Metallic Magnetic Multilayers Research Articles

Ferroelectric Control of Spin‐Orbitronics

AbstractSpin‐orbit coupling refers to the relativistic interaction between the spin and orbital motions of electrons. This interaction leads to numerous intriguing phenomena, including spin‐orbit torques, spin–momentum locking, topological spin textures, etc., that have recently gained prominence in the field of spin‐orbitronics. In particular, the emerging ferroelectric control is recognized and validated as an effective means to enhance energy efficiency across a broad spectrum of spin‐orbitronic devices. Here, cutting‐edge research on ferroelectric control of spin‐orbitronics (FECSO) by means of spontaneous polarization, reversible ferroelectric switching, and multiferroic coupling, are comprehensively reviewed. Two fascinating topics are mainly discussed: topological spin texture and spin‐charge interconversion. The classification of control mechanisms for different interactions in FECSO is summarized first. Then, from the perspective of material classification, the ferroelectric‐controlled spin‐orbit coupling with tunable topological spin texture in oxide systems, magnetic metal multilayers, and 2D van der Waals materials is reviewed. Subsequently, the ferroelectric‐tunable spin‐charge interconversion on heavy metal layers, oxide interfaces, and ferroelectric Rashba semiconductors is highlighted. In the end, the challenges and forthcoming prospects of FECSO are discussed. This work may provide pertinent and forward‐thinking guidance to accelerate the ongoing advancement of this field.

Theory of giant magnetoresistance caused by magnetic-ion pairs embedded in paramagnetic metals

To clarify a mechanism of a giant magnetoresistance (MR), we study the resistivity of a paramagnetic metal in which a magnetic-ion pair is embedded as impurities. In this pair, magnetic ions at the positions ${\mathbf{r}}_{1}$ and ${\mathbf{r}}_{2}$, respectively, are assumed to have localized spins ${\mathbf{S}}_{1}$ and ${\mathbf{S}}_{2}$, which are called $d$ spins. Conducting electrons, on the other hand, are assumed to have a light mass, being called $s$ electrons. The $s$ electrons are scattered by both the $d$ spins through the $s\ensuremath{-}d$ exchange interaction. Since interferences occur in the wave functions of electrons scattered by both the $d$ spins, transition probabilities between different electronic states are shown to depend strongly on $\mathbf{R}={\mathbf{r}}_{2}\ensuremath{-}{\mathbf{r}}_{1}$ and the spin configuration in the pair $({\mathbf{S}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathbf{S}}_{2})$. The steady state of the $s$ electron system under an external electric field is obtained on the basis of the Boltzmann equation and the relaxation-time approximation. By using the obtained relaxation times, the resistivity of the metal is calculated to find its $\mathbf{R}$ and $({\mathbf{S}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathbf{S}}_{2})$ dependencies. MRs are shown to appear when an antiparallel spin configuration realized by the Ruderman-Kittel-Kasuya-Yoshida interaction is changed to be a parallel one by a magnetic field. The maximum MR ratio ${[({\ensuremath{\rho}}_{\mathrm{ap}}\ensuremath{-}{\ensuremath{\rho}}_{\mathrm{p}})/{\ensuremath{\rho}}_{\mathrm{p}}]}_{\mathrm{MR}}$ is found to give a giant MR (GMR) of the order of ${10}^{2}\phantom{\rule{4pt}{0ex}}%, {\ensuremath{\rho}}_{\mathrm{ap}}$ and ${\ensuremath{\rho}}_{\mathrm{p}}$ being the resistivities in the metals with the antiparallel and parallel spin pairs, respectively. These results are extensively applied to the GMR in the metallic magnetic multilayers by taking into account possible magnetic-ion pairs in their nonmagnetic layers. This application shows that Mott's two-fluid model can exhibit a GMR of magnetic multilayers also in the case of the current in planes (CIP).

Spin accumulation dynamics in spin valves in the terahertz regime

The notion of spin accumulation and spin relaxation in the diffusive regime was introduced by Valet and Fert in 1993 to describe the current-perpendicular-to-plane (CPP) diffusive transport in metallic magnetic multilayers. This theory has been quite successful in explaining the giant magnetoresistance of magnetic multilayers in CPP geometry in the frequency range from DC to a few gigahertz. In this paper, we investigate the dynamic aspect of spin accumulation from the theoretical point of view when reaching the terahertz (THz) frequency range. The characteristic relaxation time of electron elastic scattering is typically in the femtosecond range. However, since spin accumulation results from a diffusion process involving a very large number of individual scattering events, the characteristic time of spin accumulation variation when the current and/or the magnetic configuration are varied can be significantly longer than that of the input signal, eventually reaching the picosecond range. In spintronic devices operating in the THz range such as those based on ferrimagnetic or antiferromagnetic materials, the spin accumulation amplitude and, correlatively, the device magnetoresistance can therefore depend on the actual device operating frequency. We investigate this question by extending the Valet and Fert theory in the time domain.

Efficient Terahertz Generation Using Fe/Pt Spintronic Emitters Pumped at Different Wavelengths

Recent studies in spintronics have highlighted ultrathin magnetic metallic multilayers as a novel and very promising class of broadband terahertz radiation sources. Such spintronic multilayers consist of ferromagnetic (FM) and non-magnetic (NM) thin films. When triggered by ultrafast laser pulses, they generate pulsed THz radiation due to the inverse spin-Hall effect, a mechanism that converts optically driven spin currents from the magnetized FM layer into transient transverse charge currents in the NM layer, resulting in THz emission. As THz emitters, FM/NM multilayers have been intensively investigated so far only at 800-nm excitation wavelength using femtosecond Ti:sapphire lasers. In this work, we demonstrate that an optimized spintronic bilayer structure of 2-nm Fe and 3-nm Pt grown on 500 {\mu}m MgO substrate is just as effective as a THz radiation source when excited either at {\lambda} = 800 nm or at {\lambda} = 1550 nm by ultrafast laser pulses from a fs fiber laser (pulse width close to 100 fs, repetition rate around 100 MHz). Even with low incident power levels, the Fe/Pt spintronic emitter exhibits efficient generation of THz radiation at both excitation wavelengths. The efficient THz emitter operation at 1550 nm facilitates the integration of such spintronic emitters in THz systems driven by relatively low cost and compact fs fiber lasers without the need for frequency conversion.

Giant Magnetoresistance of Metallic Exchange-Coupled Multilayers and Spin Valves

The microstructure and the magnetic and magnetotransport properties have been investigated depending on the types of the magnetic and nonmagnetic materials, composition, nanostructure, and regimes of thermomagnetic treatments for two types of metallic nanostructures, namely, magnetic multilayers and spin valves, which exhibit the effect of a giant magnetoresistance (GMR). Magnetosensitive materials that are promising for the applications in microelectronics have been synthesized: CoFe/Cu multilayers with record values of the GMR effect (81%); NiFeCo/Cu multilayers with a sensitivity of 0.5%/Oe; and microobjects prepared on the basis of spin valves, which possess the anhysteretic field dependence of the magnetoresistance and a GMR effect equal to 9%. Special attention is given to the methods of decreasing hysteresis. A comparison of the obtained results with the results for similar types of materials published earlier has been carried out.

Spin–Orbit Torques in Metallic Magnetic Multilayers: Challenges and New Opportunities

Two decades after the discovery of the giant magnetoresistance that revolutionizes the hard disk drive, the rapid development of spin torque-based magnetic random access memory has once again demonstrated the great potential of spintronics in practical applications. While the industrial application is mainly focusing on the implementation of current-induced spin transfer torque (STT) in magnetic tunnel junctions, a new type of spin torque emerges due to the spin–orbit interaction in magnetic multilayers. A great effort has been devoted by the scientific community to study the so-called spin–orbit torque (SOT), which is not only of interest to fundamental science, but also exhibits potential for the application of current-induced magnetization switching. In this paper, we will review recent development in the SOTs including the fundamental understanding, materials development and measurement techniques. We will also discuss the challenges of using the SOT in potential applications, particularly on the switching of perpendicularly magnetized films.

Magnetorefractive Effect for Co/Ru Multilayer Film with Antiferromagnetic Interlayer Exchange Coupling

SUMMARYThe spin‐dependent scattering of conduction electron in metallic magnetic multilayers was investigated to reveal the optical properties which corresponded to the magnetization state. From the measurement of magnetorefractive effect in antiferro‐magnetically exchange coupled Co/Ru multilayers, it was found that (1) the transmittance varied around 1% at wavelength of 1550 nm during the parallel/antiparallel spin alignment changes, (2) as a result of number of repetition and Co thickness for the multilayers, transmittance difference between the parallel and antiparallel spin alignment states had trade‐off relationship with the magnitude of transmittance, and (3) asymmetric Co/Ru/Co stacking structure brought different interlayer exchange coupling fields in each layer, which resulted in magnetic field sensitive transmittance.

Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source which relies on tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photo-induced spin currents, the inverse spin-Hall effect and a broadband Fabry-P\'erot resonance. Guided by an analytical model, such spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-to-30-THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz-field amplitude, flexibility, scalability and cost.

Tuning Magnetic Anisotropy in Metallic Multilayers by Surface Charging: AnAb InitioStudy

Our abinitio studies show clear evidence that magnetic anisotropy (MA) and the direction of magnetization in metallic magnetic multilayers can be tailored at once by surface charging. By taking Fe-Pt multilayers as a representative example, we demonstrate that surface charging has a deep effect on the magnitude of the MA, which is composition dependent, achieving remarkably large values for systems featuring a single Fe layer capped with Pt. More intriguing is the behavior of the multilayers capped with iron bilayers, for which surface charging not only affects the value of the anisotropy but an easy-axis switching is also revealed. By analyzing the electronic structure of the magnetic layers and relating the MA to the orbital moment anisotropy, some insights about the origin of the MA from a local perspective can be inferred.

Effects of interfacial roughness on the magnetoresistance of Co/Cu multilayers

Using a semi-classical approach, Hood, Falicov and Penn have studied the effects of interfacial roughness on the magnetoresistance (MR) of iron based trilayers (Fe/Cr/Fe and Fe/Cu/Fe). We extend their theory to magnetic metallic multilayers composed of N bilayers ferromagnetic-normal metal. The in plane MR of Co/Cu multilayers is calculated for correlated quasiperiodic interfaces. The averaged effects due to impurities, interdiffusion, band structure, etc. are included in a simple way using two phenomenological parameters S ↑ and S ↓ for two directions of spin. MR variation with S ↑, S ↓ and relaxation time is reported. We analyse also recent experimental data giving the influence of number of bilayers on the MR of Co/Cu multilayers for different temperatures.

磁性金属多層膜を用いた量子井戸状態による近藤効果の制御

磁性金属多層膜を用いた量子井戸状態による近藤効果の制御

Spin-torque effects in metallic magnetic multilayers in the ballistic regime

We derive the current-induced torque in metallic magnetic multilayers in the ballistic regime for the case that one of the ferromagnetic layers is acting as a full polarizer. We generalize previous results to the case when transmission and reflection amplitudes of one of the ferromagnets are different from zero or one and are complex. The obtained expressions are valid for the situation when the interfaces are sufficiently flat and when there are no scatterers in the metallic spacers. Particular attention is paid to multilayers containing a perpendicular polarizer whose magnetization is pointed along the current flow. We present analytical expressions for the spin torque that may serve as a theoretical upper bound for realistic structures.

Theoretical investigation of the relationship between spin torque and magnetoresistance in spin-valves and magnetic tunnel junctions

A theoretical study is presented concerning the relationship between spin transfer torque and absolute current-perpendicular-to-plane magnetoresistance in metallic spin-valves (SV) and magnetic tunnel junctions. In a first step, using Valet and Fert Boltzmann-like theory extended to any metallic magnetic multilayers with non-collinear magnetizations, linear relationships between spin torque and absolute current-perpendicular-to-plane giant magnetoresistance are derived numerically, when varying the parameters of the structure one by one. The obtained results are compared with an extension of J.C. Slonczewski's circuit theory. The latter model gives an analytical expression of this linear dependence. In a second step, using an out-of-equilibrium perturbation formalism (Keldysh technique), we study the tunnel magnetoresistance (TMR) dependence of the spin torque amplitude in magnetic tunnel junctions when varying the parameters one by one in the junction. Once again, linear dependences are obtained, with different characteristics when the TMR vanishes. It is shown that these two equivalent behaviours are associated with different origins of the spin torque.

Electronic transport in the multilayers with very thin magnetic layers

Metallic magnetic multilayers with very thin discontinuous magnetic layers are considered. Existence of the windows (strips) in magnetic layers leads to the additional scattering mechanism for the electrons in non-magnetic spacer. Transition from antiparallel to parallel ordering of magnetic moments in nearest magnetic layers versus external magnetic field causes the change of boundary condition for the wave function of electrons in the spacer. It has to increase the magnetoresistance in such structures as compared to the multilayers without windows.

Structure Analysis of Magnetic Multilayer Using Reflectometry

The giant magnetoresistive head, consisting of magnetic and noble metal multilayers, is a key device of hard disk drives. As head properties strongly depend on the magnetic structure, the characterization is important for producing good devices. In this report, we review multi-wave x-ray reflectometry for accurate analysis of the layer thickness, and polarized neutron reflectometry for analysis of magnetic moment. We also mention a future prospect of a neutron experiment.

266 磁性・非磁性金属多層膜堆積過程の分子動力学解析(製膜・膜特性,OSO7 電子・原子・マルチシミュレーションに基づく材料特性評価)

266 磁性・非磁性金属多層膜堆積過程の分子動力学解析(製膜・膜特性,OSO7 電子・原子・マルチシミュレーションに基づく材料特性評価)

Fabrication of a nanosized current-driven spin valve device using proximity correction in electron-beam lithography

Nanosized junctions of 100×200nm2 size were successfully fabricated on magnetic metallic multilayers in a current-driven spin-valve device by the suitable choice of the electron-beam resist, as well as careful control of back-scattering effects in electron-beam lithography. ZEP520A was chosen as an e-beam sensitive resist because its dry-etching resistance is high enough to act as an etching mask during the ion milling. The severe back-scattering effect from the underlying metallic multilayers was reasonably modified by employing the doughnut-pattern method. The doughnut-pattern method gave a set of proximity parameters (α,β, and η) with which we did the simulation to obtain the effective dose factors of each segment of the pattern. The magnetotransport signals from our devices were quite comparable to previous results from those made by different methods of other research groups. Consequently, the fabrication of nanosized current driven spin valves can be performed less complicatedly by using ZEP520A as an etching mask.

Fundamental limits on the z-resolution of ion beam patterning of magnetic multilayers

Ion beam etching (i.e. ion milling) is the preferred patterning method for metallic magnetic multilayers. We explore the extendibility and fundamental limits of the technique with regard to z-resolution (i.e. etched depth).We studied the atomic relocation in Ni–Cu and Co–Cu multilayers under Ar+ bombardment, using molecular dynamics simulations. The atomistic mechanisms upon impact are significantly different for the two systems, especially on the monolayer scale: Ar bombardment of Co on Cu is found to lead to dewetting, while bombardment of Ni on Cu is found to lead mostly to formation of an interfacial alloy due to relocated atoms.

Currents and torques in metallic magnetic multilayers

A theory is given for electron transport across a very thin non-magnetic metallic spacer joining two ferromagnetic metals whose moment vectors include a general angle. An assumed condition of negligible interfacial reflection for majority-spin electrons is approached by certain multilayer compositions including the experimentally important composition Co/Cu. Analytic formulas based on a non-spin-diagonal density operator inside the spacer connect the spin-channel currents and electro-chemical voltages across the spacer. The quantum-mechanically derived torques on sublayer moments agree with a previous macroscopic relation. The single additional parameter of the resulting macroscopic magnetoelectronic transport formulation depends only on bulk electron structure of the spacer. Illustrative application of the new connection formulas to the special case of two equal thin ferromagnets predicts closely related expressions for angular dependences of magnetoresistance and current-driven torque. A simple relation between magnetoresistance and asymmetry of exciting currents holds for this case.

Development of a data acquisition and control system for thin film deposition equipment and its application to metals-MBE

A versatile data acquisition and control system (DACS) of general application in physical vapor deposition equipment for thin films growth has been designed and developed. Based on a common and inexpensive hardware, a modular conception of the software allows the system to be easily adapted to control a variety of devices and therefore to be used for a number of applications with very light adjustments. A particular configuration of the system has been successfully implemented in a molecular beam epitaxy (MBE) setup specially designed for the growth of metallic heterostructures and magnetic multilayers. The number and different features of devices and processes to be registered and controlled in this equipment demonstrate the capability of the tool, the ease of operation and the compatibility with complementary software for simulation, data storage and processing.