and tunneling characteristics of two-terminal graphene devices are observed before and after electric breakdown, respectively. The former is caused by the strong scattering under a high E-field, and the latter is due to the appearance of a tunneling barrier in graphene channel induced by a structural transformation from crystalline graphene to disordered graphene because of the breakdown. Using Raman spectroscopy and imaging, the presence of non-uniform disordered graphene is confirmed. A memory switching effect of 100000% ON/OFF ratio is demonstrated in the tunneling regime which can be employed in various applications.

cond-mat.mes-hall↗

arXiv2011

Ambipolar bistable switching effect of graphene

Reproducible current hysteresis is observed in graphene with a back gate structure in a two-terminal configuration. By applying a back gate bias to tune the Fermi level, an opposite sequence of switching with the different charge carriers, holes and electrons, is found. The charging and discharging effect is proposed to explain this ambipolar bistable hysteretic switching. To confirm this hypothesis, one-level transport model simulations including charging effect are performed and the results are consistent with our experimental data. Methods of improving the ON/OFF ratio of graphene resistive switching are suggested.

cond-mat.mes-hall↗