i. Multiferroic and ferroelectric materials and nanocomposites

Shuai Ning, a Postdoctoral Scholar at MIT, is working on Ionotronics. Electrolyte gating (see the schematic below), with an ionic liquid (IL) as the medium, has been a popular method to tune the physical properties of a variety of material systems. Besides the electrostatic effect, electrochemistry effect has been drawing intensive interest recently as the voltage-driven ion motion significantly enrich the structural, electronic and magnetic phase diagrams, and enable the voltage-control of various properties. In our recent work, we have studied the IL gating effect in WO₃ and SrCo_1-xFe_xO_3-δ thin films.

The schematic of ionic liquid gating

The selective control of electrical and thermal conductivity in WO₃ thin film, [1]. In the WO3 thin films, hydrogen intercalation is found to affect the lattice structure, electrical conductivity and thermal conductivity. By alternating the polarity of gating bias, we can reversibly insert and extract the hydrogen ions, and hence dynamically control the electrical conductivity by 5 orders of magnitude and the thermal conductivity by a factor of 1.7. In particular, combining the strain engineering via using substrates with different lattice parameters and the IL gating, we can selectively tune the electrical and thermal conductivity.

The reversible ON/OFF magnetism in SCFO film by ionic liquid gating, [2]. In the SrCo_1-xFe_xO_3-δ (SCFO) thin films, we have realized the voltage-control of ON-OFF magnetism above room temperature by IL gating with low voltages (±2 V) (see Figure c). The substitution of Co with Fe significantly changes the magnetic properties of SCFO. In particular, for the Co/Fe ratio of ∼1:1, a switch between nonmagnetic (OFF) and ferromagnetic (ON) states with a Curie temperature above room temperature is accomplished. Tuning the oxygen stoichiometry via the polarity and duration of gating enables reversible and continuous control of the magnetization between 0 and 100 emu/cm3 at room temperature.

[1]   Shuai Ning, et al, Adv. Mater. 2019, 31, 1903738, [2]   Shuai Ning, et al, ACS Nano 2020, 14, 8949−8957

Shuai is also researching self-assembled vertical aligned nanocomposites. Vertically aligned epitaxial oxide nanostructures formed by the growth of two immiscible oxide phases are attractive candidates for two-phase multiferroics. The most widely studied examples are perovskite–spinel magnetoelectric nanocomposites such as BiFeO₃–CoFe₂O₄ (BFO–CFO), which typically consist of nanoscale pillars of the ferrimagnetic CFO phase inside a matrix of the ferroelectric BFO phase grown on a (001) oriented cubic single crystal perovskite substrate. We have been working on extending the materials system and the strain-mediated magnetoelectric coupling. For instance, we have successfully incorporated SrCo_1-xFe_xO_3-δ (SCFO) into self-assembled two-phase vertically aligned nanocomposites, in which the reversible voltage control of magnetism above room temperature is also attained. In particular, the notable structural response of SCFO to IL gating allows large strain couplings between the two oxides in these nanocomposites, with potential for voltage-controlled and strain-mediated functionality based on couplings between structure, composition, and physical properties.

The top-view (a) and cross-section (b) of the self-assembled vertical aligned 2-phase oxide nanocomposites with SCFO as the matrix.

Eunsoo Cho, a Graduate Student at MIT, is working on manipulating properties of epitaxial thin films through ionic liquid gating. Magnetic properties of materials depend on their valence state and exchange mechanism, which can be controlled by inserting ions. Her interest also consists of the formation of nanocomposites, especially self-assembled structures. For example, the pillars in the image below are spinel, and the matrix is perovskite. Last, but not least, she studies the magnetic properties of materials through DFT calculations.

Sangho Lee, a Graduate Student at MIT, is researching Graphene-Based Epitaxy and Layer Transfer of Single-Crystalline Complex Oxide Membranes. Recent developments in graphene-based epitaxy and layer transfer techniques allow for generating a wide range of freestanding complex oxide membranes with high-crystallinity including perovskite, spinel, and garnet. Such single-crystalline complex oxide membranes can be easily stacked to form unique material systems, where structurally and chemically incompatible materials are interfaced with each other. Thus, the material spectrum can be greatly expanded to explore new physical phenomena by creating the artificial heterostructures as well as to enhance the material properties by avoiding the substrate clamping effect. It further enables an efficient control of physical properties at the interface through coupling to external perturbations such as strain, light, gating, and proximity. We investigate the ferroelectric domain structure of transferred BaTiO₃ thin films onto foreign substrates, which is not restricted by the host substrate. Also, a freestanding form of the self-assembled BaTiO₃-CoFe₂O₄ nanocomposites can be integrated with piezoelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) layer to systematically manipulate the magnetoelectric coupling at the interphases of ferroelectric perovskite (BaTiO₃) and ferrimagnetic spinel (CoFe₂O₄).

Graphene-based epitaxy and layer transfer of ferroelectric perovskite (BaTiO₃) and ferrimagnetic spinel (CoFe₂O₄) membranes

[1]   Hyun S. Kum, Hyungwoo Lee, Sungkyu Kim, Shane Lindemann, Wei Kong, Kuan Qiao, Peng Chen, Julian Irwin, June Hyuk Lee, Saien Xie, Shruti Subramanian, Jaewoo Shim, Sang-Hoon Bae, Chanyeol Choi, Luigi Ranno, Seungju Seo, Sangho Lee, Jackson Bauer, Huashan Li, Kyusang Lee, Joshua A. Robinson, Caroline A. Ross, Darrell G. Schlom, Mark S. Rzchowski, Chang-Beom Eom, and Jeehwan Kim, “Heterogeneous Integration of Single-Crystalline Complex-Oxide Membranes”, Nature 2020, 578, 75–81.

Tingyu Su, a Graduate Student at MIT, is researching how to integrate Yttrium Iron Garnet (YIG) hetero-epitaxially with other functional materials on appropriate substrates using Pulsed Laser Deposition (PLD). Spontaneous organization is not unique to the physical sciences since nature has been producing such systems for millions of years. The same is true for Nano-Composite systems, which could server as the platform for combination of multiferroics (e.g. ferro/ferri magnetic, ferroelectric, magnetoelastic coupling etc.). The beauty of this composite system lies in the interface between different multiferroics, which can serve as lever to tune the properties of each individual materials by interfacial strain.

Chen, A., Bi, Z., Jia, Q., Macmanus-Driscoll, J. L. & Wang, H. Microstructure, vertical strain control and tunable functionalities in self-assembled, vertically aligned nanocomposite thin films. Acta Mater. 61, 2783–2792 (2013).

 ii. Magnetic garnets for spintronics 

 Jackson Bauer, a Graduate Student at MIT, studies rare-earth iron garnets (ReIG, R₃Fe₅O₁₂) for spintronics applications. The complexity and chemical stability of garnet systems allows for tuning of the magnetic properties for different applications. In particular, I work on depositing ReIG on silicon substrates, which are technologically important, and control the magnetic anisotropy through selection of the rare-earth ion, substrate, and film growth conditions. The benefits of ReIG over transition metals such as Co and Fe are their faster magnetization dynamics, and the insulating nature of these oxides eliminates parasitic current shunting and Joule heating in the magnetic layer, reducing energy consumption in devices.  ReIG can be integrated with materials that have strong spin-orbit coupling and large spin Hall effects, including platinum and topological insulators, facilitating electrical switching of these materials and possible future integration in memory devices.

a. Hysteresis loop of DyIG on Si
b. Anomalous Hall effect of DyIG on Si
c. AFM image of very large grains of DyIG
d. Cross-sectional TEM of garnet showing perfect lattice

[1]   J. Bauer, E. R. Rosenberg, S. Kundu, K. A. Mkhoyan, P. Quarterman, A. J. Grutter, B. J. Kirby, J. A. Borchers, C. A. Ross, “Dysprosium Iron Garnet Thin Films with Perpendicular Magnetic Anisotropy on Silicon”, Adv. Elec. Mat. 6, 1900820 (2020), DOI: 101002/aelm.201900820https://onlinelibrary.wiley.com/doi/full/10.1002/aelm.201900820

[2]   J. Bauer, E. R. Rosenberg, C. A. Ross, “Perpendicular magnetic anisotropy and spin mixing conductance in polycrystalline europium iron garnet thin films”, Appl. Phys. Lett. 114, 052403 (2019), DOI: 10.1063/1.5074166 [Editor’s Choice] https://aip.scitation.org/doi/abs/10.1063/1.5074166

Ethan Rosenberg, a Graduate Student at MIT, is focusing on the growth and characterization of rare-earth garnets for spintronic applications. His main areas of expertise are PLD growth, thin-film characterization (XRD and magnetometry), and electrical measurements of spintronic devices.

Bharat Khurana, a Graduate Student at MIT, is researching how to optimize iron garnet thin films for spintronic applications. There are three different sublattices in a magnetic garnet – the dodecahedral sublattice, the tetrahedral sublattice and the octahedral sublattice. The effect of non-magnetic ion substitution in the octahedral sublattice is being studied. This will enable greater understanding of the role of different sublattices in spintronic properties of garnets. It may also enable a more efficient spin transport across a magnetic garnet/heavy metal interface.

Takian Fakhrul, a Graduate Student at MIT, is developing novel spintronic memory materials. The key bottle neck to realizing next generation spintronic memory is the formation and fast motion of spin structures like chiral domain walls (DW) in magnetic materials. For magnetic thin films in such applications perpendicular magnetic anisotropy (PMA) is essential because it facilitates scalability and higher density while low Gilbert damping facilitates higher speeds. Ferrimagnetic thin films like Bismuth substituted yttrium iron garnet (BiYIG) are interesting because they have large PMA when grown on special garnet substrates and also have very low magnetic damping. We have already demonstrated spin-orbit-torque driven DW velocities in perpendicularly magnetized BiYIG, exceeding . Currently, we are also studying the BiYIG and thulium iron garnet TmIG heterostructures for spintronic memory applications.

Structural and Chemical Analysis a) Scanning transmission electron microscopy along the (10-1) direction of the GSGG/BiYIG(6.9 nm)/Pt(4 nm) sample. Scale bar: 5 nm. b) Electron energy loss spectroscopy (EELS) across the interface along the distance, d, shown in (c). c) Two-dimensional EELS maps for each elements, Fe, Ga, Gd and Y, as well as a simultaneously acquired annular dark-field (ADF) image. Scale bar: 1 nm. 

Yabin Fan, a Postdoctoral Associate at MIT, is also studying the magnon-magnon coupling and dynamics of high-quality garnet materials that become particularly interesting when coupled with low-damping ferromagnetic metals. Depending on the interfacial spin torques, the magnon modes in the two materials can have either repulsion or attraction interactions, revealing the strong nonlinear magnon-magnon coupling in the system. The strong magnon-magnon interaction in the magnetic hybrid structures enabled by the garnet material can modulate spin current propagation, influence the damping of the specific magnetic layer, or achieve efficient coupling with on-chip circuit resonator photons. The physically compact system may have broad applications in the spin-based quantum computation technologies.

Y. Fan, et al. “Modulating Spin Transmission with Magnon-Magnon Coupling” submitted (2020);

Yabin Fan, Patrick Quarterman, Joseph Finley, Jiahao Han, Pengxiang Zhang, Justin T. Hou, Mark D. Stiles, Alexander J. Grutter, and Luqiao Liu, “Manipulation of coupling and magnon transport in magnetic metal-insulator hybrid structures”, Physical Review Applied 13, 061002 (2020)

 iii. Magnetooptical materials and devices

 Takian Fakhrul, a Graduate Student at MIT, is also researching optical isolators and circulators. Thin film magneto-optical (MO) materials provide nonreciprocal functionality and are enablers for integrated nonreciprocal photonic devices such as isolators and circulators. The nonreciprocity of a MO material can be characterized by measuring the Faraday rotation. For the near-IR optical communications bands, materials based on ferrimagnetic yttrium iron garnet (YIG, Y₃Fe₅O₁₂) offer a desirable combination of a high Faraday rotation and a low optical absorption giving them excellent MO figure of merit (FoM, the ratio of Faraday rotation to optical absorption). My work involved growing polycrystalline bismuth-substituted yttrium iron garnet (Bi:YIG) on silicon substrates and waveguide devices in which a yttrium iron garnet (YIG) seedlayer is placed either above or below the active Bi:YIG layer to promote crystallization. When a seedlayer is included, a top seed layer is preferable as it maximizes coupling of the MO garnet with the evanescent light from an underlying waveguide. The films exhibit the highest reported MO figure of merit of up to 769° dB−1 at 1550 nm wavelength.  Moreover, I have also been working on rare earth garnets such as terbium iron garnet (TbIG) that can crystallize directly on non-garnet substrates to form polycrystalline films.

The cross-sectional SEM image of top-down
crystallized Bi:YIG film grown on a waveguide
of an optical test device.

Cubic unit cell of TbIG  showing octahedral (pink) , tetrahedral (purple) and dodecahedral (yellow) sites. The cation sites are surrounded by O^2- at the vertices of the polyhedrons (indicated by black spheres). Magnetization directions of the Tb³⁺ and Fe³⁺ cations are shown using red arrows.

Yabin Fan, a Postdoctoral Associate at MIT, is studying the magneto-optical effect of the rare-earth garnet materials (Dysprosium-based or Cerium-based garnet materials) grown on Si substrate and annealed by pin-point laser beams. The advantage of laser-annealing is that it can realize the annealing locally and will be compatible with the semiconductor industry technologies which do not favor the excess heat generated during the traditional thermal annealing process of garnet materials. The magneto-optical effect enabled by the high-quality garnet on Si can result in integrated optical isolator applications based on the Si chip.

[1]   Yan Zhang, Qingyang Du, Chuangtang Wang, Wei Yan, Longjiang Deng, Juejun Hu, Caroline A. Ross, and Lei Bi, “Dysprosium substituted Ce:YIG thin films with perpendicular magnetic anisotropy for silicon integrated optical isolator applications”, APL Mater. 7, 081119 (2019);

[2]   Xue Yin Sun, Qingyang Du, Taichi Goto, Mehmet C. Onbasli, Dong Hun Kim, Nicolas M. Aimon, Juejun Hu, and Caroline A. Ross, “Single-Step Deposition of Cerium-Substituted Yttrium Iron Garnet for Monolithic On-Chip Optical Isolation”, ACS Photonics 2, 856−863 (2015).

iv. Block copolymer self-assembly

 Runze Liu, a Graduate Student at MIT, researches block copolymers. Block copolymers (BCPs) and their self-assembly behavior have been widely pursued for fabricating well-controlled monolayer patterns in order to improve throughput and resolution in nanolithography. However, the extension of controlling multilayers or 3D structures of BCP microdomains still remains limited. He is interested in designing multilayer 3D nanostructure in periodic trench based on block copolymers by using directed self-assembly. Especially care about certain BCP like PS-P2VP or PS-P4VP whose pattern could be further transformed to some metallic functional structures.

Sangho Lee, a Graduate Student at MIT, is researching three-dimensional self-assembly of triblock terpolymers and selective infiltration of functional materials into self-assembled nanostructures. Three-dimensional (3D) nanostructures of ABC triblock terpolymer thin films are challenging to resolve due to low contrast between the polymeric components, although compared to diblock copolymers, such “three-color” structures offer a wider range of geometries and potential applications. Thus, triblock terpolymers including a Si-containing block are favorably used to reveal the structure both by grazing-incidence small-angle X-ray scattering (GISAXS) and by SEM after etching. In situ GISAXS specifically provides rich details of dynamic transformations from the disordered to the ordered state during the annealing that are not observed using ex situ characterization techniques. A selective vapor-phase infiltration synthesis of ZnO is introduced to further distinguish between two remaining organic blocks and to create silica-organic-zinc oxide nanostructures. This infiltration technique generating functional nanoparticles throughout the thickness of hundreds of nm films provides a strategy for producing hierarchical multicomponent organic/inorganic nanostructures as well as visualizing the phase behavior of multiblock copolymers.

 In situ study of ABC triblock terpolymer self-assembly under solvent vapor annealing

Resolving triblock terpolymer morphologies by vapor-phase infiltration

[1]   Sangho Lee, Li-Chen Cheng, Kevin G. Yager, Muhammad Mumtaz, Karim Aissou, and Caroline A. Ross, “In Situ Study of ABC Triblock Terpolymer Self-Assembly under Solvent Vapor Annealing”, Macromolecules 2019, 52, 1853–1863.

[2]   Sangho Lee, Ashwanth Subramanian, Nikhil Tiwale, Kim Kisslinger, Muhammad Mumtaz, Ling-Ying Shi, Karim Aissou, Chang-Yong Nam, and Caroline A. Ross, “Resolving Triblock Terpolymer Morphologies by Vapor Phase Infiltration”, Chem. Mater. 2020, 32, 5309–5316.

Zehao Sun, a Graduate Student at MIT, is studying the architecture of block copolymers (BCPs), such as linear, star, or cyclic geometry, which has been shown to impact the microphase separation behavior of BCPs. In the past decade, self-assembly of bottlebrush block copolymers (BBCPs) has been extensively explored due to their unentangled nature and extremely large/small spacings not attainable by conventional coil-coil BCPs. I am interested in developing novel multiblock BBCP architectures and revealing the structure-effect relationship dictating the self-assembly morphology.

Schematic of a triblock Janus BBCP which forms yellow/blue lamellae alternating with red lamellae.