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We demonstrate that magnetic crystal anisotropy ( K u ) and spectroscopic g-factor can be modulated simultaneously by gate voltage in a Pt/Co/MgO ultrathin film with our ferromagnetic resonance (FMR) measurement based on the rectification effect. The g-factor reflects on orbital magnetic moment, indicating that the electric field induced-modulations of magnetic anisotropy in cobalt originate from the change of the anisotropy of orbital moments ( ∆ m L ). The quantitative estimation reveals that ∆ m L to K u correlation when the electric field is applied significantly differs from the one inherent at the Pt/Co/MgO interfaces. This means that the correlation itself is also modulated by the electric field. Our results show that the FMR technique is very useful for probing the electric field effect on K u .

In this study, we investigated the modulation of the spin–orbit torque (SOT) caused by inserting the NiO layer at the Pt/Co interface. A similar Pt/NiO/Co structure was deposited on two different substrates, Al2O3 and Si/SiO x substrates. We found that the damping-like torque of the Al2O3 type sample is almost independent of NiO thickness (t NiO) when t NiO x type monotonically decreased with increasing t NiO. The X-ray diffraction measurement revealed that the degree of interface roughness varies between these types. This suggests that the effect of the NiO insertion on the SOT is associated with the interface roughness.

We fabricated a Pd/Co-based spin valve structure, which has in-plane (IP) free and out-of-plane (OOP) fixed layers, on a flexible substrate. By applying %-order biaxial tensile strain to the substrate, a spin reorientation transition from IP to OOP occurs in the free layer, resulting in a drastic change in the shape of the magnetoresistance curve. This change is reversible with increasing and decreasing strain. We suggest that our system could be utilized as an in-plane “direction-independent” strain sensor and as a pressure detector.

Voltage control of current-induced spin–orbit torque (SOT) in an in-plane-magnetized Pd/Co/Pd system with a low-temperature-deposited HfO x and a gate electrode on top is studied. An application of the gate electric field to the HfO x layer is to induce a non-volatile electrochemical effect from the Pd/HfO x interface. By means of low-frequency harmonic Hall measurements, the voltage modulation of both damping-like and field-like SOT is obtained. The sign of the voltage-induced SOT modification is found to be reversed by changing the top Pd thickness. Our finding is expected to facilitate the efficient active manipulation of SOT.

We fabricated a solid-state capacitor on an organic flexible substrate, which has an ultra-thin ferromagnetic Co electrode with a HfO2 dielectric layer formed by atomic layer deposition. Tensile stress was applied parallel to the capacitor plane. The breakdown strain monotonically increases with a reduction in HfO2 layer thickness. From the sheet resistance measurement, strain endurance up to 1.5% is realized in the capacitor with 10 nm thick HfO2. A magnetic anisotropy of the Co layer is controllable by applying voltage between the Co and counter electrode. The modulation efficiency of the voltage-controlled magnetic anisotropy is almost independent of the strain.

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.

We succeeded in fabricating a CoFeB/MgO-based magnetic tunnel junction (MTJ) directly on an organic flexible substrate. The MTJ shows good strain endurance while keeping the tunnel magnetoresistance (TMR) ratio of ~100% under various strained conditions; the TMR ratio is almost unchanged up to a tensile strain of 1.2%. Because of the magnetoelastic effect, the magnetic anisotropy fields for the top and bottom CoFeB layers are linearly proportional to strain with almost the same rate as that in a single CoFeB film, suggesting that the expected strain is added on both CoFeB layers in the MTJ pillar from the stretched flexible substrate.

This study aims to investigate the creep motion of a magnetic domain wall (DW) induced by spin-Hall torque (SHT) in a Ta/Co–Ni/Pt hetero-structured thin wire using photoemission electron microscopy. The displacement of the DW was recorded every time after the duration of an electric current pulse. Results revealed that the existence of pinning centers and the inhomogeneity of the thermal diffusion property of the wire may result in an unstable DW motion. On the contrary, this data also suggested that the SHT induced from both the Ta and Pt layers can significantly improve the DW velocity, which was found to be three orders of magnitude larger than that previously reported for the MgO/Co–Ni/Pt system (Taniguchi et al., Appl. Phys. Exp. 7, 053005 (2014)).

We have investigated magnetic anisotropy and its electric field effect in ultrathin Co with a Pt cap layer. The layers were directly deposited on SrTiO3(111) and (100) surfaces without any under layer. In as-deposited samples, a stronger perpendicular magnetic anisotropy was observed in the (111) sample. A back gate configuration was used to apply the electric field to the bottom surface of Co layer through the SrTiO3 substrate. In both samples, the coercivity was enhanced with a positive gate bias, which corresponds to the direction of the electron accumulation. The efficiency of the change in the coercivity per modulated electron density was greater in the (111) sample over a wide temperature range including 2 K, at which the thermally activated magnetization reversal was expected to be suppressed. The results suggest that the surface orientation of the substrate affects both the magnetic anisotropy and its electric field effect on Co.

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.

We have demonstrated that the local magnetization in a Co microwire can be switched by an application of a gate voltage without using any external magnetic fields. The electric-field-induced reversible ferromagnetic phase transition was used to realize this. An internal stray field from a ferromagnetic gate electrode assisted the local domain reversal in the Co wire. This new concept of electrical domain switching may be useful for dramatically reducing the power consumption of writing information in a magnetic racetrack memory, in which a shift of a magnetic domain by electric current is utilized.

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.

We investigate the electric-field effect on the magnetic anisotropy in perpendicularly magnetized Pt/Co/Pd/MgO structures deposited on GaAs and Si substrates as a function of temperature. An electric field is applied to the Pd surface through a 2-nm-thick insulating MgO layer by forming an electric double layer using an ionic liquid. We find that the electric-field dependence of the anisotropy is similar but clearly different between the two samples at low temperatures. The difference is expected to be related to the difference in the internal strain of the two structures.

We demonstrate that the Faraday effect can be electrically and reversibly switched on and off at room temperature without changing the external magnetic field. A Co ultrathin film deposited on a Pt underlayer was used for the demonstration. An electric field was applied to the surface of the Co film by forming an electric double layer using an ionic liquid. The switching of the Faraday effect was caused by the electric-field-induced ferromagnetic phase transition in the system. The magnitude of the electrical change in the Faraday rotation angle and the ellipticity are also discussed.

Introduction of a high concentration of manganese in III?V semiconductors, such as InAs and GaAs, results in carrier-induced ferromagnetism, which allows us to integrate ferromagnetism in nonmagnetic heterostructures and which modifies their magnetic properties through electric-field control of carrier concentration. The properties of ferromagnetism can in many cases be semi-quantitatively understood by the p?d Zener model, which is qualitatively different from conventional ferromagnetic metals. These ferromagnetic III?V semiconductors also offer the unique opportunity of examining spin-dependent phenomena observed so far only in metallic systems. Here, we review our experimental study on electrical manipulation of magnetization in these ferromagnetic III?V semiconductors. We first describe the results of electrically assisted magnetization reversal in ferromagnetic semiconductor (In, Mn)As field-effect transistor structures. The coercivity as well as ferromagnetic transition temperature can be controlled through the modification of carrier concentration by applied electric fields in a gated structure. We then present electrical magnetization reversal by spin-transfer torque exerted by spin-polarized currents at low threshold current density (~105?A?cm?2) in (Ga, Mn)As-based magnetic tunnel junctions.

We investigated the effect of electrical currents on magnetization reversal in ferromagnetic semiconductors. We fabricated a Hall bar-shaped device with a stepped channel from (Ga,Mn)As and that with a partially gated channel from (In,Mn)As. These structures realize in-plane patterning of coercivity HC and allow us to study the dynamics of domain walls under application of currents by monitoring the Hall resistance. In both devices, we show that there exists a region, where its HC is strongly influenced by the direction of the applied current. In addition, we demonstrate the current induced domain wall movement under a fixed magnetic field. These phenomena can be observed at current density of 104 A/cm2 lower. Possible mechanisms are discussed.

A new scheme of magnetization reversal, an electrically assisted magnetization reversal, is realized in a carrier-induced ferromagnetic semiconductor (In, Mn)As structure. The demonstration has been done with field-effect transistors (FETs) having a thin (In, Mn)As channel by the application of a gate electric field to control the hole concentration p.

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.

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.

The magnetic anisotropy of the Pt/Co system under ionic liquid gating was studied. A comparison of results obtained using samples under the gating and those subjected to mild oxidization by oxygen plasma ashing suggested that the anodic oxidization of the Co layer could be one of the causes of the large modulation observed in the magnetic anisotropy. However, the charge accumulation effect was probably dominant when the Co layer was on the cathode side. The experiments presented here are expected to aid in elucidating the mechanism by which electric fields affect magnetism.

In this study, the strain-induced change in the magnetic anisotropy of perpendicularly magnetized thin metals (TbFeCo and Pt/Co/Pt) deposited on a polyethylene naphthalate flexible substrate was investigated. The in-plane uniaxial tensile strain was reversibly applied up to 2%. The magnetic anisotropy was reversibly changed in both samples with applied stress. In the TbFeCo film, a marked change in magnetic anisotropy energy of 1.2 × 105 J/m3 was observed. In the Pt/Co/Pt film, where the thickness of Co was 2–4 monolayers, the stress-induced changes in interface and volume contributions to magnetic anisotropy were individually determined.

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.

We investigated the electric field effect on magnetic anisotropy in a perpendicularly magnetized Pt/Co system with a top ultrathin layer of nonmagnetic Pd. By applying an electric field to the surface of the ferromagnetic Pd layer, we observed a clear modulation of the perpendicular magnetic anisotropy of the system. This result shows that the magnetic anisotropy can be modulated by an electric field even when nonmagnetic Pd is inserted at the interface formed by the magnetic layer and insulator. The electric field effect of the proximity-induced moment in Pd might contribute to the anisotropy modulation.

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 Lande 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.

We report the switching of a magnetic vortex core in ferromagnetic elliptical disks induced by a nanosecond field pulse. We show that the switching probability depends on both the duration and amplitude of the field pulse. The minimum magnetic field required for the core switching depends also on the ellipticity of the disk. Micromagnetic simulations reproduce this behavior and reveal that there are two mechanisms of the core switching.

The behavior of a three-terminal domain wall (DW) device with a perpendicularly magnetized CoFeB free layer and underlying hard magnets was investigated. In a Ta/CoFeB/MgO free layer without hard magnets, a current-induced DW motion in the direction of electron flow was observed. In a device having a hard magnet under each end of the free layer, we found that a DW nucleated by injecting current played an important role in the switching of magnetization. We concluded that the switching of magnetization in our device is due to the displacement in the direction of electron flow of the DW created by current. After deriving the principle of operation through experiments, we describe a way to reduce the current required for writing by increasing the thickness of the hard magnets.

We report a systematic study of the DW motion in perpendicularly magnetized Co/Ni nanowires having different thicknesses with structure inversion asymmetry. The transition from bulk to interfacial effects is confirmed as a change in the DW motion direction at a reduced layer thickness. The bias field dependence reveals that the adiabatic spin transfer torque (STT) dominates the DW motion in the thick regime, whereas the interfacial torque originating from the spin Hall effect (SHE) is responsible for the DW motion with a fixed DW chirality induced by the Dzyaloshinskii–Moriya interaction (DMI) in the thin regime.

We report the real-time observation of spin-wave propagation across a step inserted between two ferromagnetic films with different thicknesses. Because the dispersion relation of the spin wave depends on the thickness of the film, the step works as a junction to affect the spin wave propagation. When the spin wave transmits through the junction, the wavenumber undergoes modulation as per Snell's law, which states that the refraction index is proportional to the wavenumber. From the viewpoint of magnonics, the present achievement opens up new possibilities of controlling the wavenumber of spin waves.

Two important mechanisms for current-induced domain wall (DW) dynamics, namely, precessional DW motion driven by the adiabatic spin transfer torque and steady DW motion induced by the spin Hall torque, have been proposed and experimentally confirmed. However, the effect of the spin Hall torque on precessional DW motion has not been reported yet. Here, we show that the spin Hall torque affects the precessional DW motion when the in-plane field is applied. It is found that the in-plane field induces a half rotation time difference during DW precession, which gives rise to the nonvanishing spin Hall torque on the precessional DW motion.

The authors have investigated the effect of film deposition conditions on the electric field modulation of the coercivity in a perpendicularly magnetized MgO/Co/Pt system. In samples with the MgO capping layer deposited using high rf-sputtering power, the direction of coercivity change caused by applying an electric field was opposite to that in the previous results on the same system. This result indicates that the film deposition condition can strongly affect the electrical control of magnetic properties.

We show that the angle-dependent Hall measurement is an effective method to determine the current-induced effective fields by investigating MgO/Fe/Ta and MgO/Fe/Pt/Ta multilayer structures. The experimentally obtained Hall resistance under a relatively large dc electrical current is well described by considering two components of current-induced effective fields, which may be related to the spin Hall effect and the Rashba effect. The directions of the effective fields are consistent with and their magnitudes are comparable to those reported previously for similar multilayer structures.

In this study, we have investigated perpendicular magnetic anisotropy in a MgO/Co/Pt structure under an electric field. The change in the interface anisotropy energy in a MgO/Co stack upon the application of a gate voltage was determined by using a transport measurement technique. The result indicates that a decrease in the electron number at the surface of the Co layer leads to the enhancement of the perpendicular anisotropy energy at a temperature sufficiently lower than the Curie temperature.

In this study, we have investigated magnetic domain wall motion under gate electric fields in a perpendicularly magnetized Co/Pt wire. The motion under a transparent gate electrode was directly monitored using a magneto-optical Kerr effect microscope. The domain wall velocity was determined as a function of temperature and gate voltage. It was found that this velocity could be modulated by up to two orders of magnitude by changing the gate voltage.

The authors have investigated the current-induced magnetic domain wall (DW) motion in perpendicularly magnetized Co/Ni nanowire with structural inversion asymmetry (SIA). In this system, DW motion to the direction of electric current flow, not electron flow, and high DW velocity up to 110 m/s were confirmed, which have never been observed in Co/Ni systems without SIA. In addition, we found that the DW velocity showed a strong dependence on the perpendicular magnetic field in the range of ±100 Oe. These results suggest that DW in the Co/Ni nanowire with SIA moves in the steady mode, not in the precessional mode.

We show the effect of an applied electric field on the magnetization of an Fe ultrathin film. An electric double layer was formed at the interface between an ionic liquid and the Fe layer by the accumulation of ions caused by applying a gate voltage, and a large electric field was exerted on the Fe film surface. The saturation magnetization increased when the electron density at the Fe surface increased. A change in the saturation magnetization of ~50% was observed by applying a gate voltage of ±2 V.

We have investigated current-induced domain wall (DW) motion in a perpendicularly magnetized Co/Ni nanowire under in-plane (hard-axis) and perpendicular (easy-axis) external magnetic fields. The DW velocity was found to be almost independent of them in the range of ±50 Oe. The result shows that reliable device operation against an external magnetic field disturbance can be achieved using the present system.

We have investigated the effect of dc current on magnetic domain wall (DW) depinning fields in a perpendicularly magnetized Co/Ni nanowire by utilizing the giant magnetoresistance (GMR) effect. It was found that the current assisted (prevented) the domain wall depinning when the electron flow was parallel (antiparallel) to the DW depinning direction. The depinning field was found to exhibit a linear dependence on the dc current density, from which the efficiency of the effective field was estimated to be -1.5×10-14 T m2/A.

Current-induced magnetic domain wall (DW) displacement in a Co/Ni nano-wire with perpendicular magnetic anisotropy was investigated in real space by photoemission electron microscopy (PEEM) for the first time. DW velocity determined from the PEEM observation was 40 m s(-1) for the current density of 2.5 × 10(12) A m(-2), which was consistent with the result obtained by the electrical measurement used in our previous reports.

We have investigated the notch width dependence of the domain wall structure by measuring the wall resistances in Co/Ni nanowires with perpendicular magnetization. Wall resistance was abruptly increased below a notch width of 43 nm. By comparing the experimental results with theoretical calculations of wall resistance and the micromagnetic simulation of wall energy, this increase in wall resistance was found to be caused by the change in the wall structure from the Bloch to Néel wall caused by the decreased notch width.

Magnetic domain wall (DW) motion induced by currents in a perpendicularly magnetized Co/Ni nanowire has been investigated both at room temperature and low temperature. The threshold current density was almost independent of temperature, although the external magnetic field to depin a DW was doubled at low temperature. The result is consistent with the theory based on the adiabatic spin transfer model.

The authors have investigated magnetic domain wall motion induced by electric currents in ferromagnetic nano-wires made of Co/Ni multilayers. The thicknesses of Co and Ni layers were changed, whereas the numbers of layer stacks of Co and Ni were the same in all samples. The sample with thinner total Co/Ni thickness showed the lower threshold current density for the domain wall motion as an overall trend, which is qualitatively in agreement with the expectation by the theory based on the adiabatic spin-transfer model. The lowest threshold current density was 2.9×1011 A/m2 obtained in the sample with the total Co/Ni thickness of 3.4 nm and the wire width of 110 nm.

All-electrical control and local detection of multiple magnetic domain walls in perpendicularly magnetized Co/Ni nano-wires were demonstrated. A series of domain walls was reproducibly shifted in the same direction by the current, keeping the distance between the walls almost the same. Furthermore, the walls can be shifted back and forth depending on the direction of the pulsed currents.

The electrical detection of chirality in a magnetic vortex, which is the curling direction of a magnetization structure, is reported. Two electrodes, which were placed on a NiFe microdisc at asymmetric positions with respect to the center of the disc, were used to measure the resistance between them. The magnetoresistance curves with opposite asymmetries due to the anisotropic magnetoresistance effect were observed depending on the chirality of the vortex state, indicating that the asymmetrically placed electrodes are useful for the electrical detection of chirality.

The authors show experimental results on domain wall motion induced by electric current in a Co/Ni nano-wire with perpendicular magnetic anisotropy. The motion was detected electrically by using the anomalous Hall effect. Threshold current density for the domain wall motion was found to decrease with decreasing the wire width, where the minimum threshold current density of approximately 5×1011 A/m2 was observed for the wire width of 70 nm.