Carnegie Mellon University
Latest Research Presentation from Intermag 2021

Topological Magnetism in Thin Films
Fascinating developments in condensed matter research have brought a new kind of magnetism to the forefront of modern physics. Topological magnetism is the study of unique chiral winding configurations of electron spins and their associated magnetic dipole moment. This includes Skyrmions (a type of vortex), which are stabilized by a new quantum mechanical exchange called the Dzyaloshinskii-Moriya Interaction (DMI). Unlike conventional Heisenberg exchange, which favors parallel alignment of neighboring spin vectors and is the foundation of nearly all magnetic materials technology, DMI favors orthogonal alignment leading to helical spin textures. It turns out such configurations are highly susceptible to manipulation by electrical current making them promising for future computer memory and logic devices. Experimental work to identify new materials and characterize topological spin configurations emerged only recently and is the focus of this effort. Multi-layer thin films are grown in the CMU nanofab and characterized using state of the art magnetic imaging by Lorentz TEM in the materials characterization facility and by optical magnetic imaging via Kerr microscopy.
Low Power Spin Logic Devices
For nearly a century, it has been known that electrons are defined by a fundamental quantum property, known as spin, in addition to their well-known negative charge. While conventional electronics are made functional by electron charge, the past few years have seen a surge of fascinating research on a unique class of materials & devices that are made functional by the presence of electron spin. It turns out that the spin of an electron can be used to both modify and/or probe the magnetic configuration of a material, which has enabled a wide range of future electronic devices, known as spintronics, that will be more energy efficient and cheaper to make than today's semiconductor devices. In this project, we explore a new circuit scheme called mLogic that uses a device known as an mCell that was conceived here at CMU by Professors Jimmy Zhu & Larry Pileggi. This complex multi-layered device incorporates two emerging phenomena in spin physics - tunneling magnetoresistance (TMR) and current induced domain wall motion (CIDWM) - to function in a logic circuit for future computer processors.
Combinatorial Materials Exploration
The Materials Genome Initiative launched by the White House in 2011 aims to double the rate at which new functional materials are discovered. In the field of nanoscale magnetism and spintronics, it has become clear, more and more, that key challenges are related to materials discovery. In this research thrust, we are developing combinatorial techniques to rapidly explore new materials. This is done through a thin film deposition technique that enables controlled variation of composition across the surface of a silicon substrate. Through careful design of the sputtering geometry, it is possible to test more than 100 compositions in a ternary or quaternary system from a single sputter deposition. By combining this with high throughput experimental techniques (e.g. XRD), the rate at which materials are examined increases tremendously.
Perpendicular Magnetic Tunnel Junctions For Non-Volatile Memory
Electron tunneling through an insulating barrier is a purely quantum mechanical effect with no classical analog, which holds great promise for future memory and logic applications. By making the electrodes of a tunnel barrier magnetic, a range of new quantum devices have been conceived based on the tunneling magnetoresistance (TMR) and spin-transfer torque (STT) phenomena; in particular, magneto-resistive random access memory (MRAM). MRAM is a non-volatile memory meaning that even if the power supply is interrupted or turned off, no information is lost. In this project, we explore new thin film materials to improve magnetic tunnel junctions (MTJs) for future technology by evaluation of atomic structure, interfaces, diffusion, and magnetic properties using state-of-the-art nanolithography and materials characterization tools.
Ultra-Fast Current-Induced Domain Wall Motion
It was predicted as far back as 1978 that allowing electrical current to flow through a magnetic domain wall could cause it to move. Professor Luc Berger at CMU had calculated that the spin of the electrons would cause the wall to move in the same direction that the electrons were flowing through a physical phenomenon known as spin transfer torque (STT). A number of applications have been proposed that take advantage of such an effect including mLogic, racetrack memory, and DW-MRAM. However, it has only been recently that such a phenomenon has been observed and the results are fascinating. It turns out that the domain wall sometimes moves in the predicted direction based on STT, but in other cases, it is found that the domain wall moves in a direction opposite to the electron flow. The direction and velocity of motion are by the internal magnetic structure of a domain wall and a phenomenon known as the Spin Hall Effect. In this project, we explore materials that enable ultra-fast domain wall motion in magnetic nanowires by combining a strong Spin Hall Effect with an interface-engineered wall structure.
Materials for Advanced Perpendicular Magnetic Recording Media Beyond 1 Tbits/in2
The vast majority of today's data, from videos & music to medical images & digital textbooks, are stored in hard disk drives (HDDs). HDDs have seen tremendous growth in storage density from less than 100 Mbits/in2 in the early 1990s to nearly 1 Tbits/in2 today with increasing affordability. Maintaining this growth rate has been critical to meet storage needs for big business analytics & to address increasing consumer demand for smaller, higher storage capacity personal electronics. However, major materials challenges remain to further improve today's nanogranular perpendicular magnetic recording (PMR) media. In this project, we explore novel materials & thin film structures for increasing HDD storage density beyond 1Tbits/in2 while adding to a fundamental understanding of the interplay between crystallographic defects, microstructure, and magnetic properties in PMR media.
Nanogranular Soft Magnetic Thin Films for High Frequency Power Applications
Soft magnetic materials are a critical component of electric power distribution owing to their use as inductor cores in transformers. Significant energy losses occur in these inductor cores due to eddy currents and magnetic hysteresis. The ideal magnetic material would have a large magnetic moment while maintaining a low resistance and soft magnetic behavior that would allow operation at higher frequencies. State-of-the-art soft magnetic materials are nanocomposites where a conductive magnetic material such as FeCo is embedded in an insulating non-magnetic material like SiO2. The fine details of the microstructure of a composite material like this have important consequences on its electrical and magnetic properties. In this work, we are examining soft thin film nanocomposite FeCo-SiO2 materials to elucidate information about the microstructure-property relationships critical to improving performance for power distribution. Furthermore, we explore the use of such nanocomposite thin films for microelectronic inductor applications where frequencies in the 10-300 MHz range are needed.
Vincent M. Sokalski
Associate Professor
Dept. of Material Science & Engineering
Carnegie Mellon University
5000 Forbes Ave
Roberts Engineering Hall 144
Pittsburgh, PA 15213

E-mail: sokalski@cmu.edu
Office: 412-268-8628
Fax: 412-268-7596