In Situ Tuning of Magnetization and Magnetoresistance in Fe3O4 Thin Film Achieved with All-Solid-State Redox Device Takashi Tsuchiya,*,†,‡ Kazuya Terabe,*,† Masanori Ochi,‡ Tohru Higuchi,‡ Minoru Osada,† Yoshiyuki Yamashita,† Shigenori Ueda,§,∥ and Masakazu Aono† †
International Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials Science (NIMS),1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Department of Applied Physics, Faculty of Science, Tokyo University of Science, 6-3-1, Niijuku, Katsushika-ku, Tokyo 125-8585, Japan § Quantum Beam Unit, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ∥ Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan S Supporting Information *
ABSTRACT: An all-solid-state redox device composed of Fe3O4 thin film and Li+ ion conducting solid electrolyte was fabricated for use in tuning magnetization and magnetoresistance (MR), which are key factors in the creation of high-density magnetic storage devices. Electrical conductivity, magnetization, and MR were reversibly tuned by Li+ insertion and removal. Tuning of the various Fe3O4 thin film properties was achieved by donation of an electron to the Fe3+ ions. This technique should lead to the development of spintronics devices based on the reversible switching of magnetization and spin polarization (P). It should also improve the performance of conventional magnetic random access memory (MRAM) devices in which the ON/OFF ratio has been limited to a small value due to a decrease in P near the tunnel barrier. KEYWORDS: magnetite, magnetoresistance, all-solid-state, solid state ionics, nanoionics doping achieved using an all-solid-state redox device.18−20 Magnetite (Fe3O4) was used as the ferromagnetic oxide. It has been predicted to be a half-metal at room temperature and has been intensively investigated for application to MRAM devices.1,2,5−13 Extremely high carrier density doping of Fe3O4 by electrochemical Li+ insertion (e.g., 3.2 × 1021 cm−3) enables various electrical and magnetic properties including electrical conductivity, magnetization, and MR to be tuned to an extent unachievable with conventional electrostatic carrier doping using dielectric thin films.
H
igh-density magnetic storage devices (e.g., hard disk, magneto-optical disk, and magnetic random access memory (MRAM)) have become even more important due to the information explosion now underway.1−14 Since these devices operate on the basis of the tuning of magnetic properties including magnetization, magnetoresistance (MR), and Kerr rotation angle, development of a technique for the tuning in thin film devices should lead to generate technological innovations in high-density storage, low power consumption, high operating speed, and so on. One promising approach to such tuning is electrostatic carrier doping into magnetic materials because the magnetic properties originate from the density of states (DOS) near the Fermi level, which are closely related to the electronic carrier density.13−16 While this approach is effective in principle and thus has been tried, it is not straightforward because the carrier density achieved by electrostatic carrier doping with dielectric thin films is quite low compared to that for typical ferromagnetic materials (1022 cm−3).15−17 Here, we report in situ tuning of the magnetic properties of ferromagnetic oxide thin film based on electrochemical carrier © 2016 American Chemical Society
RESULTS AND DISCUSSION Electric Conduction Characteristics of All-Solid-State Redox Transistor Composed of Li4SiO4 and Fe3O4 Thin Films. Our all-solid-state redox transistor is schematically shown in Figure 1a. The device operates on the basis of Received: November 23, 2015 Accepted: December 22, 2015 Published: January 5, 2016 1655
DOI: 10.1021/acsnano.5b07374 ACS Nano 2016, 10, 1655−1661
Article
www.acsnano.org
Article
ACS Nano
Figure 1. (a) Schematic illustration of all-solid-state redox transistor with Fe3O4 and LSO lithium ion conductor. Li+ represents positively charged lithium ions. Dotted circles in LiCoO2 represent Li+ vacancies. (b) HR-TEM image of LSO/Fe3O4 interface. Inset shows electron diffraction pattern for Fe3O4 and MgO. (c) Electrical conduction characteristics of the device measured in vacuum: iD vs VG (upper panel) and iG vs VG (lower panel). Sweep rate of VG was 1 mV/s; VD was 0.1 V. (d) iD variation with time measured for 4000 s at various VG. (e) VG dependence of iD/i0 obtained from (d).
electrical conductivity modulation in the Fe3O4 channel due to Li+ ion insertion and removal and the resultant electronic carrier density modulation (i.e., electrochemical reduction and
oxidation, or redox). A 10 nm-thick Fe3O4 electron conducting thin film and a 800 nm-thick Li4SiO4 (LSO) Li+ conducting thin film were deposited on an atomically flat (100) surface of 1656
DOI: 10.1021/acsnano.5b07374 ACS Nano 2016, 10, 1655−1661
Article
ACS Nano
Figure 2. (a) MOKE θK−H loops of device measured at various DC voltages. (b) DC voltage dependence of M in saturation region (H above 10 kOe). (c) Illustrations of two cases of electrochemical reduction of Fe3O4 by Li+ insertion. (d) DC voltage dependence of MR in Fe3O4 thin film in device as a function of magnetic field (−70 to 70 kOe) at 250 K. (e) DC voltage dependence of MR at 70 kOe obtained from (d).
undoped MgO single crystal.21,22 The high-resolution transmission electron microscope (HR-TEM) image and the electron diffraction of the Fe3O4/MgO interface shown in Figure 1b reveal Fe3O4 (100) epitaxially grown on MgO (100). The Fe3O4 was further characterized as single phase using Raman spectroscopy and X-ray photoelectron spectroscopy (see section S1 in Supporting Information). The LSO thin film was characterized as almost perfect amorphous. The gate electrode was made of a 200 nm-thick LiCoO2 (LCO) thin film, enabling Li+ insertion and removal in the Fe3O4 thin film due to its large Li+ capacity.22−24 Details of the device
fabrication and characterization are in the Methods and Supporting Information. The upper and lower panels in Figure 1c show the electrical conduction characteristics (iD, iG vs VG) at 298 K in vacuum of a transistor gated by Li+ migration in LSO. The iD and VG are the current through the drain and the voltage between the gate and source, respectively. The VD is the voltage between the drain and source. The iD is plotted on the right axis as iD/i0, which is iD normalized by the initial value, i0. The device exhibited a significant increase in iD/i0, indicating that the Li+ insertion caused additional electrons into Fe3+ ions at the octahedral (B) sites, where electrical conduction occurs via electron hop1657
DOI: 10.1021/acsnano.5b07374 ACS Nano 2016, 10, 1655−1661
Article
ACS Nano ping.13−16 The 80% difference in iD/i0 (from 1.0 to 1.8) is far larger than that reported for electrostatic carrier doping using solid dielectric (8%). It is attributed to the donated carrier concentration by Li+ insertion being as high as 3.2 × 1021 cm−3, as discussed later along with the photoelectron spectroscopy observations.16 The electrical conduction characteristics were accompanied by considerable hysteresis, particularly above VG of 0.5 V. This suggests that the electronic carrier was modulated by the Li+ insertion and removal from the Fe3O4 thin film with comparably slow kinetics featuring ion-related phenomena.18−20,22 To investigate the precise VG dependence of iD (i.e., without any artifacts from such slow kinetics), iD variation with time was measured for 4000 s for various VG. The results are shown in Figure 1d. The applied VG was increased from 0 to 4 V in 0.5 V step, and subsequently returned to 0 V to recover iD to its initial value (i0). Specifically, VG of 0, 0.5, 0, 1, 0, 1.5, 0, ..., 3.5, 0, 4, and 0 V were applied one by one. For VG from 0 to 2.0 V, iD increased and saturated at a certain value. It recovered to i0 by following subsequent 0 V applications. For VG above 2.5 V, iD exhibited complex behavior and could not be recovered to i0. That is, application of VG above 2.5 V causes an irreversible electrochemical reduction of Fe3O4. The VG dependence of iD/i0 obtained from Figure 1d is plotted in Figure 1e. While iD/i0 initially increased with VG increases, it started to decrease at a VG of 2.5 V, indicating an irreversible reduction to a less conducting phase. Irreversible phase transition of Fe3O4 to rocksalt phase, Li1xFe3O4 (1 < x < 1.5), has been reported for lithiated Fe3O4 powder.25−29 The rocksalt phase is generated below 1.3 V vs Li/Li+ on the basis of the report.25−29 Given that the x in an LixCoO2 (LxCO) electrode is close to 1 and thus the potential of LixCoO2 (i.e., VG of 0 V) is 3.8 V vs Li/Li+,23,24 a VG of 2.5 V corresponds to 1.3 V vs Li/Li+, which agrees with the reported value.29 Accordingly, the irreversible reduction is attributed to the rocksalt phase generation. Although the response speed was comparably slow, iD/i0 was reversibly switched by VG pulse below 2 V (see section S2). In this study, the VG region above 0 V was examined because application of a VG lower than 0 V causes irreversible oxidation of Fe3O4 to Fe2O3. Magnetic Property Modulation in All-Solid-State Redox Device Composed of Li4SiO4 and Fe3O4 Thin Films. Magneto-optical Kerr effect (MOKE) measurement in a polar Kerr configuration was performed to observe the magnetic property modulation in the Fe3O4 thin film caused by Li+ insertion and removal using an all-solid-state device (see Methods and section S3.1). Figure 2a shows MOKE θK (Kerr rotation angle)−H (magnetic field) loops measured at various DC voltage. Typical ferromagnetic hysteresis of Fe3O4 was observed for all voltage conditions, although the coercive field was slightly smaller than that for a thick Fe3O4 reference sample (see section S3.1).7,9,10,13,16 The device exhibited a gradual decrease in θK above 10 kOe with an increasing positive voltage to LCO, i.e., Li+ insertion into Fe3O4. θK is proportional to M (magnetization) in an approximation of the relation, n QM θK ≈ ε (0n − ε ) , where n0, Q, and εxx are the refractive xx
0
decrease in M was also reported for Fe3O4 powder that had been lithiated by liquid reagents.25−28 The decrease in M for our device agrees well with those in previous reports. Moreover, a similar decrease in M for a Li+-inserted Fe3O4 thin film was observed in a separate experiment (see section S3.2). The small additional loop observed in a very small H region (
