UC Irvine

Manipulation of the spin degree of freedom in magnetic materials is a path towards a new generation of electronics known as spintronics. In traditional spintronic devices, ferromagnets are the active spin-dependent element and anisotropic magnetoresistance (AMR) is commonly used as an electrical readout method of magnetic order. Recently, research interest has shifted towards antiferromagnetic (AFM) materials as alternatives to ferromagnets: AFMs produce no stray magnetic fields, have ultrafast dynamics, and are capable of generating large magneto-transport effects.

This dissertation presents the first quantitative measurements of AMR in a compensated, high-temperature antiferromagnet PtMn. The magnitude of AMR in PtMn is revealed to be giant which is useful for potential device applications such as ultrafast non-volatile magnetic memory. In the process of reaching this goal, several preliminary developments are chronicled. These include the epitaxial growth of PtMn, AMR studies of the ferromagnet CoFeMnSi, and the development of an AC magnetoresistance measurement setup with 1 milliOhm sensitivity. Moreover, a novel detection scheme is detailed, wherein the AMR of an AFM material can be measured with low noise by exchange-coupling the AFM to a ferromagnet with zero AMR. This AMR detection method could plausibly extend to other exchange-coupled FM/AFM systems beyond the one studied in this work.

In a separate aspect of this dissertation, single-crystal films of the insulating, ferrimagnetic oxide MgAlFe2O4 are grown by reactive magnetron sputtering. Epitaxial magnetic insulators with high saturation magnetization and low Gilbert damping are highly desirable alternatives to the iron garnets typically found in spin caloritronics. The crystallographic quality of MgAlFe2O4 films is investigated by X-ray diffraction as a function of oxygen concentration during film deposition. The magnetic properties of MgAlFe2O4, including Gilbert damping and magnetic anisotropy, are also characterized by ferromagnetic resonance.

In yet another aspect of this dissertation, a nanofabrication recipe for micron-scale arrays of tall nanopillars is described. High aspect ratio nanopillars are useful in the creation of magnetic tunnel junctions (MTJs) with perpendicular magnetization, which have gained popularity due to scaling limitations associated with in-plane varieties. Arrays of pillars with 30 nm diameters are fabricated from an MTJ-like film stack using electron beam lithography and ion mill etching. Magnetization dynamics within the 10 x 10 micron arrays are probed via the anomalous Hall effect as a function of pillar diameter and array pitch.