Disorder Engineering of Ferroic Properties

Worldwide energy consumption is expected to increase 50% by the year 2050, with as much as 25% of that being lost to waste heat from electronic devices. Multiferroic materials have the potential to mitigate this heating and volatility in computational devices by allowing voltage control of a magnetic state, virtually eliminating waste heat from “always-on” Si-based technologies. This places multiferroic devices among the most competetive post-silicon technologies considering energy and delay. Multiferroic systems, however, are extremely rare, hindering the realization of new technologies based on these materials. This dearth of materials can be mitigated through use of multiferroic composite systems, where, for instance, a piezoelectric layer is coupled to a magnet through strain, but further challenges exist in maximizing the coupling between layers in the composite, an effort that has seen relatively little work. Additionally, existing engineering techniques utilize atomic-scale or crystal-scale ordering to access magnetic coupling in materials, but chemical and structural disorder is an oft explored technique that has been shown to lead to novel and colossal functional properties. This thesis explores the use of disorder as primary phenomenon to both synthesize new ferroics and enhance material properties for superior functionalities, an orthogonal vector to addressing the scarcity of state-of-the-art materials. By using low temperature epitaxial growth to stabilize the disordered, α-Fe-like, phase of Fe1-xGax out to high, metastable concentrations of Ga, both the increased spin-lattice coupling of Fe and Ga and the lattice softening associated with the phase transition can be leveraged without the formation of intermetallic phases that detract from functionality. With this technique, epitaxial kinetic freezing of disorder, I have demonstrated a means to boost magnetostrictive coupling by as much as 10x relative to bulk, allowing us to show record magnetoelectric performance in a device based on the material. Additionally, I have shown that the phase space of ferroic materials can be extended using entropy as a driving force to stabilize materials with novel chemistries. By leveraging the large configurational entropy from the inclusion of many atomic species, the formation of a random, solid solution crystal can be achieved, potentially overriding other thermodynamic considerations. At elevated temperatures, a large entropic contribution to the Gibbs’ free energy will stabilize the formation of a single phase, even in excess of an unfavorable heat of mixing. This metastability can be further controlled with modern thin film techniques, allowing further access to a large class of materials that have been shown to possess unusual and colossal functional properties. For the first time, I have shown strong magnetism in these new systems, as well as shown that it is strongly correlated to structure and chemistry. These new magnetic oxides provide a platform to investigate and tailor interplay between charge, lattice and spin via disorder for functional properties by the design of disorder. The goal of the work presented here is to understand how engineered disorder plays a role in the tunability of functional properties. I show that low-temperature epitaxy can be used as a tool to access new, disordered, regions of the phase diagram, significantly enhancing the functional properties when compared to the thermodynamic phase. Additionally, the engineering of disorder through chemistry and processing conditions can be used to further tune magnetic phenomena, introducing new order parameters to optimize the system.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/167988/1/meisep_1.pd

Galaxy-galaxy lensing studies from COMBO-17

We study the dark matter halos of galaxies with galaxy-galaxy lensing using the COMBO-17 survey. This survey offers an unprecedented data set for studying lens galaxies at z=0.2-0.7 including redshift information and spectral classification from 17 optical filters for objects brighter than R=24. So far, redshifts and classification for the lens galaxies have mainly been available for local surveys like the Sloan Digital Sky Survey (SDSS). Further, redshifts for the source galaxies have typically not been available at all but had to be estimated from redshift probability distribution which -- for faint surveys -- even had to be extrapolated. To study the dark matter halos we parametrize the lens galaxies as singular isothermal spheres (SIS) or by Navarro-Frenk-White (NFW) profiles. In both cases we find a dependence of the velocity dispersion or virial radius, respectively, on lens luminosity and colour. For the SIS model, we are able to reproduce the Tully-Fisher/Faber-Jackson relation on a scale of 150h^-1kpc. For the NFW profile we also calculate virial masses, mass-to-light ratios and rotation velocities. Finally, we investigate differences between the three survey fields used here.Comment: 6 pages, 5 figures. To be published in the proceedings of IAU Symposium No. 225: The Impact of Gravitational Lensing on Cosmology, Y. Mellier and G. Meylan, ed

Controlled Ordering of Room-Temperature Magnetic Skyrmions in a Polar Van der Waals Magnet

Control and understanding of ensembles of skyrmions is important for realization of future technologies. In particular, the order-disorder transition associated with the 2D lattice of magnetic skyrmions can have significant implications for transport and other dynamic functionalities. To date, skyrmion ensembles have been primarily studied in bulk crystals, or as isolated skyrmions in thin film devices. Here, we investigate the condensation of the skyrmion phase at room temperature and zero field in a polar, Van der Waals magnet. We demonstrate that we can engineer an ordered skyrmion crystal through structural confinement on the $\mu$m scale, showing control over this order-disorder transition on scales relevant for device applications.Comment: 26 pages; 5 main text, 8 supplementary figure

3C 220.3: a radio galaxy lensing a submillimeter galaxy

Herschel Space Observatory photometry and extensive multiwavelength followup have revealed that the powerful radio galaxy 3C 220.3 at z=0.685 acts as a gravitational lens for a background submillimeter galaxy (SMG) at z=2.221. At an observed wavelength of 1mm, the SMG is lensed into three distinct images. In the observed near infrared, these images are connected by an arc of 1.8" radius forming an Einstein half-ring centered near the radio galaxy. In visible light, only the arc is apparent. 3C 220.3 is the only known instance of strong galaxy-scale lensing by a powerful radio galaxy not located in a galaxy cluster and therefore it offers the potential to probe the dark matter content of the radio galaxy host. Lens modeling rejects a single lens, but two lenses centered on the radio galaxy host A and a companion B, separated by 1.5", provide a fit consistent with all data and reveal faint candidates for the predicted fourth and fifth images. The model does not require an extended common dark matter halo, consistent with the absence of extended bright X-ray emission on our Chandra image. The projected dark matter fractions within the Einstein radii of A (1.02") and B (0.61") are about 0.4 +/- 0.3 and 0.55 +/- 0.3. The mass to i-band light ratios of A and B, M/L ~ 8 +/- 4 Msun/Lsun, appear comparable to those of radio-quiet lensing galaxies at the same redshift in the CASTLES, LSD, and SL2S samples. The lensed SMG is extremely bright with observed f(250um) = 440mJy owing to a magnification factor mu~10. The SMG spectrum shows luminous, narrow CIV 154.9nm emission, revealing that the SMG houses a hidden quasar in addition to a violent starburst. Multicolor image reconstruction of the SMG indicates a bipolar morphology of the emitted ultraviolet (UV) light suggestive of cones through which UV light escapes a dust-enshrouded nucleus.Comment: 17 pages, 14 Figures, accepted for publication in Ap

Order-disorder transitions in a polar vortex lattice

Order-disorder transitions are widely explored in various vortex structures in condensed matter physics, that is, in the type-II superconductors and Bose-Einstein condensates. In this study, the ordering of the polar vortex phase in [Pb(Zr0.4Ti0.6)O3]n/(SrTiO3)n (PZT/STO) superlattices is investigated through phase-field simulations. With a large tensile substrate strain, an antiorder vortex state (where the rotation direction of the vortex arrays in the neighboring ferroelectric layers are flipped) is discovered for short-period PZT/STO superlattice. The driving force is the induced in-plane polarization in the STO layers due to the large tensile epitaxial strain. Increasing the periodicity leads to antiorder to disorder transition, resulting from the high energy of the head-to-head/tail-to-tail domain structure in the STO layer. On the other hand, when the periodicity is kept constant in short-period superlattices, the order-disorder-antiorder transition can be engineered by mediating the substrate strain, due to the competition between the induction of out-of-plane (due to interfacial depolarization effect) and in-plane (due to strain) polarization in the STO layer. The 3D ordering of such polar vortices is still a topic of significant current interest and it is envisioned that this study will spur further interest toward the understanding of order?disorder transitions in ferroelectric topological structuresThis work was supported by the Joint Funds of the National Natural Science Foundation of China under grant U21A2067 (Y.W.), and the Fundamental Research Funds for the Central Universities (No. 2021FZZX003-02-03, Z.H.). Z.H. also gratefully acknowledge a start-up grant from Zhejiang University. The financial support from Grant PGC2018-096955-B-C41 funded by MCIN/AEI/10.13039/501100011033 is acknowledged (J.J., P.G.-F., F.G.-O.). F.G.-O. acknowledge financial support from Grant No. FPU18/04661 funded by Spanish Ministry of Universities. The phase-field simulation was performed on the MoFang III cluster on Shanghai Supercomputing Center (SSC). S. D. is currently at Materials Research Centre, Indian Institute of Science, Bangalore, India

Time-sequenced Multi-Radio-Frequency Observations of Cygnus X-3 in Flare

Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a major radio outburst of Cygnus X-3 in 2001 September are presented, measuring the evolution of the spectrum of the source over three decades in frequency, over a period of six days. Following the peak of the flare, as the intensity declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to nu^{-0.6}, after which the spectral index remains at this latter terminal value; a trend previously observed but hitherto not satisfactorily explained. VLBA observations, for the first time, track over several days the expansion of a sequence of knots whose initial diameters are approximately 8 milliarcseconds. The light-crossing time within these plasmons is of the same order as the time-scale over which the spectrum is observed to evolve. We contend that properly accounting for light-travel time effects in and between plasmons which are initially optically thick, but which after expansion become optically thin, explains the key features of the spectral evolution, for example the observed timescale. Using the VLBA images, we have directly measured for the first time the proper motions of individual knots, analysis of which shows a two-sided jet whose axis is precessing. The best-fit jet speed is roughly beta = 0.63 and the precession period is about 5 days, significantly lower than fitted for a previous flare. Extrapolation of the positions of the knots measured by the VLBA back to zero-separation shows this to occur approximately 2.5 days after the detection of the rise in flux density of Cygnus X-3.Comment: 23 pages, 10 figures, accepted by Ap

The Star Formation and Extinction Co-Evolution of UV-Selected Galaxies over 0.05<z<1.2

We use a new stacking technique to obtain mean mid IR and far IR to far UV flux ratios over the rest near-UV/near-IR color-magnitude diagram. We employ COMBO-17 redshifts and COMBO-17 optical, GALEX far and near UV, Spitzer IRAC and MIPS Mid IR photometry. This technique permits us to probe infrared excess (IRX), the ratio of far IR to far UV luminosity, and specific star formation rate (SSFR) and their co-evolution over two orders of magnitude of stellar mass and redshift 0.1<z<1.2. We find that the SSFR and the characteristic mass (M_0) above which the SSFR drops increase with redshift (downsizing). At any given epoch, IRX is an increasing function of mass up to M_0. Above this mass IRX falls, suggesting gas exhaustion. In a given mass bin below M_0 IRX increases with time in a fashion consistent with enrichment. We interpret these trends using a simple model with a Schmidt-Kennicutt law and extinction that tracks gas density and enrichment. We find that the average IRX and SSFR follows a galaxy age parameter which is determined mainly by the galaxy mass and time since formation. We conclude that blue sequence galaxies have properties which show simple, systematic trends with mass and time such as the steady build-up of heavy elements in the interstellar media of evolving galaxies and the exhaustion of gas in galaxies that are evolving off the blue sequence. The IRX represents a tool for selecting galaxies at various stages of evolution.Comment: Accepted for publication in GALEX Special Ap.J.Suppl., December, 200