Ferromagnetic and ferrimagnetic materials as Josephson junction barriers

In this dissertation we will show experimental results on different magnetic materialsas Josephson junction barriers. Previous work by our group has shown that a Josephson junction with two magnetic layers can be engineered to allow controllable switching between a ‘0’ state and a ‘π’ state, with applications in cryogenic computing. First, that work is expanded on to show that a different combination of Ni and NiFe thicknesses can also support that 0-π switching, allowing a preliminary phase diagram of Ni and NiFe thicknesses to be outlined. Those measurements are done using a low-inductance symmetric SQUID to simplify the data analysis compared to previous experiments. We also explore other magnetic materials beyond Ni and NiFe. Fe is studied as a hard layer candidate and we show that while it does have desirable magnetic properties, the critical current of the junction is greatly reduced compared to when using Ni. We also study the rare-earth transition metal ferrimagnet CoGd. This alloy can be a compensated ferrimagnet with zero net magnetization while still supporting a spin-polarized transport current. We study the relationship between temperature, thickness, and concentra- tion needed to grow compensated films of nanometer-scale thicknesses. We present transport data on Josephson junctions with a CoGd barrier showing the 0-π transition characteristic of a ferromagnetic junction despite the near-zero magnetization.