Nitrogen-Based Magneto-Ionic Manipulation of Exchange Bias in CoFe/MnN Heterostructures

Electric field control of the exchange bias effect across ferromagnet/antiferromagnet (FM/AF) interfaces has offered exciting potentials for low-energy-dissipation spintronics. In particular, the solid state magneto-ionic means is highly appealing as it may allow reconfigurable electronics by transforming the all-important FM/AF interfaces through ionic migration. In this work, we demonstrate an approach that combines the chemically induced magneto-ionic effect with the electric field driving of nitrogen in the Ta/Co$_{0.7}$Fe$_{0.3}$/MnN/Ta structure to electrically manipulate exchange bias. Upon field-cooling the heterostructure, ionic diffusion of nitrogen from MnN into the Ta layers occurs. A significant exchange bias of 618 Oe at 300 K and 1484 Oe at 10 K is observed, which can be further enhanced after a voltage conditioning by 5% and 19%, respectively. This enhancement can be reversed by voltage conditioning with an opposite polarity. Nitrogen migration within the MnN layer and into the Ta capping layer cause the enhancement in exchange bias, which is observed in polarized neutron reflectometry studies. These results demonstrate an effective nitrogen-ion based magneto-ionic manipulation of exchange bias in solid-state devices.Comment: 28 pages, 4 figures; supporting information: 17 pages, 11 figure

High Anisotropy Magnetic Materials for Data Storage and Spintronic Memory

University of Minnesota Ph.D. dissertation. January 2018. Major: Electrical/Computer Engineering. Advisor: Jian-Ping Wang. 1 computer file (PDF); x, 122 pages.Data storage technologies that utilize magnetic materials for storage are key for both increasing areal density of storage in traditional hard disk media and providing low energy alternatives to traditional CMOS technology through spintronic memory and logic devices. Spintronic memory relies on the spin of an electron rather than charge and is a promising candidate for achieving non-volatility which can provide dramatic energy savings. A key challenge for magnetic based storage is achieving 10 nm or smaller feature sizes while retaining thermal stability. This requires development of magnetic thin films with large magnetocrystalline anisotropy. Switching the magnetization of high anisotropy magnetic materials requires large Oersted field or spin current. One way to decrease the switching energy is to lower the anisotropy during the switching process with an applied strain or heat. This scheme retains thermal stability during storage and makes write energies feasible from a technological aspect. Development of suitable high anisotropy materials at sub 10 nm scale has proved difficult due to limitations on traditional thin film growth methods, nanoscale effects, and additional requirements on materials for memory applications. The effect of a static strain on the magnetic anisotropy is well understood, but less so for application in devices which require fast switching and high cycling. The other approach to lowering switching energies is to use magnetic materials with small magnetization, such as Mn-based compounds. I will discuss my experiments to advance understanding of: development of FePt for HAMR media, effect of strain assisted switching on the spin state of FePt, and development of novel high anisotropy Mn-based materials with low magnetization. Finally, I will present my experimental realization of Ru as the 4th room temperature ferromagnetic element. Ru has been predicted to become ferromagnetic when placed into a metastable tetragonal or cubic phase. This new phase of Ru also has potential to achieve the requirements for a viable spintronic device. I will show my work on the realization of the tetragonal phase Ru using seed layer engineering in thin films, and its associated ferromagnetic properties

Nitrogen-Based Magneto-ionic Manipulation of Exchange Bias in CoFe/MnN Heterostructures

Electric field control of the exchange bias effect across ferromagnet/antiferromagnet (FM/AF) interfaces has offered exciting potentials for low-energy-dissipation spintronics. In particular, the solid-state magneto-ionic means is highly appealing as it may allow reconfigurable electronics by transforming the all-important FM/AF interfaces through ionic migration. In this work, we demonstrate an approach that combines the chemically induced magneto-ionic effect with the electric field driving of nitrogen in the Ta/Co0.7Fe0.3/MnN/Ta structure to electrically manipulate exchange bias. Upon field-cooling the heterostructure, ionic diffusion of nitrogen from MnN into the Ta layers occurs. A significant exchange bias of 618 Oe at 300 K and 1484 Oe at 10 K is observed, which can be further enhanced after a voltage conditioning by 5 and 19%, respectively. This enhancement can be reversed by voltage conditioning with an opposite polarity. Nitrogen migration within the MnN layer and into the Ta capping layer cause the enhancement in exchange bias, which is observed in polarized neutron reflectometry studies. These results demonstrate an effective nitrogen-ion based magneto-ionic manipulation of exchange bias in solid-state devices

Resonant Spin Transmission Mediated by Magnons in a Magnetic Insulator Multilayer Structure

While being electrically insulating, magnetic insulators can behave as good spin conductors by carrying spin current with excited spin waves. So far, magnetic insulators are utilized in multilayer heterostructures for optimizing spin transport or to form magnon spin valves for reaching controls over the spin flow. In these studies, it remains an intensively visited topic as to what the corresponding roles of coherent and incoherent magnons are in the spin transmission. Meanwhile, understanding the underlying mechanism associated with spin transmission in insulators can help to identify new mechanisms that can further improve the spin transport efficiency. Here, by studying spin transport in a magnetic-metal/magnetic-insulator/platinum multilayer, it is demonstrated that coherent magnons can transfer spins efficiently above the magnon bandgap of magnetic insulators. Particularly the standing spin-wave mode can greatly enhance the spin flow by inducing a resonant magnon transmission. Furthermore, within the magnon bandgap, a shutdown of spin transmission due to the blocking of coherent magnons is observed. The demonstrated magnon transmission enhancement and filtering effect provides an efficient method for modulating spin current in magnonic devices

Resonant Spin Transmission Mediated by Magnons in a Magnetic Insulator Multilayer Structure

While being electrically insulating, magnetic insulators can behave as good spin conductors by carrying spin current with excited spin waves. So far, magnetic insulators are utilized in multilayer heterostructures for optimizing spin transport or to form magnon spin valves for reaching controls over the spin flow. In these studies, it remains an intensively visited topic as to what the corresponding roles of coherent and incoherent magnons are in the spin transmission. Meanwhile, understanding the underlying mechanism associated with spin transmission in insulators can help to identify new mechanisms that can further improve the spin transport efficiency. Here, by studying spin transport in a magnetic-metal/magnetic-insulator/platinum multilayer, it is demonstrated that coherent magnons can transfer spins efficiently above the magnon bandgap of magnetic insulators. Particularly the standing spin-wave mode can greatly enhance the spin flow by inducing a resonant magnon transmission. Furthermore, within the magnon bandgap, a shutdown of spin transmission due to the blocking of coherent magnons is observed. The demonstrated magnon transmission enhancement and filtering effect provides an efficient method for modulating spin current in magnonic devices