Ferromagnetic Metal Layers Research Articles

Magnetoanisotropic Josephson effect due to interfacial spin-orbit fields in superconductor/ferromagnet/superconductor junctions

We study theoretically the effects of interfacial Rashba and Dresselhaus spin-orbit coupling in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions---with allowing for tunneling barriers between the layers---by solving the Bogoljubov-de Gennes equation for a realistic heterostructure and applying the Furusaki-Tsukada technique to calculate the electric current at a finite temperature. The presence of spin-orbit couplings leads to out- and in-plane magnetoanisotropies of the Josephson current, which are giant in comparison to current magnetoanisotropies in similar normal-state ferromagnet/normal metal (F/N) junctions. Especially huge anisotropies appear in the vicinity of $ 0 $-$ \pi $ transitions, caused by the exchange-split bands in the ferromagnetic metal layer. We also show that the direction of the Josephson critical current can be controlled (inducing $ 0 $-$ \pi $ transitions) by the strength of the spin-orbit coupling and, more crucial, by the orientation of the magnetization. Such a control can bring new functionalities into Josephson junction devices.

Theory of unidirectional spin Hall magnetoresistance in heavy-metal/ferromagnetic-metal bilayers

Recent experiments have revealed nonlinear features of the magnetoresistance in metallic bilayers consisting of a heavy-metal (HM) and a ferromagnetic metal (FM). A small change in the lon- gitudinal resistance of the bilayer has been observed when reversing the direction of either the applied in-plane current or the magnetization. We attribute such nonlinear transport behavior to the spin-polarization dependence of the electron mobility in the FM layer acting in concert with the spin accumulation induced in that layer by the spin Hall current originating in the bulk of the HM layer. An explicit expression for the nonlinear magnetoresistance is derived based on a simple drift-diffusion model, which shows that the nonlinear magnetoresistance appears at the first order of spin Hall angle (SHA), and changes sign when the current is reversed, in agreement with the experimental observations. We also discuss possible ways to control the sign of the nonlinear magnetoresistance and to enhance the magnitude of effect.

Frequency tuning of the spin-injection radiation in the magnetic contact junction

The frequency tuning of the terahertz radiation of spin-injection oscillator based on magnetic contact junction that is formed by contacting ferromagnetic metal layers with different magnetizations is analyzed. Under the current-induced spin injection from one magnetic layer (injector) to another (working) layer, the interval of radiation frequencies is determined by the current density in the magnetic contact, the mismatch angle of the magnetizations of injector and working layer, and the equilibrium spin polarization. The wide-range tuning of terahertz frequencies of the spin-injection radiation can be implemented using variations in the above parameters.

ChemInform Abstract: CPP Magnetoresistance of Magnetic Multilayers: A Critical Review

AbstractReview: 380 refs.

Spin Hall Magnetoresistance in Metallic Bilayers.

Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of a heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find a nearly tenfold increase of SMR in W/CoFeB compared to previously studied HM/ferromagnetic insulator systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively describes the HM layer thickness dependence of SMR, allowing accurate estimation of the spin Hall angle and the spin diffusion length of the HM layer, but also can account for the temperature dependence of SMR by assuming a temperature dependent spin polarization of the FM layer. These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the HM/FM interface.

Rectifying behavior and photovoltage effect in La1.3Sr1.7Mn2O7/SrTiO3-Nb heterostructure

Perovskite oxide heterostructure possesses attractive magnetic, optical and electric properties, such as superconducting interface between two insulators, two-dimensional electron gas, positive giant magnetoresistance, photoelectric response characteristic, magnetocaloric effect, and coexistent different magnetic structures. Especially for the photoelectric response behaviors of A1-xAxMnO3 (A=La, Pr etc.; A = Sr, Ca etc.) perovskite manganese oxide heterostructure, one has made a systematic study on the photoelectric conversion efficiency, the photovoltaic response speed, and the in-plane lateral photovoltage. Besides A1-xAxMnO3 structure, manganese oxides can also exhibit the double layered perovskite structure A2-2xA1+2xMn2O7. Double layered perovskite structure can be regarded as the layers of perovskite and rock salt which are alternately stacked. This double layered perovskite manganese oxide (such as La2-2xSr1+2xMn2O7) is a natural structure of the tunnel structure: ferromagnetic metal layer-insulating layer-ferromagnetic metal layer. Double layered perovskite manganese oxide has not only the characteristics of giant magnetoresistance, but also the novel physical properties, such as persistent photoconductivity, etc. However, there are few reports on the physical properties of the double layered perovskite manganite oxides, heterostructures, especially the photovoltaic properties. In this work, the La1.3Sr1.7Mn2O7 (LSMO) film is deposited on an n-type SrTiO3-Nb (NSTO) single crystal substrate by a pulsed laser deposition method. Additionally, we study the transporting properties of LSMO/NSTO heterostructure and its photovoltaic effect. The heterostructure exhibits benign rectifying and palpable photovoltaic effect. Under the 532 nm laser irradiation, the photovoltage first increases and then decreases with temperature rising. The maximal photovoltage reaches 400 mV at 150 K which is consistent with the metal-insulator transition temperature of LSMO film. It is indicated that the photovoltaic effect of the heterostructure is regulated by the inner transporting characteristics of LSMO film. The dynamical process of the heterostructure, photovoltaic response, is analyzed. Meanwhile, by analyzing the relationship between the photovoltage and time, it is found that the rising edge fits to the first order exponential function, which is related to the migration of carriers. While the falling edge of second-order exponential function indicates that the compound of carriers has two different physical processes: 1 corresponds to the neutralization process of the carriers aggregated on both junction sides through the external circuit, and 2 corresponds to the annihilation process of non-equilibrium carriers. The carrier lifetime of our p-n junction is longer, on the order of ms, than those of other manganese oxides p-n junctions. Remarkably, the time constants of both the rising edge and falling edge first increase and then decrease as temperature increases, and the maximum values occur at the metal-insulator transition temperature of LSMO film.

Spin and Heat Transport in Ferromagnetic Insulators (FI)/Normal Metal (NM) Mesoscopic Devices — A Boltzmann Approach

We build self-consistent spin-dependent Boltzmann equation in ferromagnetic insulator (FI) and metal layers. The effect of temperature gradient is considered. In this theoretical framework, we focus on the transport phenomena in FI/normal metal (NM) layers, including spin Seebeck effect (SSE). A temperature gradient can generate magnon current in FI layer, and it can converse to the spin current in NM layer through the interaction between magnon and electron at the interface. We also study the different factors of spin-dependent scattering at the interface, regarded as the boundary conditions. By adjusting the FI/NM interface parameters and a temperature gradient, we also obtain influence of diffusion length and [Formula: see text] on the final spin currents.

Effect of temperature on magnetization reversal characteristics of ferromagnetic 3d metal layers within exchange-coupled FeMn-based structures

Multilayer films Fe20Ni80/FeMn/FM (with FM = Ni, Fe11Ni89, Fe20Ni80, Co30Ni70, Fe, Co) have been prepared and their structure has been estimated. Their hysteretic properties and interlayer coupling parameters have been studied in a temperature range of 5–350 K. Regularities of the magnetization reversal of ferromagnetic layers under magnetic exchange-bias conditions have been interpreted.

Magnetostatic surface waves in a ferrite–ferromagnetic metal layered medium based on yttrium iron garnet epitaxial films and TbCo2/FeCo nanostructures

The effect of a thin ferromagnetic metal layer placed on the surface of an yttrium iron garnet surface on the spectrum and loss of magnetostatic surface waves (MSSWs) is experimentally investigated in the case of homogeneous metallization and in the case of a system of periodic metal strips. It is shown that essential attenuation is produced by a metal layer whose thickness is substantially smaller than the depth of the skin layer appearing when an MSSW propagates along a ferromagnetic–metal boundary. The presence of the MSSW reflection by the boundary between the metallic and nonmetallic parts of a ferromagnetic film is experimentally confirmed by the Mandelstam–Brillouin spectroscopy.

Temperature-induced variation of magnetoresistance and tunneling current in La2/3Sr1/3MnO3/LaMnO3 (ZrO2)/La2/3Sr1/3MnO3 junctions

Abstract We report the temperature dependence of the magnetotransport properties for the La 2/3 Sr 1/3 MnO 3 /ZrO 2 /La 2/3 Sr 1/3 MnO 3 and La 2/3 Sr 1/3 MnO 3 /LaMnO 3 /La 2/3 Sr 1/3 MnO 3 trilayer junctions. The ferromagnetic La 2/3 Sr 1/3 MnO 3 has been selected as the top and bottom ferromagnetic metallic layers, along with the antiferromagnetic LaMnO 3 spacer and nonmagnetic ZrO 2 spacer as tunneling barriers, respectively. For different tunneling barriers, the magnetoresistance and the tunneling current of the junctions show different phenomena with decreasing temperature. Temperature dependence of magnetoresistance of the La 2/3 Sr 1/3 MnO 3 /ZrO 2 /La 2/3 Sr 1/3 MnO 3 structure shows double peaks while that of La 2/3 Sr 1/3 MnO 3 /LaMnO 3 /La 2/3 Sr 1/3 MnO 3 displays one peak. Furthermore, the tunneling current is correlated with the magnetoresistance. These findings may find potential uses in magnetic data storages and magnetic switches.

Cylindrical radial superlattice conductors for low loss microwave components

Theory and experimental demonstration of a cylindrical radial superlattice (CRS) conductor composed of alternating nanoscopic non-ferromagnetic/ferromagnetic metal layers are presented with focus on low conductor loss in a K-band microwave spectrum. The dynamic frequency response of the ferromagnetic thin films has been extracted using the Landau-Lifshitz-Gilbert equation which shows a negative magnetic permeability value in the frequencies above its ferromagnetic resonance. The reduction of the conductor loss results from the eddy current canceling (ECC) effect in the CRS conductors, where the negative-permeability ferromagnetic and positive-permeability non-ferromagnetic metal layers produce a zero effective permeability, resulting in virtually infinite skin depth at the targeted frequency. The closed and uniform boundary conditions inherent in the radial shape conductors preclude discontinuity effects occurring at the edges of the planar superlattice conductor and end up with a more effective ECC effect in practice. The design aspects with regards to the CRS materials and structural configuration are discussed. Simulations using a full-wave finite element method high frequency structure simulator are performed to show the ECC effect inside the CRS conductors. An air-lifted inductor made of the CRS conductor has been implemented to prove the effectiveness of the conductor loss reduction with the CRS conductor. The inductor shows an inductance value of 1–2 nH and a Q-factor of 45 at 18 GHz, which is the highest value reported at the frequency by now.

Resonant Magnetoresistance in Asymmetric Double-Barrier Magnetic Tunnel Junctions

Resonant tunneling is studied theoretically for the planar asymmetrical double-barrier magnetic tunnel junction (DMTJ) when a dc bias field is applied. The spin-polarized conductance and tunnel magnetoresistance (TMR) through the DMTJ have been calculated. In DMTJ nanostructure the magnetization of middle ferromagnetic metal layer can be aligned parallel or antiparallel with respect to the fixed magnetizations of the top and bottom ferromagnetic electrodes. Analytical expression for the transmission coefficient of the DMTJ is received, which is expressed through the single-barrier transmission coefficients taking into account the voltage drop on each barrier and spin degrees of freedom of the electron. The dependencies of the tunnel conductance and TMR on the applied voltage have been calculated for the case of resonant transmission.

Tunnel magnetoresistance in asymmetric double-barrier magnetic tunnel junctions

Abstract The spin-polarized tunnel conductance and tunnel magnetoresistance (TMR) through a planar asymmetric double-barrier magnetic tunnel junction (DBMTJ) have been calculated using quasi-classical model. In DBMTJ nanostructure the magnetization of middle ferromagnetic metal layer can be aligned parallel or antiparallel with respect to the fixed magnetizations of the top and bottom ferromagnetic electrodes. The transmission coefficients of an electron to pass through the barriers have been calculated in terms of quantum mechanics. The dependencies of tunnel conductance and TMR on the applied voltage have been calculated in case of non-resonant transmission. Estimated in the framework of our model, the difference between the spin-channels conductances at low voltages was found relatively large. This gives rise to very high magnitude of TMR.

Interplay of Magnetic Anisotropies in Epitaxial Ferromagnetic Hybrids of Fe and (Ga,Mn)As

The combined magnetometric and microwave spectroscopy study of Fe/(Ga,Mn)As hybrids is reported. Temperature-dependent remnant magnetization data revealed unexpected non-monotonic behavior when measured along one of the in-plane crystallographic directions of (Ga,Mn)As, with a slope-sign change at the Curie point of the ferromagnetic semiconductor. Subsequent ferromagnetic resonance measurements presented uniaxial and cubic+uniaxial symmetries of in-plane magnetic anisotropy in Fe and (Ga,Mn)As, respectively. The apparent non-monotonicity of magnetization was explained as a manifestation of competition between the applied magnetic field and temperature-altered magnetic anisotropy in ferromagnetic metal layer.

Strong Eddy-Current Shielding of Ferromagnetic Resonance Response in Sub-Skin-Depth-Thick Conducting Magnetic Multilayers

Exchange-coupled nonmagnetic metal (NM) and ferromagnetic metal (FM) multilayers are crucial for microwave magnonic and spintronic devices. These layered materials usually have total thicknesses smaller than the microwave skin depth. By using a stripline broadband ferromagnetic resonance spectroscopy technique, we experimentally demonstrate that the amplitude of the magnetization precession in the FM layer is strongly diminished by the shielding effect of microwave (612GHz) eddy currents circulating in the NM capping layers.

A Wide Band and High Confinement Surface Plasmon Polariton Mode Converter Based on Magneto-Optic Effects

In this paper, we have analyzed the magneto-optic (MO) effects in three layer surface plasmon polariton (SPP) slab waveguides in a longitudinal configuration which the applied magnetic field is parallel to the interfaces and the longitudinal wave propagation direction. Different configurations of layers for metal-insulator-metal (MIM) with ferromagnetic dielectric or metal layers are analyzed. We have derived the exact dispersion relation with considering MO effects for all three layers by separation of variables method. Because of coupling between SPP modes, all components of the electromagnetic field are created, hence we do not have pure TM modes in this configuration. We have shown the conversion of the transverse magnetic (TM) to transverse electric (TE) SPP mode in these anisotropic waveguides in a wide band wavelength range with high field confinement. These configurations could be used to design devices, such as isolators, modulators and switches in photonic integrated circuits (PICs).

Enhanced transmission and giant Faraday effect in magnetic metal–dielectric photonic structures

Due to their large electrical conductivity, stand-alone metallic films are highly reflective at microwave (MW) frequencies. For this reason, it is nearly impossible to observe Faraday rotation in ferromagnetic metal layers, even in films just tens of nanometres thick. Here, we show using numerical simulations that a stack of cobalt nano-layers interlaced between dielectric layers can become highly transmissive and display a large Faraday rotation in a finite frequency band. A 45° Faraday rotation commonly used in MW isolators can be achieved with ferromagnetic metallic layers as thin as tens of nanometres.

Investigation of ZnO thin films deposited on ferromagnetic metallic buffer layer by molecular beam epitaxy toward realization of ZnO-based magnetic tunneling junctions

Deposition of ZnO thin films on a ferromagnetic metallic buffer layer (Co3Pt) by molecular beam epitaxy technique was investigated for realization of ZnO-based magnetic tunneling junctions with good quality hexagonal ZnO films as tunnel barriers. For substrate temperature of 600 °C, ZnO films exhibited low oxygen defects and high electrical resistivity of 130 Ω cm. This value exceeded that of hexagonal ZnO films grown by sputtering technique, which are used as tunnel barriers in ZnO-MTJs. Also, the effect of oxygen flow during deposition on epitaxial growth conditions and Co3Pt surface oxidation was discussed.

Self-Biased Low Loss Conductor Featured With Skin Effect Suppression for High Quality RF Passives

We present experimental data for artificial metaconductors exhibiting skin effect suppression at microwave frequencies. The metaconductor consists of a stack comprising twelve periods of alternating ferromagnetic (Permalloy) and normal metal (Cu) layers. Near the effective antiferromagnetic resonant frequency the average in-plane magnetic permeability of the stack approaches zero, leading to an increase in the skin depth. Compared to a Cu-based device, up to 70% loss reduction has been achieved by a metaconductor based coplanar wave guide at ~ 10 GHz without changing the propagation wavelength. Moreover, unlike conventional magnetic devices, no external magnetic bias is required due to the large magnetic anisotropy present in the ferromagnetic layers.

Non-equilibrium spatial distribution of Rashba spin torque in ferromagnetic metal layer

We study the spatial distribution of spin torque induced by a strong Rashba spin-orbit coupling (RSOC) in a ferromagnetic (FM) metal layer, using the Keldysh non-equilibrium Green's function method. In the presence of the s-d interaction between the non-equilibrium conduction electrons and the local magnetic moments, the RSOC effect induces a torque on the moments, which we term the Rashba spin torque. A correlation between the Rashba spin torque and the spatial spin current is presented in this work, clearly mapping the spatial distribution of Rashba spin torque in a nano-sized ferromagnetic device. When local magnetism is turned on, the out-of-plane (Sz) Spin Hall effect (SHE) is disrupted, but rather unexpectedly an in-plane (Sy) SHE is detected. We also study the effect of Rashba strength (αR) and splitting exchange (Δ) on the non-equilibrium Rashba spin torque averaged over the device. Rashba spin torque allows an efficient transfer of spin momentum such that a typical switching field of 20 mT can be attained with a low current density of less than 107A/cm2.