Antiparallel Magnetization Research Articles

Chirality-induced tunneling asymmetry at a semiconductor interface

The chiral tunneling asymmetry for the transmission through a semiconductor interface with spin-orbit coupling (SOC) and antiparallel magnetization is presented from the perspective of the transfer Hamiltonian approach. We explicitly show that the expression for the tunneling rate contains terms which are odd in the momentum along the interface thus contributing to the skew tunneling and the transverse current along the interface. These terms contain the scalar chirality that is the mixed vector product of the effective SOC and exchange fields acting on the tunneling electron or hole. The crucial role of the evanescent states in the tunneling asymmetry is revealed. It is shown that the chiral Hall effect also results in a spin injection across the interface. We finally come up with a more general view on the chiral tunneling as a universal phenomenon expected in various systems.

Manipulation of Coupling and Magnon Transport in Magnetic Metal-Insulator Hybrid Structures

Ferromagnetic metals and insulators are widely used for generation, control and detection of magnon spin signals. Most magnonic structures are based primarily on either magnetic insulators or ferromagnetic metals, while heterostructures integrating both of them are less explored. Here, by introducing a Pt/yttrium iron garnet (YIG)/permalloy (Py) hybrid structure grown on Si substrate, we studied the magnetic coupling and magnon transmission across the interface of the two magnetic layers. We found that within this structure, Py and YIG exhibit an antiferromagnetic coupling field as strong as 150 mT, as evidenced by both the vibrating-sample magnetometry and polarized neutron reflectometry measurements. By controlling individual layer thicknesses and external fields, we realize parallel and antiparallel magnetization configurations, which are further utilized to control the magnon current transmission. We show that a magnon spin valve with an ON/OFF ratio of ~130% can be realized out of this multilayer structure at room temperature through both spin pumping and spin Seebeck effect experiments. Thanks to the efficient control of magnon current and the compatibility with Si technology, the Pt/YIG/Py hybrid structure could potentially find applications in magnon-based logic and memory devices.

High resonance frequencies induced by in-plane antiparallel magnetization in NiFe/FeMn bilayer

Continuous exchange biased bilayers with in-plane antiparallel magnetization were prepared by a pulsed-current, fast-annealing method. The exchange-bias field and ferromagnetic resonance frequency of the bilayers, annealed with different voltages, were obtained using a vibrating sample magnetometer and vector network analyzer, respectively. All annealed bilayers had two exchange-bias fields and two resonance frequencies. The exchange-bias field decreased or even reversed direction, while the resonance frequency increased after annealing. We argue that the transition zone between antiparallel magnetizations had a greater number of rotatable antiferromagnetic spins, and thus, greater rotatable anisotropy, resulting in higher resonance frequencies in the annealed film. Moreover, for samples annealed at different voltages, the narrow area had a greater proportion of rotatable antiferromagnetic spins with a low exchange-bias field and high resonance frequency in comparison with the broad area. A model was proposed to describe these phenomena, which was supported by ion-etching tests.

Spin-polaronic effects in electric shuttling in a single molecule transistor with magnetic leads

Current-voltage characteristics of a spintromechanical device, in which spin-polarized electrons tunnel between magnetic leads with anti-parallel magnetization through a single level movable quantum dot, are calculated. New exchange- and electromechanical coupling-induced (spin-polaronic) effects that determine strongly nonlinear current-voltage characteristics were found. In the low-voltage regime of electron transport the voltage-dependent and exchange field-induced displacement of quantum dot towards the source electrode leads to nonmonotonic behavior of differential conductance that demonstrates the lifting of spin-polaronic effects by electric field. At high voltages the onset of electron shuttling results in the drop of current and negative differential conductance, caused by mechanically-induced increase of tunnel resistivities and exchange field-induced suppression of spin-flips in magnetic field. The dependence of these predicted spin effects on the oscillations frequency of the dot and the strength of electron-electron correlations is discussed.

Submicron-scale light modulation device driven by current-induced domain wall motion for electro-holography

A submicron-scale magneto-optical light modulation device driven by current-induced domain wall motion (CIDWM) was fabricated for an electro-holographic display application. The device consists of a Gd–Fe nanowire for light modulation and two Co/Pd hard magnets (HMs) with different coercivities. We succeeded in fabricating two HMs with different coercivities from a Co/Pd film by simply changing their length and realizing an anti-parallel magnetization direction configuration for the HMs, which is very important for initial domain nucleation and the CIDWM. We confirmed successful domain wall motion and light modulation when a pulse current of ±0.8 mA was injected into the device, whose size was as small as 0.5 × 2 μm2. The current value was lower than that required for the previously reported light modulation device driven by spin-transfer switching.

Magnetization dynamics in synthetic antiferromagnets: Role of dynamical energy and mutual spin pumping

We investigate magnetization dynamics in asymmetric interlayer exchange coupled Py/Ru/Py trilayers using both vector network analyzer-based and electrically detected ferromagnetic resonance techniques. Two different ferromagnetic resonance modes, in-phase and out-of-phase, are observed across all three regimes of the static magnetization configurations, through antiparallel alignment at low fields, the spin-flop transition at intermediate fields, and parallel alignment at high fields. The nonmonotonic behavior of the modes as a function of the external field is explained in detail by analyzing the interlayer exchange and Zeeman energies and is found to be solely governed by the interplay of their dynamical components. In addition, the linewidths of both modes were determined across the three regimes and the different behaviors of the linewidths versus external magnetic field are attributed to mutual spin pumping induced in the samples. Interestingly, the difference between the linewidths of the out-of-phase and in-phase modes decreases at the spin-flop transition and is reversed between the antiparallel and parallel aligned magnetization states.

Bias-Dependent Magnetoresistance Device Based on a Magnetically Confined GaAs/AlxGa1– xAs Heterostructure

We apply a bias to a magnetoresistance (MR) device, which is constructed on surface of GaAs/AlxGa1– xAs heterostructure by patterning two asymmetric ferromagnetic stripes. Using improved transfer matrix method and Landauer–Büttiker theory, bias-dependent transmission, conductance and magnetoresistance ratio are calculated numerically. An obvious MR effect appears, because of a significant difference of transmission or conductance between parallel (P) and antiparallel (AP) magnetization configurations. MR ratio can be tuned by adjusting magnitude or direction of applied bias. These interesting features not only propose an alternative way to control MR effect, but also put forward an electrically-tunable MR device for magnetic information storage.

Magnetic dynamics of quasi-antiferromagnetic layer fabricated by 90° magnetic coupling

In this study, we fabricated quasi-antiferromagnetic (AFM) layers and investigated the magnetic dynamics of quasi-AFM in both an experiment and simulation. Quasi-AFM has multi domains with alternating antiparallel magnetization, which can be realized by using 90° magnetic coupling between two ferromagnetic layers. In magnetic resonance measurement, a frequency and damping constant of CoFe-based quasi-AFM are higher than those of conventional CoFe. Likewise, in micromagnetic simulation, we obtained the spin torque oscillation in Quasi-AFM with high frequency. This suggests the possibility of a novel material with the high frequency wave generator without a stray field, like antiferromagnetic materials.

Electrically-tunable magnetoresistance effect in magnetically modulated semiconductor heterostructure

We report on a theoretical study of magnetoresistance (MR) effect in a magnetically modulated semiconductor heterostructure (MMSH) under an applied bias, which can be constructed on surface of [Formula: see text] heterostructure by depositing two asymmetric ferromagnetic (FM) stripes. Bias-dependent transmission and conductance are calculated numerically, on the basis of both improved transfer matrix method (ITMM) and Landauer–Büttiker conductance theory. An obvious MR effect appears because of a significant difference of transmission between parallel and antiparallel (AP) magnetization configurations. Moreover, MR ratio can be tuned by the bias. These interesting features not only provide an alternative way to manipulate MR effect, but also may lead to an electrically-controllable MR device.

Manipulation of free-layer bias field in giant-magnetoresistance spin valve by controlling pinned-layer thickness

The manipulation of the bias field of the free-layer in giant magnetoresistance spin-valves is of great importance in sensor applications because this feature dominantly affects the low-field sensitivity of magnetoresistance. In this study, it is demonstrated that the bias field of the free-layer can be manipulated by controlling the thickness of the pinned-layer deposited afterward. The key to success is the utilization of the magnetostatic interactions between the free-poles formed on the Néel walls in both free- and pinned-layers. Magnetostatic interactions play a role in stabilizing the antiparallel magnetization state and hence in suppressing the magnetization switching of the free-layer from an antiparallel to a parallel state. A nearly zero bias field is achieved for a Ta-buffered sample with a pinned-layer thickness of 1.75 nm, where a very high low-field sensitivity of 7.7 mV/mA·Oe is obtained.

Numerical analysis of the spin dynamics in bilayer nanodots with interlayer antiferromagnetic coupling

Synthetic antiferromagnetic nanodots with perpendicular magnetic anisotropy are promising candidates for improving the performance of magnetic random-access memory or spin torque nano-oscillators; however, the mechanism for the interlayer antiferromagnetic coupling is still not completely understood. Therefore, we numerically investigated the ferromagnetic resonance characteristics of perpendicularly magnetized bilayer nanodots with interlayer antiferromagnetic coupling. The results show that the resonance frequency strongly depends on the interlayer antiferromagnetic coupling intensity and the individual layer thickness. It was found that external fields induce opposite resonance peak shifts, reflecting the contradicting Zeeman energy effect on individual layers with opposite magnetization directions. The resonance properties were successfully reconfigured by adjusting the uniaxial anisotropy and coupling intensity. Moreover, bistable (parallel and antiparallel) magnetization states were controlled by applying an external field sweep. The difference between the resonance frequencies of two bistable states was enhanced by decreasing the layer thickness and increasing the antiferromagnetic coupling intensity. Our numerical results demonstrate the potential ability of currently available strong interlayer exchange coupling for further increasing of high resonance frequencies in the synthetic antiferromagnet system with perpendicular anisotropy.

First Principle Study of Temperature-Dependent Magnetoresistance and Spin Filtration Effect in WS2 Nanoribbon.

An applicable use of density functional theory (DFT) along with nonequilibrium Green's function (NEGF) is done for exploring the temperature-dependent spin electron transport nature in a ferromagnetic tungsten disulfide (WS2) nanoribbon. To demonstrate the effect of temperature on spin filtration and spin Seebeck effect, we evaluated vital parameters such as spin-polarized current and spin filtration efficiency. Spin filtration efficiency of around ∼95% is obtained in the high-temperature difference range. The high temperature (TL) of the left electrode in comparison to the high temperature (TR) of the right electrode results in higher and lower spin filtration efficiency in parallel magnetization (PM) and antiparallel magnetization (APM), respectively. Transmission spectrum plots at equilibrium are also calculated in PM and APM to justify the temperature-dependent spin transport behavior in the WS2 nanoribbon. Giant thermal magnetoresistance around 1.934 × 103% is achieved. The temperature-dependent negative differential resistance behavior of the current plot has been observed. Huge value of thermal magnetoresistance (MR) and excellent spin filtration obtained for WS2 nanoribbon suggests the potential application of this material in spin caloritronic devices.

All-optical detection and evaluation of magnetic damping in synthetic antiferromagnet

Synthetic antiferromagnets (SyAFs), which consist of a thin nonmagnetic spacer sandwiched by two nanolayer ferromagnets with antiferromagnetic coupling, are promising artificial magnets for spintronic memory and have attracted attention for use in future ultrafast spintronics devices. Here, we report an observation of the magnetization dynamics in a SyAF with nearly antiparallel magnetizations using an all-optical pump-probe technique. High- and low-frequency precessional dynamics of the SyAF were clearly observed. The damping of both modes was explained theoretically in terms of the dynamic exchange coupling induced by the spin current.

Nodal superconducting exchange coupling.

A superconducting spin valve consists of a thin-film superconductor between two ferromagnetic layers. A change of magnetization alignment shifts the superconducting transition temperature (ΔΤc) due to an interplay between the magnetic exchange energy and the superconducting condensate. The magnitude of ΔΤc scales inversely with the superconductor thickness (dS) and is zero when dS exceeds the superconducting coherence length (ξ). Here, we report a superconducting spin-valve effect involving a different underlying mechanism in which magnetization alignment and ΔΤc are determined by nodal quasiparticle excitation states on the Fermi surface of the d-wave superconductor YBa2Cu3O7-δ sandwiched between insulating layers of ferromagnetic Pr0.8Ca0.2MnO3. We observe ΔΤc values that approach 2 K with the sign of ΔΤc oscillating with dS over a length scale exceeding 100ξ and, for particular values of dS, the superconducting state reinforces an antiparallel magnetization alignment. These results pave the way to all-oxide superconducting memory in which superconductivity modulates the magnetic state.

Quasi-antiferromagnetic multilayer stacks with 90 degree coupling mediated by thin Fe oxide spacers

We fabricated quasiantiferromagnetic (quasi-AFM) layers with alternating antiparallel magnetization in the neighboring domains via 90° magnetic coupling through an Fe-O layer. We investigated the magnetic properties and the relationship between the magnetic domain size and the 90° magnetic coupling via experiments and calculations. Two types of samples with a Ru buffer and a (Ni80Fe20)Cr40 buffer were prepared, and we found that with the NiFeCr buffer, the sample has a flatter Fe-O layer, leading to stronger 90° magnetic coupling and a smaller domain size compared with the Ru buffer sample. This trend is well explained by the bilinear and biquadratic coupling coefficients, A12 and B12, in Landau–Lifshitz–Gilbert simulations, suggesting the possibility of using both AFM and FM properties by controlling the quasi-AFM domain size.

Direct observation of magneto-Peltier effect in current-in-plane giant magnetoresistive spin valve

We report on the direct observation of the magneto-Peltier effect in a current-in-plane giant magnetoresistive (CIP-GMR) spin valve. By means of the recently developed thermoelectric imaging technique based on lock-in thermography, we demonstrate that thermoelectric cooling and heating are generated by applying a local magnetic field to the CIP-GMR spin-valve film, confirming the different Peltier coefficients of the spin valve between the parallel and antiparallel magnetization configurations. The cooling and heating positions are found to be tuned simply by changing the magnitude of the local magnetic field. This versatile and reconfigurable thermoelectric conversion functionality may provide a thermal management method for CIP-GMR magnetic sensors.

Negative Differential Conductance Assisted by Optical Fields in a Single Quantum Dot with Ferromagnetic Electrodes.

In a single quantum dot (QD) system connected with ferromagnetic electrodes, the electron transport properties, assisted by the thermal and Fock state optical fields, are theoretically studied by the Keldysh nonequilibrium Green’s function approach. The results show that the evolution properties of the density of state and tunneling current assisted by the Fock state optical field, are quite different from those of the thermal state. The photon sideband shift decreases monotonously with the increase in the electron–photon coupling strength for the case of the thermal state, while the shift is oscillatory for the case of the Fock state. Negative differential conductance (NDC) appears obviously in a QD system contacted with parallel (P) and antiparallel (AP) magnetization alignment of the ferromagnetic electrode leads, assisted by the Fock state optical field in a wide range of electron–photon interaction parameters. Evident NDC usually only arises in an AP configuration QD system assisted by the thermal state optical field. The results have the potential to introduce a new way to actively manipulate and control the single-electron tunneling transport on a QD system by the quantum states of the optical field.

Long-range chiral exchange interaction in synthetic antiferromagnets.

The exchange interaction governs static and dynamic magnetism. This fundamental interaction comes in two flavours-symmetric and antisymmetric. The symmetric interaction leads to ferro- and antiferromagnetism, and the antisymmetric interaction has attracted significant interest owing to its major role in promoting topologically non-trivial spin textures that promise fast, energy-efficient devices. So far, the antisymmetric exchange interaction has been found to be rather short ranged and limited to a single magnetic layer. Here we report a long-range antisymmetric interlayer exchange interaction in perpendicularly magnetized synthetic antiferromagnets with parallel and antiparallel magnetization alignments. Asymmetric hysteresis loops under an in-plane field reveal a unidirectional and chiral nature of this interaction, which results in canted magnetic structures. We explain our results by considering spin-orbit coupling combined with reduced symmetry in multilayers. Our discovery of a long-range chiral interaction provides an additional handle to engineer magnetic structures and could enable three-dimensional topological structures.

Unidirectional-propagating surface magnetoplasmon based on remanence and its application for subwavelength isolators

Ferrimagnetic material with remanence holds the potential to realize unidirectional propagation of the electromagnetic field by taking advantage of magnetoplasmon in the subwavelength regime. Here, we theoretically investigate magnetoplasmons in a layered structure consisting of a dielectric sandwiched by two magnetic materials with anti-parallel remanent magnetization directions, which shows a complete unidirectional propagating region for both even and odd symmetry modes when the thickness of the dielectric is smaller than a certain value. Additionally, the even symmetry mode supported by such a one-way waveguide can be effectively, with low insertion loss, excited by the fundamental transverse-electric mode of a traditional metal slab waveguide. Relying on low insertion loss and a one-way propagation feature, we propose a broadband and subwavelength isolator working at the microwave region. Our results demonstrate that remanence based magnetoplasmons provide a promising way to realize devices below the diffraction limit with new functionalities.

Tailoring of magnetic properties of giant magnetoresistance spin valves via insertion of ultrathin non-magnetic spacers between pinned and pinning layers

The low-field sensitivity of a giant magnetoresistance (GMR) spin valve can be enhanced by tailoring the bias field of the free layer because this sensitivity and bias field are known to show a strong correlation. In this study, the free-layer bias field is reduced considerably to almost zero via the insertion of an ultrathin nonmagnetic spacer between the pinned layer and the pinning layer. The spacer promotes an increase in the density of Néel walls in the pinned layer. This increase, in turn, induces domain-wall-induced magnetostatic interactions of the free poles formed on the Néel walls inside the free and pinned layers. The magnetostatic interactions result in the formation of flux closures that act as pinning sites during the magnetization reversal process and stabilize the antiparallel magnetization state between the free layer and the pinned layer by suppressing the switching of the free layer from the antiparallel state to the parallel state. Furthermore, the spacer offers an additional advantage of increasing the GMR ratio by inducing a specular scattering effect at its top and bottom interfaces. A highly improved low-field sensitivity of 12.01 mV/mA·Oe is achieved in the sample with a Cu/Pt dual spacer.