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Magnetism is a workhouse for electric power generation at the macroscale, responsible for a large fraction of the electricity generation today. However, conventional approaches to energy conversion using magnets fail at a small scale due to Joule heating from electric currents. Recently, a method has emerged – room-temperature composite multiferroics coupling electrics and magnetics - that allows for control of magnetism via voltage as opposed to current. With this newfound capability, industrial & commercial application spaces are rapidly opening up for domains, including ultra-low-power spintronics devices, microwave devices, and even magnetic particle and cell sorting platforms. Meanwhile, personalized medical therapies hold the potential of new forms of highly effective therapies. One example of personalized medicine is CAR T-cell therapy, which uses patients’ cells for cancer treatments. However, such an approach requires technology that can analyze and sort these cells in high quantity, selecting only the cells that will be the most effective cancer fighters. Magnetic cell sorting is a popular approach for high throughput cell sorting. Still, there is no current method capable of capturing and culturing arrays of cells and then selectively releasing the desired ones.Recent advances in room-temperature multiferroic devices, such as devices where magnetism is controlled by electric fields, provide a path for capture, culture, and selective release of cells. However, work remains to understand how to manipulate the magnetic structures that capture these cells.This work aims to develop a multiferroics-based cell manipulation platform with high scalability and can achieve cell control locally. This work first conducts an in-depth study of the magnetization and multiferroic properties of various magnetostrictive layers, including Ni, FeGa, Ni/CoFeB, and Terfenol-D micromagnets on Pb(Mg1/3Nb2/3)O3]0.69[PbTiO3]0.31 (PMN-PT) piezoelectric substrates. It then selects the highly magnetostrictive Terfenol-D micromagnets in the same size scale as human cells as the candidate for life science applications. It also investigates the interaction between these micromagnets and cells before and after a voltage is applied across the PMN-PT substrate. The key questions addressed include how to create structures from a magnetoelastic material that are in the same size scale as human cells (20 \mu m) and control the magnetization of these structures to release cells on-demand via electric fields. Furthermore, this work demonstrates the potential of using patterned surface electrodes to generate localized strain in order to control the behavior of the micromagnets both locally and selectively.

Strain-coupled multiferroic heterostructures provide a path to energy-efficient, voltage-controlled magnetic nanoscale devices, a region where current-based methods of magnetic control suffer from Ohmic dissipation. Magnetoelectric coupling behavior in such composite heterostructures has thus been of substantial interest for scientific research and applications. As the dimension of the devices scale down, novel physical phenomenon emerges and thus also requires further understanding of both the magnetization and strain behavior at micro- and nanoscale. When it comes to the magnetization behavior, there has been a growing interest in highly magnetoelastic materials, such as Terfenol-D, prompting a more accurate understanding of their magnetization behavior. To address this need, we simulate the strain-induced magnetization change with two modeling methods: the commonly used unidirectional model and the recently developed bidirectional model. Unidirectional models account for magnetoelastic effects only, while bidirectional models account for both magnetoelastic and magnetostrictive effects. We found unidirectional models are on par with bidirectional models when describing the magnetic behavior in weakly magnetoelastic materials (e.g., Nickel), but the two models deviate when highly magnetoelastic materials (e.g., Terfenol-D) are introduced. These results suggest that magnetostrictive feedback is critical for modeling highly magnetoelastic materials, as opposed to weaker magnetoelastic materials, where we observe only minor differences between the two methods’ outputs. To our best knowledge, this work represents the first comparison of unidirectional and bidirectional modeling in composite multiferroic systems, demonstrating that back-coupling of magnetization to strain can inhibit formation and rotation of magnetic states, highlighting the need to revisit the assumption that unidirectional modeling always captures the necessary physics in strain-mediated multiferroics. In terms of the strain behavior, there hasn’t been a system-level work that quantifies the strain distribution as a function of the electric field at these so-called mesocales level (100 nm- 10 um), in the range of the constitutive grain size, etc. To obtain mechanical properties at such length scale, including strain information, we used synchrotron polychromatic scanning x-ray diffraction (micro-diffraction) on beamline 12.3.2 at the Advanced Light Source of the Lawrence Berkeley National Lab. With given ferromagnetic and ferroelectric components, it is the magnetoelectric coupling between the two that governs the interaction. In this work, we also demonstrate a method to enhance the coupling behavior between two existing components by interposing a polymer layer.

There has been much effort to develop next generation refrigeration systems that do not use harmful fluids. Magnetocaloric, electrocaloric and mechanocaloric materials have been studied independently as potential replacement of these fluids refrigerants but each has its own limitations. More recently, multicaloric materials that respond to multiple fields have been proposed in order to overcome individual limitations. Here, I studied three different multicaloric materials. The first study focused on extrinsic magnetocaloric effects in La0.7Ca0.3MnO3 (LCMO) driven using strain. LCMO’s intrinsic caloric effect near its ferromagnetic transition is small (-1.2 J kg−1 K−1 T−1 ) [1]. Moya et al. have discovered that LCMO thin films grown on BaTiO3 (BTO) substrates can produce extrinsic magnetocaloric effects as large as -9 J kg−1 K−1 T−1, driven by a structural phase transition in the substrate, and strain-mediated feedback [2]. However, producing high-quality thin films can be challenging, and thin films do not provide large thermal mass for cooling. Here, LCMO || BTO core-shell particles with different compositions were investigated to see whether they present similar behaviour to the film-substrate heterostructures, while simultaneously providing larger thermal mass. The second study focused on surface magnetocaloric effect in near stoichiomet- ric Ni-Mn-Ga (NMG) thin films. NMG is interesting as its ferromagnetic transition is accompanied by a structural transition, producing large magnetocaloric effects that can be driven using magnetic field and/or stress. In this study, the surface magnetisation was studied on 200-nm-thick NMG thin films grown on MgO sub- strate. Previous studies have reported that magnetisation is suppressed at the surface in NMG thin films, therefore compromising their magnetocaloric perfor- mance. Here, we observed that suppression of the magnetisation can be avoided by optimising growth conditions. Furthermore, using synchrotron techniques, we confirmed the presence of the magnetostructural phase transition at the surface, thus demonstrating that the surface is magnetocalorically active. The third study focused on barocaloric effects in fluorinated ferroelectric poly- mers of polyvinylidene fluoride – co – trifluoroethylene (PVDF-TrFE) with different compositions. P(VDF-TrFE) polymers have been previously investigated for elec- trocalorics effects driven by electric fields; here their barocaloric performance on applying and removing pressure is studied. Other ferroelectric materials have been studied for their barocaloric effects, but their performance was limited due to brittleness, low transition temperature, or small entropy changes. PVDF-TrFE is interesting as it is ductile, inexpensive and a good electrocaloric material. Here, we achieved the maximum reversible entropy change (|∆S| = 221.86 J kg−1 K−1) and temperature change (|∆T | = 29.05 K) in PVDF-TrFE 20 and PVDF-TrFE 25, respec- tively, with a pressure change of 2.7 kbar. These are much larger than the values obtained from the electrocaloric effect. [1] C. M. Xiong, J. R. Sun, Y. F. Chen, B. G. Shen, J. Du, and Y. X. Li, “Relation between magnetic entropy and resistivity in La 0.67Ca0.33MnO3,” IEEE Trans. Magn., vol. 41, no. 1 I, pp. 122–124, 2005. [2] X. Moya, L. E. Hueso, F. Maccherozzi, A. I. Tovstolytkin, D. I. Podyalovskii, C. Ducati, L. C. Phillips, M. Ghidini, O. Hovorka, A. Berger, M. E. Vickers, E. Defay, S. S. Dhesi, and N. D. Mathur, “Giant and reversible extrinsic magne- tocaloric effects in La0.7Ca0.3MnO3 films due to strain,” Nat. Mater., vol. 12, no. 1, pp. 52–58, 2012.

Strain-mediated magnetoelastic composite materials are being considered for communication in lossy environments. Their consideration is attributable to predictions stating order of magnitude improvements over current antenna technology. The magnetic antenna design considered herein consists of three layers: 1) a piezoelectric layer, 2) a linear elastic layer, and 3) a magnetoelastic layer. The antenna operates by mediating strain through the device in a resonant bending mode. The magnetoelastic layer is stressed which results in a changing magnetization ultimately leading to a changing magnetostatic field in free space which acts as a signal for information transfer. In order to prove the efficacy of this approach finite element models have been developed to aid in the design and optimization process. Where these models fall short is in their overall run-time to fully resolve the coupled dynamics. It is for this reason that the work presented in this thesis focuses on the development of a figure of merit capable of predicting optimal bias conditions and geometries needing only the data from a static bias study from FEA. The material level magnetomechanical coupling factor is chosen as the foundation for the figure of merit. The figure of merit is then augmented to include structure level information regarding the demagnetizing field and the non-uniform stress distribution. The main results presented are the effects of including demagnetization and stress distributions, and most importantly the ability of the metric to predict the change in magnetization of the device. It is shown that for aspect ratios greater than roughly 2.5 the metric trends the same as the change in magnetization predicted by finite element simulations. The region of disagreement between the metric and the fully resolved finite element simulation is explained by tying back to underlying assumptions made during the formulation of the magnetometric demagnetization factor used in the analysis. The case is made for the figure of merit to be included in the analysis of strain-mediated antennas for its ability to find optimum designs while reducing the overall simulation run-time by an order of magnitude. Master of Science Many communication devices are readily available however there are a few key gaps in communication technology that are yet to be filled. Notably, communication in lossy environments using small scale, low frequency, devices has proven difficult due to the fundamental limits of antennas (a cell phone cannot communicate into a mine shaft for search and rescue operations, nor can they communicate underwater to submarines or divers for instance). A promising new approach of communication using smart magnetic materials is under consideration in this thesis. Specifically, the goal herein is to develop an analysis tool capable of predicting device performance without having to run computationally expensive/time consuming finite element simulations. In this thesis it is shown that the analysis tool is capable of predicting device performance while reducing the necessary simulation run-time by an order of magnitude. Using this tool, researches will be able to design better prototypes; moving one step closer to portable communication in lossy environments.