Fabrication and Characterization of Magnetic Microstructures

Abstract

This thesis is an experimental study of spin dependent transport in Magnesium oxide (MgO) magnetic tunnel junctions (MTJ) and width modulated ferromagnetic waveguides. The magnetoimpedance of micron sized magnetic tunnel junction sensors with 1.7 nm MgO tunnel barrier was investigated using ac impedance spectroscopy (IS). We performed impedance spectroscopy in the frequency range between 100 Hz and 40 MHz as a function of applied magnetic field in the sensing direction. The results were evaluated using equivalent circuit models. We did not observe any magnetocapacitance in single MgO MTJ devices. Multiple single MTJs connected in series (array junctions) were also measured by tunnel magnetoresistance (TMR) and IS techniques. We have observed non zero magnetocapacitance and magnetoinductance for array junctions. We have shown that TMC can be used to sense external dc magnetic fields. We havealso shown that the raw magnetoimpedance measurements can be used in sensing external dc magnetic field without equivalent circuit analysis with better sensitivity compared to sensitivity via dc-TMR. The dc voltage bias dependence of single MgO MTJs was also studied. These results provide improvements and new ways to measure magnetic fields and they are valuable to the magnetic sensor industry.Magnonic crystal structures which included a coupled reservoir and a waveguide were fabricated and measured using time and space resolved scanning magneto-optic Kerr microscopy (TSRSKM). Two kinds of devices were fabricated. A rectangular reservoirconnected to an unmodulated waveguide and to a width modulated waveguide. Both the reservoir and the waveguide were sputter deposited (110 nm Permalloy). We have observed propagating spin wave injection under uniform global ac magnetic field between 3.6 and 4.4 GHz. The reservoir acted as a broad-band antenna and injected spin waves into the waveguides. Unlike the unmodulated waveguide, the modulated waveguide also showed a standing wave which was dictated by the modulation. We didnot observe stop/pass bands for spin waves (magnonic crystal). The results provide better understanding of spin wave creation and propagation.