Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S43: Electric Field and Optical Effects in Magnetic Oxide HeterostructuresFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Matt Brahlek, Penn State University Room: 390 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S43.00001: Photon-gated spin transistor via optical control of magnetism in manganite Li Fan, Song Cheng, Pan Feng The optical control of magnetism of La1/2Sr1/2MnO3-$\delta $ (LSMO) was observed according to the variation of magnetoresistance under the illumination of a common used light emitting diode.[1] An increase of saturation field under light illumination is observed, which is ascribed to the photon-induced spin transition from up spins to down spins, considering the half-metallic electronic structure given by first-principles calculations. Then we designed and prepared a new type of photon-gated spin transistor via partial illumination of the device channel, while an effective transient gate operation of the device conductance via optical method is demonstrated.[2] This optical gating effect has an obvious enhancement with the increase of the light intensity and shows a good repeatable cycling properties. Besides the fundamental significance, our finding would offer a twist for gate operation of spin FET and advance the application of optical manipulation of magnetism in spintronics for ultrafast data processing. [1] Li et al, Optical control of magnetism in manganite film, Phys. Rev. B 2016, 93, 024406. [2] Li et al, Photon-gated spin transistor, Adv. Mater. doi: 10.1002/adma.201604052. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S43.00002: Ultrafast photo-induced hidden phases in strained manganite thin films Jingdi Zhang, A. S. McLeod, Gu-Feng Zhang, Vladimir Stoica, Feng Jin, Mingqiang Gu, Venkatraman Gopalan, John W. Freeland, Wenbin Wu, James Rondinelli, Haidan Wen, D. N. Basov, R. D. Averitt Correlated transition metal oxides (TMOs) are particularly sensitive to external control because of energy degeneracy in a complex energy landscape that promote a plethora of metastable states. However, it remains a grand challenge to actively control and fully explore the rich landscape of TMOs. Dynamic control with pulsed photons can overcome energetic barriers, enabling access to transient or metastable states that are not thermally accessible. In the past, we have demonstrated that mode-selective single-laser-pulse excitation of a strained manganite thin film La2/3Ca1/3MnO3 initiates a persistent phase transition from an emergent antiferromagnetic insulating ground state to a ferromagnetic metallic metastable state [1]. Beyond the photo-induced insulator to metal transition, we recently discovered a new peculiar photo-induced hidden phase, identified by an experimental approach that combines ultrafast pump-probe spectroscopy, THz spectroscopy, X-ray diffraction, cryogenic near-field spectroscopy and SHG probe. [1] J. Zhang and R.D. Averitt et al., Nat. Mater. 15, 956 (2016) [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S43.00003: Ultrafast switching of the magnetic ground state in $d^{1}$ titanates though nonlinear phononic coupling Mingqiang Gu, James M Rondinelli LaTiO3 and YTiO3 are isostructure $d^{1}$ titanates, which exhibit distinct magnetic and orbital properties: The former is a G-type antiferromagnet with a 150 K Neel temperature whereas the latter is a rare ferromagnetic (FM) insulator with a 30 K Curie temperature. With first-principles density functional theory calculations, we identify the local structural origin of the magnetic order difference in these orthorhombic perovskites. By increasing the tilt and rotation angles in LaTiO3, respectively, LaTiO3 is predicted to undergo a magnetic phase transition to an FM state. Similarly, decreasing the tilt and rotation angles in YTiO3 leads to a FM-to-AFM phase transition. The underlying physics is attributed to the change in the superexchange coupling between Ti-sites. Last, we propose a route to switch the magnetism in the titanates by controlling the octahedral distortions through dynamical nonlinear phononic coupling. The proposed experiment requires the use of static strain to position the crystal structure in proximity to the structural transition combined with readily achievable fluencies in an ultrafast optical pump-probe geometry [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S43.00004: Percolation via combined electrostatic and chemical doping in complex oxide films Peter P. Orth, Rafael M. Fernandes, Jeff Walter, C. Leighton, B. I. Shklovskii Electric field control of magnetism in complex oxide thin films provides many opportunities for novel storage and information processing devices with low power consumption. Electrolyte gating was successfully employed to electrostatically induce and control large charge densities in these systems. Attainment of sufficient densities to induce magnetic/electronic phase transitions, however, remains a challenge. One obvious strategy is to employ a combination of chemical and electrostatic doping. Stimulated by experimental advances in electrolyte gating methods, we theoretically investigate percolation in thin films of inhomogenous complex oxides, such as La$_{1-x}$Sr$_{x}$CoO$_3$ (LSCO), induced by a combination of bulk chemical and surface electrostatic doping. We identify two mechanisms that describe how bulk dopants reduce the amount of surface charge required to reach percolation: (i) bulk-assisted surface percolation, and (ii) surface-assisted bulk percolation. We show that thin films can be driven across the percolation transition by modest surface charge densities. We further show that if percolation is associated with the onset of ferromagnetism, the presence of critical magnetic clusters extending from the surface into the bulk leads to enhanced saturation magnetization [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S43.00005: Electrostatic~\textit{vs.}~Electrochemical Doping and Control of Ferromagnetism in Ion-Gel-Gated La$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$CoO$_{\mathrm{3-\delta }}$. J. Walter, B. Yu, G. Yu, H. Wang, B. Luo, Z. Zhang, H. Zhou, J. Freeland, A. Grutter, J. Borchers, B. Kirby, C. D. Frisbie, M. Greven, C. Leighton Recently developed ionic liquid/gel gating techniques have proven remarkably expedient in the study of charge density effects in a variety of conductors. Much remains to be learned, however, about the exact gating mechanisms ($i.e., $electrostatic \textit{vs}. electrochemical), particularly in oxides, where oxygen vacancy formation and diffusion is facile. In this work we demonstrate that in ion-gel-gated La$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$CoO$_{\mathrm{3-\delta }}$ (LSCO) films, transport, AFM, and XPS measurements indicate that negative gate biases induce reversible electrostatic hole accumulation, whereas positive biases irreversibly induce oxygen vacancies [1]. This is rationalized in terms of the known redox stability of LSCO, with broad implications for electrolyte gating of hole- \textit{vs}. electron-doped oxides. Clear voltage-control of magnetic and transport properties is then demonstrated under hole accumulation, including a 12 K shift in $T_{C} $probed \textit{via} anomalous Hall effect [1]. Further to this, \textit{in operando} probes have also been applied, including synchrotron X-ray diffraction directly revealing expansion in unit cell volume due to oxygen vacancy formation under positive bias, and polarized neutron reflectometry to probe the gate-voltage-dependent depth-profile of chemistry and magnetization. [1] Walter \textit{et al}. ACS Nano. (2016). [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S43.00006: In operando measurement of structural effects accompanying electrostatic and electrochemical doping in ion-gel-gated La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta}$ thin films Biqiong Yu, Guichuan Yu, Martin Greven, Jeff Walter, Chris Leighton, Zhan Zhang, Hua Zhou, John Freeland Electrolyte gating techniques employing ionic liquid/gels in electric double-layer transistors provide an effective way to continuously vary the charge carrier densities in various materials. However, it is unclear whether the doping mechanism in such devices is electrostatic or electrochemical, particularly in oxides. Our recent investigation of ion-gel-gated La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta}$ (LSCO) thin films indicated the presence of both processes: Negative gate biases were found to cause simple electrostatic accumulation of holes, whereas positive gate bias led to irreversible electrochemical changes due to formation of oxygen vacancies [1]. Here we report a hard X-ray diffraction study of ion-gel-gated LSCO films. We find that, in electrostatic charging (negative bias), the c lattice constant of the films decreases slightly and is restored to its initial value upon subsequent removal of the gate bias. On the other hand, in electrochemical charging (positive bias) a large irreversible increase of the c lattice constant occurs. From a separate measurement of the lattice volume dependence on oxygen deficiency in bulk LSCO, we confirm that the observed large irreversible changes are due to oxygen vacancies created under positive bias. [1] Walter et al., ACS Nano (2016) [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S43.00007: Voltage-induced entropy change in complex oxide thin films via electrostatic doping. Prakash Giri, Dhananjay Kumar, Christian Binek The quest for cost-effective, energy efficient and environmental friendly cooling system has driven unconventional refrigeration technology by utilizing magnetocaloric, electrocaloric, barocaloric and elastocaloric-materials. For the latter class of materials strain, e.g., electrically induced via adjacent piezoelectric materials is used to change the structure and thereby entropy of the material. We in contrast propose to use voltage to change the magnetic order and thereby entropy. In a complex oxide thin film such as La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$, strain and electric field can change anisotropy and magnitude of magnetization. Only the latter has the potential to lead to entropy change. In heterostructures of LSMO and ferroelectrics it is difficult to disentangle both effects because all ferroelectrics are piezoelectric. We therefore employ pure electrostatic doping in the absence of strain to achieve isothermal entropy change in LSMO from voltage-induced change of its magnetic state. We use the quantum paraelectric SrTiO$_{\mathrm{3}}$ with high dielectric constant but no spontaneous polarization to fabricate LSMO/STO heterostructures via pulsed laser deposition. We measure the voltage-induced variation in magnetization via SQUID magnetometry and determine the isothermal entropy change with the help of Maxwell's relation. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S43.00008: Ferroelectric field effect tuning of planar Hall effect in epitaxial La$_{\mathrm{0.8}}$Sr$_{\mathrm{0.2}}$MnO$_{\mathrm{3\thinspace }}$thin films Anil Rajapitamahuni, Xia Hong We report the ferroelectric field effect modulation of planar Hall effect in ultra-thin La$_{\mathrm{0.8}}$Sr$_{\mathrm{0.2}}$MnO$_{\mathrm{3\thinspace }}$(LSMO) films. We fabricated LSMO thin filmsand Pb(Zr,Ti)O$_{\mathrm{3}}$ (PZT)/LSMO heterostructures on (001) SrTiO$_{\mathrm{3}}$ substrates via off-axis RF magnetron sputtering, with high crystallinity and smooth surfaces. We worked with LSMO thin films with thickness close to the electric dead layer thickness (\textasciitilde 4 nm). The resistivity-peak temperature (T$_{\mathrm{p}})$ is \textasciitilde 170 K, significantly lower than the bulk value, with magnetoresistance (MR) ratio of 8.6 observed at 150K. We employed planar Hall effect (PHE) to study the in-plane magnetocrystalline anisotropy (MCA). The PHE resistance of LSMO films exhibits sinusoidal angular dependence in an in-plane magnetic field and shows four-fold resistance switching below a critical magnetic field of 500 Oe. This yields a biaxial magnetic anisotropy energy density of \textasciitilde 1.09 x 10$^{\mathrm{5}}$ erg/cm$^{\mathrm{3}}$, with the easy axis along \textless 110\textgreater directions. We then modulate the carrier density in the PZT/LSMO heterostructure via ferroelectric polarization switching. We will discuss the effect of electric field doping on the magnetotransport properties such as T$_{\mathrm{p}}$, MR, and MCA of the LSMO thin films. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S43.00009: Voltage control of spin polarization in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/C$_{5}$H$_{2}$O$_{5}$/SiO$_{2}$/Co spin valves with organic ferroelectric barrier Yuewei Yin, Xuanyuan Jiang, Xiaoshan Xu Voltage control of spin degree of freedom is a great challenge in modern spintronics research. Here, an organic spin valve, employing a thin organic croconic acid (C$_{5}$H$_{2}$O$_{5})$ as a ferroelectric barrier sandwiched between two ferromagnetic electrodes (La$_{0.7}$Sr$_{0.3}$MnO$_{3\, }$and Co), has been successfully fabricated, in which a control of the spin polarization by purely electrical method was realized. Both the tunneling magnetoresistance (TMR) effect reflecting the magnitude of spin polarization and the tunneling electroresistance (TER) effect possibly related to the ferroelectric polarization reversal were observed. More importantly, not only the magnitude but also the sign of the TMR appears to be correlated to the ferroelectric polarization reversal. With ferroelectric polarization pointing to the Co, the resistance is low and the sign of the TMR is negative; while after switching the ferroelectric polarization towards the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, the resistance is significantly increased and the sign of the \textit{TMR} is negative at positive biases and positive at negative biases. Possible mechanisms based on ferroelectricity controlled band alignment and interfacial redox of Co will be discussed. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S43.00010: Tuning the effective anisotropy in a voltage-susceptible exchange bias heterosystem Will Echtenkamp, Mike Street, Ather Mahmood, Christian Binek Voltage and temperature tuned ferromagnetic hysteresis is investigated by SQUID and Kerr-magnetometry in a thin film heterostructure of a perpendicular anisotropic Co/Pd ferromagnet exchange coupled to the magnetoelectric antiferromagnet $Cr_{2}O_{3}$. An abrupt disappearance of exchange bias with a simultaneous more than two-fold increase in coercivity is observed and interpreted as a competition between the effective anisotropy of $Cr_{2}O_{3}$ and the exchange coupling energy between boundary magnetization and the adjacent ferromagnet. The effective anisotropy energy is given by the intrinsic anisotropy energy density multiplied by the effective volume separated from the bulk through a horizontal antiferromagnetic domain boundary. Kerr measurements show that the anisotropy of the interfacial $Cr_{2}O_{3}$ can be tuned, isothermally, and in the absence of an external magnetic field, by application of an electric field. A generalized Meiklejohn-Bean model accounts for the change in exchange bias and coercivity as well as the asymmetric evolution of the hysteresis loop. In support of this model, the reversal of the boundary magnetization is experimentally confirmed as a contribution to the magnetic hysteresis loop. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S43.00011: Low dc leakage Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ thin films towards mili-volt switching of exchange bias Salinporn Kittiwatanakul, Yuhan Wang, Congli Sun, Paul Voyles, Jiwei Lu Reactive bias target ion beam deposition (RBTIBD) was used to synthesize single phase highly textured antiferromagnetic Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ on Pt/sapphire substrate. The as-deposited substrate temperature and oxygen flow rate were explored to optimize the phase, crystallinity, and surface morphology of the Cr$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$thin films with Neel temperature of 300 K. A very low electric leakage (3E-5 A/cm$^{\mathrm{2}})$ in single-phase chromia films less than 10 nm thick was observed. Dielectric permittivity and loss were measured as a function of film thickness. These new results demonstrate the potential of milli-volt switching voltage for exchange bias of magnetic structures using antiferromagnetic chromia, for practical low power switching of magnetic tunnel junctions. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S43.00012: Enhanced resistivity in voltage-controlled exchange-bias devices Ather Mahmood, Will Echtenkamp, Mike Street, Christian Binek, Chun Kwan, Jonathan Bird Manipulation of magnetism by electric field is important for energy-efficient devices in information technology. Voltage-controlled switching of magnetization is manifested through exchange bias (EB) and promises non-volatile spintronic memory and logic devices. Earlier, we demonstrated robust isothermal voltage control of EB near room temperature using a heterostructure of Co/Pd thin film and an exchange coupled bulk single crystal of the antiferromagnetic magnetoelectric Cr2O3 (Chromia). A major obstacle in the display of EB in patterned Chromia thin-film devices is significant leakage current at high electric fields. Electrical measurements on patterned devices and conductive Atomic Force Microscopy of Chromia thin-films reveal the nature of defects which form conducting paths, impeding the application of sufficient voltage. By replacing the base metal Pd with Pt, we demonstrate the mitigation of conduction paths due to reduced lattice mismatch of Chromia over Pt. X-ray diffraction show the suppression of 60 degree domains and electrical measurements on patterned devices confirm the enhanced resistivity. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S43.00013: Tuning N\'{e}el temperature and anisotropy of magnetoelectric Cr$_{2}$O$_{3}$ via doping for enhanced performance in voltage-controlled spintronic devices Michael Street, Will Echtenkamp, Takashi Komesu, Shi Cao, Jian Wang, Peter Dowben, Christian Binek Spintronic devices have been considered a promising route to revolutionizing current logic and memory technologies. This work is an effort to realizing such spintronic devices by voltage-control of the magnetoelectric Cr$_{2}$O$_{3}$. The electrically switchable boundary magnetization of Cr$_{2}$O$_{3\, }$can be used to voltage-control the magnetic states of an adjacent ferromagnet. For this technique to be utilized in a spintronic device, the N\'{e}el temperature of Cr$_{2}$O$_{3\, }$must be increased above the bulk value of T$_{N}=$307K, Previously, B-doped Cr$_{2}$O$_{3\, }$thin films were fabricated via PLD showing boundary magnetization at elevated temperatures via magnetometry and spin polarized inverse photoemission spectroscopy (SPIPES). Temperature dependent exchange bias measurements of B-doped Cr$_{2}$O$_{3}$ were also investigated using VSM and MOKE. The data indicate a substantial increase in the blocking temperature by about 100K accompanied, however, by a detrimental change in the anisotropy of Cr$_{2}$O$_{3.\, }$Conclusions from magnetometry are supported by SPIPES. Chemical straining is investigated to recover anisotropy while maintaining increased blocking temperature. This project was supported by SRC through CNFD, an SRC-NRI Center under Task ID 2398.001, and by C-SPIN, part of STARnet, an SRC program sponsored by MARCO and DARPA (SRC 2381.001). [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S43.00014: Correlated electric-field induced reversal of antiferromagnetic order and surface magnetization in magnetoelectric Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ Junlei Wang, Uday Singh, Christian Binek The electric-field-induced Faraday effect in magnetoelectrics comprises a superimposition of linear electric field responses with temperature dependencies of the linear magnetoelectric susceptibility and the antiferromagnetic order parameter. The tunability of the relative strength between the two contributions leads to a table-top set-up allowing to measure voltage-controlled selection and temperature dependence of the antiferromagnetic order parameter. Simultaneous measurement of the polar Kerr effect and the electric-field-induced Faraday effect is utilized to investigate correlated formation and switching of the surface magnetization and bulk antiferromagnetic order in Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$ The correlated reversal of surface or boundary magnetization in response to voltage-controlled reversal of the bulk antiferromagnetic order parameter is of key importance for applications in spintronic devices such as the magnetoelectric MRAM. The Faraday rotation per applied voltage is independent of the sample thickness making the method scalable and versatile for thin film investigations. Scalability, compactness, and simplicity of the data analysis combined with low photon flux requirements make the Faraday approach advantageous for the investigation of the otherwise difficult to access voltage-controlled switching of antiferromagnetic domain states in magnetoelectric thin films. [Preview Abstract] |
Thursday, March 16, 2017 2:03PM - 2:15PM |
S43.00015: Determining the sign of exchange coupling in a chromia based perpendicular exchange bias heterostructure Uday Singh, Mike Street, Will Echtenkamp, Christian Binek, Shireen Adenwalla Exchange bias arises from the coupling at the AFM/FM interface and, has been observed and studied in a wide range of systems. A key property of exchange bias systems is the sign of the coupling between the ferromagnet spins and the interfacial antiferromagnet spins, which may be aligned either ferromagnetically (parallel) or antiferromagnetically (antiparallel). Antiferromagnetic exchange coupling is known to be the generic cause of positive exchange bias. Determining the sign of exchange coupling is straight forward in system where the coupling is weak and can be overcome by Zeeman energy on field -cooling. It is, however, a challenging task when the available magnetic field is low or the magnitude of the exchange coupling is high. Here, we present a technique to determine the sign of the exchange coupling using low fields. We measure the exchange bias field as a function of ferromagnet magnetization during field cooling and the resultant behavior of the exchange bias vs. the magnetization uniquely determines the sign of the coupling. We use this to measure the sign of the exchange coupling in a Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$(300 nm)/Pd(0.5 nm)/[Co(0.3 nm)/Pd(1 nm)]$_{\mathrm{3}}$ heterostructure thin film system and verify our results with the conventional high field method. [Preview Abstract] |
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