Oblique evaporation of CO80Ni20 films for magnetic recording
Authors:   , Abelmann Leon 
Published: April 2022

Tape and disc media have their own specific fields of application in magnetic, magneto-optic and optical recording. Tape media are commonly used where a large storage capacity is needed. To meet the increasing demands for larger storage capacities industry has investigated the possibility to use the method of vacuum evaporation for the production of magnetic recording tape. This has resulted in a commercial product called Metal Evaporated tape. In ME-tape production one takes advantage of the special properties of magnetic films evaporated under oblique vapour incidence, a method which has been discovered in 1959 in laboratory experiments. The difference however between industrial production of ME-tape and laboratory scale evaporation of thin film is extreme. The results obtained in the 35 years of experiments with oblique evaporation cannot be used unconditionally for the industrial production of ME-tape. Therefore the necessity for a model relating process conditions to structural and magnetic film properties is evident. In this thesis such a model is constructed using theoretical and experimental results already published in literature, combined with new theory and experiments. A number of experiments with Co80Ni20 evaporated at high angles of incidence are presented, including the first results on experiments with small-scale ME-tape production. In chapter 2 the relation between process conditions and film structure is discussed. The special structure of obliquely evaporated films has its origin in shadowing phenomena during film growth. Because of shadowing the film consists of bundles of inclined columns, the bundles being aligned perpendicularly to the vapour incidence direction. The column inclination angle lies in-between the film normal and the vapour incidence direction. Different models found in literature relating process parameters and film structure are discussed. It is shown that surface diffusion plays an important role, especially the difference between random and directional surface diffusion. The latter is induced by the oblique evaporation process. An attempt is made to give a quantitative expression for the relation between process conditions and surface diffusion including the influence of substrate temperature, rate and contamination with residual gasses. Using the models found in literature and adding the new calculations the relation between surface diffusion and film structure is discussed in detail and compared with measurements found in literature. In chapter 3 the relation between film structure and magnetic properties of obliquely evaporated magnetic thin films is treated. The tilted columnar structure results in a magnetic shape anisotropy. Two methods to calculate the shape anisotropy are compared, one is especially developed for this purpose. Next to the columnar structure also the anisotropic texture orientation and stress distribution contribute to the magnetic anisotropy. All these effects lead to an easy axis of magnetisation tilted out of the film plane. To determine the magnetic anisotropy in such films a measurement method using a torque-magnetometer is presented, leading to a general formulation of the anisotropy energy in arbitrary systems. The magnetic reversal is strongly affected by the magnetic anisotropy and the exchange between spins in adjacent columns. In films where the exchange is continuous throughout the film the tilted easy axis leads to specific magnetic domain structures which are only found in obliquely evaporated magnetic films. With increasing columnar separation the magnetic reversal turns to a mechanism of rotation of magnetisation in magnetically isolated particles, accompanied by a strong increase in coercivity. Furthermore the tilted easy axis causes a dependence of the recording properties on the tape running direction. In chapter 4 and 5 several experiments performed in the CNRS and MESA laboratories are discussed. Where possible a relation with the theory of chapter 2 and 3 is given. For each evaporation set-up used an estimate of the run-to-run reproducibility is made. It is concluded that the run-to-run scatter is mainly caused by small variations in the angle of incidence. This could be prevented by using an improved substrate holder. It is shown that the structural and magnetic properties of obliquely evaporated Co80Ni20 films depend on layer thickness up to a thickness of about 150 nm. Above this thickness the film properties remain constant. It was concluded that the difference is not caused by the change in substrate temperature during evaporation but by the film growth process itself. In preparation of the tape deposition set-up the rate of evaporation had to be increased. Therefore a short investigation on the effect of increase in evaporation on films prepared on Si substrates has been performed. An increase in rate from 0.5 to 20 nms-1 resulted in an increase in coercivity and anisotropy of the film. Since the columnar inclination and the apparent saturation magnetisation remain almost constant, it was concluded that the changes are mainly caused by an increase in crystal anisotropy. The influence of stress however should not be excluded. The observed change in film properties with increasing evaporation rate might be caused by the expected decrease in contamination with H2O molecules. To enable recording experiments, Co80Ni20 is evaporated on tape by the method of continuously varying angle of vapour incidence. For this a special "mini roll coater" was constructed which resembles the much larger roll-coaters used in industry for the production of ME-tape. This mini roll coater is used to study the effect of addition of oxygen on the film structure and its magnetic and recording properties. The recording performance of the tape prepared with this mini-roll coater are still bad in comparison with commercially available ME tape. The aim of the experiment however is not to produce commercial tape but to investigate the relation between process conditions, film structure, magnetic and recording properties. From this perspective the first results are encouraging.