Cryogenic Memory Elements using Rare Earth Nitrides

The rare earth nitrides (RENs) are a series of intrinsically ferromagnetic semiconductors with a range of contrasting magnetic properties across the series due to the filling of their 4f electron orbitals. The electrical conductivity of the RENs can be tuned via nitrogen vacancy doping to achieve conductivities ranging from insulating to metallic-like. It is these properties that have prompted their use in memory device structures in recent years, with the potential for integration with superconducting computing in the future. We report on the potential use of the RENs as ferromagnetic (FM) layers in tunnelling magnetoresistance (TMR) and giant magnetoresistance (GMR) device structures for non-volatile memory storage devices.

In this thesis, we present a study on the use of gadolinium nitride and dysprosium nitride as the ferromagnetic layers in a range of device structures including magnetic tunnel junctions (MTJs) and in-plane conduction devices based on the giant magnetoresistance effect. First, we have fabricated and characterised GdN/AlN/GdN/Gd and GdN/GaN/GdN/Gd MTJ structures with a maximum TMR of 135% at 5 K for magnetic fields of ±8 T. The maximum difference in resistance between the high- and low-resistance states – corresponding to the anti-aligned and aligned magnetisation states – at 0 T was ~0.53%.

Following this, magnetic tunnel junctions and GMR-style devices have been fabricated using gadolinium nitride and dysprosium nitride as the two FM layers. In the MTJ, lutetium nitride was used as the tunnel barrier material (GdN/LuN/DyN), while in the GMR-style devices, Lu and Al were separately used as the conductive exchange blocking layers (GdN/Lu/DyN and GdN/Al/DyN). A maximum difference in resistance states of ~1.2% and ~0.04% at 0 T was observed at 5 K for the MTJ and GMR-style devices respectively. This is the first time that a difference in resistance between the aligned and anti-aligned configuration has been observed for these device structures in the absence of an external magnetic field, demonstrating the use of GdN and DyN in such non-volatile memory elements.

We also report electrical transport and optical spectroscopy measurements on a series of lutetium nitride thin films variously doped with nitrogen vacancies, along with the computed band structures of stoichiometric and nitrogen vacancy doped LuN. Here, we bridge the void between computation and experiment with a combined study of LuN focusing on its electronic properties. We find that stoichiometric LuN is a semiconductor with an optical bandgap of ∼1.7 eV. Its conductivity can be controlled by doping with nitrogen vacancies, which results in defect states at the conduction band minimum and valence band maximum. These results not only provide information on LuN but also help underpin understanding of the electronic properties of the entire rare earth nitride series.