Magnetic Junctions Research Articles

Kondo peak splitting and Kondo dip in single molecular magnet junctions

Many factors containing bias, spin–orbit coupling, magnetic fields applied, and so on can strongly influence the Kondo effect, and one of the consequences is Kondo peak splitting (KPS). It is natural that KPS should also appear when another spin degree of freedom is involved. In this work we study the KPS effects of single molecular magnets (SMM) coupled with two metallic leads in low-temperature regime. It is found that the Kondo transport properties are strongly influenced by the exchange coupling and anisotropy of the magnetic core. By employing Green's function method in Hubbard operator representation, we give an analytical expression for local retarded Green's function of SMM and discussed its low-temperature transport properties. We find that the anisotropy term behaves as a magnetic field and the splitting behavior of exchange coupling is quite similar to the spin–orbit coupling. These splitting behaviors are explained by introducing inter-level or intra-level transitions, which account for the seven-peak splitting structure. Moreover, we find a Kondo dip at Fermi level under proper parameters. These Kondo peak splitting behaviors in SMM deepen our understanding to Kondo physics and should be observed in the future experiments.

Stimulated terahertz emission

The observation of the stimulated terahertz radiation in magnetic junctions at the room temperature resulting from the current injection of nonequilibrium spins is reported. Contacts between two ferromagnetic layers and between a ferromagnetic and an antiferromagnetic layer are investigated. As the current is increased, first, a narrow isolated radiation peak at terahertz frequencies is observed, which is analogous to the laser peak; then, the peak broadens and its intensity increases by more than an order of magnitude.

Charge Transport in Magnetic Semiconductor p-n Heterojunctions

Previously, the p-n-p bipolar magnetic junction transistor was demonstrated using a magnetic semiconductor InMnAs as the collector. A current gain β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> as high as 20 of the transistor is observed at 300 K. A negative magnetoamplification of -150% is obtained when the applied magnetic field is 8 T. In order to assess the gain mechanism for such transistors, we measured the minority carrier lifetime in a p-n InMnAs/InAs heterojunction diode. A minority carrier lifetime of 320 ns was obtained at room temperature. For the p-n-p MJT, a decrease in the emitter injection efficiency with the magnetic field is observed for the various base currents, which is attributed to the positive magnetoresistance of the p-type InMnAs. The emitter injection efficiency decreases with the magnetic field leading to the observed negative magnetic amplification.

Spin-Injection Stimulated Radiation of Terahertz Waves in Magnetic Junctions (“Twaser” Action)

We report on apparently the first observation of terahertz stimulated radiation in magnetic junctions due to nonequilibrium spins injected by current at room temperatures. The contact between ferromagnetic layers is investigated and so the contact between ferromagnetic and antiferromagnetic layers. The current flowing in the structure creates an inversed population of spin energy levels. After a placing of such a structure in a resonator positive feedback and stimulated emission appears. When current is growing, we observe the relatively narrow (in frequencies) peak - analogous to laser generation but at terahertz frequencies. For the higher currents we observe the picture of electromagnetic turbulence.

Spin-injection generators of terahertz waves based on metal magnetic structures

Fundamentally new devices for the generation of terahertz waves due to the effect of the spin-polarized current in ferromagnetic and antiferromagnetic metal structures (magnetic junctions) are considered. The spin injection and the sd exchange and its effect on the interaction with electromagnetic waves play the main role in such devices. The effects that are induced by the polarized current in the antiferromagnetic materials (exchange instability, induced magnetization, and spin-injection antiferromagnetic resonance) are discussed. The principles and effects of the generation of the terahertz electromagnetic oscillations are considered, and the characteristics of the experimental prototypes of the corresponding devices are analyzed.

Mo1643 Early Postoperative Outcomes of Magnetic Gastroesophageal Junction Reinforcement Compared to Laparoscopic Nissen Fundoplication

Mo1643 Early Postoperative Outcomes of Magnetic Gastroesophageal Junction Reinforcement Compared to Laparoscopic Nissen Fundoplication

Negative differential conductance and super-Poissonian shot noise in single-molecule magnet junctions.

Molecular spintroinic device based on a single-molecule magnet is one of the ultimate goals of semiconductor nanofabrication technologies. It is thus necessary to understand the electron transport properties of a single-molecule magnet junction. Here we study the negative differential conductance and super-Poissonian shot noise properties of electron transport through a single-molecule magnet weakly coupled to two electrodes with either one or both of them being ferromagnetic. We predict that the negative differential conductance and super-Poissonian shot noise, which can be tuned by a gate voltage, depend sensitively on the spin polarization of the source and drain electrodes. In particular, the shot noise in the negative differential conductance region can be enhanced or decreased originating from the different formation mechanisms of negative differential conductance. The effective competition between fast and slow transport channels is responsible for the observed negative differential conductance and super-Poissonian shot noise. In addition, we further discuss the skewness and kurtosis properties of transport current in the super-Poissonian shot noise regions. Our findings suggest a tunable negative differential conductance molecular device, and the predicted properties of high-order current cumulants are very interesting for a better understanding of electron transport through single-molecule magnet junctions.

Scalable and thermally robust perpendicular magnetic tunnel junctions for STT-MRAM

Thermal budget, stack thickness, and dipolar offset field control are crucial for seamless integration of perpendicular magnetic junctions (pMTJ) into semiconductor integrated circuits to build scalable spin-transfer-torque magnetoresistive random access memory. This paper is concerned with materials and process tuning to deliver thermally robust (400 °C, 30 min) and thin (i.e., fewer layers and integration-friendly) pMTJ utilizing Co/Pt-based bottom pinned layers. Interlayer roughness control is identified as a key enabler to achieve high thermal budgets. The dipolar offset fields of the developed film stacks at scaled dimensions are evaluated by micromagnetic simulations. This paper shows a path towards achieving sub-15 nm-thick pMTJ with tunneling magnetoresistance ratio higher than 150% after 30 min of thermal excursion at 400 °C.

Magnetotransport in La0.7Sr0.3MnO3/CuCr2O4/Fe3O4 magnetic junctions

We demonstrate distinct magnetic and resistive switching with junction magnetoresistance up to −6% in magnetic tunnel junctions with a CuCr2O4 barrier. Junction magnetoresistance is inversely related to barrier thickness and reveals a maximum at a finite applied bias that converges to zero bias at low temperatures for all barrier thicknesses. The non-monotonic bias dependence is attributed to a charge gap from the Fe3O4 electrode and possible spin filtering from the spin-split conduction band of the ferrimagnetic CuCr2O4 barrier.

Interlayer dependent polarity of magnetoresistance in graphene spin valves

We have studied the polarity of magnetoresistance (MR) in three types of magnetic junctions. While the NiFe/single layer graphene (SLG)/Co and NiFe/Al2O3/Co junctions showed spin valve signals with positive MR, the NiFe/Al2O3/SLG/Co junction revealed negative MR values.

Cotunneling spectroscopy and the properties of excited-state spin manifolds ofMn12single molecule magnets

We study charge transport through single molecule magnet (SMM) junctions in the cotunneling regime as a tool for investigating the properties of the excited-state manifolds of neutral ${\mathrm{Mn}}_{12}$ SMs. This study is motivated by a recent transport experiment [S. Kahle et al., Nano Lett. 12, 518 (2012)] that probed the details of the magnetic and electronic structure of ${\mathrm{Mn}}_{12}$ SMMs beyond the ground-state spin manifold. A giant spin Hamiltonian and master equation approach is used to explore theoretically the cotunneling transport through ${\mathrm{Mn}}_{12}$-Ac SMM junctions. We identify SMM transitions that can account for both the strong and weak features of the experimental differential conductance spectra. We find the experimental results to imply that the excited spin-state manifolds of the neutral SMM have either different anisotropy constants or different $g$ factors in comparison with its ground-state manifold. However, the latter scenario accounts best for the experimental data.

Deposition of cobalt atoms ontoAlq3films: A molecular dynamics study

The charge and spin injections into semiconductors are determined by the quality of the electrode/semiconductor interface. The latter is defined by the atomic penetration depth when growing metal electrodes on organic semiconductor thin films by the physical deposition method. Although the interfaces between top electrodes and a typical organic material, tris(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{3})$ thin films, have been intensively investigated by several groups, their results on the atomic penetration depth into ${\mathrm{Alq}}_{3}$ film diverged from each other. In this paper we study the deposition of cobalt atoms onto ${\mathrm{Alq}}_{3}$ thin films using the molecular dynamics method. The intermixing between Co and ${\mathrm{Alq}}_{3}$ is calculated to be limited to the first ${\mathrm{Alq}}_{3}$ layer for low Co initial kinetic energies, but spread to a few layers for high initial energies. The results of our calculation indicate that a proper deposition method with low injection energy helps to reduce the amount of intermixing at interfaces and improve the magnetoresistance of an organic-based magnetic junction.

Transient Magnetic Tunneling Mediated by a Molecular Bridge

This paper extends our recent theoretical study Kalvova et al. (J. Supercon. and Novel Mag. 26, 773 2013) of transient currents through molecular bridges to magnetic tunneling. We calculate the excess magnetization on a molecular bridge shunting the magnetic junction. The system is represented by two ferromagnetic electrodes bridged by a molecular size island with a few discrete electronic levels and a local Hubbard type correlation. The island is linked to the electrodes by tunneling junctions. One of the junctions is permanent, the coupling strength of the other one is assumed to undergo rapid changes modulating the connectivity of the system. The sudden switching events give rise to transient magnetic currents resulting in changes of magnetization at the island. They vary with the constant galvanic bias between the electrodes. Depending on the time scale of the switching series, the transients are dominated by the initial quantum correlations, or reach the kinetic stage controlled by a simple relaxation mechanism. We employ the nonequilibrium Green’s functions. The finite-time correlated initial conditions are taken into account using the partitioning-in-time method we developed previously Velický et al. (Phys. Rev. B 81, 235116 2010). The numerical solution is obtained solving the real-time Dy-son equation in the integro-differential form self-consistently. For long time asymptotic, a generalized master equation is matched to the initial stage solution.

Spin-injection stimulated emission of terahertz waves in magnetic junctions

The stimulated emission of terahertz radiation in magnetic junctions at room temperature resulting from the current injection of nonequilibrium spins is observed. Contacts between two ferromagnetic layers as well as between a ferromagnetic and an antiferromagnetic layer are investigated. The current produces the population inversion of spin energy levels. As the system is placed in a cavity, positive feedback and stimulated emission appear. The development of a dominant emission peak and onset of electromagnetic turbulence are observed.

Electrical control of valley and spin polarized current and tunneling magnetoresistance in a silicene-based magnetic tunnel junction

Abstract We investigate the electronic transport in a silicene-based ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal tunnel junction. The results show that the valley and spin transports are strongly dependent on local application of a vertical electric field and effective magnetization configurations of the ferromagnetic layers. In particular, it is found that the fully valley and spin polarized currents can be realized by tuning the external electric field. Furthermore, we also demonstrate that the tunneling magnetoresistance ratio in such a full magnetic junction of silicene is very sensitive to the electric field modulation.

Magnetoresistance in half-metallic/metallic ferromagnetic junction through silicon

We report spin transport through the silicon in novel magnetic junction with half metallic as free layer and metallic as pinned layer. We used La0.7Sr0.3MnO3 as free layer, FeCo as pinned layer and studied the magnetoresistance through silicon as spacer layer. We fabricated this magnetic tunnel junction using RF/DC sputtering technique over SrTiO3 substrate. Tunneling magnetoresistance (TMR) measurement for this junction at room temperature was found to be 1.1 %. At 2 K, we found a large magnetoresistance of 396 %. TMR found to be increased with decreasing temperature. The results are discussed.

Nanoscale Spin Seebeck Rectifier: Controlling Thermal Spin Transport across Insulating Magnetic Junctions with Localized Spin

The spin Seebeck effect is studied across a charge insulating magnetic junction, in which thermal-spin conjugate transport is assisted by the exchange interactions between the localized spin in the center and electrons in metallic leads. We show that, in contrast with bulk spin Seebeck effect, the figure of merit of such nanoscale thermal-spin conversion can be infinite, leading to the ideal Carnot efficiency in the linear response regime. We also find that in the nonlinear spin Seebeck transport regime, the device possesses the asymmetric and negative differential spin Seebeck effects. In the last, the situations with leaking electron tunneling are also discussed. This nanoscale thermal spin rectifier, by tuning the junction parameters, can act as a spin Seebeck diode, spin Seebeck transistor and spin Seebeck switch, which could have substantial implications for flexible thermal and information control in molecular spin caloritronics.

Spin Injection Mechanisms for Radiation and Detection of Terahertz Waves Based on Magnetic Nano Junction

Spin injection mechanisms for radiation and detection of terahertz waves based on magnetic nano junction are considered. High efficiency of terahertz radiation is theoretically explained and the method of the detection of this radiation is offered. The device of the spin injected terahertz radiators is presented.

Generation of terahertz waves by a current in magnetic junctions

The terahertz region of the electromagnetic spectrum (approximately 0.3–30 THz) is still insufficiently mastered primarily because of the absence of compact and controllable emitters (oscillators) and receivers (detectors) reliably operating in this range in a wide temperature range, including room temperature. The corresponding recent studies in this field, which were supported by the Russian Foundation for Basic Research, have been reviewed. New physical effects have been proposed and principles of the operation of terahertz devices based on these effects have been implemented. These effects refer to the physics of ferromagnetic and/or antiferromagnetic conducting layers assembled in micro- and nanostructures, which are called magnetic junctions. These effects are as follows: the formation of a quasiequilibrium distribution of current-injected electrons over the energy levels and the possibility of inverted population of levels, induction of the macroscopic magnetization by a spin-polarized current in an antiferromagnetic layer in the absence of external magnetic field, the appearance of current-induced contribution to antiferromagnetic resonance, and the experimental observation and study of the properties of terahertz radiation in ferromagnet-ferromagnet and ferromagnet-antiferromagnet junctions.

A Study of Coronal Holes Observed by SOHO/EIT and the Nobeyama Radioheliograph

Abstract Coronal holes (CHs) are areas of reduced emission in EUV and X-ray images that show bright patches of microwave enhancements (MEs) related to magnetic network junctions inside the CHs. A clear correlation between the CH size and the solar wind (SW) speed is well known, but we have less information about the relationship between MEs and other CH and SW properties. We studied the characteristics of 21 equatorial CHs associated with corotating interaction regions (CIRs) during 1996 to 2005. Our CHs were divided into two groups according to the intensity of the associated geomagnetic storms: Dst≤-100 nT (10 events) and &amp;gt; -100 nT (11 events). Using EUV 284 Å images obtained by SOHO/EIT and 17 GHz microwave images obtained by the Nobeyama Radioheliograph (NoRH), we found a linear correlation not only between the maximum SW speed and the area of EUV CH (r = 0.62), but also between the maximum SW speed and the area of the ME (r = 0.79). We also compared the EUVCH areas with and without an overlapping ME. The area of the CHs with an ME is better correlated with the SW speed (r = 0.71) than the area of those without an ME (r = 0.36). Therefore, the radio ME may play an important role in understanding the origin of SW.