Antiparallel Magnetization Research Articles

Electrical detection of multiple ferromagnetic resonance modes in interlayer exchange coupled Fe20Ni80/Ru/Fe20Ni80 multilayers

We report the electrical detection of multiple ferromagnetic (FM) resonance modes in multilayers of Pt/Fe20Ni80/Ru/Fe20Ni80 using microwave spin pumping and the inverse spin Hall effect (ISHE) as a function of Ru layer thickness tRu. The electrically detected voltage signals were analyzed for the anti-parallel magnetization state as well as the “spin-flop” magnetization state of the two Fe20Ni80 layers. When the two FM layers are antiferromagnetically (AFM) coupled (tRu = 1 nm), both the optic and acoustic modes were observed. The amplitude of the acoustic mode voltage signals obtained increases with frequency. However, when the two magnetic layers are ferromagnetically coupled (tRu = 1.4 nm), only acoustic mode was observed whose voltage signals decreases as frequency increases. We demonstrate the tunability of the sign of the ISHE voltage in the AFM coupled system by controlling the polarization of the spin currents.

Effects of ferromagnetic and Schottky metal stripes on electronic transport in a GMR device

Based on transfer matrix method, the giant magnetoresistance (GMR) effect of 2-dimensional electron gas (2DEG) under magnetic and electrical potential is investigated. Such system can be obtained easily by depositing two ferromagnetic (FM) stripes and one schottky metal (SM) on the surface of semiconductor heterostructure. Our study shows a great difference in electrons tunneling through parallel (P) and antiparallel (AP) magnetization of FM stripes. The corresponding magnetoresistance ratio (MRR) depends strongly on the magnetic intensity of magnetic barrier (MB) and the height (U(x)) of electric barrier (EB). Furthermore, we also study the effect of SM stripe width (ds) on MRR for a given value of U(x). It indicates that the MRR of the device is sensitive to the ds. These results may offer an alternative to get a tunable GMR device by adjusting U(x) and width of the SM stripe.

Tunable magnetoresistance in spin-orbit coupled graphene junctions

Using the Landauer-B\"utikker formalism, we study the graphene magneto-transport in the presence of Rashba spin-orbit interaction (RSOI). We show that the angle resolved transmission probability in the proposed structures can be tuned by the RSOI strength. The transmission spectrum show Klein tunneling in the parallel (P) magnetization configuration which can be blocked by the RSOI. This effect is also observable for the anti-parallel (AP) magnetization configuration in different incident angle. The numerical results shows that the spin-polarized conductance strongly depends on the strength of the RSOI and can be generated by tuning the magnetic exchange field and RSOI strength. This spin-polarized conductance is a sensitive oscillatory function of the thickness of the RSO region. Because of the spin-flip effect, the junction shows a spin-valve effect with large and negative magnetoresistance (MR) and spin-magnetoresistance (SMR) in the presence of RSOI. When the RSOI is on, the frequency and amplitude of shot-noise and Fano factor's oscillations are also increased. These results can provide a way to extending the application of graphene-based junctions in spintronics.

Telegraphic switching signals by magnet tunnel junctions for neural spiking signals with high information capacity

Magnetic tunnel junctions (MTJs) operating in the superparamagnetic regime produce telegraphic signals that emulate neural spiking signals. Previous studies have characterized the random spiking signals produced by MTJs in terms of the percentage of time spent in the anti-parallel (AP) magnetization state (referred to as the “AP rate”) but ignore the switching rate of the MTJ. In this work, we demonstrate that with proper tuning of both an external bias field and a bias voltage, we can control the average dwell time in the AP-state and P-state pulses separately. Our data show that the AP rate can be tuned with bias voltages ranging from 310 mV to 460 mV and bias fields from −200 Oe to −230 Oe. The average dwell times in each state ranged from 225 ns to 285 μs and could be controlled separately. This suggests that neural spiking signals produced by MTJs can be decoded by both the spike rate and the spike count, which creates the possibility for increasing the information capacity in the rate coding scheme.

Tailoring the magnetic and magneto-transport properties of Pd/Co multilayers and pseudo-spin valve antidots

Magnetic multilayers and pseudo-spin valves (PSVs) based on Pd/Co were tailored to present well-defined antiparallel magnetization states while maintaining a high perpendicular magnetic anisotropy (PMA). The magneto-electrical properties were measured as a function of intermediate Cu spacers to determine the values that optimize magnetoresistance and the Hall effect. Magnetic multilayers were also deposited on flat porous alumina membranes, forming antidots. The flat antidots shows good PMA and PSVs were studied in these systems for the first time, showing improved magnetoresistance and the extraordinary Hall effect (EHE). Anomalous peaks were obtained in the EHE curves of the PSVs on both continuous films and 40 nm antidots, with four well-defined remnant Hall resistance states.

Calculated tunneling magnetoresistance ratio of FeRh/MgO/FeRh (001) magnetic tunnel junction

The electronic transport properties across the FeRh/MgO/FeRh (001) magnetic tunnel junction have been investigated under an external voltage bias employing the density functional theory in conduction with the non-equilibrium Green's formalism as implemented in TranSIESTA. The results show that by increasing the external voltage bias the current increases nonlinearly. It's claimed that the variation of the electronic coupling between the oxide and the electrodes with the external bias leads to this behavior. The large difference of the k||-resolved transmission at the Fermi level between the parallel (P) and antiparallel (AP) magnetizations of the electrodes can be explained by the k||-resolved density of states. It is also shown that the transmission is not dominated by the contribution at the “Γ” point like in Fe/MgO/Fe MTJ. This large difference between the P and AP transmissions led to a huge tunneling magnetoresistance ratio at zero bias.

Mechanically induced thermal breakdown in magnetic shuttle structures

A theory of a thermally induced single-electron ‘shuttling’ instability in a magnetic nano-mechanical device subject to an external magnetic field is presented in the Coulomb blockade regime of electron transport. The model magnetic shuttle device considered comprises a movable metallic grain suspended between two magnetic leads, which are kept at different temperatures and assumed to be fully spin-polarized with anti-parallel magnetizations. For a given temperature difference shuttling is found to occur for a region of external magnetic fields between a lower and an upper critical field strength, which separate the shuttling regime from normal small-amplitude ‘vibronic’ regimes. We find that (i) the upper critical magnetic field saturates to a constant value in the high temperature limit and that the shuttle instability domain expands with a decrease of the temperature; (ii) the lower critical magnetic field depends not only on the temperature-independent phenomenological friction coefficient used in the model but also on intrinsic friction (which vanishes in the high temperature limit) caused by magnetic exchange forces and electron tunneling between the quantum dot and the leads. The feasibility of using thermally driven magnetic shuttle systems to harvest thermal breakdown phenomena is discussed.

Quantitative Assessment of Xenon Exchange Kinetics with Cucurbit[6]uril in Physiological Saline.

Cucurbit[6]uril and xenon form supramolecular complexes that are of great potential for biosensing by NMR. This host-guest system acts alike a signaler in sensors facilitating the ultrasensitive detection of biomarkers by saturation transfer of chemically exchanging, hyperpolarized 129 Xe. Here, the exchange process is evaluated by NMR exchange spectroscopy utilizing the preparation of anti-parallel longitudinal magnetization with respect to free and host-bound xenon and the variation of xenon concentration. Evidence for dissociative as well as degenerate exchange mechanisms is revealed by a linear regression analysis of the determined exchange rates resulting in rate coefficients of 1131±11 s-1 (2390±70 s-1 ) and 108500±4900 M-1 s-1 (174200±13900 M-1 s-1 ), respectively, and an affinity constant of 289±8 M-1 (278±14 M-1 ) in physiological saline at 298 K (310 K). The results elucidate the supramolecular exchange and underpin the high efficacy for biosensing of this host-guest system. The approach is generally applicable to enhanced host-xenon exchange dynamics, yet slow on the NMR timescale, for quantitative kinetics and biosensing analyses.

A multi-state synthetic ferrimagnet with controllable switching near room temperature

Ferrite composites with temperature-induced magnetization reversal, and synthetic ferrimagnets and antiferromagnets have been of great interest to the scientific community due to their uncommon thermal properties and potential applications in magnetic storage, spintronic devices, and several other fields. One of the advantages of these structures is the strong antiferromagnetic coupling, which stabilizes the magnetization state and gives access to interesting static and dynamical magnetic behaviors. Some of their drawbacks lie in that it is difficult to induce temperature-induced magnetization reversal at room temperature in composites, and that the strong interaction makes it difficult to induce a parallel magnetization state (and thus a high magnetic moment). In this work, we study numerically the magnetization behaviour of a Cu(1 0 0)/Ni/Pt/[Co/Pt]4 synthetic ferrimagnet and show that is possible to revert the sign of its magnetization by varying the temperature in ranges around room temperature. We also show that the four parallel and antiparallel magnetization states are stable at temperatures up to 360 K, and demonstrate that it is possible to change deterministically between these states by increasing the temperature of the device and/or applying a magnetic field, showcasing simultaneous non-hysteretic and hysteretic switching processes induced by temperature. Thus, this structure opens the possibility to have reconfigurable magnetic devices with multiple purposes based on the nature of the different switching events and the interplay between them.

Arbitrary helicity control of circularly polarized light from lateral-type spin-polarized light-emitting diodes at room temperature

We demonstrate arbitrary helicity control of circularly polarized light (CPL) emitted at room temperature from the cleaved side facet of a lateral-type spin-polarized light-emitting diode (spin-LED) with two ferromagnetic electrodes in an antiparallel magnetization configuration. Driving alternate currents through the two electrodes results in polarization switching of CPL with frequencies up to 100 kHz. Furthermore, tuning the current density ratio in the two electrodes enables manipulation of the degree of circular polarization. These results demonstrate arbitrary electrical control of polarization with high speed, which is required for the practical use of lateral-type spin-LEDs as monolithic CPL light sources.

Spin polarized current in a quantum dot connected to a spin-orbit interacting Fermi sea

In this study, we investigate the transport properties of a device comprising a quantum dot (QD) connected to a Rashba spin-orbit interacting nanoribbon, which is spontaneously magnetically polarized or deposited on a ferromagnetic substrate. The QD is as well connected to two other lateral contacts along which the electronic current circulates. One contact comprising the source of electrons is ferromagnetic, whereas the other contact comprising , the drain, could be a non-magnetic or a ferromagnetic metal with anti-parallel magnetization relative to the source contact polarization. The connection of the QD local state to the Fermi sea under the effect of spin-orbit coupling (SOC) creates a spin-flip mechanism that affects the electronic spins circulating along the device. This process is tuned by the QD gate voltage and it is strongly enhanced when the Fermi energy of the system is in the neighborhood of a Van-Hove singularity of the SOC Fermi sea band. The spin-flip of the incident polarized current can be controlled by manipulating the gate potential of the QD. We show that this system possesses all the requirements necessary to operate as a spin transistor.Due to the local character of the QD level, the Coulomb interaction plays an important role. It enhances the parameter region within which the spin transistor can operate. The effects of the intra-dot Coulomb interaction are analyzed within the context of the alloy analogy Hubbard III approximation, which provides an adequate understanding of the Coulomb blockade transport properties above the Kondo temperature.

Spin-polarized transport properties of a pyridinium-based molecular spintronics device

By applying a first-principles approach based on non-equilibrium Green's functions combined with density functional theory, the transport properties of a pyridinium-based “radical-π-radical” molecular spintronics device are investigated. The obvious negative differential resistance (NDR) and spin current polarization (SCP) effect, and abnormal magnetoresistance (MR) are obtained. Orbital reconstruction is responsible for novel transport properties such as that the MR increases with bias and then decreases and that the NDR being present for both parallel and antiparallel magnetization configurations, which may have future applications in the field of molecular spintronics.

Synthetic Antiferromagnetic Spintronics.

Spintronic and nanomagnetic devices often derive their functionality from layers of different materials and the interfaces between them. This is especially true for synthetic antiferromagnets - two or more ferromagnetic layers that are separated by metallic spacers or tunnel barriers and which have antiparallel magnetizations. Here, we discuss the new opportunities that arise from synthetic antiferromagnets, as compared to crystal antiferromagnets or ferromagnets.

Large Magnetoresistance in Fe3O4/4,4′-Bipyridine/Fe3O4 Organic Magnetic Tunnel Junctions

Organic magnetic tunnel junctions (OMTJs) are promising systems thanks to their chemically tunable electronic property, long spin lifetime and easy functionalizations. Here, the spin-dependent electronic transport properties in Fe3O4/4,4’-bipyridine/Fe3O4 OMTJs are investigated by first-principles quantum transport calculations. Since the transport properties of junctions are sensitive to the device details, two types of terminations of Fe3O4 electrodes are considered. The device with tetrahedral Fe termination shows an anomalous negative tunnel magnetoresistance (TMR), i.e., which has a higher and lower junction resistance in the parallel and anti-parallel magnetization configurations, respectively. When the contact termination is octahedral Fe, a large positive TMR of 180% appears. The difference in TMR sign of two OMTJs originates from the electrons transmission mediated by frontier molecular levels coupled differently to Fe d states. Furthermore, TMR can be effectively controlled by applied electrical...

Controllable Interfacial Coupling Effects on the Magnetic Dynamic Properties of Perpendicular [Co/Ni]5/Cu/TbCo Composite Thin Films.

Dynamic magnetic properties in perpendicularly exchange-coupled [Co/Ni]5/Cu (tCu = 0-2 nm)/TbCo structures show strong dependences on the interfacial antiferromagnetic strength Jex, which is controlled by the Cu interlayer thickness. The precession frequency f and effective damping constant αeff of a [Co/Ni]5 multilayer differ distinctly for parallel (P)and antiparallel (AP) magnetization orientation states. For samples with a thin tCu, f of the AP state is apparently higher, whereas αeff is lower than that in the P state, owing to the unidirectional exchange bias effect (HEB) from the TbCo layer. The differences in f and αeff between the two states gradually decrease with increasing tCu. By using a uniform precession model including an additional HEB term, the field-dependent frequency curves can be well-fitted, and the fitted HEB value is in good agreement with the experimental data. Moreover, the saturation damping constant α0 displays a nearly linear correlation with Jex. It decreases significantly with Jex and eventually approaches a constant value of 0.027 at tCu = 2 nm where Jex vanishes. These results provide a better understanding and effective control of magnetization dynamics in exchange-coupled composite structures for spintronic applications.

Voltage induced artificial ferromagnetic-antiferromagnetic ordering in synthetic multiferroics

This paper presents numerical and experimental data for dipole-dipole coupled Ni nanodots on a piezoelectric [Pb(Mg1/3Nb2/3)O3]0.68[PbTiO3]0.32 substrate. Simulation results show that the dipole coupling produces artificial ferromagnetic (parallel magnetization alignment in the nanodot arrays) behavior that can be modified to artificial antiferromagnetic behavior with an applied voltage. Experimental results show the trends in Mr and Hc predicted by the model, but discrepancies arise due to geometric defects present in the fabricated samples. Geometric defects are introduced into the Ni nanodot models, thus dramatically improving the correlation between experiments and analysis. This work shows, through numerical simulations, that artificial multiferroic nanostructures can be designed to produce switching from parallel (artificial ferromagnetic) to antiparallel (artificial antiferromagnetic) magnetization ordering by leveraging dipole coupling with voltage induced changes in magnetic anisotropy.

Current control of time-averaged magnetization in superparamagnetic tunnel junctions

This work investigates spin transfer torque control of time-averaged magnetization in a small 20 nm × 60 nm nanomagnet with a low thermal stability factor, Δ ∼ 11. Here, the nanomagnet is a part of a magnetic tunnel junction and fluctuates between parallel and anti-parallel magnetization states with respect to the magnetization of the reference layer generating a telegraph signal in the current versus time measurements. The response of the nanomagnet to an external field is first analyzed to characterize the magnetic properties. We then show that the time-averaged magnetization in the telegraph signal can be fully controlled between +1 and −1 by voltage over a small range of 0.25 V. NIST Statistical Test Suite analysis is performed for testing true randomness of the telegraph signal that the device generates when operated at near critical current values for spin transfer torque. Utilizing the probabilistic nature of the telegraph signal generated at two different voltages, a prototype demonstration is shown for multiplication of two numbers using an artificial AND logic gate.

A spin current rectifier

Current modulation and rectification is an important subject of electronics as well as spintronics. In this paper, an efficient rectifying mesoscopic device is introduced. The device is a two terminal device on the 2D plane of electron gas. The lateral contacts are half-metal ferromagnetic with antiparallel magnetizations and the central channel region is taken as ferromagnetic or normal in the presence of an applied magnetic field. The device functionality is based on the modification of spin-current by tuning the strength of the magnetic field or equivalently by the exchange coupling of the channel to the substrate. The result is that the (spin-) current depends on the polarity of the bias voltage. Converting an alternating bias voltage to direct current is the main achievement of this model device with an additional profit of rectified spin-current. We analyze the results in terms of the spin-dependent barrier in the channel. Detecting the strength of the magnetic field by spin polarization is also suggested.

Spin transport through a junction entirely consisting of molecules from first principles

Using first-principles calculations based on density functional theory combined with the nonequilibrium Green's function formalism, we studied the spin transport through a single molecular junction which consists of a single 1,4-benzenedithiolate (BDT) molecule and two ferromagnetic electrodes [(Ge5)Fe]∞. A large magnetoresistance ratio (MR) of 21100% was found in the [(Ge5)Fe]∞-BDT-[(Ge5)Fe]∞ molecular junction at small bias voltage, and the MR value decreased with the increase in the applied bias voltage. For the parallel magnetization configuration, the molecular junction showed outstanding spin injection effects. Negative differential resistance was observed for the antiparallel magnetization configuration. Spin dependent transmission spectra at different bias voltages were used to explain the calculated results.

Electron tunneling through double magnetic barriers in Weyl semimetals

We theoretically investigate the transport in a magnetic/normal/magetic hybrid structure on the surface of a Weyl semimetal. We find a directional-dependent tunneling which is sensitive to the magnetic field configuration and the electric gate voltage. The momentum filtering behavior becomes more significant for two-delta-function-shaped magnetic barriers. There are many Fabry-Pérot resonances in the transmission determined by the distance between the two magnetic barriers. The combined effects of the magnetic field and the electrostatic potential can enhance the difference in the transmission between the parallel and antiparallel magnetization configurations, and consequently lead to a giant magnetoresistance.