Perovskite oxides can have a range of electronic, optical, and magnetic properties that make them valuable in devices. In this project we explore the room temperature ferromagnetism induced in SrTiO₃ (STO) thin films by substituting the B-site Ti with transition metals (Cr, Mn, Fe, Co, and Ni) during growth by pulsed laser deposition on various substrates. In the SrTi_1-xFe_xO₃ (STF) and the SrTi_1-xCo_xO₃ (STC) systems with less than about 50% Fe or Co, the materials show ferromagnetism up to 1000 K with ~0.5 – 1 µ_B per magnetic ion and a strong out-of-plane anisotropy, which is attributed to magnetoelastic effects ^[1] because the films are strained by epitaxial growth on the substrates.  Above 50% substituent, the magnetic moment decreases (Figure 1). The magnetism of these materials is reduced if the films are grown in oxygen, compared to vacuum, so is associated with the presence of oxygen vacancies and mixed valence Fe or Co. We are investigating these effects with density functional theory simulations. High throughput study of a wider range of systems involving other transition metals (Mn, Cr, Ni) was demonstrated using combinatorial pulsed laser deposition (CPLD), which allows the deposition of thin films with a spatially varying concentration of the substituent ^[2] . The perovskite films were grown on Si using buffers of CeO₂/YSZ, enabling integration on a Si platform. We demonstrated self-assembly of STF grown on buffered Si, in which strain relaxation and surface charge minimization drive the formation of (110) nanopillars in an (100) matrix. Further understanding of these materials may facilitate their incorporation into nonvolatile logic or memory or in nonreciprocal optical devices.

Multifunctionality can be tailored by assembling materials with distinct properties in a single composite material. Bottom-up approaches such as self-assembly are very desirable because of their low cost and usually high throughput. We grew dual phase materials by codeposition, including composites of perovskites and spinel phases. BiFeO₃/CoFe₂O₄ (BFO/CFO) two-phase perovskite-spinel nanocomposites were grown by self-assembly using CPLD, resulting in ferrimagnetic CFO pillars embedded in a ferroelectric BFO matrix (Figure 2). Epitaxial strain in the highly magnetoelastic CFO phase dramatically influences the magnetic anisotropy of the films ^[3] . It is also possible to induce strain in the magnetoelastic phase by ferroelectrically straining the matrix with an applied voltage, making these nanocomposites very attractive for applications requiring a strong magnetoelectric coupling, such as magnetic logic and memory devices.

1. D. H. Kim, L. Bi, P. Jiang, G. F. Dionne, and C. A., Ross, (2011) “Magnetoelastic effects in SrTi_{1−x}M_{x}O_{3} (M = Fe, Co, or Cr) epitaxial thin films,” Physical Review B, vol. 84, no. 1, pp. 1-9.
2. D. H. Kim, L. Bi, N. M. Aimon, P. Jiang, G. F. Dionne, and C. A.  Ross, (2012). “Combinatorial pulsed laser deposition of Fe, Cr, Mn and Ni- substituted SrTiO3 films on Si substrates,” ACS Combinatorial Science, vol. 14, no. 3, pp. 179-90.
3. N. M. Aimon, D. H. Kim, H. K. Choi, and C. A. Ross, (2012)“Deposition of epitaxial BiFeO3/CoFe2O4 nanocomposites on (001) SrTiO3 by combinatorial pulsed laser deposition,” Applied Physics Letters, vol. 100, no. 9, pp. 092901.

We are investigating the fabrication, material, and circuit properties of magnetic memory and logic devices.  The research is divided into two main subjects. The first is the study of 360° domain walls (360 DWs), which show novel characteristics under DC and AC applied magnetic fields and currents and are a promising data token in racetrack memory applications.  The second subject is designing and fabricating an instantiation of magnetic logic that has the potential to be more energy efficient than today’s transistors.

In 360 DWs, the magnetization makes a full in-plane 360° turn in a localized region of a magnetic wire, while the rest of the wire is magnetized parallel to its edges. Figure 1 shows a 360 DW in a Co nanowire ^[1] . Simulations ^[2] indicate that 360 DWs have a response to a current that is qualitatively different from the behavior of 180 DWs. The 360 DWs move at a velocity independent of applied magnetic fields and can be destroyed by a burst of applied current. The stability of the domain wall can be controlled by an externally applied field. These features make the 360 DW a data token candidate in novel domain wall logic and memory devices.

The heat dissipated per switching operation by logic transistors can potentially be greatly reduced by using a collective effect, such as the collective switching of magnetic moments.  Thus, we are researching a device that uses the current-induced switching of a 180 DW in a soft ferromagnet to perform logic.  The state of the logic gate is read out using a magnetic tunnel junction.  Modeling of the device performance shows its potential for beyond-CMOS: it is nonvolatile, it is compatible with CMOS, it has a fanout greater than one, a single device acts as a universal NAND gate, and the power dissipation per device operation scales well with device size ^[3] .  Figure 2 shows a device prototype, using NiFe as the soft ferromagnetic wire, with ends exchange biased with IrMn antiferromagnets to ensure a single 180 DW in the wire.

1. Y. Jang, S. R. Bowden, M. Mascaro, J. Unguris, and C.A. Ross, “Formation and structure of 360 and 540 degree domain walls in thin magnetic stripes,” Appl. Phys. Lett., vol. 100, p. 062407, 2012.
2. M. D. Mascaro and C. A. Ross, “AC and DC Current-Induced Motion of a 360˚ Domain Wall,” Phys. Rev. B vol. 82, p. 214411, 2010.
3. J. A. Currivan, Y. Jang, M. D. Mascaro, M. A. Baldo, and C. A. Ross, “Low energy magnetic domain wall logic in short, narrow ferromagnetic wires, IEEE Magnetics Letters, vol. 3, p. 3000104, 2012.

Self-organized macromolecular materials provide an alternative to conventional lithography for fabricating devices on the nanometer scale. Self-assembly of the microdomains of diblock copolymers within lithographically-defined templates to create patterns with long range order has attracted much attention; advantages include cost-effectiveness, large-area coverage, and compatibility with pre-established top-down patterning technologies. Block copolymers consist of two covalently bound polymer chains of chemically distinct polymer materials. The chains can self-assemble to form small-scale domains whose size and geometry depend on the molecular weights of the two types of polymer and their interaction. We showed that cylindrical morphology poly(styrene-b-dimethylsiloxane) (PS-PDMS) block copolymers, which have a large interaction parameter and high etch-contrast between two blocks, can be templated using an array of nanoscale topographical elements acting as surrogates for the minority domains of the block copolymer ^[1] and that combining the self-assembly of block-copolymer thin films with minimal top-down templating can generate complex nanoscale patterns. A sparse array of nanoscale HSQ posts accurately dictated the assembly of a cylindrical PS-PDMS diblock copolymer into complex, aperiodic features, as in Figure 1 ^[2] .

Recently, we have found strategies to control the self-assembly in three dimensions by functionalizing the HSQ posts with a majority-block brush. This brush allows us to guide multilayer films of PS-PDMS as shown in Figure 2a ^[3] , where the self-assembly of a crossbar structure is shown. This strategy can make cross-point structures with junctions and connections between the layers of cylinders. We have also developed a continuous flow solvent annealing system which allows extensive control over the morphologies of the block copolymers, so that spheres, cylinders, lamellae, etc. can be formed from one polymer simply by adjusting the solvent vapor pressures.

Beyond the rather limited morphologies of diblock copolymers, ABC triblock polymer thin films provide a diversity of new structures such as high-density nano-ring structures ^[3] , and square arrays of dots or holes can also be achieved from a self-assembled PI-PS-PFS triblock terpolymer, as shown in Figure 2b ^[4] . Topographical templating enables these structures to be ordered.

1. I. Bita, J. K. W. Yang, Y. S. Jung, C. A. Ross, E. L. Thomas, and K. K. Berggren, Science, vol. 321, pp. 939-943, 2008.
2. J. K. W. Yang, Y. S. Jung, J.-B. Chang, C. A. Ross, and K. K. Berggren, Nature Nanotechnology, vol. 5, pp. 256-260, 2010.
3. A. T. K. Ghariehali, K. W. Gotrik, A. F. Hannon, A. Alexander-Katz, C. A. Ross, K. K. Berggren, Science, vol. 336, pp. 1294-1298, 2010.
4. J. G. Son, J. Gwyther, J.-B. Chang, K. K. Berggren, I. Manners, and C. A. Ross, Nano Letters, vol. 11, pp. 2849-55, 2011.

We have experimentally demonstrated a monolithically integrated ultra-compact optical isolator on silicon. This device allows a significant device footprint reduction from the centimeter or millimeter scale down to the 10s-of-micrometers level. As a first experimentally demonstrated monolithic nonreciprocal optical component on silicon, this device can also be developed into a variety of integrated nonreciprocal photonic devices including optical circulators, modulators, or switches, which opens a new dimension of functionality for silicon photonics.

The optical isolator, as shown in Figure 1, has been fabricated on silicon on insulator (SOI) substrate with part of the SiO₂ top cladding layer etched away to expose the underlying silicon waveguide. Then a magneto-optical oxide thin film layer was deposited to fill this etched region. When an in-plane magnetic field is applied perpendicular to the light propagation direction in the exposed resonator region shown in the figure, a nonreciprocal magneto-optical phase shift is observed for TM polarized light in the resonator. The effective indices between forward and backward propagating TM mode lights are different, and the resonance wavelengths are non-degenerate. This effect is demonstrated in a sketch of the transmission spectra of forward and backward propagating TM polarized light near resonant wavelengths, shown in Figure 2 (a). The different resonant wavelengths allow different transparencies of the device at the given wavelength range, enabling the device to operate as an optical isolator. Experimental demonstration of the device performance was carried out by using an Y₃Fe₅O₁₂ buffered Ce₁Y₂Fe₅O₁₂ polycrystalline thin film as the magneto-optical oxide layer, fabricated by pulsed laser deposition ^[1] . The experimental and theoretical TM mode transmission spectra with oppositely applied magnetic fields near the resonance wavelength of 1549.5 nm are shown in Figure 2 (a). This device achieved an isolation ratio of 19.5±2.9 dB, an insertion loss of 18.8±1.1 dB, and a 10 dB isolation bandwidth of 1.6 GHz. We are now investigating multiple resonator configurations for enhanced optical isolation and tunable bandwidth that arises due to the coupling regime, as shown in Figure 2 (b). Tunability range is 100 nm in wavelength ^[2] .

1. L. Bi, J. Hu, P. Jiang, D. H. Kim, G. F. Dionne, L. C. Kimerling, and C. A. Ross, “On-chip optical isolation in monolithically integrated non-reciprocal optical resonators,” Nature Photonics, vol. 5, p. 758, 2011.
2. M. Onbasli, J. Hu, L. Bi, G. F. Dionne, and C. A. Ross, “Cascaded magneto-optical ring resonator structures for tunable faraday rotation and reduced isolator footprint,” presented at American Physical Society Meeting, Boston, MA, 2012.

Collaborators

• H.I. Smith
• C.V Thompson
• E.L. Thomas
• Y. Shao-Horn
• K.K. Berggren
• A. Alexander-Katz
• L.C. Kimerling
• G. Dionne (Lincoln Lab)
• J. Borchers (NIST)
• A. Adeyeye (Nat. Univ. Singapore)
• W.C. Choi (Nat. Univ. Singapore)
• J.Y. Cheng (IBM)
• M. Vazquez (Madrid)
• J.L. Vicent (Madrid)

Postdocs/Visitors

• Dr Dong Hun Kim
• Dr Hong Kyoon Kim
• Dr Juan Manuel Florez
• Dr Larysa Tryputen
• Dr Karim Aissou

Graduate Students

• Kevin Gotrik
• Mark Mascaro
• Adam Hannon
• Nicholas Aimon
• Frank Liu
• Mehmet Onbasli
• Jean Anne Currivan
• Jin Young Kim
• Jinshuo Zhang

Support Staff

• G. Joseph, Administrative Assistant II

Publications

Effect of the exchange bias coupling strength on the magnetoimpedance of IrMn/NiFe films, C. Garcia, J. M. Florez, P. Vargas, and C. A. Ross, J. Appl. Phys. 109, 07D735 (2011).

Spectral Origins of Large Faraday Rotation at 1.5-mm Wavelength from Fe and Co in SrTiO₃ Films, Gerald F. Dionne, Lei Bi, H.-S. Kim and C.A. Ross, J. Appl. Phys. 109 07B761 (2011).

Highly tunable self-assembled nanostructures from a poly(2-vinylpyridine-b-dimethylsiloxane) block copolymer, Jae Won Jeong, Woon Ik Park, Mi-Jeong Kim, C. A. Ross, and Yeon Sik Jung, Nano Letts. 11 (10), pp 4095–4101 (2011).

360-degree domain wall-mediated reversal in rhombic multilayer magnetic rings, Mark D. Mascaro, Helmut S. Körner*, Chunghee Nam, Bryan G. Ng, and C. A. Ross, APL 98, 252506 (2011).

Magnetoelastic effects in SrTi1−xMxO3 (M = Fe, Co, or Cr) epitaxial thin films, Dong Hun Kim, Lei Bi, Peng Jiang, Gerald F. Dionne, C.A. Ross, Phys. Rev. B 84 014416 (2011).

Enhancement of the magnetooptical performance of Sr(Ti_0.6-xGa_xFe_0.4)O₃ perovskite films by Ga substitution, Dong Hun Kim, Peng Jiang, Lei Bi, Gerald F. Dionne, C.A. Ross, Appl. Phys. Lett. 98 231909 (2011).

Ferromagnetism in Single Crystal and Nanocomposite Sr(Ti,Fe)O₃ Epitaxial Films, Hyun-Suk Kim, Lei Bi, Dong Hun Kim, Dae-Jin Yang, Yoon Jeong Choi, Jung Woo Lee, Jeung Ku Kang, Yun Chang Park, Gerald F. Dionne, and Caroline A. Ross, J. Mater. Chem. 21, 10364-10369 (2011).

Domain Wall Induced Magnetoresistance in a Superconductor/Ferromagnet Nanowire, G.X. Miao, M.D. Mascaro, Chunghee Nam, C.A. Ross, J.S. Moodera, Appl. Phys. Lett. 99, 032501 (2011).

Highly Ordered Square Arrays from a Templated ABC Triblock Terpolymer, Jeong Gon Son, Jessica Gwyther, Jae-Byum Chang, Karl K. Berggren, Ian Manners, and Caroline A. Ross, Nano Lett. 11 2849-55 (2011).

Angular dependence  of  ferromagnetic resonance and magnetization configuration of thin film permalloy nanoellipse arrays, M. Pardavi-Horvath, B.G. Ng, F. J. Castaño, H.S. Körner, C. Garcia, C.A. Ross, J. Appl. Phys. 110, 053921 (2011).

On-chip optical isolation in monolithically integrated nonreciprocal optical resonators, Lei Bi, Juejun Hu, Peng Jiang, Dong Hun Kim, Gerald F. Dionne, Lionel C. Kimerling and C. A. Ross, Nature Photonics 5, 758–762  (2011).

Magnetic pinning of the flux lattice in superconducting-nanomagnet hybrids, D. Perez de Lara, F. J. Castaño, B. G. Ng, H. S. Körner, R. K. Dumas, E. M. Gonzalez, Kai Liu, C. A. Ross, Ivan K. Schuller and J. L. Vicent, Appl. Phys. Lett. 99 182509 (2011).

Block copolymer assembly of sub-10 nm patterns with mixed morphology and period Using Electron Irradiation and Solvent Annealing, Jeong Gon Son,Jae-Byum Chang,Karl B. Berggren, and Caroline A. Ross, Nano Letters 11 (11), pp 5079–5084 (2011).

Magnetic Frustration in Circular Arrays of Dipoles, Seungha Yoon, Youngman Jang, Chunghee Nam, Jean Anne Currivan, B. K. Cho, and C. A. Ross, IEEE Magnetics Letters 3 4000104 1-4 (2012).

Comparative study of magnetization reversal process between rectangular and circular thin film rings, G. Shimon, A. O. Adeyeye, and C. A. Ross, J. Appl. Phys. 111 013909 (2012).

The role of deposition conditions on the structure and magnetic properties of SrTi_xFe_1-xO₃ films, Dong Hun Kim, Nicolas M. Aimon, Lei Bi, Gerald F. Dionne, C.A. Ross, J. Appl. Phys. 111, 07A918 (2012).

Combinatorial pulsed laser deposition of Fe, Cr, Mn and Ni-substituted SrTiO₃ filmson Si substrates, Dong Hun Kim, Lei Bi, Nicolas Aimon, Peng Jiang, Gerald F. Dionne, and C. A. Ross, ACS Combinatorial Science 14 179-190 (2012).

Formation and structure of 360 and 540 degree domain walls in thin magnetic stripes, Youngman Jang, S.R. Bowden, Mark Mascaro, J. Unguris, C.A. Ross, Appl. Phys. Lett. 100 062407 (2012).

Deposition of epitaxial BiFeO3/CoFe2O4 nanocomposites on (001) SrTiO3 by combinatorial pulsed laser deposition, by combinatorial pulsed laser deposition, Nicolas M. Aimon, Dong Hun Kim, Hong Kyoon Choi, and C. A. Ross, Appl. Phys. Lett. 100, 092901 (2012).

Low Energy Magnetic Domain Wall Logic in Short, Narrow Ferromagnetic Wires, Jean Anne Currivan, Youngman Jang, Mark D. Mascaro, Marc A. Baldo, and Caroline A. Ross, IEEE Magnetics Letters 3 3000104 (2012).

Aligned Sub-10-nm Block Copolymer Patterns Templated by Post Arrays, Jae-Byum Chang, Jeong Gon Son, Adam F. Hannon, Alfredo Alexander-Katz, Caroline A. Ross, and Karl K. Berggren, ACS Nano (2012).

Current-driven domain wall motion in heterostructured ferromagnetic nanowires, Youngman Jang, Mark D. Mascaro, G. S. D. Beach, and C. A. Ross, Appl. Phys. Lett. 100, 112401 (2012).

A.T.K. Ghariehali, K.W. Gotrik, A.F. Hannon, A. Alexander-Katz, C.A. Ross, K. K. Berggren, “Templating Three-Dimensional Self-Assembled Structures in Bilayer Block Copolymer Films” Science (in Press, 2012).

Monolithic Integration of Chalcogenide glass/Iron Garnet Waveguides and Resonators for On-chip Nonreciprocal Photonic Devices, Lei Bi, Juejun Hu, Gerald F. Dionne, Lionel Kimerling and C. A. Ross, Proc. SPIE 7941, 794105 (2011).

Magnetoelastic Effects in Nanostructures, J.I. Arnaudas, A. Badia-Majós, L. Berbil-Bautista, M. Bode, F.J. Castaño, M. Ciria, C. de la Fuente, J.L. Diez-Ferrer, S. Krause, B.G. Ng, R.C. O’Handley, C.A. Ross and R. Wiesendanger, Solid State Phenomena Vols. 168-169 (2011) pp 177-184.