Microscopic mechanisms of the electric-field effect on proximity-induced magnetism in Pt revealed by X-ray absorption spectroscopy

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Chemical Science Research Frontiers 2018 82 Microscopic mechanisms of the electric-field effect on proximity-induced magnetism in Pt revealed by X-ray absorption spectroscopy Carrier doping in condensed matter by an electric field is a principal technology for operating electronic devices. Electric fields are also used to control the magnetic properties of not only ferromagnetic semiconductors [1] but also thin ferromagnetic metals [2], such as Fe and Co. In particular, electric-field- assisted magnetization switching in ferromagnetic metals [3] is promising for the energy-efficient operation of magnetic memory devices. However, despite the potential applications, the mechanisms of the electric-field effect on ferromagnetism are still controversial because of the lack of experimental study. In this research [4], we unraveled the microscopic mechanisms of the electric-field effect on proximity- induced magnetism in Pt by X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) performed at SPring-8 BL39XU . Pt atoms in the proximity of a ferromagnetic layer can be ferromagnetic. “Ferromagnetic Pt” has interesting characters: a small exchange splitting and strong spin- orbit interaction. Because the coexistence of the two factors can enhance the electric-field effect on the magnetic anisotropy [5], ferromagnetic Pt has been attracting attention. XMCD and XAS spectroscopy are powerful tools for scrutinizing the electric-field effect. An XMCD spectrum contains information about the magnetic polarization of a specific element and, importantly, enables us to evaluate the spin and orbital magnetic moments by using the sum rules. By analyzing the XAS spectrum obtained under the application of an electric field, one can examine how the applied electric field affects the unoccupied electronic states. In our experiments, we employed an ionic liquid to apply a large electric field to a Pt channel of an electric-double-layer transistor formed on a synthetic quartz glass substrate (see Fig. 1). By applying a gate voltage ( V G ) between the channel and lateral gate electrodes, the ions move onto the channel surface to form an electric double layer, equivalent to a nanogap capacitor. Here, applying a positive V G causes the accumulation of electrons at the Pt/MgO interface. The experimental configuration is schematically shown in Fig. 1 . A magnetic field was applied perpendicular to the device surface and a circularly polarized X-ray propagated parallel to the magnetic field direction. We measured the fluorescence yields of the Pt L α and L β lines as a function of X-ray energy at the Pt L 3 and L 2 edges, respectively. The XAS and XMCD intensities are defined as I XAS = [ I ( σ + ) + I ( σ – )]/2 and I XMCD = I ( σ + ) – I ( σ – ), where I ( σ + ) and I ( σ – ) indicate the intensities of X-ray fluorescence when the incident photon momentum and magnetization vectors are antiparallel and parallel, respectively. Throughout the X-ray measurements, the device was located in an evacuated chamber of a He-flow cryostat and the measurement temperature was fixed at 100 K. Figure 2 shows I XMCD and I XAS at V G = +6 and –4 V together with their differences, Δ I XMCD and Δ I XAS , between the two voltages. The significant changes in I XMCD and I XAS indicate that the applied electric field modulates the magnetic moments and electronic states of Pt. By applying the sum rules to values of the I XMCD measured at different V G ’ s, the perpendicular components of the effective spin and orbital moments are estimated to be 0.222 ± 0.001 (0.232 ± 0.002) μ B and 0.043 ± 0.001 (0.047 ± 0.001) μ B at V G = +6 (–4) V, respectively. The voltage-induced changes in I XAS indicate that the electronic states above the Fermi Fig. 1. Schematic illustration of the experimental configuration. Research Frontiers 2018 83 energy ( E F ), i.e., the number of 5 d holes, change upon the application of an electric field. The total number of 5 d holes of Pt increases by ~0.009 when V G is switched from +6 V to –4 V. Note that the spectral profile of the XAS difference ( Δ I XAS ) is not monotonic with the X-ray energy; Δ I XAS changes sign around 11.571 and 13.290 eV for the L 3 and L 2 absorption edges, respectively. The reductions of Δ I XAS near the absorption edges are mostly due to the electric- field-induced shift of E F , simply corresponding to the electrostatic charge accumulation. Then, Δ I XAS becomes positive at the high-energy sides of the absorption edges, which cannot be explained by the electrostatic charge accumulation. The electric field can also alter the hybridizations between the sp and d orbital of Pt. This mechanism is considered to increase (decrease) the number of 5 d holes of the Pt atoms under a positive (negative) electric field. The changes in I XMCD and I XAS were well reproduced by a density functional theory calculation . Scrutinizing the electronic structures under electric fields, we find that the two mechanisms (electric-field-induced shift of E F and changes in the orbital hybridizations) indeed give rise to the nonmonotonic changes in I XAS and are relevant to the modulated magnetic moments of the Pt atom. The experimentally unveiled mechanisms of the electric-field effects in ferromagnetic Pt are expected to give us a fundamental model applicable to a wide range of ferromagnetic metals, represented by Fe and Co. Furthermore, the highly sensitive XMCD and XAS measurements under the application of electric fields enabled us to correlate the changes in the magnetism with the microscopic picture. This research has demonstrated the significant benefit of XAS for investigating magnetic phenomena induced by external fields. Fig. 2. ( a ) XMCD ( I XMCD ) and ( b ) XAS ( I XAS ) intensities at Pt L 3 and L 2 edges for V G = +6 and – 4 V and their difference Δ I XMCD(XAS) [= I XMCD(XAS) (+6 V) – I XMCD(XAS) ( – 4 V)]. References [1] H. Ohno et al. : Nature 408 (2000) 944. [2] M. Weisheit et al .: Science 315 (2007) 349. [3] Y. Shiota et al .: Nat. Mater. 11 (2012) 39. [4] K. T. Yamada, M. Suzuki, A.-M. Pradipto, T. Koyama, S. Kim, K.-J. Kim, S. Ono, T. Taniguchi, H. Mizuno, F. Ando, K. Oda, H. Kakizakai, T. Moriyama, K. Nakamura, D. Chiba and T. Ono: Phys. Rev. Lett. 120 (2018) 157203. [5] S. Miwa et al .: Nat. Commun. 8 (2017) 15848. Kihiro Yamada a,b, *, Motohiro Suzuki c and Teruo Ono a,d a Institute for Chemical Research, Kyoto University b Institute for Molecules and Materials, Radboud University, The Netherlands c Japan Synchrotron Radiation Research Institute (JASRI) d Center for Spintronics Research Network (CSRN), Osaka University *Email: K.Yamada@science.ru.nl (a) (b) X-ray Energy (keV) I XAS (arb. units) Δ I XAS (arb. units) I XMCD (arb. units) Δ I XMCD (arb. units) 11.54 11.56 11.58 11.60 11.54 0.00 0.03 1 0 2 3 – 0.03 – 0.06 0.00 0.02 – 0.3 – 0.2 – 0.1 0.0 0.1 11.56 Pt L 3 edge Pt L 3 edge Pt L 2 edge Pt L 2 edge +6 V – 4 V +6 V – 4 V +6 V – 4 V +6 V – 4 V 11.58 11.60 11.54 11.56 11.58 11.60 13.26 13.28 13.30 13.26 13.28 13.30 13.26 13.28 13.30