Magnetic thin films For spintronic memory

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 107-128).Domain walls are regions of spatially non-uniform magnetizations in magnetic materials. They form the boundaries between two or more uniformly magnetized regions called domains. Skyrmions are circular magnetic domains with chiral domain walls that are interesting due to their stability and potential for fast motion. These spin structures can be used to encode Os and Is in spintronic memory. Chiral domain walls and skyrmions have been seen in magnetic thin films sandwiched between non-identical non-magnetic materials which have high spin-orbit coupling and Dzyaloshinskii-Moriya interaction. An optimization of the different physical interactions involved in magnetic thin films can result in stripe and labyrinth domain patterns which can then be transformed into skyrmion lattices. In this thesis, we present a detailed understanding of single- and multi-layer magnetic thin films along with all the relevant physical interactions. We show that inplane magnetic fields stabilize domain walls in thin films with Dzyaloshinskii-Moriya interaction. The application of in-plane magnetic fields is shown to create multi-domain patterns in films where the ground state is uniform magnetization. Next, we study the formation of stripe and labyrinth domain patterns in magnetic films. The domain widths obtained are compared with the predictions of several theoretical models developed over the last 50 years. The appropriate model that works for thin films with strong Dzyaloshinskii-Moriya interaction is identified with the help of micromagnetic simulations. The appropriate model includes effects of finite domain wall width and volume charges inside Neel domain walls. This model is then used to measure the Dzyaloshinskii-Moriya interaction in experimentally grown magnetic thin films. Thereafter, we highlight the role of other design variables such as the thickness of magnetic and non-magnetic layers, the choice of materials, and the role of geometrical confinement in controlling the length scale of the domain patterns. This work generates the necessary knowledge and develops techniques to engineer chiral spin textures in single- and multi-layer magnetic thin films.by Parnika Agrawal.Ph. D

An Assessment of Automated Quantitative Structure-Activity Relationship Modeling on Drug Discovery for Novel Treatment of Blood Disorders

The MYND domain of the ETO2 protein is a novel target for drugs aimed at treating sickle cell disease and related blood disorders.1,2 This study explored the application of automated quantitative structure-activity relationship (QSAR) modeling, a machine learning application of in-silico drug discovery, to this target protein system using Schrödinger’s AutoQSAR software. The protein target in this study currently has no known drug-like binders, allowing the assessment of conducting every stage of lead discovery in-silico. A training set was generated using a preliminary docking study, from which QSAR models were built and verified across varying data splitting ratios. The most favorable of these models was subject to further testing to assess overfitting and ligand-inclusion/exclusion dependency, and a test set of QSAR predictions was evaluated for accuracy. The use of AutoQSAR modeling for this system was found to be unsuccessful, likely associated with the lack of verified drug-like binders in the training set.Bachelor of Science in Public Healt

Spin Hall torque magnetometry of Dzyaloshinskii domain walls

Current-induced domain wall motion in the presence of the Dzyaloshinskii-Moriya interaction (DMI) is experimentally and theoretically investigated in heavy-metal/ferromagnet bilayers. The angular dependence of the current-induced torque and the magnetization structure of Dzyaloshinskii domain walls are described and quantified simultaneously in the presence of in-plane fields. We show that the DMI strength depends strongly on the heavy metal, varying by a factor of 20 between Ta and Pa, and that strong DMI leads to wall distortions not seen in conventional materials. These findings provide essential insights for understanding and exploiting chiral magnetism for emerging spintronics applications

Magneto-ionic control of interfacial magnetism

In metal/oxide heterostructures, rich chemical electronic magnetic and mechanical properties can emerge from interfacial chemistry and structure. The possibility to dynamically control interface characteristics with an electric field paves the way towards voltage control of these properties in solid-state devices. Here, we show that electrical switching of the interfacial oxidation state allows for voltage control of magnetic properties to an extent never before achieved through conventional magneto-electric coupling mechanisms. We directly observe in situ voltage-driven O{superscript 2−] migration in a ​Co/metal-oxide bilayer, which we use to toggle the interfacial magnetic anisotropy energy by >0.75 erg cm[superscript −2] at just 2 V. We exploit the thermally activated nature of ion migration to markedly increase the switching efficiency and to demonstrate reversible patterning of magnetic properties through local activation of ionic migration. These results suggest a path towards voltage-programmable materials based on solid-state switching of interface oxygen chemistry.National Science Foundation (U.S.) (NSF-ECCS-1128439)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (DMR-0819762)Samsung (Firm) (Samsung Global MRAM Innovation program

Nanomedicine: Big Promise from a Tiny World

19-22The day is not far when nanomedicine will provide us drugs and devices that would take us from blunderbuss treatment to target-specific and efficient therapy of incurable cancers and life-threatening multi drug-resistant bacterial infections

Spontaneous domain nucleation under in-plane fields in ultrathin films with Dzyaloshinskii-Moriya interaction

In this paper, we show that applying a hard axis field reduces the energy barrier for the spontaneous formation of a multi-domain state in magnetic ultrathin films sandwiched between a heavy metal and an oxide. This provides a simple technique to generate a metastable multi-domain state in magnetic films where the ground state is uniform. This approach could be particularly interesting in materials with strong Dzyaloshinskii-Moriya interaction as a means to realize metastable chiral textures.National Science Foundation (U.S.) (NSF-CCS-1408172

Temperature dependence of the Dzyaloshinskii-Moriya interaction in Pt/Co/Cu thin film heterostructures

© 2018 Author(s). Magnetic materials that exhibit chiral domain walls are of great interest for spintronic devices. In this work, we examine the temperature-dependent behavior of the Dzyaloshinskii-Moriya interaction (DMI) in Pt/Co/Cu thin film heterostructures. We extract the DMI strength, D, from static domain spacing analysis between 300 K and 500 K and compare its temperature dependence to that of the magnetic anisotropy, Ku, and saturation magnetization, Ms. Consistent with expected scaling in thin films, Ms exhibits Bloch-law temperature scaling and Ku scales as Ms2.1±0.1. However, D varies more strongly with temperature than expected, scaling as D-Ms4.9±0.7, indicating that interfacial DMI is more sensitive to thermal fluctuations than bulk magnetic properties. We suggest that this may be related to the temperature dependence of locally induced magnetic moments in the Pt underlayer and the 3d-5d orbital interactions at the interface. While we observe stable domain widths in the studied temperature range, a strongly temperature dependent DMI may have significant consequences for potential devices based on the chiral domain wall or skyrmion motion

Measurement of interfacial Dzyaloshinskii-Moriya interaction from static domain imaging

Perpendicularly magnetized thin films with a strong Dzyaloshinskii-Moriya interaction (DMI) exhibit chiral spin structures such as Néel domain walls and skyrmions. These structures are promising candidates for next-generation magnetic memory devices. Determining the magnitude of the DMI accurately is key to engineering materials for such applications. Existing approaches are based on quantities extracted either from magnetization dynamics, which present experimental and theoretical challenges, or from measurements of quasistatic domain spacing, which so far have been analyzed using incomplete models or prohibitively slow micromagnetic simulations. Here, we use a recently developed analytical model of stripe domain widths in perpendicularly magnetized multilayers to extract the DMI from domain images combined with magnetometry data. Our approach is tested on micromagnetically simulated domain patterns, where we achieve a 1% agreement of the extracted DMI with the DMI used to run the simulation. We then apply our method to determine the thickness-dependent DMI in two experimental materials, one with ([Pt(2.5-7.5nm)/Co60Fe20B20(0.8nm)/MgO(1.5nm)]13) and one without ([Pt(2.5-7.5nm)/Co(0.8nm)/Pt(1.5nm)]13) inversion symmetry breaking. We discuss the means to obtain realistic error bars with our method. Our results demonstrate that analytical domain spacing analysis is a powerful tool to extract the DMI from technologically relevant multilayer materials.United States. Department of Energy. Office of Basic Energy Sciences (Award #DE-SC0012371)National Science Foundation (U.S.) (Awards DMR-1419807, 1541959

Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets

Magnetic skyrmions are topologically protected spin textures that exhibit fascinating physical behaviours and large potential in highly energy-efficient spintronic device applications. The main obstacles so far are that skyrmions have been observed in only a few exotic materials and at low temperatures, and fast current-driven motion of individual skyrmions has not yet been achieved. Here, we report the observation of stable magnetic skyrmions at room temperature in ultrathin transition metal ferromagnets with magnetic transmission soft X-ray microscopy. We demonstrate the ability to generate stable skyrmion lattices and drive trains of individual skyrmions by short current pulses along a magnetic racetrack at speeds exceeding 100 m s-1as required for applications. Our findings provide experimental evidence of recent predictions and open the door to room-temperature skyrmion spintronics in robust thin-film heterostructures.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0012371