Magnetic and Physical Characterisation of Soft Thin Films for Use in Perpendicular Recording Heads
The increasing demand for greater storage densities poses constant challenges for the magnetic recording industry. The move to perpendicular recording occurred recently as this provides one way of increasing the storage density. For further advance in the technology it is important to gain an understanding of the micromagnetic and microstructural properties of the materials used in the hard drive. The material used in the write head is extremely important as the storage density is increased. CoFe alloys are currently of interest as they have a high magnetic moment and therefore have the potential to write to the high anisotropy materials required for writing at the high densities. Transmission electron microscopy is an ideal tool to gain such information as it provides a way of extracting magnetic information of the nanometre scale and structural information on the atomic scale.
Chapter 1 of this thesis provides an introduction to ferromagnetic materials and the magnetic energy contributions which are present. The principles of magnetic recording are then discussed along with the advantages of switching to perpendicular recording. The requirements for the write head material are discussed at the end of chapter 1 and a review of previous work on CoFe materials is given.
The main experimental techniques used for the work in this thesis are described in chapter 2. Firstly the sputter deposition of samples is discussed along with the characterisation of the bulk films with a BH looper. A detailed description of transmission electron microscopy (TEM) and the various imaging modes which have been used is given. The Fresnel mode and the differential phase contrast (DPC) mode of Lorentz microscopy have been used to study the magnetisation reversal behaviour of the films and these are described. Electron energy loss spectroscopy (EELS), which was used to investigate the elemental distribution in the films, is also discussed. A dual beam SEM/FIB was used to investigate the surface topology of the films and a brief description of the instrument is given. The final part of chapter 2 deals with the preparation of cross-sectional specimens with the encapsulation method and the patterning of thin films with electron beam lithography for use in the TEM.
By way of an introduction to the various reversal behaviour which is observed within magnetic films two soft films, NiFeCuMo and CoFeB, have been investigated. The results from these films are discussed in chapter 3.
In chapter 4 results from four CoFe thin films are presented. While all films were of similar total thickness, 50 nm, the differences were the inclusion or otherwise of a seedlayer and the introduction of nonmagnetic spacer layers to form laminate films. The detailed mechanisms for easy and hard axis reversals of the films were investigated. As expected cross-tie walls were observed in the films with thicker CoFe layers and wall displacements were seen with the introduction of the spacer layers. Magnetisation dispersion was reduced as multilayering was introduced and a significant reduction in the grain size from 12.5±2.8 nm to 7.9±1.5 nm was observed. In the laminated films with three spacer layers defect areas where the magnetisation distribution differed markedly from that in the surrounding film were observed and the formation of 360° domain walls was noted. Cross-sectional TEM showed that the layer roughness increased throughout the film thickness and this was thought to be the probable cause of the localised anomalies.
Chapter 5 follows on from chapter 4 with an investigation into the origin of the defect areas and 360° domain walls. The easy and hard axis magnetisation reversals for four CoFe laminate films were investigated. The main differences between these films and those investigated in chapter 4 was the reduction in spacer layer thickness and variation of CoFe layer thickness. In this case the presence of physical defects which significantly influenced the reversal behaviour were noted. A high density of 360° domain walls which persisted up to fields of a few hundred oersteds were observed. The walls remained throughout reversal and the likely processes which occur during the reversal are presented. A quantitative investigation with DPC revealed that although some of the 360° domain walls exist in only one or two layers of the laminate films there are occasions where the walls exist throughout the whole thickness of the film. However, the end points of the 360° domain walls are still not understood.
In an attempt to gain an understanding of the physical structure of the defects and some insight into what happened where the 360° domain walls ended, SEM, cross-sectional TEM and EELS have been carried out and the results are presented at the end of chapter 5 and in chapter 6. The defects were found to originate from particles present on the substrate before the deposition of the CoFe films. The identity of the particles is still unknown, however, due to the regularity of the particles they are thought to be caused from some process after the etching of the substrate and not from handling. Cross-sectional TEM revealed the spacer layers are effective at separating the magnetic layers although there are some instances where the growth in one layer may be affected by that in an underlying layer.
In chapter 7 the final experimental work which explores the reversal mechanisms occurring in patterned films is presented. Rather than doing the investigation on the complex CoFe multilayers, six permalloy shapes with similar dimensions to a write head were studied. It was found that pole tips with widths of 500 nm and 1 µm were influenced significantly by the reversal occurring within the whole shape. This study revealed the obstacles which the CoFe films with well defined anisotropy have to overcome when patterned down to write head dimensions. Finally, the conclusions from the experimental studies are discussed in chapter 8 along with future work which could be carried out.