Resent Research and Education Highlights
Ferroelectric Tunnel Junctions Enhanced by a
Polar Oxide Barrier Layer
Qiong Yang, Lingling Tao, Evgeny Tsymbal, and Vitali Alexandrov
Ferroelectric tunnel junctions (FTJs) consist of two metal electrodes separated by a thin ferroelectric barrier layer. The electric resistance of an FTJ changes when the electric polarization of this layer is reversed by an applied voltage. This property, known as the tunneling electroresistance (TER) effect, can be used for applications of FTJs in random-access memories. The enhancement of TER is beneficial for the applications.
Nebraska MRSEC researchers have proposed a new concept to design high-performance FTJs with enhanced TER. This design exploits property of a polar oxide material to create an ionic charge at the interface. When used in a composite barrier, it pins polarization of the adjacent ferroelectric layer which strongly reduces resistance of one of the FTJ states but does not affect the other. Using first-principles calculations, the researchers predicted that an ultrathin lanthanum aluminate (LaAlO3) layer enhances the TER of an FTJ with resistance ratio exceeding a factor of ten thousand. Such enhanced performance of the proposed FTJ can be exploited for device applications.
Picture: Schematic showing a ferroelectric tunnel junction (FTJ) with a composite barrier consisting of a ferroelectric layer (red) and a polar lanthanum aluminate (LaAlO3) layer (brown). The LaAlO3 layer creates a positive ionic charge at the interface which pins ferroelectric polarization (P) away from it. This property makes one state having a uniform polarization (top panel) and the other state having a head-to-head polarization (bottom panel). As a result the resistance of the FTJ corresponding to the two polarization states differs significantly, resulting in the enhanced TER effect.
Nanoscale Properties of MXene Membranes
Alexander Sinitskii and Alexei Gruverman
MXenes are two-dimensional (2D) ceramics made of transition metal carbides and nitrides. Unlike other 2D ceramics, MXenes have inherently good conductivity and thus are promising for various applications. Probing the local physical properties of MXenes monolayers is important for the understanding of their functional performance.
Nebraska MRSEC researchers in collaboration with their colleagues at Drexel University have developed an improved method for synthesis of monolayer membranes of Nb4C3Tx MXene. Using an approach based on Atomic Force Microscopy (AFM) they tested the electrical properties of the MXene membranes, such as electron mobility and conductance. AFM nanoindentation measurements facilitated evaluation of their elastic modulus, which turned out to be the highest among the solution-processed 2D materials. These results open a possibility of using Nb4C3Tx MXenes for nanomechanical applications and provide guidance for a search of new MXenes with improved functionalities.
The results are published: A. Lipatov, M. Alhabeb, H. Lu, S. Zhao, M. J. Loes, N. S. Vorobeva, Y. Dall'Agnese, Y. Gao, A. Gruverman, Y. Gogotsi, A. Sinitskii, Electrical and elastic properties of individual single‐layer Nb4C3Tx MXene flakes. Advanced Electronic Materials 1901382 (2020).
Picture: Atomic Force Microscopy (AFM) image of an Nb4C3Tx MXene flake covering an 820 nm microwell in a Si/SiO2 substrate. The color map reflects AFM measured height on the sample. The blue spot indicates an AFM-indented area of the flake atop of the microwell.
Intrinsic Exchange Bias in Cobalt Ferrite Thin Films
Xiaoshan Xu and Peter Dowben
Exchange bias describes the effect of a “harder” magnetic material on the magnetization of a “softer” magnetic material via their interface. The state of the “harder” magnetic material, which is less susceptible to the magnetic field, can pin the state of the “softer” magnetic material, as if there is an additional bias magnetic field. In magnetic recording, the exchange bias plays an essential role in stabilizing the magnetic state of the read head.
Nebraska MRSEC researchers have studied thin films of magnetic cobalt ferrite (CoFe2O4) on a non-magnetic alumina (Al2O3) substrate and showed that exchange bias exists with only one magnetic material. A temperature and thickness dependence of this “intrinsic” exchange bias indicated that the CoFe2O4 at the interface or surface exhibited different magnetic properties compared to the bulk, playing the role of a “harder” magnetic material. These results indicated that thin-film heterostructures only one magnetic material can be employed to generate a large exchange bias which may be useful for magnetic storage devices.
Picture: Model of atomic arrangement at the interface between ferromagnetic cobalt ferrite (CoFe2O4) and non-magnetic alumina (Al2O3).
Controlling Magnets with Sound
Anil Adhikari and Shireen Adenwalla
Computer hard drives are fast spinning discs of magnetic thin films, with regions of magnetization pointing either up or down, interpreted as 0s and 1s. Editing a file rewrites the 0s and 1s, using a miniscule magnet to change the magnetization direction. Finding alternative ways to control magnetization could reduce the power or space requirements.
One way is to use sound. In the figure, the two sets of inter-penetrating fingers on the left and right represent electrodes that launch a sound wave. The stripes between the two are magnetic films with the magnetization pointing up and down in the orange and blue regions, respectively. Where the two meet is a domain wall.
The entire pattern is etched onto a piezoelectric material, which converts voltage to movement. When the fingers are excited by a high-frequency voltage, oscillating at about one hundred million times a second, they launch a wave that alternately compresses and pulls apart the surface atoms. This alternating squeezing and stretching moves the domain walls, reversing an up domain to a down domain or vice versa. These results demonstrate that a high-frequency sound may serve as a viable approach to control thin-film magnetization for magnetic data recording and storage.
Picture: Magnetic thin-film stripes between two sets of finger electrodes. The electrodes launch a high-frequency sound wave into the magnetic stripes and move the boundaries between the regions of magnetization pointing up and down.
Probing Negative Capacitance
Xia Hong and Xiaoshan Xu
Ferroelectric materials possess ferroelectric polarization P, which can be switched by an electric field due to a double-well structure in the free energy profile. Around P = 0, the curvature of the energy vs. polarization relation is negative. It can be described as a negative capacitance (NC) effect, which can be used to reduce supply voltage for field-effect transistors (FET).
To understand how to stabilize the NC mode in epitaxial oxide thin films, Nebraska MRSEC researchers have fabricated high quality capacitor stacks composed of ferroelectric lead zirconium titanate (PZT) and dielectric strontium titanate (STO), and examined the transient switching behavior in samples with different layer thickness ratios r between PZT and STO. The Landau theory modeling shows that the free energy of the stack capacitor evolves from a ferroelectric-like double well to a dielectric-like single minimum behavior with reducing r, consistent with the transient switching results of the stack capacitors. This study can facilitate the development of NC-FETs for low-power nanoelectronics.
Figure: The free energy G vs. polarization P of a STO/PZT stack capacitor evolves from a double well to a single energy minimum as the thickness ratio r between the PZT and STO layers decreases from ∞ to 1.
Fabrication and Characterization of Free-Standing Magnetic Oxide Membranes
Complex oxide thin films and heterointerfaces are versatile playgrounds for designing new functional behaviors that are not accessible in bulk materials. The conventional method for realizing these materials systems builds on epitaxial thin film deposition on substrates with similar crystal structures, which limits the choice of viable constituent materials, as well as altering the properties of the thin film samples due to the clamping effect of the substrate. Such constrains can be overcome by the fabrication and controlled transfer of free-standing complex oxide membranes.
Working with a water soluble buffer layer Sr3Al2O6, Nebraska researchers have successfully suspended free-standing membranes of ferrimagnetic oxide NiCo2O4, a promising spin injection material. The suspended thin films can be transferred to any designated substrates, and have been fabricated into devices for electrical characterization. This work lifts the restriction on the structural match for developing high performance epitaxial spintronic devices, enabling a plethora of functional oxides as viable candidates.
Picture: Flow chart of the device fabrication process.
Nano Week: “Spark”-ing an Interest in Materials Science
Krista Adams, Rebecca Lai, and Jocelyn Bosley
In the third year of a collaboration with the Foundation for Lincoln Public Schools (LPS), Nebraska MRSEC partnered with elementary educators in the Spark Summer Learning program to offer lab tours and research-based explorations of nanoscience concepts to studnts in grades K-5. In 2018, nine elementary teachers were selected to participate in a workshop and residency, where they were introduced to lessons on nanomagnetism developed by a Nebraska MRSEC graduate student in the Department of Teaching, Learning, and Teacher Education. In 2019, during the Spark Summer Learning program’s ”Nano Week,” these lesson plans were implemented and expanded as MRSEC researchers and LPS teachers guided 72 K-5 students through interactive tours and activities at the University of Nebraska. This sustained collaboration between researchers and elementary educators links MRSEC research objectives with the LPS science curriculum to inspire the next generation of materials scientists.
Picture: Graduate student Luis Jauregui talks with an elementary student during the Spark Summer Learning program’s Nano Week.
Science! With Friends
Jocelyn Bosley and Rebecca Lai
In July 2019, Nebraska MRSEC launched the podcast Science! With Friends, created and co-hosted by Assistant Director for Education and Outreach Jocelyn Bosley. Podcasts are an increasingly popular and effective medium to reach large, diverse audiences. The goal of this podcast is to make scientific research meaningful through interviews, storytelling, and engaging conversation, while also promoting the educational and outreach programs of Nebraska MRSEC and other Centers. To maximize its listening base and social relevance, Science! With Friends takes a broadly interdisciplinary approach, showing how materials science contributes to and benefits from research in a wide variety of fields. The podcast has featured interviews with Nebraska MRSEC faculty, postdocs, and graduate and undergraduate alumni. All episodes can be found on Spotify, Spreaker, Google Podcasts, iHeartRadio, and on Apple Podcasts: https://podcasts.apple.com/us/podcast/science-with-friends/id1471423633.
Picture: Bosley (left) interviews Nebraska MRSEC adjunct faculty Dr. Axel Enders of the University of Bayreuth, Germany, who was featured on episode #16, “Go Small or Go Home!”.
First Observation of a Native Ferroelectric Metal
P. Sharma and J. Seidel
University New South Wales
D.-F. Shao and E. Y. Tsymbal
Ferroelectric materials are insulators which possess an electrically switchable spontaneous polarization. It was predicted half a century ago that a polar ferroelectric-like structure can also exists in a metal, which might have interesting functional properties. However, an experimental demonstration of such a “ferroelectric metal” in a room-temperature single-phase material has remained elusive.
Recently Nebraska MRSEC researchers collaborated with their colleagues at University of New South Wales in Australia who experimentally observed coexistence of native metallicity and ferroelectricity in bulk crystalline material for the first time. The switchable spontaneous polarization and an intrinsic ferroelectric domain structure was found at room temperature in van der Waals semimetal tungsten telluride WTe2. This material has a layered structure (shown in the figure) with ferroelectric polarization pointing up or down perpendicular to the WTe2 layers and is conductive along the layers. This new class of materials with unique properties has potential for novel nanoelectronics applications.
The results are published: P. Sharma, F.-X. Xiang, D.-F. Shao, D. Zhang, E. Y. Tsymbal, A. R. Hamilton, and J. Seidel, A room temperature ferroelectric semimetal, Science Advances 5, eaax5080 (2019).
Picture: The atomic structure of van der Waals ferroelectric semimetal tungsten telluride WeTe2 with polarization up (left) and polarization down (right).
All Research and Education Highlights:
- Interfacial Charge Engineering in Ferroelectric-Gated Mott Transistors
- A Viable Material for Topological Antiferromagnetic Spintronics
- Control of Spin State at the Molecular Level
- Defect-Assisted Tunneling across Ferroelectric Tunnel Junctions
- Giant Electrostriction of Halide Perovskites Discovered
- New Hybrid Heterostructure Nanophotonic Materials
- Capturing Structural Dynamics of Materials with Ultrafast Electron Diffraction
- Science Slams: Seeding a Community of Science Communicators
- Tenth Annual Conference for Undergraduate Women in Physical Sciences (WoPhyS)
- Ferroelectric Domain Wall as a Memristor
- Decoupling of Magnetization and Electric Polarization in Hexagonal Ferrites
- Nebraska MRSEC Facility: Synthesis and Characterization of Graphene-Like Boron-Carbon-Nitrogen Monolayers
- Characterizing Complex Nanostructured Materials with Atomic-Scale Resolution
- UNL-NCAT Joint Workshop in Materials Science and Engineering
- Nebraska MRSEC Puts a “Spark” in Summer Learning
- NanoThermoMechanical Thermal Computing
- Solar Cell Enhancement by Ionic Defect Passivation
- Optical Control of Polarization in Hybrid 2D-Ferroelectric Structures
- Ferroelectrically-Controlled Magnetic Anisotropy
- Direct Observation of Ferrimagnetism in a Multiferroic Hexagonal Ferrite
- Inducing Magnetism by Proximity
- Hybrid 2D-Ferroelectric Structures for Information Technology
- Artificial Magnetism with a Metamaterial
- Science Night Live! : Bringing Science to the Public
- Job Start: Preparing Students for STEM Careers
- Nebraska MRSEC Research on Cover Pages
- Trap Passivation to Enhance Solar Cell Efficiency
- Detecting Magnetic Order when Magnetization is Absent
- Room-Temperature Ferroelectricity in Croconic Acid Films
- Science Slams: The Future of Science Communication
- Nucleation Control of Conjugated Polymers
- Elucidating Single-ion Magnetic Anisotropy in LuFeO3
- P-SPINS Research in K-6 Science Curriculum Development
- Nebraska Nanoscale Facility
- Nebraska MRSEC Strengthens International Partnerships
- Nanoscience Vlog – a New Way to Communicate P-SPINS Research
- Nebraska MRSEC Partnership with Universities of Strasbourg and Bordeaux
- Search for Ferroelectricity in Two-Dimensional Molecular Crystals
- Magnetic Textures Stabilized by Strong Spin-Orbit Interactions
- Strain on the Fly
- Shared MRSEC Facility Strengthening Collaboration
- The P-SPINS Bridge Program – A Pipeline for Minority Students to University of Nebraska’s Graduate Program
- Nebraska MSREC WoPhyS14 Conference
- Structural and Magnetic Evolution of Bimetallic MnAu Clusters
- Nebraska MRSEC Partnership with the Hiroshima Synchrotron Radiation Center
- A Nickelate Channel for a Ferroelectric Field Effect Transistor
- Graphene-Enhanced Ferroelectric Tunnel Junctions
- Laves Phase Ferromagnetism and Core-Shell Nanoparticles
- Polarization Control of the Magnetic State of a Molecule
- Electromechanical Switching in Graphene Nanoribbons
- Center for NanoFerroic Devices
- Universality of Voltage-Controlled Boundary Magnetization
- Magnetic Domain Structure of Cobalt Nanospirals
- Hybrid Ferroelectric/Graphene Devices
- Nebraska MSREC WoPhyS13 Conference
- Nebraska MRSEC Professor/Student Pairs Program
- Mechanical Writing of Polarization
- Hydroxyl-Induced Magnetism in TiO Nanoclusters
- Designer Superlattices with Multifunctional Properties
- Nanostructures with a Twist
- Spin-Polarized Scanning Tunneling Microscopy
- Epitaxial Magnetoelectric Interfaces
- Nebraska MRSEC/Industry Workshop
- Nebraska MRSEC Professor/Student Pairs Program
- Organic Ferroelectric Photovoltaics
- Theoretical and Experimental Characterization of Structures of MnAu Nanoclusters
- Organic Molecular Layers for Efficient Charge Injection
- Electric Field Control of Magnetization
- Enhanced Ferroelectric Stability by Interface Engineering
- Nebraska MSREC WoPHY11 Conference
- Nebraska MRSEC Professor/Student Pairs Program
- High-Sensitivity Detector for Molecular Sensing using Magnetic Particles
- Cluster Synthesis of High-Anisotropy Nanomagnets
- Putting an Organic Spin on Ferroelectric Tunnel Junctions
- Quantum Entanglement in Magnetic Nanoparticles
- A Low-Cost Scanning Tunneling Microscope for Education and Outreach
- Visualization of Quantum States
- Optical Hall-Effect in Graphene
- Molecular Legos
- Robust Isothermal Electric Control of Exchange Bias at Room Temperature
- Electronic DNA Detection via Magnetic Nanoparticles Preconcentration
- Resistive Ferroelectric Switching at the Nanoscale
- Low-Power Picotesla Magnetoresistive Sensor
- Magnetic Doping of Golden-Cage Clusters
- Electrically Controlled Surface Magnetism
- Sculptured Cobalt Nanomagnets
- Stripes are Stars! Pt Helps Fe to Stay Magnetized
- How Strong is It?
- Lessons and Activities for Science Teachers
- Reaching a New Level of Nanostructuring: Two-Phase Clusters
- Correlation Effects and Electronic Structure of Gd@C60
- Ferroelectrically Controlled Magnetism
- Collective Buckling of Periodic Soft Nanostructures on Surfaces
- Summer Programs Share Nanoscale Research
- Tunneling Anisotropic Magnetoresistance in Magnetic Break Junctions
- Defect Mediated Properties of Magnetic Tunnel Junctions
- Nanostructuring for High-Energy Magnets
- Fourth Graders Study Optical Properties of Solids
- Magnetization Reversal of Patterned Submicron-Sized Dot Arrays
- New Magnetoresistive Phenomenon Discovered at the Nanoscale
- Electric Field Control of Magnetic Nanostructures
- Magnetic Domain Walls at the Nanoscale: Rigid or Soft?
- Controlling Magnetism through a Ferroelectric Switch
- Light Used as a Magnetic Hammer
- Transition Metal Oxides as Tunnel Barriers
- Ballistic Anisotropic Magnetoresistance
- Anisotropic Magnetism in Dilute Magnetic Semiconductors
- Microcantilever Torque Magnetometer (MTM)
- Magnetic Tunnel Junctions
- Template-Mediated Self-Assembly of Patterned Nanomagnets
- Bonding of Biphenyldimethyldithiol on Co Surface
- Spin Polarization at Oxidized Co Surface
- Research Experiences for Teachers
- Seventh Graders Make Nanowires
- Nanotube Magnetism
- Domain Wall Resistance in Magnetic Nanojunctions
- Co and Ni Magnetic Nanocontacts
- Materials Science for Elementary/Secondary Students