Domains in Co-Cr layers for perpendicular magnetic recording

In this thesis the magnetic properties of RF and magnetron sputtered CoCr (81/19 at%) films for perpendicular recording are studied. In chapter 1 an introduction is given to perpendicular recording, the properties of CoCr are briefly reviewed and compared with other layers and finally several magnetic models, divided into particulate and continuous models, are briefly described. In tbe literature a controversy exists whether or not CoCr is a particulate or a continuous medium. In cbapter 2 the situation is compared with the dual particle-wave nature of light and it is shown tbat tbe problem bas to be defined more precisely. Arguments for and against a particulate reversal mechanism and a reversal by domain-wall displacements are summarized. Neither of the models can give a satisfactory explanation for all the observed phenomena, and a strict application of one of the models does not seem to be very fruitful. A new concept would be tbe existence of an antiferromagnetic coupling between tbe colurnnar particles. (The presence of the Meiklejobn effect would be convincing evidence for sucb a coupling, but as yet this bas not been found in CoCr films. ) In the results of the experimental work, that can be divided into tbe magneto-optic surface measurements (chapter 4) and tbe neutron depolarization experiments (chapter 6) evidence fqr a brancbed domain structure with reversed spike domains is found. These chapters are each preceded by a theoretical conspectus in whicb the techniques are introduced. Tbe coercivity at the surface is found to be lower than the bulk coercivity measured by means of a Vibrating Sample Magnetometer for films thicker than a critical thickness. This bebaviour is almost discontinuous for RF-sputtered films. Tbe critical thickness is correlated with the minimum film thickness at wbich spike domains can be formed. In neutron depolarization an effective film thickness is introduced to account for the lower experimentally found rotation of tbe polarization vector than expected on the basis of a perpendicular domain structure with straight parallel domains. Tbe effective film thickness approaches the real one for films near the critical thickness, while in the thicker films (-4- -5 ~m), the effective film thickness is only half of tbe real one. Again these results are consistent with the formation of reversed domains. Angular-dependent neutron depolarization showed that the domain width is proportional to hx, h being the film thickness and with the exponent x between e.6 and e.8. This value is closer to the calculated power of 2/3 for a branched domain structure than 1/2 predicted for a Kittel structure with perpendicular domains. The reversal mechanism in a branched domain structure would be the nucleation and growth of reversed spike domains. This could well take place inside each column. In that case it is a type of particle reversal mechanism although domain-wall motion is involved hence both contradictary models, viz. the particulate and the continuous type are united.