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M.Uenohara, H.Nishio, K.Toyoki, et al. Magnetic properties and magnetic domain observation of ndfeb sintered magnets treated by grain boundary diffusion process with dyal co-sorption. Materials Transactions 62, 1216 (2021); https://doi.org/10.2320/matertrans.MT-M2020389., Effective grain boundary diffusion (GBD) process with DyAl co-sorption is applied to enhance the coercivity of NdFeB sintered magnets. The coercivity of the magnet (HcJ = 1789 kA m⁻¹) subjected to the present GBD treatment was observed to be superior to that of the untreated magnet (HcJ = 1003 kA m⁻¹) and the conventional GBD magnet (HcJ = 1661 kA m⁻¹) treated with DyAl alloy. In the present GBD magnet, the DyAl co-sorption process facilitated Dy diffusion into the center region of the magnet (thickness: 3.5 mm), resulting in high coercivity. Further, magnetic domain observations were made using magnetic force microscopy (MFM) to observe the thermal demagnetization behavior of the present GBD magnet. The present GBD magnet suppressed the continuous domain reversal of adjacent grains; thus, the partially persistent single-domain structure remained, even at 453 K.

To overcome a bottleneck in spintronic applications such as those of ultralow-power magnetoresistive random-access memory devices, the electric-field control of magnetization vectors in ferromagnetic electrodes has shown much promise. Here, we show the giant converse magnetoelectric (CME) effect in a multiferroic heterostructure consisting of the ferromagnetic Heusler alloy Co2FeSi and ferroelectric-oxide Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) for electric-field control of magnetization vectors. Using an in-plane uniaxial magnetic anisotropy of polycrystalline Co2FeSi film grown on PMN-PT(011), the nonvolatile and repeatable magnetization vector switchings in remanent states are demonstrated. The CME coupling coefficient of the polycrystalline Co2FeSi/PMN-PT(011) is over 1.0 × 10−5 s/m at room temperature, comparable to those of single-crystalline Fe1-xGax/PMN-PT systems. The giant CME effect has been demonstrated by the strain-induced variation in the magnetic anisotropy energy of Co2FeSi with an L21-ordered structure. This approach can lead to a new solution to the reduction in the write power in spintronic memory architectures at room temperature.

Hirofumi Ekawa, Jiaqi Shen, Kentaro Toyoki, Ryoichi Nakatani, and Yu Shiratsuchi, "Gate-induced switching of perpendicular exchange bias with very low coercivity in Pt/Co/Ir/Cr2O3/Pt epitaxial film", Appl. Phys. Lett. 122, 062404 (2023) https://doi.org/10.1063/5.0131695., We investigate magnetoelectric switching of perpendicular exchange bias with very low coercivity in a Pt/Co/Ir/Cr2O3/Pt epitaxial film. We also optimize the suitable Ir spacer thickness so that the film exhibits the perpendicular exchange bias greater than the coercivity up to the vicinity of the Néel temperature. Main impact of the Ir spacer layer is the significant reduction of coercivity less than 0.5 mT in maintaining both the perpendicular magnetic anisotropy and the perpendicular exchange bias. For the suitable structure, the perpendicular exchange bias was isothermally switched by the gate voltage in combination with the magnetic field. Analysis of the hysteresis of the exchange bias field as a function of the gate voltage suggested that the magnetoelectric coefficient was comparable to the reported value. This implies that the Ir layer does not degrade the efficiency to transfer the magnetoelectrically controlled antiferromagnetic order parameter to the ferromagnetic Pt/Co/Ir trilayer.

We investigate the magneto-optic Kerr effect in perpendicularly magnetized Pt/Co/Ir/Cr2O3/Pt thin films, associated with the antiferromagnetic–paramagnetic transition of the Cr2O3 layer. The magneto-optic Kerr rotation angle (θ K) shows oscillatory behavior as a function of the photon energy of incident light owing to interference in the Cr2O3 layer. The temperature dependence of θ K at 2.67 eV (λ = 465 nm), at which the largest θ K is obtained, shows a sharp dip at 287.0 K. The dip temperature is similar to the reported Néel temperature for Cr2O3 thin films. Although the θ K spectra measured at several temperatures are generally explained by the classical interference model, θ K is enhanced at 2.36–2.79 eV (λ = 525–445 nm) close to the dip temperature. This peculiar enhancement in θ K is discussed on the basis of the anomaly in the optical parameters of the Cr2O3 layer associated with the antiferromagnetic–paramagnetic transition.

Increase in the Néel temperature ( TN) of the 10-nm thick Cr2O3(0001) thin films by the lattice strain was experimentally investigated. TN was determined based on the zero-offset anomalous Hall measurements for the Pt/Cr2O3/Pt epitaxial trilayer. The lattice strain was altered by the Pt buffer layer thickness and was evaluated by the lattice parameters. TN was increased from 241.5 to 260.0 K by varying the Pt buffer layer thickness from 0 to 20 nm. For the film without the Pt buffer layer, the apparent critical exponent near TN increased suggesting the distribution of TN due to the inhomogeneous strain. The TN value was weakly correlated with the axial ratio c/ a and the lattice volume of the Cr2O3 layer V; TN increases with decreasing c/ a or increasing V. The increase in TN by the reduction in c/ a (or by increasing V) implies that the exchange coupling between the nearest neighbor Cr3+ spins has the significant role in the strain effect.

Magnetoelectric (ME) effect is a type of cross-coupling between unconjugated physical quantities, such as the interplay between magnetization and electric field. The ME effect requires simultaneous breaking of spatial and time inversion symmetries, and it sometimes appears in specific antiferromagnetic (AFM) insulators. In recent years, there has been a growing interest for applying the ME effect to spintronic devices, where the effect is utilized as an input method for the digital information. In this article, we review the recent progress of this scheme mainly based on our own achievements. We particularly focus on several fundamental issues, including the ME control of the AFM domain state, which is detectable through the perpendicular exchange bias polarity. The progress made in understanding the switching mechanism, interpretation of the switching energy, switching dynamics, and finally, the future prospects are included.

We have investigated an analytic formula of the 1-dimensional magnetic skyrmion dynamics under external magnetic field gradient. We find excellent agreement between the analytical model and micromagnetic simulation results for various magnetic parameters such as the magnetic field gradient, Gilbert damping constant. We also observe much faster velocity of the chiral domain wall (DW) motion. The chiral DW is exist with smaller interfacial Dzyaloshinskii-Moriya interaction energy density cases. These results provide to develop efficient control of skyrmion for spintronic devices., 15 pages, 5 figures, 1 table

The interface between a magnetic material and a heavy metal that has a large spin-orbit interaction is at the root of various spin-related phenomena. In this paper, we address the peculiar spin-dependent transport at a $\mathrm{Pt}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interface by exploring the origin of the nonlinear anomalous Hall effect (AHE) in $\mathrm{Pt}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ bilayers. X-ray magnetic circular dichroism (XMCD) measurements show no appreciable magnetic moment at the interface originating from $\mathrm{Cr}$ 3d and $\mathrm{Pt}$ 5d orbitals, which could be associated with the AHE response. A possible interfacial magnetic moment M at the $\mathrm{Pt}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interface, assumed from the detection limit of the XMCD measurements, yields an anomalous Hall conductivity ($\ensuremath{\sigma}\mathrm{AHE}$) per unit net magnetic moment (M), \ensuremath{-}${\ensuremath{\sigma}}_{\mathrm{AHE}}$/M, of 0.57 ${\mathrm{V}}^{\ensuremath{-}1}$, which is extraordinary large compared with that for general magnetic materials. Together with first-principles calculations, the results suggest the possibility of an intrinsic AHE in the $\mathrm{Pt}/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ interface that does not rely on the net magnetic moment.

The magnetoelectric (ME) effect is one of the methods for electrically controlling the magnetization direction. In this study, we investigated the ME-driven domain wall creep and depinning using a Pt/Co/Au/ME-Cr2O3/Pt thin film. The domain switching process is governed by domain wall propagation rather than the nucleation of reversed domains, similar to a pure ferromagnet. The domain wall velocity v increases exponentially with the ME pressure, that is, the simultaneous application of magnetic H and electric E fields. The v– E curve under a constant H can be scaled by the ME pressure with the assistance of the exchange bias. We determined the depinning threshold, pinning energy scale, and depinning velocity based on the model for the magnetic domain wall creep. Compared with the depinning velocity in various other systems, it was suggested that the ME-driven mechanism could yield a fast domain wall velocity utilizing the low pressure.

Yu Shiratsuchi, Yiran Tao, Rou Tsutsumi, Kentaro Toyoki, and Ryoichi Nakatani, Journal of Applied Physics 130, 193902 (2021); https://doi.org/10.1063/5.0060606., The magnetization of a nanosized magnet, such as an ultrathin film, thermally fluctuates and can become superparamagnetic. In ferromagnetic/antiferromagnetic thin films, superparamagnetism can be suppressed in accordance with antiferromagnetic ordering. The exchange bias can also be induced at the ferromagnetic/antiferromagnetic interface, and it is nontrivial whether the superparamagnetic blocking temperature (TB_SPM) can match either the onset temperature of the exchange bias (TB_EB) or the Néel temperature (TN). In this study, we investigated the temperature dependence of parameters such as coercivity, exchange bias field, magneto-optic Kerr rotation (θK), and AC magnetization (MAC) to elucidate the matching of TB_EB, TB_SPM, and TN in a Pt/Co/Au/Cr₂O₃/Pt thin film. Based on the temperature dependences of MAC, TB_SPM was yielded as about 283 K. TB_EB and TN, which were determined based on the temperature dependence of θK, were 278 and 282 K, respectively. TB_SPM was almost equal to TN but TB_EB was smaller. This temperature lag was caused by the difference in the magnetic anisotropy energy required to induce the exchange bias and suppress superparamagnetism.

Antiferromagnetic Cr1-xAlx with, x ~ 0.25, is a pseudo-gap semimetal. This study focuses the relationship between the pseudo-gap and the dominant carriers by comparing two films with the same composition but different chemical ordering. The film deposited at high temperatures with C11b ordering exhibited a weak temperature dependence of resistivity and a positive Hall coefficient. In contrast, the film deposited at low temperature exhibited negative temperature dependence of resistivity and a change in the Hall coefficient from negative to positive with increasing temperature. The temperature derivative of the Seebeck coefficient, dαs/dT, was positive for the C11b ordered film, but reversed from negative to positive for the film without C11b ordering. These results indicate that pseudo-gap formation is relevant to the change in the dominant carrier to electrons at low temperatures. Additionally, the change in the dominant carriers by temperature should be related to a low effective mass of holes.

Magnetoelectric (ME) effect is a type of cross-coupling between unconjugated physical quantities, such as the interplay between magnetization and electric field. The ME effect requires simultaneous breaking of spatial and time inversion symmetries, and it sometimes appears in specific antiferromagnetic (AFM) insulators. In recent years, there has been a growing interest for applying the ME effect to spintronic devices, where the effect is utilized as an input method for the digital information. In this article, we review the recent progress of this scheme mainly based on our own achievements. We particularly focus on several fundamental issues, including the ME control of the AFM domain state, which is detectable through the perpendicular exchange bias polarity. The progress made in understanding the switching mechanism, interpretation of the switching energy, switching dynamics, and finally, the future prospects are included.

M. Yamada, Y. Shiratsuchi, H. Kambe, K. Kudo, S. Yamada, K. Sawano, R. Nakatani, and K. Hamaya, Journal of Applied Physics 129, 183901 (2021); https://doi.org/10.1063/5.0048321., Using artificially controlled ferromagnet (FM)-semiconductor (SC) interfaces, we study the decay of the nonlocal spin signals with increasing temperature in SC-based lateral spin-valve devices. When more than five atomic layers of Fe are inserted at the FM/SC interfaces, the temperature-dependent spin injection/detection efficiency (P inj /det) can be interpreted in terms of the T3/2 law, meaning a model of the thermally excited spin waves in the FM electrodes. For the FM/SC interfaces with the insufficient insertion of Fe atomic layers, on the other hand, the decay of P inj /det is more rapid than the T3/2 curve. Using magneto-optical Kerr effect measurements, we find that more than five atomic layers of Fe inserted between FM and SC enable us to enhance the ferromagnetic nature of the FM/SC heterointerfaces. Thus, the ferromagnetism in the ultra-thin FM layer just on top of SC is strongly related to the temperature-dependent nonlocal spin transport in SC-based lateral spin-valve devices. We propose that the sufficient ferromagnetism near the FM/SC interface is essential for high-performance FM-SC hybrid devices above room temperature.

We experimentally show a giant magnetoelectric (ME) effect at room temperature in an interfacial multiferroic heterostructure consisting of L21-ordered Co2FeSi and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). Molecular beam epitaxy growth at 400 °C enables us to obtain epitaxial and L21-ordered Co2FeSi films on PMN-PT(001). For the epitaxial Co2FeSi/PMN-PT heterostructure, the remanent magnetization state can be largely modulated by varying electric fields. We note that the room-temperature ME coupling coefficient (α) is estimated to be 6.0–6.3 × 10 − 6 s/m, comparable to the highest α value reported previously. Nonvolatile and repeatable magnetization changes in remanent states are also demonstrated. These results will pave the way for room-temperature electric-field control of the magnetization of half-metallic Heusler alloys in high-performance spintronic devices.

We propose the use of artificial spin ice (ASI) for reservoir computing and evaluate its performance from simulation results. Our ASI reservoir is formed by 72 magnets arranged in a honeycomb lattice and the temperature is 0 K. A pseudo-random binary sequence is sent to the reservoir and its status is updated by external magnetic fields. Short-term memory capacity of 3.5 and nonlinear computational capacity of 2.9 are achieved when the strength of the magnetic fields is near the switching field of the magnets. The performance can be optimized by tuning the aspect ratios of the magnets.

Magnetic domain pattern and magnetic domain wall motion are particularly important to understand the magnetization process. Here, we investigated the magnetization process of perpendicularly exchange-biased Pt/Co/Au/Cr2O3/Pt stacked films based on observations of the magnetic domain. In particular, in contrast to previous studies which use fully exchange-biased state, we used the bi-exchange-biased state. We found that the magnetic domain pattern at the remanent state was robust against magnetic-field cycling, which is relevant to the absence of the training effect. The magnetization process was followed by domain wall propagation in the increasing branch of the magnetization curve. In the decreasing branch, both nucleation of the reversed domain and domain wall propagation were involved. The former was accompanied by latency, suggesting that thermal activation played a significant role in the nucleation of the reversed domain.

The proximity effect in a heavy metal attached to a ferromagnetic (FM) layer results in varied interface magnetism. In this study, we investigated the critical behavior of the perpendicular exchange bias in a Ru/Pd/Co/Pt/Cr2O3/Pd stacked film, with focus on the influence of the Pt spacer layer. The apparent critical exponent β increases with increasing Pt thickness. We discussed the possible mechanism from the perspective of the induced magnetic moment of Pt and interfacial exchange coupling. The former was explored based on X-ray magnetic circular dichroism, which revealed that the magnetic moment of Pt was almost temperature-independent within the studied temperature regime. This suggests that the interfacial FM moment is stable. The latter, i.e., the reduction of the interfacial exchange coupling by the Pt spacer layer, is a possible cause of the increase in β, as theoretically predicted.

The magnetic domain wall energy density σ W of a Ni/Co superlattice possessing perpendicular magnetic anisotropy was determined using the magnetic domain theory derived by Kooy and Enz (1960). To determine σ W , we obtained the saturation magnetization, magnetic domain period, and perpendicular magnetic anisotropy energy by individual measurements. Using the magnetic domain period and the ferromagnetic layer thickness, we first determined the dipolar length. The estimated dipolar length was about 15–25 nm, which is in good agreement with the change in the magnetization curve with the ferromagnetic layer thickness. By using the dipolar length and saturation magnetization, the σ W was calculated to be 4–7 erg/cm 2 .

Reservoir computing (RC), which utilizes physical phenomena, is a candidate for low energy consumption recurrent neural networks. Recently, we proposed RC which uses a dipole-coupled nanomagnet array as a reservoir, which applies the direction of the static magnetic moment of each nanomagnet as a node state in the array. It is possible to learn binary tasks with one-bit input data using this reservoir. Moreover, more complex tasks such as motion detection utilize multi-bit input data. This study simulates a two-bit binary input RC by employing the dipole coupled nanomagnet array as a reservoir. We apply a binary task with two-bit input data, which requires a memory capacity and non-linear computing capability of the reservoir. Due to the simulations with a macrospin model at a temperature of 0 K, this RC was able to learn these binary tasks by utilizing 24 nanomagnets.

We demonstrate reservoir computing using a dipole-coupled magnetic-dot array by simulations with a macro-spin model at 0 K. The magnetic moment of the dot is used for the node state of the reservoir. We perform short-term-memory (STM) and parity check (PC) tasks to evaluate the performance of the reservoir. Binary information was written to the input magnetic dots as a direction of the magnetic moment. A voltage-induced magnetic-anisotropy change was used to update the node state. We investigated the STM capacity (STMC) and PC capacity (PCC) dependence on the number of magnetic dots by means of randomly generated node-update procedures. As a result, the STMC and PCC are strongly dependent on the node-state update procedure.

ZnF2 thin films were deposited on SiO2-glass and Al2O3-single crystal substrates using electron-beam evaporation, and the morphology of the films was studied in relation to the kind of substrate and deposition conditions. While the films deposited on SiO2-glass, (11 2 ¯ 0)-Al2O3 substrates exhibited an almost non-oriented polycrystalline nature, the films deposited on (0001)-Al2O3 substrates were epitaxially grown (100)-oriented films. The full-width at half maximum of the rocking curve of (200)-diffraction for the (100)-oriented film deposited at 400 °C was 150″. This value was comparatively small compared with previously reported (100)-oriented SnO2 and TiO2 films and showed the high orientation and crystallinity of the ZnF2 film. From the transmission spectra from near-infrared (NIR) to vacuum ultraviolet (VUV), the optical band gap of the ZnF2 was evaluated to be 6.7 eV, and the films exhibited high transparency in the wide range from NIR to VUV. We discuss the potential of the film as a transparent conducting material in the VUV region.

We investigated the perpendicular exchange anisotropy of Pt(111)/Co1-x Ni x(111)/α-Cr2 O 3(0001) thin films. In order to address the inherent influence of the ferromagnetic layer composition, we also investigated the structure and the change in the perpendicular magnetic anisotropy (PMA) with the ferromagnetic layer composition. From the ferromagnetic layer thickness dependence of the PMA energy density, the interface contribution to the PMA was determined. The results suggest that the PMA is induced at the Co1-x Ni x(111)/α-Cr2 O 3(0001) interface as well as at the Pt(111)/Co 1-xNix(111) interface. Below the Ni composition of 0.5, the high perpendicular exchange anisotropy of about 0.34 erg/cm 2 was observed. The perpendicular exchange anisotropy energy is insensitive to the ferromagnetic layer composition, which should be explained by the interfacial uncompensated antiferromagnetic spins.

We investigated the control of the perpendicular exchange bias (PEB) in Pt/Co/α-Cr2O3 (0001) thin films that exhibit a unique temperature dependence of the PEB. Previously, we demonstrated that the onset of the PEB is explained by the competition between the interface coupling energy and the magnetic anisotropy energy in the α-Cr2O3 layer. Based on this finding, in this study, we investigated the influence of a Pt spacer layer at the Co/α-Cr2O3 interface to control the interface exchange coupling. By inserting a Pt spacer layer, the unidirectional magnetic anisotropy energy JK was reduced because of the weakened exchange coupling between Co and α-Cr2O3. The weakened direct exchange coupling caused an increase in the onset temperature of JK, resulting in a reduction in the α-Cr2O3 layer thickness necessary to maintain the PEB up to the Neel temperature. In addition, the change in the interface exchange coupling mechanism between Co and α-Cr2O3 is discussed with respect to the temperature dependence of JK.

The effect of the crystalline quality of ultrathin Co films on perpendicular exchange bias (PEB) has been investigated using a Au/Co/Au/α-Cr 2 O 3 thin film grown on a Ag-buffered Si(1 1 1) substrate. Our investigation is based on the effect of the Au spacer layer on the crystalline quality of the Co layer and the resultant changes in PEB. An α-Cr 2 O 3 (0 0 0 1)layer is fabricated by the thermal oxidization of a Cr(1 1 0) thin film. The structural properties of the α-Cr 2 O 3 (0 0 0 1) layer including the cross-sectional structure, lattice parameters, and valence state have been investigated. The fabricated α-Cr 2 O 3 (0 0 0 1) layer contains twin domains and has slightly smaller lattice parametersthan those of bulk-Cr 2 O 3 . The valence state of the Cr 2 O 3 (0 0 0 1) layer is similar to that of bulk Cr 2 O 3 . The ultrathin Co film directly grown on the α-Cr 2 O 3 (0 0 0 1) deposited by an e-beam evaporator is polycrystalline. The insertion of a Au spacer layer with a thickness below 0.5 nm improves the crystalline quality of Co, probably resulting in hcp-Co(0 0 0 1). Perpendicular magnetic anisotropy (PMA) appears below the Neel temperature of Cr 2 O 3 for all the investigated films. Although the PMA appears independently of the crystallinequality of Co, PEB is affected by the crystalline quality of Co. For the polycrystalline Co film, PEB is low, however, a high PEB is observed for the Co films whose in-plane atom arrangement is identical to that of Cr 3+ in Cr 2 O 3 (0 0 0 1). The results are qualitatively discussed on the basis of the direct exchange coupling between Cr and Co at the interface as the dominant coupling mechanism.

Yu Shiratsuchi, Hiroaki Yoshida, Yoshinori Kotani, Kentaro Toyoki, Thi Van Anh Nguyen, Tetsuya Nakamura, and Ryoichi Nakatani, APL Materials 6, 121104 (2018); https://doi.org/10.1063/1.5053928., We observed the magnetoelectric induced domain wall propagation in a Pt/Co/Au/Cr₂O₃/Pt stacked thin film based on magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. The antiferromagnetic (Cr₂O₃) domain wall velocity was estimated by a quasi-static approach using a pulsed voltage. At a pulse voltage amplitude of -12 V, corresponding to an electric field of -8.0 × 10²kV/cm, the domain wall velocity was very low, at 0.3 m/s. The domain wall velocity increased with increasing voltage amplitude, reaching 22 m/s at -20 V (-1.3 × 10³kV/cm). The change in the domain wall velocity with the applied voltage amplitude indicates the creep motion of the domain wall. Using a phenomenological model, we estimated the domain wall depinning energy, and found that the bulk and interface terms of the magnetic anisotropy affect the effective magnetic field to the same degree, suggesting that the magnetic domain wall motion may be controllable by the antiferromagnetic layer thickness.

Yu Shiratsuchi, Shunsuke Watanabe, Hiroaki Yoshida, Noriaki Kishida, Ryoichi Nakatani, Yoshinori Kotani, Kentaro Toyoki, and Tetsuya Nakamura, Appl. Phys. Lett. 113, 242404 (2018); https://doi.org/10.1063/1.5053925., We investigated the switching process of the perpendicular exchange bias, which is driven by the magnetoelectric effect, by conducting magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. Isothermal and simultaneous application of magnetic and electric fields switches the perpendicular exchange bias polarity. The switching process proceeds by the nucleation and growth of reversed domains. The correspondence among the ferromagnetic/antiferromagnetic domains and exchange bias polarity indicates that interfacial antiferromagnetic spin/domain reversal is responsible for the magnetoelectric switching of the perpendicular exchange bias polarity.

We have investigated interface magnetism of Au-capped ultrathin Co film grown on antiferromagnetic Cr2O3(0001) thin film. In particular, perpendicular magnetic anisotropy and perpendicular exchange bias are investigated. To strengthen the analysis, structural investigations are also conducted. Ultrathin Co film shows both perpendicular magnetic anisotropy and perpendicular exchange bias. Perpendicular magnetic anisotropy is mainly caused by Au capping layer. However, Co/Cr2O3 interface also affects the perpendicular magnetic anisotropy through the collinear exchange coupling of Co spin and Cr spin. The collinear exchange coupling gives rise to the perpendicular exchange bias. This is explained by aligned Cr spin direction owing to epitaxial growth and small lattice misfit.