61 research outputs found
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Energetics of Nanomaterials
This project, ''Energetics of Nanomaterials'', represents a three-year collaboration among Alexandra Navrotsky (University of California at Davis), Brian Woodfield and Juliana Boerio-Goates (Brigham Young University) and Frances Hellman (University of California at San Diego). Its purpose has been to explore the differences between bulk materials, nanoparticles, and thin films in terms of their thermodynamic properties, with an emphasis on heat capacities and entropies, as well as enthalpies. We used our combined experimental techniques to address the following questions: How does energy and entropy depend on particle size and crystal structure? Do entropic differences have their origins in changes in vibrational densities of states or configurational (including surface configuration) effects? Do material preparation and sample geometry, i.e., nanoparticles versus thin films, change these quantities? How do the thermodynamics of magnetic and structural transitions change in nanoparticles and thin films? Are different crystal structures stabilized for a given composition at the nanoscale, and are the responsible factors energetic, entropic, or both? How do adsorption energies (for water and other gases) depend on particle size and crystal structure in the nanoregime? What are the energetics of formation and strain energies in artificially layered thin films? Do the differing structures of grain boundaries in films and nanocomposites alter the energetics of nanoscale materials? Of the several directions we first proposed, we initially concentrated on a few systems: TiO(sub 2), CoO, and CoO-MgO. In these systems, we were able to clearly identify particle size-dependent effects on energy and vibrational entropy, and to separate out the effect of particle size and water content on the enthalpy of formation of the various TiO(sub 2) polymorphs. With CoO, we were able to directly compare nanoparticle films and bulk materials; this comparison is important because films can be either 2 dimensional structures, limited by thickness, or can be dominated by nanoparticle granular behavior. These materials represent good model systems which are relevant to technological and geochemical applications as well as to the fundamental underlying science. The collaboration was both congenial and fruitful. We exchanged both samples and scholars among the laboratories. We met several times a year, rotating these meetings among the three institutions. We had frequent conference calls and were in constant email contact. We learned an immense amount from each other because we brought not just different methodologies but different disciplines to the project. In particular, the interplay of physics (Hellman), chemistry (Woodfield, Boerio-Goates, Navrotsky) and geochemistry (Navrotsky) viewpoints has been very enriching. The result has been a number of publications already in print, and several more in preparation, graduate student PhD and MS degrees, and undergraduate research students supported, as well as a well-developed collaboration that will lead to even more fruitful and important science in the coming years
Spin-Orbit Torques in ferrimagnetic GdFeCo Alloys
The spin-orbit torque switching of ferrimagnetic
Gd(FeCo) films was studied for both transition
metal (TM)-rich and rare earth (RE)-rich configurations. The spin-orbit torque
driven magnetization switching follows the same handedness in TM-rich and
RE-rich samples with respect to the total magnetization, but the handedness of
the switching is reversed with respect to the TM magnetization. This indicates
that the sign of the spin-orbit-torque-driven magnetic switching follows the
total magnetization, although transport based techniques such as anomalous Hall
effect are only sensitive to the transition metal magnetization. These results
provide important insight into the physics of spin angular momentum transfer in
materials with antiferromagnetically coupled sublattices
Disorder-driven localization and electron interactions in BiTeI thin films
Strong disorder has a crucial effect on the electronic structure in quantum
materials by increasing localization, interactions, and modifying the density
of states. BiTeI films grown at room temperature and \SI{230}{K} exhibit
dramatic magnetotransport effects due to disorder, localization and electron
correlation effects, including a MIT at a composition that depends on growth
temperature. The increased disorder caused by growth at 230K causes the
conductivity to decrease by several orders of magnitude, for several
compositions of BiTeI. The transition from metal to insulator with
decreasing composition is accompanied by a decrease in the dephasing length
which leads to the disappearance of the weak-antilocalization effect.
Electron-electron interactions cause low temperature conductivity corrections
on the metallic side and Efros-Shklovskii (ES) variable range hopping on the
insulating side, effects which are absent in single crystalline BiTeI. The
observation of a tunable metal-insulator transition and the associated strong
localization and quantum effects in BiTeI shows the possibility of tuning
spin transport in quantum materials via disorder.Comment: 7 pages, 4 figure
Exploring the origins of perpendicular magnetic anisotropy in amorphous Tb-Co via changes in medium-range ordering
Amorphous thin films of TbCo (a-Tb-Co) grown by magnetron
co-sputtering exhibit changes in magnetic anisotropy with varying growth and
annealing temperatures. The magnetic anisotropy constant increases with
increasing growth temperature, which is reduced or vanishes upon annealing at
temperatures above the growth temperature. The proposed explanation for this
growth-induced anisotropy in high orbital moment Tb-based transition metal
alloys such as a-Tb-Co is an amorphous phase texturing with preferential
in-plane and out-of-plane local bonding configurations for the rare-earth and
transition metal atoms. Scanning nanodiffraction performed in a transmission
electron microscope (TEM) is applied to a-TbCo films deposited
over a range of temperatures to measure relative changes in medium-range
ordering (MRO). These measurements reveal an increase in MRO with higher growth
temperatures and a decrease in MRO with higher annealing temperatures. The
trend in MRO indicates a relationship between the magnetic anisotropy and local
atomic ordering. Tilting select films between 0 and 40 in
the TEM measures variations in the local atomic structure a function of
orientation within the films. The findings support claims that preferential
ordering along the growth direction results from temperature-mediated adatom
configurations during deposition, and that oriented MRO correlates with the
larger anisotropy constants.Comment: 13 pages, 9 figure
Recommended from our members
Energetics of Nanomaterials
This project, "Energetics of Nanomaterials," represents a three-year collaboration among Alexandra Navrotsky (UC Davis), Brian Woodfield and Juliana Boerio-Goates (BYU), and Frances Hellman (UC Berkeley). It's purpose has been to explore the differences between bulk materials, nanoparticles, and thin films in term of their thermodynamic properties, with an emphasis on heat capaacities and entropies, as well as enthalpies. the three groups have brought very different expertise and capabilities to the project. Navrotsky is a solid-state chemist and geochemist, with a unique Thermochemistry Facility emphasizing enthalpy of formation measurements by high temperature oxide melt and room temperatue acid solution calorimetry. Boerio-Goates and Woodfield are calorimetry. Hellman is a physicist with expertise in magnetism and heat capacity measurements using microscale "detector on a chip" calorimetric technology that she pioneered. The overarching question of our work is "How does the free energy play out in nanoparticles?", or "How do differences in free energy affect overall nanoparticle behavior?" Because the free energy represents the temperature-dependent balance between the enthalpy of a system and its entropy, there are two separate, but related, components to the experimental investigations: Solution calorimetric measurements provide the energetics and two types of heat capacity measurements the entropy. We use materials that are well characterized in other ways (structurally, magnetically, and chemically), and samples are shared across the collaboration
Reconfigurable ferromagnetic liquid droplets.
Solid ferromagnetic materials are rigid in shape and cannot be reconfigured. Ferrofluids, although reconfigurable, are paramagnetic at room temperature and lose their magnetization when the applied magnetic field is removed. Here, we show a reversible paramagnetic-to-ferromagnetic transformation of ferrofluid droplets by the jamming of a monolayer of magnetic nanoparticles assembled at the water-oil interface. These ferromagnetic liquid droplets exhibit a finite coercivity and remanent magnetization. They can be easily reconfigured into different shapes while preserving the magnetic properties of solid ferromagnets with classic north-south dipole interactions. Their translational and rotational motions can be actuated remotely and precisely by an external magnetic field, inspiring studies on active matter, energy-dissipative assemblies, and programmable liquid constructs
Evidence for topological surface states in amorphous BiSe
Crystalline symmetries have played a central role in the identification of
topological materials. The use of symmetry indicators and band representations
have enabled a classification scheme for crystalline topological materials,
leading to large scale topological materials discovery. In this work we address
whether amorphous topological materials, which lie beyond this classification
due to the lack of long-range structural order, exist in the solid state. We
study amorphous BiSe thin films, which show a metallic behavior and
an increased bulk resistance. The observed low field magnetoresistance due to
weak antilocalization demonstrates a significant number of two dimensional
surface conduction channels. Our angle-resolved photoemission spectroscopy data
is consistent with a dispersive two-dimensional surface state that crosses the
bulk gap. Spin resolved photoemission spectroscopy shows this state has an
anti-symmetric spin-texture resembling that of the surface state of crystalline
BiSe. These experimental results are consistent with theoretical
photoemission spectra obtained with an amorphous tight-binding model that
utilizes a realistic amorphous structure. This discovery of amorphous materials
with topological properties uncovers an overlooked subset of topological matter
outside the current classification scheme, enabling a new route to discover
materials that can enhance the development of scalable topological devices.Comment: 40 pages (21 main + 19 supplemental), 15 figures (4 main + 11
supplemental
Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films
Ultrafast control of the magnetization in ps timescales by fs laser pulses
offers an attractive avenue for applications such as fast magnetic devices for
logic and memory. However, ultrafast helicity-independent all-optical switching
(HI-AOS) of the magnetization has thus far only been observed in Gd-based,
ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal
(\textit{a}-RE-TM) systems, and a comprehensive understanding of the reversal
mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic
\textit{a}-GdTbCo thin films, from x = 0 to x = 18, and
elucidate the role of Gd in HI-AOS in \textit{a}-RE-TM alloys and multilayers.
Increasing Tb content results in increasing perpendicular magnetic anisotropy
and coercivity, without modifying magnetization density, and slower
remagnetization rates and higher critical fluences for switching but still
shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on
the two-temperature model reproduce these results qualitatively and predict
that the lower damping on the RE sublattice arising from the small spin-orbit
coupling of Gd (with ) is instrumental for the faster dynamics and lower
critical fluences of the Gd-rich alloys. Annealing
\textit{a}-GdTbCo leads to slower dynamics which we argue
is due to an increase in damping. These simulations strongly indicate that
acounting for element-specific damping is crucial in understanding HI-AOS
phenomena. The results suggest that engineering the element specific damping of
materials can open up new classes of materials that exhibit low-energy,
ultrafast HI-AOS
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