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Masaaki Tanaka

Publications and source records attributed to Masaaki Tanaka.

At least 37 records · Page 2Linked to original sources
arXiv2024

Mechanism of ferromagnetism enhancement in a La$_{2/3}$ Sr$_{1/3}$ MnO$_3$ membrane released from epitaxial strain

Recent studies have shown that the magnetic properties of the ferromagnetic perovskite oxide La$_{2/3}$ Sr$_{1/3}$ MnO$_3$ (LSMO) grown on an SrTiO3 (STO) substrate, such as its magnetic moment and Curie temperature, can be improved by releasing the film from the substrate. However, the microscopic origin of this enhancement is not yet well understood. In this study, we use synchrotron radiation measurements to investigate the mechanism of ferromagnetism enhancement in an LSMO membrane released from an STO substrate by dissolving a water-soluble Sr$_4$Al$_2$O$_7$ buffer layer. Using resonant photoemission spectroscopy on the as-grown LSMO film and LSMO membrane, we elucidate that the strain release from the STO substrate enhances the itineracy of the Mn-3d electrons via p-d hybridization, and this strengthens the double-exchange interaction. The reinforcement of the double-exchange interaction, in turn, improves the ferromagnetism of LSMO.

cond-mat.mtrl-sci↗
arXiv2023

Allotropic transition of Dirac semimetal α-Sn to superconductor β-Sn induced by irradiation of focused ion beam

Diamond-type structure allotrope α-Sn is attracting much attention as a topological Dirac semimetal (TDS). In this study, we demonstrate that α-Sn undergoes a phase transition to another allotrope β-Sn with superconductivity at low temperature by irradiating with a focused Ga ion beam (FIB). To clarify the transition mechanism, we performed X-ray photoemission spectroscopy (XPS) measurements on an α-Sn thin film irradiated with FIB and an as-grown α-Sn thin film. The XPS results suggest that the local annealing, which is one of the side effects of FIB, causes the transformation from α-Sn into β-Sn. Furthermore, the difference in the chemical states between α-Sn and β-Sn can be quantitatively explained by the crystal structures rather than the degree of metallicity reflecting the conductivity. These results propose a new way of fabricating TDS/superconductor in-plane heterostructures based on α-Sn and β-Sn.

cond-mat.mtrl-sci↗
arXiv2023

Giant superconducting diode effect in ion-beam patterned Sn-based superconductor nanowire / topological Dirac semimetal planar heterostructures

Superconductor/topological material heterostructures are intensively studied as a platform for topological superconductivity and Majorana physics. However, the high cost of nanofabrication and the difficulty of preparing high-quality interfaces between the two dissimilar materials are common obstacles that hinder the observation of intrinsic physics and the realisation of scalable topological devices and circuits. Here, we demonstrate an innovative method to directly draw nanoscale superconducting beta-tin (beta-Sn) patterns of any shape in the plane of a topological Dirac semimetal (TDS) alpha-tin (alpha-Sn) thin film by irradiating a focused ion beam (FIB). We utilise the property that alpha-Sn undergoes a phase transition to superconducting beta-Sn upon heating by FIB. In beta-Sn nanowires embedded in a TDS alpha-Sn thin film, we observe giant non-reciprocal superconducting transport, where the critical current changes by 69% upon reversing the current direction. The superconducting diode rectification ratio reaches a maximum when the magnetic field is applied parallel to the current, distinguishing itself from all the previous reports. Moreover, it oscillates between alternate signs with increasing magnetic field strength. The angular dependence of the rectification ratio on the magnetic field and current directions is similar to that of the chiral anomaly effect in TDS alpha-Sn, suggesting that the non-reciprocal superconducting transport may occur at the beta-Sn/alpha-Sn interfaces. The ion-beam patterned Sn-based superconductor/TDS planar structures thus show promise as a universal platform for investigating novel quantum physics and devices based on topological superconducting circuits of any shape.

cond-mat.supr-con↗
arXiv2023

Scattering-dependent transport of SrRuO3 films: From Weyl fermion transport to hump-like Hall effect anomaly

Recent observation of quantum transport phenomena of Weyl fermions has brought much attention to 4d ferromagnetic perovskite SrRuO3 as a magnetic Weyl semimetal. Besides, the hump-like Hall effect anomaly, which might have a topological origin, has also been reported for this material. Here, we show that the emergence of such phenomena is governed by the degree of scattering determined by the defect density (Ru-deficiency- and/or interface-driven-defect scattering) and measurement temperature (phonon scattering), where the former is controlled by varying the growth conditions of the SrRuO3 films in molecular beam epitaxy as well as the film thickness. The resulting electronic transport properties can be classified into three categories: clean, intermediate, and dirty regimes. The transport of Weyl fermions emerges in the clean regime, whereas that of topologically trivial conduction electrons in the ferromagnetic metal state prevail in the intermediate and dirty regimes. In the clean and intermediate regimes, anomalous Hall resistivity obeys a scaling law incorporating the intrinsic Karplus-Luttinger (Berry phase) and extrinsic side-jump mechanisms. The hump-like Hall effect anomaly is observed only in the dirty regime, which is contrary to the scaling law between anomalous Hall resistivity and longitudinal resistivity. Hence, we conclude that this anomaly is not inherent to the material and does not have a topological origin. We also provide defect- and temperature-dependent transport phase diagrams of stoichiometric SrRuO3 and Ru-deficient SrRu0.7O3 where the appearance of Weyl fermions and hump-like Hall effect anomaly are mapped. These diagrams may serve as a guideline for designing SrRu1-xO3-based spintronic and topological electronic devices.

cond-mat.mtrl-sci↗
arXiv2023

Origin of magnetically dead layers in spinel ferrites $M\text{Fe}_2\text{O}_4$ grown on $\text{Al}_2\text{O}_3$: Effects of post-deposition annealing studied by XMCD

We study the electronic and magnetic states of as-grown and annealed $M\text{Fe}_2\text{O}_4$(111)/$\text{Al}_2\text{O}_3$(111) ($M=\text{Co, Ni}$) thin films with various thicknesses grown on Si(111) substrates with the $γ$-$\text{Al}_2\text{O}_3$(111) buffer layers by using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), to investigate magnetically dead layers in these films. Although the magnetically dead layers in the as-grown samples are formed near the interface with the $\text{Al}_2\text{O}_3$ buffer layer, we reveal that ferrimagnetic order is partially recovered by post-deposition annealing at 973 K for 48 hours in air. By analyzing the line shapes of the XAS and XMCD spectra, we conclude that, in the dead layers, there are a significant number of vacancies at the $T_d$ sites of the spinel structure, which may be the microscopic origin of the degraded ferrimagnetic order in the $M\text{Fe}_2\text{O}_4$(111) thin films.

cond-mat.mtrl-sci↗
arXiv2022

Giant gate-controlled odd-parity magnetoresistance in one-dimensional channels with a magnetic proximity effect

According to Onsager's principle, electrical resistance $R$ of general conductors behaves as an even function of external magnetic field $B$. Only in special circumstances, which involve time reversal symmetry (TRS) broken by ferromagnetism, the odd component of $R$ against $B$ is observed. This unusual phenomenon, called odd-parity magnetoresistance (OMR), was hitherto subtle (< 2%) and hard to control by external means. Here, we report a giant OMR as large as 27% in edge transport channels of an InAs quantum well, which is magnetized by a proximity effect from an underlying ferromagnetic semiconductor (Ga,Fe)Sb layer. Combining experimental results and theoretical analysis using the linearized Boltzmann's equation, we found that simultaneous breaking of both the TRS by the magnetic proximity effect (MPE) and spatial inversion symmetry (SIS) in the one-dimensional (1D) InAs edge channels is the origin of this giant OMR. We also demonstrated the ability to turn on and off the OMR using electrical gating of either TRS or SIS in the edge channels. These findings provide a deep insight into the 1D semiconducting system with a strong magnetic coupling.

cond-mat.mes-hall↗
arXiv2022

Ferromagnetism induced by hybridization of Fe 3d orbitals with ligand InSb bands in n-type ferromagnetic semiconductor (In,Fe)Sb

Fe-doped III-V ferromagnetic semiconductor (FMS) (In,Fe)Sb is a promising material for spintronic device applications because of the n-type carrier conduction and the ferromagnetism with high Curie temperature (TC > 300 K). To clarify the mechanism of the high-TC ferromagnetism, we have investigated the electronic structure and magnetic properties of an (In,Fe)Sb thin film by performing x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe L2,3 edges. The magnetic-field dependence of the XMCD spectra reveals that there are ferromagnetic-like Fe and paramagnetic-like Fe components in the (In,Fe)Sb thin film. The XAS and XMCD spectra of the ferromagnetic-like and paramagnetic-like Fe components resemble those of other Fe-doped FMSs and extrinsic oxides, respectively. The finite value of the ratio between the orbital and spin magnetic moments estimated by applying the XMCD sum rules indicates that the valence state of the Fe ions substituting for the In sites in (In,Fe)Sb is not purely ionic Fe3+, but intermediate between Fe3+ and Fe2+. The qualitative correspondence between the magnetic-field dependence of the visible-light magnetic circular dichroism intensity and that of the XMCD intensity demonstrates that the Zeeman splitting of the InSb band is proportional to the net magnetization of the doped Fe. These results suggest that the ferromagnetism of (In,Fe)Sb originates from the Fe 3d orbitals hybridized with the host InSb bands.

cond-mat.mtrl-sci↗
arXiv2021

High-mobility two-dimensional carriers from surface Fermi arcs in magnetic Weyl semimetal films

High-mobility two-dimensional carriers originating from surface Fermi arcs in magnetic Weyl semimetals are highly desired for accessing exotic quantum transport phenomena and for topological electronics applications. Here, we demonstrate high-mobility two-dimensional carriers that show quantum oscillations in magnetic Weyl semimetal SrRuO3 epitaxial films by systematic angle-dependent, high-magnetic field magnetotransport experiments. The exceptionally high-quality SrRuO3 films were grown by state-of-the-art oxide thin film growth technologies driven by machine learning algorithm. The quantum oscillations for the 10-nm SrRuO3 film show a high quantum mobility of 3500 cm2/Vs, a light cyclotron mass, and two-dimensional angular dependence, which can be attributed to the surface Fermi arcs. The linear thickness dependence of the phase shift of the quantum oscillations provides evidence for the non-trivial nature of the quantum oscillations mediated by the surface Fermi arcs. In addition, at low temperatures and under magnetic fields of up to 52 T, the quantum limit of SrRuO3 manifests the chiral anomaly of the Weyl nodes. Emergence of the hitherto hidden two-dimensional Weyl states in a ferromagnetic oxide pave the way to explore novel quantum transport phenomena for topological oxide electronics.

cond-mat.mtrl-sci↗
arXiv2021

Gate-controlled proximity magnetoresistance in In1-xGaxAs/(Ga,Fe)Sb bilayer heterostructures

The magnetic proximity effect (MPE), ferromagnetic coupling at the interface of magnetically dissimilar layers, attracts much attention as a promising pathway for introducing ferromagnetism into a high-mobility non-magnetic conducting channel. Recently, our group found giant proximity magnetoresistance (PMR), which is caused by MPE at an interface between a non-magnetic semiconductor InAs quantum well (QW) layer and a ferromagnetic semiconductor (Ga,Fe)Sb layer. The MPE in the non-magnetic semiconductor can be modulated by applying a gate voltage and controlling the penetration of the electron wavefunction in the InAs QW into the neighboring insulating ferromagnetic (Ga,Fe)Sb layer. However, optimal conditions to obtain strong MPE at the InAs/(Ga,Fe)Sb interface have not been clarified. In this paper, we systematically investigate the PMR properties of In1-xGaxAs (x = 0%, 5%, 7.5%, and 10%) / (Ga,Fe)Sb bilayer semiconductor heterostructures under a wide range of gate voltage. The inclusion of Ga alters the electronic structures of the InAs thin film, in particular changing the effective mass and the QW potential of electron carriers. Our experimental results and theoretical analysis of the PMR in these In1-xGaxAs/(Ga,Fe)Sb heterostructures show that the MPE depends not only on the degree of penetration of the electron wavefunction into (Ga,Fe)Sb but also on the electron density. These findings help us to unveil the microscopic mechanism of MPE in semiconductor-based non-magnetic/ferromagnetic heterojunctions.

cond-mat.mes-hall↗
arXiv2021

Theoretical analysis of the inverse Edelstein effect at the LaAlO$_3$ / SrTiO$_3$ interface with an effective tight-binding model: Important role of the second $d_{xy}$ subband

The two-dimensional electron gas formed at interfaces between SrTiO$_3$ and other materials has attracted much attention since extremely efficient spin-to-charge current conversion has been recently observed at these interfaces. This has been attributed to their complicated quantized multi-orbital structures with a topological feature. However, there are few reports quantitatively comparing the conversion efficiency values between experiments and theoretical calculations at these interfaces. In this study, we theoretically explain the experimental temperature dependence of the spin-to-charge current conversion efficiency using an 8x8 effective tight-binding model considering the second $d_{xy}$ subband, revealing the vital role of the quantization of the multi-band structure.

cond-mat.mtrl-sci↗
arXiv2021

Development of magnetism in Fe-doped magnetic semiconductors: Resonant photoemission and x-ray magnetic circular dichroism studies of (Ga,Fe)As

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both n- and p-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-doped III-V FMSs, shows paramagnetism. The origin of the different magnetic properties in the Fe-doped III-V semiconductors remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and x-ray magnetic circular dichroism (XMCD) to investigate the electronic and magnetic properties of the Fe ions in a paramagnetic (Ga$_{0.95}$,Fe$_{0.05}$)As thin film. The observed Fe 2$p$-3$d$ RPES spectra show that the Fe 3$d$ states are similar to those of ferromagnetic (Ga,Fe)Sb. The estimated Fermi level is located in the middle of the band gap in (Ga,Fe)As. The Fe $L_{2,3}$ XMCD spectra of (Ga$_{0.95}$,Fe$_{0.05}$)As show pre-edge structures, which are not observed in the Fe-doped FMSs, indicating that the minority-spin ($\downarrow$) $e_\downarrow$ states are vacant in (Ga$_{0.95}$,Fe$_{0.05}$)As. The XMCD results suggest that the carrier-induced ferromagnetic interaction in (Ga$_{0.95}$,Fe$_{0.05}$)As is short-ranged and weaker than that in the Fe-doped FMSs. The experimental findings suggest that the electron occupancy of the $e_\downarrow$ states contributes to the appearance of ferromagnetism in the Fe-doped III-V semiconductors, for p-type as well as n-type compounds.

cond-mat.mtrl-sci↗
arXiv2021

Reduced magnetocrystalline anisotropy of CoFe$_2$O$_4$ thin films studied by angle-dependent x-ray magnetic circular dichroism

Spinel-type CoFe$_2$O$_4$ is a ferrimagnetic insulator with the Néel temperature exceeding 790 K, and shows a strong cubic magnetocrystalline anisotropy (MCA) in bulk materials. However, when a CoFe$_2$O$_4$ film is grown on other materials, its magnetic properties are degraded so that so-called magnetically dead layers are expected to be formed in the interfacial region. We investigate how the magnetic anisotropy of CoFe$_2$O$_4$ is modified at the interface of CoFe$_2$O$_4$/Al$_2$O$_3$ bilayers grown on Si(111) using x-ray magnetic circular dichroism (XMCD). We find that the thinner CoFe$_2$O$_4$ films have significantly smaller MCA values than bulk materials. The reduction of MCA is explained by the reduced number of Co$^{2+}$ ions at the $O_h$ site reported by a previous study [Y. K. Wakabayashi $\textit{et al.}$, Phys. Rev. B $\textbf{96}$, 104410 (2017)].

cond-mat.mtrl-sci↗
arXiv2021

Room-temperature spin injection and spin-to-charge conversion in a ferromagnetic semiconductor / topological insulator heterostructure

Spin injection using ferromagnetic semiconductors at room temperature is a building block for the realization of spin-functional semiconductor devices. Nevertheless, this has been very challenging due to the lack of reliable room-temperature ferromagnetism in well-known group IV and III-V based semiconductors. Here, we demonstrate room-temperature spin injection by using spin pumping in a (Ga,Fe)Sb / BiSb heterostructure, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature (TC) and BiSb is a topological insulator (TI). Despite the very small magnetization of (Ga,Fe)Sb at room temperature (45 emu/cc), we are able to detect spin injection from (Ga,Fe)Sb by utilizing the inverse spin Hall effect (ISHE) in the topological surface states of BiSb with a large inverse spin Hall angle of 2.5. Our study provides the first demonstration of spin injection as well as spin-to-charge conversion at room temperature in a FMS/TI heterostructure.

cond-mat.mtrl-sci↗
arXiv2021

Growth and characterization of quaternary-alloy ferromagnetic semiconductor (In,Ga,Fe)Sb

We study the growth and properties of quaternary-alloy ferromagnetic semiconductor (FMS) (In0.94-x,Gax,Fe0.06)Sb (x = 5% - 30%, Fe concentration is fixed at 6%) grown by low temperature molecular beam epitaxy (LT-MBE).Reflection high-energy electron diffraction (RHEED) patterns, scanning transmission electron microscopy (STEM) lattice images, and X-ray diffraction (XRD) spectra indicate that the (In0.94-x,Gax,Fe0.06)Sb layers have a zinc-blende crystal structure without any other second phase. The lattice constant of the (In0.94-x,Gax,Fe0.06)Sb films changes linearly with the Ga concentration x, indicating that Ga atoms substitute In atoms in the zinc-blend structure. We found that the carrier type of can be systematically controlled by varying x, being n-type when x \le 10% and p-type when x \ge 20%. Characterizations using magnetic circular dichroism (MCD) spectroscopy indicate that the (In0.94-x,Gax,Fe0.06)Sb layers have intrinsic ferromagnetism with relatively high Curie temperatures (TC = 40 - 120 K). The ability to widely control the fundamental material properties (lattice constant, bandgap, carrier type, magnetic property) of (In0.94-x,Gax,Fe0.06)Sb demonstrated in this work is essential for spintronic device applications.

cond-mat.mtrl-sci↗
arXiv2021

Elemental topological Dirac semimetal α-Sn with high quantum mobility

α-Sn with a diamond-type crystal structure provides an ideal avenue to investigate novel topological properties owing to its rich diagram of topological phases and simple elemental material structure. Thus far, however, realisation of high-quality α-Sn remains a challenge, which limits our understanding of its quantum transport properties and device applications. Here, we present epitaxial growth of α-Sn on InSb (001) with the highest quality thus far and reveal that it is a topological Dirac semimetal (TDS) by quantum transport investigations together with first-principles calculations. We realise unprecedentedly high quantum mobilities of both the surface state (30000 cm^2/Vs), which is ten times higher than the previously reported values, and the bulk heavy-hole (HH) state (1700 cm^2/Vs), which has never been obtained experimentally. These excellent features allow us, for the first time, to quantitatively characterise the nontrivial interfacial and bulk band structure of α-Sn via Shubnikov-de Haas oscillations at various temperatures and under various magnetic field directions. These results reveal the existence of a topological surface state (TSS) and a bulk HH band, both with nontrivial phase shifts, indicating that the TDS phase of α-Sn is established. Furthermore, we demonstrate a crossover from the TDS to a two-dimensional topological insulator (2D-TI) and a subsequent phase transition to a trivial insulator when varying the thickness of α-Sn. Our work indicates that α-Sn is an excellent model system to study novel topological phases and a prominent material candidate for topological devices.

cond-mat.mtrl-sci↗
arXiv2021

Ferromagnetism and giant magnetoresistance in zinc-blende FeAs monolayers embedded in semiconductor structures

Material structures containing tetrahedral FeAs bonds, depending on their density and geometrical distribution, can host several competing quantum ground states ranging from superconductivity to ferromagnetism. Here we examine structures of quasi two-dimensional (2D) layers of tetrahedral Fe-As bonds embedded with a regular interval in a semiconductor InAs matrix, which resembles the crystal structure of Fe-based superconductors. Contrary to the case of Fe-based pnictides, these FeAs/InAs superlattices (SLs) exhibit ferromagnetism, whose Curie temperature (Tc) increases rapidly with decreasing the InAs interval thickness t_InAs (Tc ~ t_InAs^-3), and an extremely large magnetoresistance up to 500% that is tunable by a gate voltage. Our first principles calculations reveal the important role of disordered positions of Fe atoms in the establishment of ferromagnetism in these quasi-2D FeAs-based SLs. These unique features mark the FeAs/InAs SLs as promising structures for spintronic applications.

cond-mat.mtrl-sci↗
arXiv2021

Inter-valence charge transfer and charge transport in the spinel ferrite ferromagnetic semiconductor Ru-doped CoFe$_2$O$_4$

Inter-valence charge transfer (IVCT) is electron transfer between two metal $M$ sites differing in oxidation states through a bridging ligand: $M^{n+1} + M'^{m} \rightarrow M^{n} + M'^{m+1}$. It is considered that IVCT is related to the hopping probability of electron (or the electron mobility) in solids. Since controlling the conductivity of ferromagnetic semiconductors (FMSs) is a key subject for the development of spintronic device applications, the manipulation of the conductivity through IVCT may become a new approach of band engineering in FMSs. In Ru-doped cobalt ferrite CoFe$_2$O$_4$ (CFO) that shows ferrimagnetism and semiconducting transport properties, the reduction of the electric resistivity is attributed to both the carrier doping caused by the Ru substitution for Co and the increase of the carrier mobility due to hybridization between the wide Ru $4d$ and the Fe $3d$ orbitals. The latter is the so-called IVCT mechanism that is charge transfer between the mixed valence Fe$^{2+}$/Fe$^{3+}$ states facilitated by bridging Ru $4d$ orbital: Fe$^{2+}$ + Ru$^{4+}$ $\leftrightarrow$ Fe$^{3+}$ + Ru$^{3+}$. To elucidate the emergence of the IVCT state, we have conducted x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES) measurements on non-doped CFO and Co$_{0.5}$Ru$_{0.5}$Fe$_2$O$_4$ (CRFO) thin films. The observations of the XAS and RPES spectra indicate that the presence of the mixed valence Fe$^{2+}$/Fe$^{3+}$ state and the hybridization between the Fe $3d$ and Ru $4d$ states in the valence band. These results provide experimental evidence for the IVCT state in CRFO, demonstrating a novel mechanism that controls the electron mobility through hybridization between the $3d$ transition-metal cations with intervening $4d$ states.

cond-mat.mtrl-sci↗