Quantitative magnetic imaging of thin films with reduced dimensions

The work described in this thesis is based on the high spatial resolution magnetic imaging, which can be realised at the University of Glasgow on two transmission electron microscopes (TEMs). Both machines are highly modified to optimise magnetic imaging conditions. One is based on a JEOL 2000 FX and the other one on a Philips CM20 FEG microscope. Central to the thesis is the option to investigate samples in zero external field as well as to carry out in-situ magnetising experiments.

The aim was to study the magnetisation reversal behaviour of two groups of magnetic thin films with reduced dimensions which are not only interesting in terms of fundamental micromagnetic studies but also for applications (field sensors, MRAM). The first group were so-called small magnetic elements (patterned thin films of permalloy) and the second group were Co/Cu multilayers. The main focus of investigation for the former group lay on the effects of structuring the edges of acicular elements whilst in the case of the Co/Cu multilayers the influence of different degrees of gas-damage was evaluated. Furthermore experimental data of the small magnetic elements was compared with simulation results of a commercially available software package (LLG Micromagnetics Simulator™).

The first chapter gives an overview of basic ferromagnetism and discusses the energy considerations governing domain configurations in ferromagnetic thin films. Subsequently special emphasis was paid to the energy considerations with respect to the two groups of samples, which were investigated in this work, i.e. small magnetic elements and Co/Cu multilayers.

The second chapter deals with the instrumental requirements of a TEM in order to image the magnetic microstructure of thin film samples. The main focus hereby lies on the modified Philips CM20 FEG microscope. Different magnetic imaging modes, which were applied in this work are described and the magneto-optical Kerr and Faraday effects are also briefly discussed.

In chapter 3 the fabrication processes for small magnetic elements and electron transparent substrates (Si3N4 membranes) are described. The in-plane dimensions of the small magnetic elements studied here lie in the micron to sub-micron range and are similar in size to state-of-the-art magnetic sensors. As the aim of this work is to assess the influence of structuring the edges of such elements, three types of periodic repeat structures were designed and are introduced in this chapter. No magnetising experiments were carried out with the test patterns, only the as-grown and ac-demagnetised states have been investigated.

Chapter 4 focuses on the physical and magnetic microstructure of acicular elements and their magnetisation reversal behaviour. The length of the elements was in the micron-range and their width was mainly in the sub-micron range. Standard elements with nominally straight edges were compared with elements with structured edges. Two different lengths as well as two widths of elements were investigated in the first pattern. Three different types of periodic repeat structures were designed and the period of the structure features was varied over a wide range. The magnetic microstructure of the elements in their as-grown and ac-demagnetised state was investigated and in-situ magnetising experiments were carried out.

Results are presented in chapter 5 of a second and third pattern which were also designed with the intention to investigate the influence of structured edges on the micromagnetic states and reversal behaviour of acicular elements. The effects of two different tips ratios of the double pointed elements were evaluated and the height of the structure features was varied. A description of the second and third patterns is given and the physical microstructure of the elements is dealt with. The magnetic microstructure in the as-grown and ac-demagnetised state is discussed and the results of the magnetising experiments are presented.

The aim of chapter 6 is to determine the effects of variation of overall width of the elements with structured edges which was present in the former patterns. Therefore small magnetic elements were designed with structured edges but nominally constant width in the fourth and fifth patterns. All other parameters were the same as in the previous chapter. A description of the two patterns is given before the physical microstructure of the elements is evaluated. The magnetic microstructure of the elements in their as-grown and ac-demagnetised states is dealt with and the development of the domain configurations under the influence of external fields is described.

The magnetic microstructure during magnetising cycles was also modelled for a number of elements which were investigated experimentally using commercially available software (LLG Micromagnetics Simulator™) and the results are presented in chapter 7. A brief description of the theory underlying LLG is given and the set up of problems is discussed. A selection of simulation results is presented and compared with experimental findings.

Chapter 8 deals with another group of magnetic materials with reduced dimensions which are multilayer systems of alternate layers of magnetic and non-magnetic materials with layer thicknesses in the nanometer range. The effects of so-called gas-damage on the GMR performance of different multilayer systems are discussed and the preparation of three Co/Cu multilayers samples with different degrees of gas-damage which were investigated in this work is described. In the results sections GMR measurements and MOKE hysteresis loops are presented before the investigation of the samples with several applied TEM investigation techniques is discussed. By these means the so-called biquadratic coupling was verified and quantified.

In the final chapter this thesis concludes with the discussion of possible future experiments following on from this work.