Effects of Focused Ion Beam Irradiation on Thin Ferromagnetic Films
Focused ion beam (FIB) systems are relatively new research tools, having only become widely commercially available to the scientific community and industry in the past twenty years. Current FIB systems utilising a Ga+ ion source, offer exciting opportunities for analytical investigation, fabrication, property modification or performance of other novel tasks on materials at length scales which approach 10 nm. In this thesis the effects of Ga+ ion irradiation using a FIB on thin exchange biased and unbiased ferromagnetic layers have been investigated. A ferromagnetic layer can be exchange biased by depositing it in intimate contact with an antiferromagnetic layer. Provided an aligning field is present during the deposition stage, or applied as part of an annealing process afterwards, then the moments in the ferromagnetic layer become pinned along a unique direction. The strength of this pinning is reflected in a quantity known as the exchange bias field.
In chapter 1 of this thesis an introduction to the concept of ferromagnetism is presented along with descriptions of the various magnetic energy contributions. The concept of antiferromagnetism is also presented prior to a description of the phenomenon and theoretical aspects of exchange biasing. A description of magnetoresistance for spin-valves or magnetic tunnel junctions which possess exchange biased ferromagnetic layers is also presented. At the end of chapter 1 a short review is given concerning previous work performed by other researchers on the ion irradiation of thin film ferromagnetic systems. Chapter 2 mainly describes the instrumentation and experimental techniques used to irradiate thin ferromagnetic films and examine alterations to their properties. The chapter begins with a description of the technique of sputtering and the apparatus employed for thin film deposition. Transmission electron microscopy (TEM) has been used extensively to investigate the physical microstructure of the thin films prior to and after irradiation. Aspects of the theory and operation of the TEM are presented along with descriptions of the main imaging modes used. The magnetisation reversals of the films were studied extensively using Lorentz TEM techniques and descriptions of the Fresnel and differential phase contrast (DPC) imaging modes are given. The optical technique of Kerr magnetometry, which was also used to gain information about the magnetic properties of the films, is also described. Finally in chapter 2, the theory and operation of the FIB system used to carry out the irradiation are detailed along with a description of computer simulation software used to provide information on the penetration of the ions into the films.
Prior to their irradiation, the physical microstructure and magnetisation reversal properties of the unbiased CoFe layer and exchange biased CoFe/IrMn bilayers were investigated and are described in chapter 3. Chapter 4 follows, where the results of a detailed study into the effects of 30 keV Ga+ ion irradiation on the CoFe/IrMn exchange biased bilayer are presented. As a result of the irradiation, changes to the physical microstructure of the exchange biased bilayer were noted. At low ion doses increases in the mean grain size and layer texture were observed, but at larger doses the polycrystalline grain structure became destroyed with the formation of an unusual structural phase containing needles; at a slightly higher dose amorphisation of the bilayer occurred. Irradiation was also observed to alter the magnetic properties of the bilayer with increasing dose resulting in the monotonic attenuation of the bias field strength and coercivity in irradiated regions. Other changes to the magnetic properties that were noted were alterations to the magnetisation reversal mechanism and magnetic moment decrease in the ferromagnetic layer. These observations and results from other experiments, such as annealing and irradiating with Ga+ and He+ ions of lower energies, suggested that at low doses 30 keV Ga+ irradiation, led simply to atomic mixing at the interface between the antiferromagnetic and ferromagnetic layers. At larger doses other significant damage processes also occurred.
The irradiation of the CoFe/IrMn exchange biased bilayer with 30 keV Ga+ ions in the presence of a magnetic field is reported in chapter 5. The magnetic field was supplied by a custom built in-situ magnetising stage for the FIB; a description of this is given. Upon the bilayer were irradiated a series of 10 x 10 µm2 boxes at different doses in the zero field condition and with a strong field applied parallel or antiparallel to the bias direction. The boxes irradiated in zero field and with the field parallel to the bias direction possessed similar reversal properties. These boxes showed that irradiation was effective in causing magnetic property modification in regions with micron sized dimensions. For the boxes irradiated with the field antiparallel to the bias direction, stronger but variable effects were observed. These ranged from the reduction to zero of the bias field to reversals of the bias direction. In chapter 6 the structural and magnetic properties of CoFe/PtMn bilayers were explored. PtMn is a chemically ordered antiferromagnet and requires a post-deposition annealing at 275°C for 4 hours to induce such ordering and thus exchange biasing. Microstructural alterations such as, the formation of regions containing large crystallites up to 100 nm in size and a decrease in the strength of the texture in the bilayer, were observed for the annealed CoFe/PtMn bilayer. The rather inhomogeneous nature of the structural transformations were probably responsible for local variations in the bias field strength. This in turn, resulted in the magnetisation reversal of the annealed bilayer taking place over large field ranges and involving complicated domain processes. The influence of 30 keV Ga+ irradiation on the CoFe/PtMn bilayer was also examined in chapter 6, with the bias field strength being reduced as a result of irradiation damage leading to a decrease in chemical order in the PtMn layer.
The patterning by ion irradiation of both the exchange biased CoFe/IrMn bilayers and the unbiased CoFe layers by 30 keV Ga+ irradiation was explored in chapter 7. For the exchange biased bilayer, it was found that irradiated stripe features with widths of 500 nm or less were poorly reproduced during the magnetisation reversal of the ferromagnet. This suggested that there was a limit to the lengthscale for which patterning of an exchange biased ferromagnet could be performed. For unbiased CoFe layers, irradiation induced ferromagnetic patterning of magnetic wire features with widths as small as 30 nm was demonstrated. A pattern was also irradiated containing 70 nm wide parallel magnetic wires separated by 35 nm wide irradiated lines.
The final chapter of the thesis, chapter 8, contains conclusions on the experimental findings and discusses future work that could be performed.