Session X19: Magnetic Oxide Thin Films and Heterostructures: Electrostatic, Ionic, and Magnetoelectric Coupling

Electric-Field Coupling to Spin Waves in a Centrosymmetric Ferrite

A systematic control of spin waves via external electric fields has been a long standing issue for the design of magnonic devices, and is of fundamental interest. One way to attain such control is to use multiferroics [1], whose electric and magnetic polarizations are inherently coupled. The lack of electric polarization in a centrosymmetric ferrite, however, makes direct coupling of its magnetization to external electric fields a challenge. Indirect electric control of spin waves has been accomplished by hybridizing yttrium iron garnet (YIG), a centrosymmetric ferrite, with a piezoelectric material [2]. Here, we predict direct control of spin waves in YIG by {\it a flexoelectric interaction}, which couples an electric field to the spatial gradient of the magnetization, and thus the spin waves [3]. Based on a superexchange model, which describes the antiferromagnetic coupling between two nearest neighbor iron ions through an oxygen ion, including spin-orbit coupling, we estimate the coupling constant and predict a phase shift linear in the applied electric fields [4]. The theory is then confirmed by experimental measurement of the electric-field-induced phase shift in a YIG waveguide [5]. In addition to the flexoelectric effect, another electric effect is observed, which couples the electric field directly with the magnetization of YIG. We call this a magnetoelectric effect. By adjusting the direction of the electric field, the two effects can be well separated. Experimental results agree quantitatively with the theoretical prediction. A phenomenological coupling constant for the magnetoelectric effect is also obtained. Our findings point to an important avenue for manipulating spin waves and developing electrically tunable magnonic devices. [1] P. Rovillain \textit{et al}., Nat. Mater. {\bf 9}, 975 (2010). [2] M. Bao \textit{et al}., Appl. Phys. Lett. {\bf 101}, 022409 (2012). [3] T. Liu and G. Vignale, J. Appl. Phys. {\bf 111}, 083907 (2012). [4] T. Liu and G. Vignale, Phys. Rev. Lett. {\bf 106}, 247203 (2011). [5] X. Zhang \textit{et al}., Phys. Rev. Lett. {\bf 113}, 037202 (2014). [6]The author gratefully acknowledges collaborations with G. Vignale, M.E. Flatte', X. Zhang and H. X. Tang. This work is supported by DARPA MESO and an ARO MURI.   

Enhanced magneto-ionic switching of interface anisotropy in Pt/Co/GdOx films

Voltage control of magnetic anisotropy is of great interest for reducing the switching energy barrier in spintronic devices. It has recently been shown that electric field-driven oxygen ion migration near the interface of ferromagnet/oxide bilayers can lead to very large changes in magnetic anisotropy at elevated temperature. Here, we examine magneto-ionic switching in ultrathin Pt(3nm)/Co(0.9nm)/GdOx(t$_{ox})$/Au(t$_{Au})$ films with perpendicular anisotropy, in which the GdOx layer and gate structure are optimized for efficient room-temperature oxygen conduction. We study voltage-induced switching dynamics as a function of the GdOx stoichiometry and the thickness of the Au gate layer. We find that for optimally oxidized GdOx, a positive bias voltage applied to the Au electrode results in a transition from PMA to in-plane magnetization, and at zero bias, the PMA spontaneously returns. The rate of this transition depends on the thickness of the Au gate which suggests that the rate-limiting step is removal and reintroduction of oxygen by gate voltage. This toggling of PMA under positive bias does not require oxidation of the Co layer, in contrast to earlier work by Uwe et al . We demonstrate that by optimizing the electrode materials, extremely fast room-temperature switching can be achieved in these devices.   

Giant enhancement of magnetocrystalline anisotropy in ultrathin manganite films via nanoscale 1D periodic depth modulation

We report a unusual giant enhancement of in-plane magnetocrystalline anisotropy (MCA) in ultrathin colossal magnetoresistive oxide films due to 1D nanoscale periodic depth modulation. High quality epitaxial thin films of La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$ (LSMO) of thickness 6 nm were grown on (001) SrTiO$_{\mathrm{3}}$ substrates via off-axis radio frequency magnetron sputtering. The top 2 nm of LSMO films are patterned into periodic nano-stripes using e-beam lithography and reactive ion etching. The resulting structure consists of nano-stripes of 2 nm height and 100-200 nm width on top of a 4 nm thick continuous base layer. We employed planar Hall effect measurements to study the in-plane magnetic anisotropy of the unpatterned and nanopatterned films. The unpatterned films show a biaxial anisotropy with easy axis along [110]. The extracted anisotropy energy density is \textasciitilde 1.1 x 10$^{\mathrm{5}}$ erg/cm$^{\mathrm{3}}$, comparable to previously reported values. In the nanopatterned films, a strong uniaxial anisotropy is developed along one of the biaxial easy axes. The corresponding anisotropy energy density is \textasciitilde 5.6 x 10$^{\mathrm{6}}$ erg/cm$^{\mathrm{3\thinspace }}$within the nano-striped volume, comparable to that of Co. We attribute the observed uniaxial MCA to MnO$_{\mathrm{6}}$ octahedral rotations/tilts and the enhancement in the anisotropy energy density to the strain gradient within the nano-stripes.   

Magnetoelectric Dead Layer and Uncompensated Spins in Magnetic/Ferroelectric Heterostructures

Interfacial magnetoelectricity across a multilayer system is known to sometimes result in much larger coupling between electric and magnetism than in single phase systems. We compared the magnetic domains in LaSrMnO3 thin films, ferroelectric domains in PbZrTiO3 and observed uncompensated spin at the interface. Several techniques to quantify image contrast switching between left and right circularly polarized x-ray absorption spectra of magnetic domains and uncompensated spin were developed and gave similar results. Not surprisingly, the magnetic domain switching increased with magnetic film thickness, but the uncompensated spin did as well. This results suggests that there may be an effective magnetoelectric dead layer at the interface between coupled magnetic and ferroelectric layers, which is likely linked to at least the magnetic dead layer in the magnetic film. These measurements were taken by L-edge spectromicroscopy at the PEEM3 beamline of the Advanced Light Source.   

Magnetoelectric Coupling Characteristics of the La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$(001) Interface

Multiferroic heterostructures composed of thin layers of ferromagnetic and ferroelectric perovskites have attracted considerable attention in recent years. We apply \textit{ab initio} computational methods based on density functional theory to study the characteristics of the magnetoelectric coupling at the (001) interface between La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) and PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$(PZT). The calculations are carried out using the Quantum ESPRESSO electronic structure code combined with Vanderbilt ultrasoft pseudopotentials. Our study shows that the interfacial interaction between LSMO and PZT and the polarization of PZT have a strong influence on the distribution of magnetization within the LSMO layer. A significant change in the magnetization of the LSMO layer adjacent to PZT is observed after reversal of the direction of polarization of PZT.   

\textbf{Large enhancement of magnetic anisotropy and laser induced resistive switching effect in La}$_{\mathbf{0.7}}$\textbf{Sr}$_{\mathbf{0.3}}$\textbf{MnO}$_{\mathbf{3}}$\textbf{ films due to strain from BaTiO}$_{\mathbf{3}}$\textbf{ substrates}

Multifunctional oxide materials are interesting for their fundamental physical properties and technological applications. Epitaxial films of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) on BaTiO$_{3}$ (BTO) show intriguing properties such as a giant magnetoelectric effect due to strain from BTO substrate. The LSMO film shows sharp jumps in magnetization M(T) and resistance R(T) at first-order structural phase transitions of BTO (T$_{R-O}$ 200K and T$_{O-T}$ 270 K) due to strain coupling from BTO. A temperature evolution of effective in-plane anisotropy field (H$_{K})$ measured using the radio-frequency transverse susceptibility (TS) shows a sharp increase in H$_{K}$ around T$_{R-O}$, which vanishes around T$_{O-T.\, }$The in-plane magnetic anisotropy plays an important role in changing the magnetic and resistive states around T$_{O-T}$. A switchable laser-induced resistive change of up to 300 {\%}, which is about 10 times greater than those of conventional oxide systems, has been achieved in LSMO films using a 0.5 W violet laser just below the T$_{O-T}$.$_{\, }$The repeatability and stability of the laser-induced resistive switching effect reveal potential applications of LSMO/BTO heterostructures in developing new type of temperature sensors and memory devices.   

Reversible Control of Magnetism in La0.67Sr0.33MnO3 through Chemically-Induced Oxygen Migration

There has been a surge of interest in controlling magnetism through oxygen migration for applications in hybrid ionic/magnetoelectric device architectures. With a rich magnetic and electronic phase diagram, the colossal magnetoresistive perovskite (La,Sr)MnO3 (LSMO) is an ideal candidate for achieving large modulations in magnetic properties with small changes in oxygen content. We demonstrate reversible control of magnetism in LSMO films through interfacial oxygen migration. Gd metal capping layers deposited onto LSMO leach oxygen from the film to form porous Gd$_2$O$_3$. X-ray absorption and polarized neutron reflectometry measurements show Mn valence alterations consistent with high oxygen vacancy concentrations, resulting in suppressed magnetization and increased coercive fields. Oxygen migration is observed both at the interface and also throughout the majority of a 40 nm thick film, suggesting extensive oxygen vacancy diffusion. After Gd-capped LSMO is exposed to atmospheric oxygen for a prolonged period of time, oxygen diffuses through the Gd$_2$O$_3$ layer and the magnetization of the LSMO returns to the uncapped value. These findings showcase perovskite heterostructures as ideal candidates for developing functional interfaces through chemically-induced oxygen migration.   

Detecting interfacial defects at magnetic/non-magnetic junctions

Recent three terminal (3T) measurements in Co/LaAlO3/SrTiO3 show that spin-dependent transport through an interfacial defect is occurring instead of Hanle dephasing [1]. We propose extending 3T measurements into a coherent regime where single defects are detected by their local fields. The setup involves defects being situated between biased non-magnetic (NM) and ferromagnetic (FM) contacts. Spin torque on the FM drives an AC magnetization. Due to the large exchange interaction, the ability for charge to enter the FM depends on its spin and FM's relative orientation. As the FM precesses, the spin is dynamically filtered and a precessing spin accumulation remains at the defect. Local fields also precess the defect spin and interfere with the dynamic spin filtering. If the AC and local field are resonant, the spin accumulation is locked anti-parallel to the FM and leads to a dip in current. By adjusting the AC frequency, information on the local field is ascertained which, for hyperfine local fields, tells which nuclei are present at the defect and aids in identifying the defect. In the DC limit, defect spin accumulation leads to modifications in Hanle signals. [1] H. Inoue, A.G. Swartz, N.J. Harmon, M.E. Flatt\'e, T. Tachikawa, Y. Hikita, and H.Y. Hwang. In press at Phys. Rev. X.   

Electronic and Magnetic Properties of Transition-Metal Oxide Nanocomposites: A Tight-Binding Modeling at Mesoscale.

Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. In this talk, I will present an overview on our recent efforts in theoretical understanding of the electronic and magnetic properties TMO nanocomposites. In particular, I will introduce our recently developed tight-binding modeling of these properties arising from the interplay of competing interactions at the interfaces of planar and pillar nanocomposites. Our theoretical tool package will provide a unique capability to address the emergent phenomena in TMO nanocomposites and their mesoscale response to such effects like strain and microstructures at the interfaces, and ultimately help establish design principles of new multifunctionality with TMOs.   

Direct measurement of voltage-controlled reversal of the antiferromagnetic spin structure in magnetoelectric Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$

The frequency dependence of the electric field induced magneto-optical Faraday effect is investigated in the magnetoelectric antiferromagnet chromia. Two electrically induced Faraday signals superimpose in proportion to the linear magnetoelectric susceptibility and the antiferromagnetic order parameter. The relative strength of these contributions is determined by the frequency of the probing light beam. It allows tuning the Faraday signal between extreme characteristics which follow the temperature dependence of the magnetoelectric susceptibility or solely that of the antiferromagnetic order parameter. The frequency dependence is analyzed in terms of electric dipole transitions of perturbed Cr$^{\mathrm{3+\thinspace }}$crystal-field states. The results lead to a table-top set-up allowing to measure voltage-controlled selection and temperature dependence of the antiferromagnetic order parameter. 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.   

Voltage Control of Exchange Bias in a Chromium Oxide Based Thin Film Heterostructure

Controlling magnetism by electrical means is a key challenge in the field of spintronics, and electric control of exchange bias is one of the most promising routes to address this challenge. Isothermal electric control of exchange bias has been achieved near room temperature using bulk, single crystal, magnetoelectric Cr$_{2}$O$_{3}$[1,2]. In this study the electrically-controlled exchange bias is investigated in an all thin film Cr$_{2}$O$_{3}$/PdCo exchange bias heterosystem where an MBE grown ferromagnetic and perpendicular anisotropic Pd/Co multilayer has been deposited on a PLD grown (0001) Cr$_{2}$O$_{3}$ thin film. Prototype devices are fabricated using lithography techniques. Using a process of magnetoelectric annealing, voltage control of exchange bias in Cr$_{2}$O$_{3}$ heterostructures is demonstrated with significant implications for scalability of ultra-low power memory and logical devices. In addition, the dependence of the exchange bias on the applied electric and magnetic fields are independently studied at 300K and isothermal voltage-controlled switching is investigated. [1] Xi He, et. al., Nature Mater.9, 579–585 (2010). [2] W. Echtenkamp and Ch. Binek, Phys. Rev. Lett. 111, 187204 (2013).   

Eliminating leakage current in voltage-controlled exchange-bias devices

Manipulation of magnetism by electric field has drawn much attention due to the technological importance for low-power devices, and for understanding fundamental magnetoelectric phenomena. A manifestation of electrically controlled magnetism is voltage control of exchange bias (EB). Robust isothermal voltage control of EB was demonstrated near room temperature using a heterostructure of Co/Pd thin film and an exchange coupled single crystal of the antiferromagnetic Cr$_{2}$O$_{3}$ (Chromia)~[1,2]. A major obstacle for EB in lithographically patterned Chromia based thin-film devices is to minimize the leakage currents at high electric fields (\textgreater 10 kV/mm). By combining electrical measurements on patterned devices and conductive Atomic Force Microscopy of Chromia thin-films, we investigate the defects which form conducting paths impeding the application of sufficient voltage for demonstrating the isothermal EB switching in thin film heterostructures. Technological challenges in the device fabrication will be discussed. [1] Xi He, \textit{et.al}, Nat. Mater. 9, 579-585 (2010) [2] W. Echtenkamp, Ch. Binek, Phys. Rev. Lett. 111, 187204 (2013)   

Electrostatic doping limits and control of magnetism in electrolyte gated LaAlO$_{\mathrm{3}}$(001)/La$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$CoO$_{\mathrm{3-\delta }}$ thin films

Recently developed ionic liquid/gel gating techniques have proven remarkably expedient in the study of charge density effects in a variety of conductors, ranging from organics to complex oxides. Here we present electrolyte gate control of magnetism in ultrathin (8 u.c.) La$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$CoO$_{\mathrm{3-\delta }}$ (LSCO) films, using ion gels in electric double layer transistors. The LSCO films are initially metallic and ferromagnetic ($T_{c}\approx $ 170 K), with anomalous Hall conductivity up to 40 S/cm, and strong perpendicular magnetic anisotropy. Based on extensive temperature and gate voltage dependences we first determined the limits for electrostatic \textit{vs}. electrochemical operation, concluding that negative bias enables reversible hole accumulation, whereas positive bias irreversibly induces oxygen vacancies. Following this we demonstrated clear voltage-control of resistivity, magnetoresistance, and$ T_{c}$. Utilizing the anomalous Hall conductivity as an exceptional probe of the magnetic order parameter in the gated surface region, a 12 K shift in $T_{c}$ is obtained. This compares favorably to the state-of-the-art and exhibits potential for much larger modulation in films of lower Sr content. Work supported by NSF MRSEC.