Program Highlights

Ferroelectric Tunnel Junctions Enhanced by a
Polar Oxide Barrier Layer

Qiong Yang, Lingling Tao, Evgeny Tsymbal, and Vitali Alexandrov

Nebraska MRSEC

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
Nebraska MRSEC

Y. Gogotsi
Drexel University

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 Nb₄C₃T_x 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 Nb₄C₃T_x 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 Nb₄C₃T_x MXene flakes. Advanced Electronic Materials 1901382 (2020).

Picture: Atomic Force Microscopy (AFM) image of an Nb₄C₃T_x 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
Nebraska MRSEC

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
Nebraska MRSEC

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
Nebraska MRSEC

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 

Xia Hong
Nebraska MRSEC

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
Nebraska MRSEC

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
Nebraska MRSEC

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
Nebraska MRSEC

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).