An XMCD study of magnetism and valence state in iron-substituted strontium titanate

Room temperature ferromagnetism was characterized for thin films of SrTi$_{0.6}$Fe$_{0.4}$O$_{3-{\delta}}$ grown by pulsed laser deposition on SrTiO$_{3}$ and Si substrates under different oxygen pressures and after annealing under oxygen and vacuum conditions. X-ray magnetic circular dichroism demonstrated that the magnetization originated from Fe$^{2+}$ cations, whereas Fe$^{3+}$ and Ti$^{4+}$ did not contribute. Films with the highest magnetic moment (0.8 {\mu}B per Fe) had the highest measured Fe$^{2+}$:Fe${^3+}$ ratio of 0.1 corresponding to the largest concentration of oxygen vacancies ({\delta} = 0.19). Post-growth annealing treatments under oxidizing and reducing conditions demonstrated quenching and partial recovery of magnetism respectively, and a change in Fe valence states. The study elucidates the microscopic origin of magnetism in highly Fe-substituted SrTi$_{1-x}$Fe$_x$O$_{3-{\delta}}$ perovskite oxides and demonstrates that the magnetic moment, which correlates with the relative content of Fe$^{2+}$ and Fe$^{3+}$, can be controlled via the oxygen content, either during growth or by post-growth annealing

Design of an Integrated Cotton Picking System for Small-Scale Indian Agriculture

India, the world's largest producer of cotton, contains more than 4 million cotton farms that are less than 5 acres. These farms are incapable of large-scale mechanization due to small farm size and irregular farm shape. A previous team developed a handheld, roller-based picking device that demonstrated increased performance over similar products. However, a significant improvement in productivity requires increasing picking speed through mechanization as well as increasing worker cotton carrying capacity. We present a system that utilizes the roller-based picking device in tandem with a compressive storage bag and an efficient carrier. Through modeling and initial testing, the system demonstrates a two times (2X) improvement in worker productivity over current methods. This paper characterizes the cotton picking process, details the modules of the integrated system, and suggests further procedural improvements for greater increases in worker productivity

Tm3Fe5O12/Pt Heterostructures with Perpendicular Magnetic Anisotropy for Spintronic Applications

With recent developments in the field of spintronics, ferromagnetic insulator (FMI) thin films have emerged as an important component of spintronic devices. Ferrimagnetic yttrium iron garnet in particular is an excellent insulator with low Gilbert damping and a Curie temperature well above room temperature, and has been incorporated into heterostructures that exhibit a plethora of spintronic phenomena including spin pumping, spin Seebeck, and proximity effects. However, it has been a challenge to develop high quality sub-10 nm thickness FMI garnet films with perpendicular magnetic anisotropy (PMA) and PMA garnet/heavy metal heterostructures to facilitate advances in spin-current and anomalous Hall phenomena. Here, robust PMA in ultrathin thulium iron garnet (TmIG) films of high structural quality down to a thickness of 5.6 nm are demonstrated, which retain a saturation magnetization close to bulk. It is shown that TmIG/Pt bilayers exhibit a large spin Hall magnetoresistance (SMR) and SMR-driven anomalous Hall effect, which indicates efficient spin transmission across the TmIG/Pt interface. These measurements are used to quantify the interfacial spin mixing conductance in TmIG/Pt and the temperature-dependent PMA of the TmIG thin film

Effect of nonstoichiometry on the magnetism and ferroelectricity of Fe- and Co-substituted strontium titanate thin films

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019Cataloged from PDF version of thesis. "Feb 2019."Includes bibliographical references (pages 170-185).Multiferroic and magnetoelectric materials, in which the magnetic properties can be controlled via electric field and vice versa, hold the potential to be useful in several emerging memory technologies, including spin-wave devices and multi-state memory. Room temperature ferromagnetism and ferroelectricity are of great interest to the multiferroic and magnetoelectric community, as a key challenge in the field is engineering a material exhibiting room temperature ferroelectricity and ferromagnetism. Pulsed laser deposition grown oxygen-deficient Fe- (STF) and Co- (STC) substituted strontium titanate thin films are investigated for their microstructural, magnetic and ferroelectric properties. The films demonstrate room temperature ferromagnetism when grown under highly reducing conditions and in the case of STF, appear to exhibit room temperature ferroelectricity, though the magnetoelectric coupling is quite small. The impact of double-epitaxy on magnetism is investigated in STC.Double-epitaxy is a strain relaxation mechanism and microstructural feature observed in several perovskite oxides, and its effect on strain can in turn reduce the observed ferromagnetism of STC thin films. X-ray magnetic circular dichroism and x-ray absorption spectroscopy for STF thin films are also investigated. The results clarify the source of the observed ferromagnetism to be Fe²⁺ cations, which can be controlled via oxygen vacancy content, and a corresponding mechanism is suggested to explain how the ferromagnetism arises. The strong correlation between magnetic moment and strain is attributed to magnetoelasticity, and a preliminary study into the effect of epitaxial strain on oxygen-deficient STF is presented. Finally, attempts to alter the magnetic properties post-growth, with varying levels of success, are discussed.Although a method to manipulate the magnetic properties post-growth could prove useful to enabling easier processing and new applications for these materials, this work finds that post-growth manipulation of the magnetic properties is not completely reversible. The results presented in this work may provide insight into room temperature magnetism and ferroelectricity of related material systems.by Astera S. Tang.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineerin

Thermal conductivity and characterization of compacted, granular silica aerogel

Monolithic silica aerogels are well known for their low thermal conductivity (approximately 15 mW/(m K)) (Aegerter et al. (Eds.), 2011. Aerogels Handbook, first ed., Springer-Verlag New York, LLC, New York, NY). Their low relative density (typically less than 5%) reduces conduction through the solid and their small pore size, typically less than one hundred nanometers, on the order of the mean free path of air, reduces conduction through air, as well as convection and radiation. As they are fragile and brittle, they are often used in a granular form in thermal insulation, with some increase in their thermal conductivity from the air between the granules. Here, we describe a technique for compacting a bed of granular silica aerogel that reduces the thermal conductivity from 24 mW/(m K) (when uncompacted) to 13 mW/(m K) (after compaction). We find that there is an optimum level of compaction to minimize the thermal conductivity: at higher levels of compaction, the contact area between the granules increases and the granules densify, increasing conduction through the solid.DuPont MIT Allianc

Magnetic domain wall based synaptic and activation function generator for neuromorphic accelerators

Magnetic domain walls are information tokens in both logic and memory devices and hold particular interest in applications such as neuromorphic accelerators that combine logic in memory. Here, we show that devices based on the electrical manipulation of magnetic domain walls are capable of implementing linear, as well as programmable nonlinear, functions. Unlike other approaches, domain-wall-based devices are ideal for application to both synaptic weight generators and thresholding in deep neural networks. Prototype micrometer-size devices operate with 8 ns current pulses and the energy consumption required for weight modulation is ≤16 pJ. Both speed and energy consumption compare favorably to other synaptic nonvolatile devices, with the expected energy dissipation for scaled 20 nm devices close to that of biological neurons.National Science Foundation (U.S.) (Award 1639921

Mechanical and thermal performance of aerogel-filled sandwich panels for building insulation

Monolithic silica aerogels provide exceptional thermal insulation but have low mechanical properties. For instance, their flexural strength is typically 0.03–0.08 MPa. Here, we report on the development of a truss-core sandwich panel filled with compacted aerogel granules, designed to provide both mechanical support and thermal insulation. Mechanical and thermal properties of the sandwich panel prototype were measured and compared with theoretical models available in the literature. The models give a good description of the properties of aerogel-filled truss-core sandwich panels.DuPont MIT Allianc

Thickness-Dependent Double-Epitaxial Growth in Strained SrTi_0.7Co_0.3O_3−δ Films

Perovskite-structured SrTi_0.7Co_0.3O_3−δ (STCo) films of varying thicknesses were grown on SrTiO₃(001) substrates using pulsed laser deposition. Thin films grow with a cube-on-cube epitaxy, but for films exceeding a critical thickness of about 120 nm, a double-epitaxial microstructure was observed, in which (110)-oriented crystals nucleated within the (001)-oriented STCo matrix, both orientations being epitaxial with the substrate. The crystal structure, strain state, and magnetic properties are described as a function of film thickness. Both the magnetic moment and the coercivity show maxima at the critical thickness. The formation of a double-epitaxial microstructure provides a mechanism for strain relief in epitaxially mismatched films