Tomohiro Koyama

• Ph.D
• Professor (Associate) at Osaka University

The application of % order biaxial tensile strain can control the perpendicular magnetic anisotropy (PMA) in a CoFeB/MgO system deposited on a flexible substrate. Magnetic anisotropy switch from perpendicular to in-plane was observed when increasing the strain. Reversibility of the anisotropy change has been confirmed. The mechanism of the anisotro...

Pt ultrathin films on ferromagnetic insulators have been widely studied for spintronics applications, and magnetic moments of interface Pt atoms were considered to be ferromagnetically ordered due to a magnetic proximity effect (MPE). An anomalous Hall effect (AHE) is usually used to examine an out-of-plane magnetic moments of the Pt layer. To tune...

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 measurement based on the rectification effect. The g-factor reflects on orbital magnetic moment, indicating that the electric field induced-modulations of m...

In this study, we investigated the modulation of the spin orbit torque (SOT) caused by inserting NiO layer at Pt/Co interface. The similar Pt/NiO/Co structure was deposited on two different substrates, Al 2 O 3 and Si/SiO x substrates. We found that the damping-like torque of the Al 2 O 3 type sample is almost independent of NiO thickness (t NiO )...

In a perpendicularly magnetized (PM) material with the interfacial Dzyaloshinskii–Moriya interaction (iDMI), a chirality-induced effective magnetic field (EMF) acts on a magnetic domain wall, which is a naturally formed in-plane magnetized (IM) region sandwiched by perpendicular (PM) regions. In this study, we artificially created a Pt/Co/MgO syste...

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....

We show that a CoFeB/MgO-based magnetic tunnel junction (MTJ) formed directly on a flexible substrate has considerable potential as a high-sensitivity strain gauge. A gauge factor of ∼1000, which represents the highest sensitivity reported, thus far, for a film-type strain gauge and is ∼500 times larger than that of the most prevalent metal-foil st...

We demonstrate current-induced switching of perpendicular magnetization without any external magnetic field by introducing lateral structural asymmetry through gate-induced redox reactions. A gate electrode was fabricated to cover only half of a heavy metal/ferromagnet/oxide wire, allowing a gate voltage to form an in-plane oxidation gradient. The...

We investigate current-induced spin-orbit torque (SOT) in Co/Pt/oxide systems by varying a sort of the oxide layer. When the Pt thickness is less than ∼2 nm, Pt at the interface of the oxide layer is magnetically polarized owing to the magnetic proximity effect. In these systems, fieldlike (FL) SOT depends significantly on the adjacent oxide materi...

This study investigates the electric-field effect on magnetic moments in perpendicularly magnetized Co ultra-thin films on a Pt underlayer. By applying electric fields to the Co surface through MgO/HfO2 insulating layers, we determined the change in the magnetic moment per one electrically increased electron. From systematic experiments at temperat...

This study investigates the effect of strain on the compensation temperature of ferrimagnetic Tb–Fe films formed on a flexible substrate. The compensation temperature is determined by the anomalous Hall measurement, and an application of 1.2% tensile strain reduces the compensation temperature by 12 K. X-ray magnetic circular dichroism reveals that...

Spin-dependent scattering at the nonmagnet/ferromagnet interface plays a key role in determining the amplitude of unidirectional spin Hall magnetoresistance (USMR), similar to giant magnetoresistance (GMR). We report the enhancement of USMR by inserting a thin Cu interlayer into the Pt/Co interface, where the Cu/Co system is well known to exhibit a...

Uniaxial-strain-induced switching between cooling and heating generated by the anisotropic magneto-Peltier effect (AMPE) is demonstrated using a Ni thin film deposited on an organic flexible substrate, where 1%-order strain can be introduced by stretching the substrate. By means of the lock-in thermography technique, we observed the reversible sign...

We have demonstrated a reversible control of magnetic anisotropy in Pt/Co and Pd/Co structures formed on a polyethylene naphthalate flexible substrate by stretching the substrate biaxially. Compared with the case of the uniaxial tensile strain, which has been previously studied, the in-plane biaxial tensile strain produces much larger compressive s...

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 har...

Electric field modulations of magnetism in ferromagnetic metals are in general ascribed to the electric field effects on the orbitals. However, a direct observation of the electric-field-induced changes in the orbital states of a ferromagnetic metal is lacking. In this Rapid Communication, we demonstrate that an electric field applied to an ultrath...

Spin-orbit torques (SOTs) acting in a Py/Pt/Co tri-layer, where the Py and Co layers are in-plane and perpendicularly magnetized, respectively, are investigated via the ferromagnetic resonance method. The resonance linewidth of the Py layer is clearly modulated by the DC bias current due to the damping-like SOT originated from the spin Hall effect...

This study investigates element-specific atomic distances and strains in ferromagnetic (FM) nano-thin films deposited on a flexible substrate, which can be modulated by stretching the substrate. Extended x-ray absorption fine structure (EXAFS) spectroscopy was used for the experiments. Both in-plane tensile and out-of-plane compressive strains were...

In creep regime, a magnetic domain wall (DW) is regarded as a one-dimensional object that consists of many small segments. However, observation of such a specific segment motion has remained a challenge due to its small size. Here we present direct evidence of individual DW segment motion by employing electrical Hall measurement. When the DW is mov...

In this report, current-induced spin–orbit torques (SOTs) in a permalloy (Py)/Pt/Co tri-layer system are investigated. In addition to the conventional SOTs generated from the spin Hall effect in the Pt layer, we observed a distinct existence of unconventional SOT acting on the in-plane magnetized Py layer in which the polarity of SOT reverses with...

The electric field (EF) modulation of magnetic domain wall (DW) creep velocity v in the Pt/Co/Pd structure with perpendicular magnetic anisotropy (MA) has been studied. The structures with different Co thicknesses tCo up to ∼1 nm are investigated. In all samples, applying a gate voltage induces a clear change in v. Thicker samples provide a higher...

We demonstrated the electric field (EF) effect on exchange bias (EB) in a perpendicularly magnetized Co/CoOx layered structure. The antiferromagnetic CoOx layer was formed by naturally oxidizing the Co surface. The modulation of the EB field and the coercivity by gate voltage application through a dielectric layer was clearly observed below the blo...

We have investigated the electric field effect on magneto-thermoelectric conversion in ultrathin Co films with a naturally-oxidized surface and a solid-state capacitor structure. By means of the thermoelectric imaging technique based on the lock-in thermography, we demonstrate the reversible on–off switching of heat currents generated by the anomal...

This study investigates the effect of high-temperature (350–500 °C) annealing on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) directly formed on a flexible polyimide substrate, which has superior thermal tolerance. As the annealing temperature increases, the tunnel magnetoresistance (TMR) ratio enhances and reaches up to ∼200% at an annealing t...

The evolution of information technology has been driven by the discovery of new forms of large magnetoresistance, such as giant magnetoresistance1,2 and tunnelling magnetoresistance3,4, in magnetic multilayers. Recently, new types of this effect have been observed in much simpler bilayers consisting of ferromagnetic and non-magnetic thin films5–10....

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 th...

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...

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 cent...

The evolution of information technology has been driven by the discovery of new forms of large magnetoresistance (MR), such as giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) in magnetic multilayers. Recently, new types of MR have been observed in much simpler bilayers consisting of ferromagnetic (FM)/nonmagnetic (NM) thin film...

There is a need to control magnetic properties at a desired location in a magnetic film towards a realization of fundamental devices, such as domain wall logic or magnonic applications. Here, we demonstrate the formation of a magnetic domain structure at a desired location in a Pt/Co film, using electrical gating with a meshed gate electrode and sw...

We show that the electric field (EF) can control the domain wall (DW) velocity in a Pt/Co/Pd asymmetric structure. With the application of a gate voltage, a substantial change in DW velocity up to 50 m/s is observed, which is much greater than that observed in previous studies. Moreover, modulation of a DW velocity exceeding 100 m/s is demonstrated...

The electric field effect on the magnetism in a MgO/Pd/Co system, in which a magnetic moment is induced in the Pd layer owing to the ferromagnetic proximity effect, has been investigated using various experimental methods. An electric field was applied to the surface of the Pd layer through a solid-state HfO2/MgO dielectric bilayer by applying a ga...

We report that the spin-orbit torque (SOT) in a Co/Pt structure is significantly enhanced by partial oxidation of the Co-side interface. The interface-oxidized and unoxidized Co/Pt samples were prepared to compare the SOTs, which were determined using the harmonic Hall measurement. Even though an insulating CoOx layer exists at the Co/Pt interface,...

The Curie temperature is one of the most fundamental physical properties of ferromagnetic materials and can be described by Weiss molecular field theory with the exchange interaction of neighboring atoms. Recently, the electric-field-induced modulation of the Curie temperature has been demonstrated in transition metals. This can be interpreted as i...

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...

A vertical spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) is a promising low-power device for the post scaling era. Here, using a ferromagnetic-semiconductor GaMnAs-based vertical spin MOSFET with a GaAs channel layer, we demonstrate a large drain-source current IDS modulation by a gate-source voltage VGS with a modulation rat...

Chiral spin textures of a ferromagnetic layer in contact to a heavy non-magnetic metal, such as Néel-type domain walls and skyrmions, have been studied intensively because of their potential for future nanomagnetic devices. The Dyzaloshinskii-Moriya interaction (DMI) is an essential phenomenon for the formation of such chiral spin textures. In spit...

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 rever...

Electric field effects on magnetism in metals have attracted widespread attention, but the microscopic mechanism is still controversial. We experimentally show the relevancy between the electric field effect on magnetism and on the electronic structure in Pt in a ferromagnetic state using element-specific measurements: x-ray magnetic circular dichr...

We have investigated the electric field (EF) effect on magnetism in a Co thin film with a naturally oxidized surface. The EF was applied to the oxidized Co surface through a gate insulator layer made of HfO2, which was formed using atomic layer deposition (ALD). The efficiency of the EF effect on the magnetic anisotropy in the sample with the HfO2...

A vertical spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) is a promising low-power device for the post scaling era. Here, using a ferromagnetic-semiconductor GaMnAs-based vertical spin MOSFET with a GaAs channel layer, we demonstrate a large drain-source current IDS modulation by a gate-source voltage VGS with a modulation rat...

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 t...

We studied the effect of electric field (EF) on the coercivity of two Pt/Co structures with different Co layer thicknesses. Opposite signs of the coercivity change by the gate voltage application are observed in these samples, whereas the sign of the magnetic anisotropy change is the same. We performed direct observations of the domain structures d...

Manipulation of magnetization using current-induced torque is crucial for magnetic recording devices. Recently, the spin-orbit torque (SOT) that emerges in a ferromagnetic thin film on a heavy metal is focused as a new scheme for magnetization switching in perpendicularly magnetized systems. Since the SOT provides a perpendicular effective field to...

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 ioni...

The Curie temperature is one of the most fundamental physical properties of ferromagnetic materials and can be described by Weiss molecular field theory with the exchange interaction of neighboring atoms. Recently, the electric-field-induced modulation of the Curie temperature has been demonstrated in transition metals. This can be interpreted as i...

We investigated the effect of surface oxidation of a ferromagnetic Co layer on current-induced spin-orbit torque. A Co thin film was deposited on a Pt underlayer, for which the spin current was expected to be generated through the spin Hall effect. Both the damping- and field-like torques quantitatively determined using harmonic Hall voltage measur...

The authors have investigated magnetic domain wall (DW) motion induced by electrical current in the perpendicularly magnetized Pt/Co/Pd and Pd/Co/Pt structures. In these systems, owing to the spin-orbit torque (SOT) and the interfacial Dzyaloshinskii-Moriya interaction (DMI), the DW moves in the current flowing direction. On the other hand, the thr...

An asymmetric magnetoresistance (MR) is investigated in Py/Pt bilayers. The asymmetric MR linearly increases with current density up to a threshold, and increases more rapidly above the threshold. To reveal the origin of threshold behavior, we investigate the magnetic field dependence of the asymmetric MR. It is found that the magnetic field strong...

An asymmetric magnetoresistance (MR) is investigated in Py/Pt bilayers. The asymmetric MR linearly increases with current density up to a threshold, and increases more rapidly above the threshold. To reveal the origin of threshold behavior, we investigate the magnetic field dependence of the asymmetric MR. It is found that the magnetic field strong...

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 va...

Chiral spin textures at the interface between ferromagnetic and heavy nonmagnetic metals, such as Neel-type domain walls and skyrmions, have been studied intensively because of their great potential for future nanomagnetic devices. The Dyzaloshinskii-Moriya interaction (DMI) is an essential phenomenon for the formation of such chiral spin textures....

We used x-ray absorption spectroscopy and x-ray magnetic circular dichroism to investigate the effects of inserting Cu into Co/Pt interfaces, and found that a 0.4-nm-thick inserted Cu layer showed perpendicularly magnetized properties induced by the proximity effect through the Co and Pt layers. The dependence of the magnetic properties on the thic...

The switching of the magnetization direction induced by the spin-orbit torque in a perpendicularly magnetized Pt/Co/Pd structure has been investigated using X-ray magnetic circular dichroism. An electrical current injected parallel to the layers generates a spin current in a direction that is normal to the current in the Pt layer, and this spin cur...

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 ma...

The dielectric and magnetic properties of electric double layer (EDL) capacitor structures with a perpendicularly magnetized Pt/Co/Pt electrode and an insulating cap layer (MgO) are investigated. An electric field is applied through a mixed ionic liquid/MgO barrier to the surface of the top Pt layer, at which the magnetic moment is induced by the f...

We propose a current-in-plane spin-valve field-effect transistor (CIP-SV-FET), which is composed of a ferromagnet / nonferromagnet / ferromagnet trilayer structure and a gate electrode. This is a promising device alternative to spin metal-oxide-semiconductor field-effect transistors. Here, we fabricate a ferromagnetic-semiconductor GaMnAs-based CIP...

We propose a current-in-plane spin-valve field-effect transistor (CIP-SV-FET), which is composed of a ferromagnet / nonferromagnet / ferromagnet trilayer structure and a gate electrode. This is a promising device alternative to spin metal-oxide-semiconductor field-effect transistors. Here, we fabricate a ferromagnetic-semiconductor GaMnAs-based CIP...

A tensile strain on the order of a few percent was created in Ni thin films deposited on a flexible polyethylene naphthalate substrate, and the strain-induced change in the magnetic anisotropy was investigated. The magnetic easy axis was reversibly switched by 90° by the application of the stress. The easy axis was orthogonal to the applied stress....

We report on the temperature dependence of the magnetic anisotropy in Co/Pd/MgO system, in which magnetic moment in Pd is induced by the magnetic proximity effect. We demonstrate that the magnetic anisotropy is modulated by applying an electric field to the Pd surface. At temperatures below 100K, we find the nonlinear electric-field dependence of t...

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 anisotrop...

Electric field effect on magnetism is an appealing technique for manipulating the magnetization at a low cost of energy. Here, we show that the local magnetization of the ultra-thin Co film can be switched by just applying a gate electric field without an assist of any external magnetic field or current flow. The local magnetization switching is ex...

The electric field (EF) effect on the magnetic domain structure of a Pt/Co system was studied, where an EF was applied to the top surface of the Co layer. The width of the maze domain was significantly modified by the application of the EF at a temperature slightly below the Curie temperature. After a detailed analysis, a change in the exchange sti...

The electric field (EF) effect on the magnetic domain structure of a Pt/Co system was studied, where an EF was applied to the top surface of the Co layer. The width of the maze domain was significantly modified by the application of the EF at a temperature slightly below the Curie temperature. After a detailed analysis, a change in the exchange sti...

Current-induced magnetization excitation is a core phenomenon for next-generation magnetic nanodevices, and has been attributed to the spin-transfer torque (STT) that originates from the transfer of the spin angular momentum between a conduction electron and a local magnetic moment through the exchange coupling. However, the same coupling can trans...

Electric field effect on magnetism is an appealing technique for manipulating the magnetization at a low cost of energy. Here, we show that the local magnetization of the ultra-thin Co film can be switched by just applying a gate electric field without an assist of any external magnetic field or current flow. The local magnetization switching is ex...

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 app...

We investigated the effect of electric current on the magnetic-field-driven magnetic domain wall (DW) creep velocity in ultrathin Co with perpendicular magnetic anisotropy deposited on a Pt underlayer. The DW velocity was considerably modulated by the current, and its field dependence deviated from the scaling law with the critical exponent of 1/4,...

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 magn...

Spin orbit torque has been intensively investigated because of its high energy efficiency in manipulating a magnetization. Although various methods for measuring the spin orbit torque have been developed so far, the measurement results often show inconsistency among the methods, implying that an electromotive force, such as Nernst effect, irrelevan...

Several magnetic properties have recently become tunable with an applied electric field. Particularly, electrically controlled magnetic phase transitions and/or magnetic moments have attracted attention because they are the most fundamental parameters in ferromagnetic materials. In this study, we showed that an electric field can be used to control...

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 Landg-factor, the effective demagnetizing field,...

The interaction of propagating dipolar spin waves with magnetic domain walls is investigated in square-shaped microstructures patterned from the Heusler compound Co$_2$Mn$_{0.6}$Fe$_{0.4}$Si. Using magnetic force microscopy, the reversible preparation of a Landau state with four magnetic domains separated by N\'eel domain walls is confirmed. A loca...

We report the relation between current induced effective fields and ferromagnetic layer thickness. Hall measurements with rotating magnetic field show that the transverse and perpendicular effective fields have linear relations to the inverse of the magnetic moment m per area S. The results imply that both of these effective fields may originate fr...

The Pt thickness dependence of the Curie temperature of perpendicularly magnetized ultra-thin (Pt/)Co/Pt films has been investigated by magnetization measurements. The Curie temperature and the saturation magnetic moment increase with the Co layer thickness and even with the Pt layer thickness. The Curie temperature is found to have linear dependen...

A cobalt ultrathin film, which shows a large change in Curie temperature by an electric field application, has been studied by X-ray reflectometry with applying electric fields. The cobalt film was made by the sputtering method on top of a Pt buffer layer, and capped with a MgO layer. X-ray reflectometry shows that the change in Co thickness caused...

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...

The bit-to-bit distribution of a critical current density for magnetic domain wall (DW) motion is studied using Co/Ni wires with various wire widths (ws). The distribution inherently decreases with the w, and the ratio of standard deviation to average is 9.8% for wires with w = 40 nm. It is found that a self-distribution within one device, which is...

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. He...

We report the magnetoresistance fluctuation in a submicrometer-sized Hall-bar made of epitaxial Bi 2 Se 3 thin film. We observed the magnetoresistance fluctuation below 18 K and analyzed the fluctuation as universal conductance fluctuation (UCF). According to our results, the temperature dependence is quantitatively explained by the conventio...

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 th...