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We provide a macroscopic theory and experimental results for magnetic resonances of antiferromagnetically-coupled ferrimagnets. Our theory, which interpolates the dynamics of antiferromagnets and ferromagnets smoothly, can describe ferrimagnetic resonances across the angular momentum compensation point. We also present experimental results for spin-torque induced ferrimagnetic resonance at several temperatures. The spectral analysis based on our theory reveals that the Gilbert damping parameter, which has been considered to be strongly temperature dependent, is insensitive to temperature. We envision that our work will facilitate further investigation of ferrimagnetic dynamics by providing a theoretical framework suitable for a broad range of temperatures. [ABSTRACT FROM AUTHOR]

We studied perpendicular magnetic anisotropy (PMA) and magnetization compensation temperature (TM) in Tb/CoFeB/MgO films. Interlayer antiferromagnetic coupling between Tb and CoFeB layers enables useful antiferromagnetic properties containing a CoFeB/MgO junctions, which are important for the practical applications to obtain giant tunnel magnetoresistance and voltage-control of magnetic anisotropy. While the PMA is mainly attributed to the interface at CoFeB/MgO, TM can be controlled by the thickness ratio of Tb and CoFeB layers as the magnetic moments of them are antiferromagnetically coupled. Our results provide many possibilities to add exciting new functions to ferrimagnetic spintronics. [ABSTRACT FROM AUTHOR]

We investigate temperature, temporal and magnetic-field dependence of unidirectional magnetoresistance (UMR) in metallic bilayers. The UMR is found to decrease rapidly with reducing temperature and converges to a finite value at low-temperature limit. The temporal dependence shows that the UMR emerges in a nanosecond time scale, which depends on the current amplitude. The magnetic-field dependence shows that the UMR is almost constant up to 5 T. These experimental results imply that the high-energy magnons, which are not considered in existing theory, can be involved in the observed UMR. Our results therefore suggest that a more extended theory is required for the complete understanding of UMR. [ABSTRACT FROM AUTHOR]

We investigate the possible emergence of topological Hall effect (THE) in a driven magnetic domain wall (DW). Numerical simulations based on the Landau–Lifshitz-Gilbert-Slonczewski equation shows that the emergent magnetic flux appears when the DW is in a non-equilibrium state. The magnitude of magnetic flux is modulated by the Dzyaloshinskii–Moriya interaction (DMI), providing an experimental test of the predicted THE by a voltage controlled DMI modulation. These results indicate that the THE can be observed even in a topologically trivial magnetic DW, and therefore open up new possibility to electrically detect the dynamical spin structure. [ABSTRACT FROM AUTHOR]

In this study, we investigated the electric field effect on interfacial perpendicular magnetic anisotropy (i-PMA) and interfacial Dzyaloshinskii–Moriya interaction (i-DMI) by propagating spin wave spectroscopy with a high degree of accuracy in Pt/Co/MgO structures. We estimated i-PMA and i-DMI energy densities of 0.71 and 0.45 mJ/m2 from the nonreciprocity of spin wave propagation, and the electric field effect on them of 2.5 and 2.0 fJ/(V·m) from symmetrical and asymmetrical electrical modulations of the spin wave frequency depending on the propagation direction. We also discussed the possibility of an electric-field-controlled phase shifter for spin wave logic devices. [ABSTRACT FROM AUTHOR]

Spin current transmission in antiferromagnetic materials is one of the important and intriguing problems in the recently emerged field of antiferromagnetic spintronics. We investigate the spin current transmission in polycrystalline NiO by referencing the effective Gilbert damping constant in NiO/FeNi bilayers. The results are discussed with reference to the spin pumping theory assuming that the NiO layer is spin diffusive. We extracted the spin diffusion length of ∼22 nm for the NiO and the mixing conductance of ∼6 nm−2 at the NiO/FeNi interface. The obtained spin diffusion length and the mixing conductance are within reasonable ranges compared with previous reports. [ABSTRACT FROM AUTHOR]

The Curie temperature TC is one of the most fundamental physical properties of ferromagnetic materials and can be described by the Weiss molecular field theory with the exchange interaction of neighboring atoms. Here, we demonstrate the electrical control of exchange coupling in cobalt films through direct magnetization measurements. We find that the reduction in magnetization with temperature, which is caused by thermal spin wave excitation and scales with Bloch's law, clearly depends on the applied electric field. Furthermore, we confirm that the correlation between the electric-field-induced modulation of TC and that of exchange coupling follows the Weiss molecular field theory. [ABSTRACT FROM AUTHOR]

The domain-wall (DW) motion of ferrimagnets is investigated with respect to the angular-momentum-compensation temperature TA for a wide range of magnetic fields μ0Hz, ranging from the depinning to flow regimes. While the DW velocity v exhibits a maximum at TA in the flow regime regardless of μ0Hz, the temperature at which v is maximized depends on μ0Hz because of the temperature dependence of the depinning field μ0Hdep, causing difficulty in determining TA. To solve this problem, we propose a reduced coordinate μ0Hz − μ0Hdep, which reveals that v is maximized at TA even in the depinning regime. Our work suggests that μ0Hdep should be identified prior to determining TA. [ABSTRACT FROM AUTHOR]

Electric-field modulations of magnetic properties have been extensively studied not only for practical applications but also for fundamental interest. In this study, we investigated the electric field effect on the exchange interaction in ultrathin Co films with ionic liquids. The exchange coupling J was characterized from the direct magnetization measurement as a function of temperature using Pt/ultrathin Co/MgO structures. The trend of the electric field effect on J is in good agreement with that of the theoretical prediction, and a large change in J by applying a gate voltage was observed by forming an electric double layer using ionic liquids. [ABSTRACT FROM AUTHOR]

Ferromagnetic resonance (FMR) is one of the most popular techniques to characterize dynamic properties of ferromagnetic materials. Among various FMR measurement techniques, the homodyne FMR detection has been frequently used to characterize thin-film ferromagnetic multilayers owing to its high sensitivity. However, a drawback of this technique was considered to be the requirement for a structural inversion asymmetry, which makes it unsuitable to characterize a single layer of ferromagnet. In this study, we demonstrate a homodyne FMR detection of the Kittel’s mode FMR dynamics of a single layer of FeNi by creating a non-uniform radio-frequency excitation current. [ABSTRACT FROM AUTHOR]

We report a systematic experimental study on the refraction and reflection of magnetostatic spin-waves at a thickness step between two Permalloy films of different thickness. The transmitted spin-waves for the transition from a thick film to a thin film have a higher wave vector compared to the incoming waves. Consequently, such systems may find use as passive wavelength transformers in magnonic networks. We investigate the spin-wave transmission behavior by studying the influence of the external magnetic field, incident angle, and thickness ratio of the films using time-resolved scanning Kerr microscopy and micro-focused Brillouin light scattering. [ABSTRACT FROM AUTHOR]

We propose an alternative method of determining the magnetic anisotropy field μ0HK in ferro-/ferrimagnets. On the basis of the droplet nucleation model, there exists linearity between domain-wall (DW) energy density and in-plane magnetic field. We find that the slope is simply represented by μ0HK and Dzyaloshinskii–Moriya interaction (DMI). By measuring the in-plane magnetic field dependence of the coercivity field, closely corresponding to the DW energy density, a robust value for μ0HK can be quantified. This robust value can be used to determine μ0HK over a wide range of values, overcoming the limitations caused by the small strength of the external magnetic field typically used in experiments. [ABSTRACT FROM AUTHOR]

An X-ray tomographic technique was developed to investigate the internal magnetic domain structure in a micrometer-sized ferromagnetic sample. The technique is based on a scanning hard X-ray nanoprobe using X-ray magnetic circular dichroism (XMCD). From transmission XMCD images at the Gd L3 edge as a function of the sample rotation angle, the three-dimensional (3D) distribution of a single component of the magnetic vector in a GdFeCo microdisc was reconstructed with a spatial resolution of 360 nm, using a modified algebraic reconstruction algorithm. The method is applicable to practical magnetic materials and can be extended to 3D visualization of the magnetic domain formation process under external magnetic fields. [ABSTRACT FROM AUTHOR]

Spin injection into 3d transition metals and their alloys has been investigated intensively in recent years. In this study, we focus on the FeRh alloy, which is often used in the context of antiferromagnetic spintronics. We explore the interaction between the spin current and magnetic moments in FeRh by means of spin-torque ferromagnetic resonance. We obtain a spin Hall efficiency of ξSH = 24 ± 3%, which indicates that the spin current from Pt interacts with the magnetic moments in FeRh. [ABSTRACT FROM AUTHOR]

Current-controlled magnetic domain wall motion has opened the possibility of a novel type of shift register memory device, which has been optimistically predicted to replace existing magnetic memories. Owing to this promising prospect, intensive work has been carried out during the last few decades. In this article, we first review the progress in the study of current-induced magnetic domain wall motion. Underlying mechanisms behind the domain wall motion, which have been discovered during last few decades, as well as technological achievements are presented. We then present our recent experimental results on the real-time detection of current-driven multiple magnetic domain wall motion, which directly demonstrates the operation of a magnetic domain wall shift register. [ABSTRACT FROM AUTHOR]

The asymmetric magnetoresistance (MR) in Py/Pt bilayers is investigated. It increases linearly with respect to the current density up to a threshold and increases more rapidly above this threshold. To reveal the origin of the threshold behavior, we investigate the magnetic field dependence of the asymmetric MR. It is found that the magnetic field strongly suppresses the asymmetric MR only above the threshold current density. Micromagnetic simulation reveals that the reduction of magnetization due to the spin-torque oscillation can be the origin of the threshold behavior of the asymmetric MR. [ABSTRACT FROM AUTHOR]

We study the perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO bilayers in contact with W, Ta, IrMn, and Ti, which have been suggested for use as the spin–orbit-torque-related underlayers. The saturation magnetization of CoFeB depends on the underlayer material used owing to the formation of a dead layer that affects the PMA strength of each film. The X-ray magnetic circular dichroism measurement reveals that interfacial intermixing suppresses only the perpendicular orbital moment of Fe, whereas it simultaneously suppresses both the perpendicular and in-plane orbital moments of Co. [ABSTRACT FROM AUTHOR]

A creep motion of the magnetic domain wall (DW) in a perpendicularly magnetized Co wire, where the DW energy is artificially varied by applying a sloped electric field, is studied. Under the sloped electric field and a constant external magnetic field, the DW velocity gradually changes according to the position of the wire owing to the spatially varying DW energy. Although the sloped DW energy can be a source to drive a DW, no clear electric-field-induced DW motion is observed, most likely because the effective magnetic field induced by the sloped electric field is very small in the present system. [ABSTRACT FROM AUTHOR]

We present an energy-efficient magnetic domain-writing scheme for domain wall (DW) motion-based memory devices. A cross-shaped nanowire is employed to inject a domain into the nanowire through current-induced DW propagation. The energy required for injecting the magnetic domain is more than one order of magnitude lower than that for the conventional field-based writing scheme. The proposed scheme is beneficial for device miniaturization because the threshold current for DW propagation scales with the device size, which cannot be achieved in the conventional field-based technique. [ABSTRACT FROM AUTHOR]

The electric field control of superparamagnetism is realized using a Cu/Ni system, in which the deposited Ni shows superparamagnetic behavior above the blocking temperature. An electric double-layer capacitor (EDLC) with the Cu/Ni electrode and a nonmagnetic counter electrode is fabricated to examine the electric field effect on magnetism in the magnetic electrode. By changing the voltage applied to the EDLC, the blocking temperature of the system is clearly modulated. [ABSTRACT FROM AUTHOR]

We investigate the temperature dependence of the spin Hall magnetoresistance (SMR) in a W/CoFeB bilayer. The SMR is found to increase with decreasing temperature. An analysis based on the SMR theory suggests that the spin Hall angle of W and/or the spin polarization of CoFeB can be the origin of the temperature dependence of the SMR. We also find that the spin diffusion length and the resistivity of W do not significantly vary with temperature, which indicates the necessity of further study on the electron transport mechanism in W films to reveal the origin of the spin Hall effect in W. [ABSTRACT FROM AUTHOR]

The temperature dependence of magnetoresistance is investigated in ferrimagnetic GdFeCo/Pt heterostructures. An anomalous Hall effect (AHE) shows a monotonic behavior in temperature even across the magnetization compensation temperature TM, implying that the FeCo moment is responsible for the magnetotransport properties. An anisotropic magnetoresistance (AMR) exhibits a steep increase at low temperatures, which we ascribe to the contribution of a weak antilocalization in an amorphous GdFeCo layer. A spin Hall magnetoresistance (SMR) is found to exist in ferrimagnet/Pt systems and shows a moderate temperature dependence, in contrast to the SMR in YIG/Pt where a significant temperature dependence was observed. These results provide a basic understanding of the magnetotransport in amorphous ferrimagnets/heavy metal heterostructures. [ABSTRACT FROM AUTHOR]

Exchange bias is a unidirectional magnetic anisotropy developed in a bilayer of ferromagnetic and antiferromagnetic layers. Its technical importance as a “fix layer” is seen in various spintronic devices. The exchange bias can also be a probe to investigate the antiferromagnetic layer as it partly reflects the magnetic state of the antiferromagnet. In this work, we investigated the modulation of the exchange bias by a flow of electric current in Pt/Fe50Mn50/FeNi and Cu/Fe50Mn50/FeNi. We show that the exchange bias can be modulated just by applying the current due to interplay among the Joule heating, Ampere field, and current-induced effective field. [ABSTRACT FROM AUTHOR]

Electric field effect on magnetism is an appealing technique for manipulating magnetization at a low energy cost. Here, we show that the local magnetization of an ultrathin Co film can be switched by simply applying a gate electric field without the assistance of any external magnetic field or current flow. The local magnetization switching is explained by nucleation and annihilation of magnetic domains through domain wall motion induced by the electric field. Our results lead to external-field-free and ultralow-energy spintronic applications. [ABSTRACT FROM AUTHOR]

Spin motive force (SMF) is induced by a time and spatial derivative of magnetizations and is dependent on spin polarization. We compare the SMF in FeNi with positive spin polarization with that in a magnetite (Fe3O4) with negative spin polarization. We observe the SMF induced by a nonuniform ferromagnetic resonance in Fe3O4 and find that the SMF in Fe3O4 is opposite to that in FeNi. This result originates from the negative spin polarization of Fe3O4. Our clear observation of the SMF depending on the sign of the spin polarization agrees well with the framework of the SMF theory. [ABSTRACT FROM AUTHOR]

We show that our ferromagnetic resonance (FMR) measurement based on the rectification effect is sufficiently sensitive for characterizing various static and dynamic magnetic properties of a sub-nanometer ferromagnetic film where the interfacial effects dominate. The extracted properties, such as the Landé g-factor, the effective demagnetizing field, and the Gilbert damping parameter, are reasonably well scaled with the film thickness, indicating that our measurements clearly capture the interfacial properties of the sub-nanometer-thick film. In particular, the capability of the g-factor extraction in the ultrathin film will be very helpful for characterizing the various interfacial effects involved with interfacial orbit moments and spin–orbit interactions. [ABSTRACT FROM AUTHOR]

Magnetic domain wall (DW) motion induced by nanosecond current pulses is investigated in Co/Ni nanowires where the DW motion is driven by either adiabatic spin transfer torque (STT) or spin Hall torque (SHT). The DW displacement, including transient displacement and steady-state displacement, is found to be linearly related to the current pulse duration for both current-induced torques. The transient displacement is found to be less than 10% of the DW displacement. This result implies that the DW position can be controlled by tuning the duration of the current pulse, which enables a robust operation of racetrack memory. [ABSTRACT FROM AUTHOR]