Research and education highlights are brief and digestible summaries of recent significant research results or education activities, chosen for their potential interest to a broad audience.




Resent Research and Education Highlights


Polarization Control of the Magnetic State of a Molecule

Xin Zhang and Peter A. Dowben
Nebraska MRSEC

Spin crossover molecules form a vast class of materials for which the magnetic structure can be altered at the atomic level by an external stimulus. Diamagnetic low spin to paramagnetic high spin transitions can be induced by pressure, temperature, illumination with light, or magnetic pulses. Nebraska MRSEC researchers have shown for the first time that ferroelectric polarization can be used to stabilize spin cross-over molecules in either the high spin state or the low spin state over a wide temperature range including room temperature [Chemical Communications 50, 2255 (2014)]. Using a molecular thin film of [Fe(H2B(pz)2)2(bipy)] adsorbed on an organic ferroelectric (polyvinylidene fluoride with trifluoroethylene) substrate they showed that the polarization of the organic ferroelectric mediates the spin state of the cross-over system. The possibility to induce a spin crossover transition by electric fields could lead to heteromolecular magneto-electric systems where the magnetic properties of the molecular overlayer are altered by an applied electric field. This is very important because molecular systems are among the very few systems that could potentially deliver low power GHz nonvolatile magneto-electric logic operations. 

Figure: The ferroelectric polarization of an organic ferroelectric (polyvinylidene fluoride with trifluoroethylene) substrate mediates the spin state of an adsorbed spin cross-over molecular thin film [Fe(H2B(pz)2)2(bipy)].


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Center for NanoFerroic Devices

Evgeny Tsymbal
Nebraska MRSEC

The University of Nebraska-Lincoln (UNL) leads a new $7.125 million research collaboration involving six universities to develop a new generation of electronic devices in partnership with an industry consortium. The Center for NanoFerroic Devices (CNFD) is one of the three centers sponsored by the Nanoelectronics Research Initiative (NRI) and the National Institute of Standards and Technology (NIST).  The NRI is a consortium of companies in the Semiconductor Research Corporation (SRC) seeking to propel technology beyond its current limits.
This joint research with five other universities will help transform basic university discoveries and knowledge into actual devices, in collaboration with industries. UNL is partnering with researchers at the University of California at Irvine; University of Wisconsin-Madison; University at Buffalo, SUNY; University of Delaware; and Oakland University. CNFD targets non-conventional, low-energy technologies based on innovative functional materials systems and conceptually novel approaches for device operation. Research involves exploration of properties, materials, structures, and phenomena non-traditional for existing technologies, such as magnetoelectricity, ferroelectricity, and spin dynamics.
Establishing of the CNFD is largely due to a long standing collaboration of the Nebraska MRSEC and NRI. Two MRSEC supplements have been supported by the NRI, and two research thrusts of the CNFD are the direct result of innovative concepts born and developed within the Nebraska MRSEC. The Nebraska MRSEC research addresses the NRI’s core objectives and has the potential for further strong and mutually beneficial interaction with the industrial companies participating in the NRI. 


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Universality of Voltage-Controlled Boundary Magnetization

Christian Binek and Peter A. Dowben
Nebraska MRSEC

Roughness insensitive boundary magnetization is a new concept discovered by Nebraska MRSEC researchers. It is a unique feature of magnetoelectric antiferromagnets, i.e. materials in which the application of electric field induces a net magnetic moment. An important property of the boundary magnetization is that it can be switched between two states by applying an electrical voltage. This property can be used in nanoelectronic devices, such as low power magnetic random access memories and spin-based field effect transistors. Although the boundary magnetization is predicted to be a generic property of any magnetoelectric antiferromagnet, its experimental evidence is scarce and has only been provided for the Cr2Osurface. To bring the concept of voltage-controlled boundary magnetization into a broader context, Nebraska MRSEC researchers explored magnetoelectric Fe2TeO6and showed the evidence of surface magnetization that can be controlled by applied voltage. They fabricated this oxide material for the first time as highly textured thin films using pulsed laser deposition methodology. X-ray magnetic circular dichroism – photoemission electron microscopy (XMCD-PEEM) confirmed that am antiferromagnetic multi-domain state can be brought into an antiferromagnetic single domain state with voltage-controlled boundary magnetization. The finding establishes magnetoelectric antiferromagnets as a broader class of materials with significant implications for voltage-controlled spintronics.

Picture: Atomic and spin structure of the magnetoelectric antiferromagnet Fe2TeO6 (left) and XMCD-PEEM image of antiferromagnetic Fe2TeO6 in an multidomain state (top) and a single-domain state (bottom), showing that local boundary magnetization can be controlled by voltage. 



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Magnetic Domain Structure of Cobalt Nanospirals

Eva Schubert, Daniel Schmidt, and Ralph Skomski
Nebraska MRSEC

Charudatta Phatak and Amanda Petford-Long
Argonne National Laboratory

Nanoscale three-dimensional (3D) structures are building blocks for the fabrication of miniature switching devices and can be used as functional units in nanorobotics. The functionality of the 3D structures is affected by their size and shape and may significantly differ depending on the properties of the bulk material. Nebraska MRSEC researchers use deposition under oblique angles of incidence to produce manifold ensembles of magnetic 3D nanostructures with different shapes and sizes, such as cobalt nanospirals shown in the figure.  Collaboration with researchers from Argonne National Laboratory allowed visualizing the magnetic domain structure of individual nanospirals, using Lorentz transmission electron microscopy. The magnetic behavior was found to be of single domain character along the spiral wire, with strong magnetic coupling among the nanospirals due to an alternating direction of magnetization given by the spatial geometry, and the close packed arrangement of the nanomagnets within an array. The findings were recently published in Nano Letters, present the first observation of the complex magnetic domain structure in 3D chiral nanoobjects, and are important for tailoring nanomagnets towards their desired properties.  

Picture: Magnetization map of a cobalt nanospiral (left) showing variations in the magnetization direction (right).


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Hybrid Ferroelectric/Graphene Devices

Alexandra Fursina, Alexey Lipatov, Haidong Lu, Alexei Gruverman, and Alexander Sinitskii
Nebraska MRSEC

Graphene is a two-dimensional material that consists of carbon atoms arranged in a hexagonal lattice. Graphene has a very high electronic conductivity that could be tuned by external electric field. Ferroelectrics comprise an important group of materials, which possess a permanent electric polarization. When graphene and a ferroelectric material are combined in a hybrid ferroelectric/graphene heterostructure, the polarization of a ferroelectric could be used to control the conductivity of graphene.  MRSEC researchers have grown graphene on ferroelectric Pb(Zr0.2Ti0.8)O3 (PZT) films and showed that ferroelectric polarization switching leads to a significant change in the conductivity of graphene. They found that the critical role in the performance of these hybrid devices is played by water molecules adsorbed at the interface between graphene and a PZT film. An accurate chemical control of the molecular water layer at the interface improves ferroelectric polarization stability and its effect on the conductivity of graphene. This study demonstrates a functional field-effect transistor with a capability of non-volatile performance, which makes such a device promising for applications in data storage and computer logics.   

Picture: Schematic (left) and scanning electron microscopy image (right) of a graphene (G) field-effect transistor with gold (Au) electrodes fabricated on a ferroelectric Pb(Zr0.2Ti0.8)O3 (PZT) substrate. The inset shows a water molecule at the interface between graphene and PZT.



Nebraska MSREC WoPhyS13 Conference

Axel Enders
Nebraska MRSEC

The University of Nebraska held its fifth Conference for Undergraduate Women in Physical Sciences on October 24-26, 2013 under the scientific theme “Nano Trek.”
WoPhyS, chaired by MRSEC faculty Axel Enders, provides participants with a unique opportunity to discuss scientific work and research experiences with their peers. WoPhyS attracts approximately 75 students per year from across the U.S. Within the last five years, WoPhyS has become a phenomenal success, growing in stature (as indicated by the speakers we have been able to draw from U.S. universities, national laboratories, industry, and NASA), popularity (in 2013 we had to close the online registration four weeks before the deadline due to high demand, and reputation (in 2013 we drew students from 26 states in the U.S.). WoPhyS has become a highly effective recruitment tool.

Highlights of the WoPhyS13 Conference included:

  • plenary talks by accomplished women scientists from the U.S.
  • invited presentations by undergraduate students upon nomination by their faculty mentor
  • Podium discussions on careers in science
  • Networking opportunities for students during poster session, conference banquets, and social events
  • lab tours at the Physics, Chemistry, and Engineering Department 


Nebraska MRSEC Professor/Student Pairs Program

Nebraska MRSEC Professor/Student Pairs Program

Jeffrey Shield
Nebraska MRSEC

The Nebraska MRSEC Professor/Student Pairs Program brings in a professor and a student from non-research intensive four-year institutions to conduct research with Nebraska MRSEC scientists. The goal is to offer a research experience which benefits both the participants and the MRSEC projects. This program provides opportunities for the professor to conduct new research, access to facilities typically unavailable at their home institution, and make strong and lasting connections with MRSEC researchers; for the student to acquire new expertise and training; for the MRSEC scientist to establish new collaborations and mentoring practices.
In summer 2013, Professor Dhananjay Kumar and student David Thompson of North Carolina A&T University worked with Nebraska MRSEC researcher Professor Jeff Shield. The research focused on creating extensive (i.e., non-equilibrium) solid solution alloys between Fe and W or Ta, and then studying the magnetic behavior. David fabricated the alloys using high-energy mechanical alloying, and discovered that Ta dissolved into the bcc Fe much more readily than did W. He and Prof. Kumar also fabricated alloys using arc melting, which were utilized as targets for pulsed laser deposition of thin films back in Prof. Kumar’s lab at NC A&T.  The complementary expertise of both synthesis and characterization between the two research groups bodes well for a productive long-term collaboration.

Picture: Student David Thompson from North Carolina A&T working in Nebraska MRSEC laboratory.



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