Theoretical and experimental investigations on short-wavelength recording

In this thesis CoCr and CoCr/NiFe double layers are studied by ferromagnetic resonance. The coercivity and the initial susceptibility of these layers are measured. An approximation of the Kooy and Enz model which is suitable for calculating the initial suceptibility is presented [3.36]. A theoretical model [3.34] for magnetization curves which includes hysteresis is given. Neutron depolarization [4.5] is used to check a model [4.1] which describes the interaction between the two layers of the double layer. Chapter 1 gives a brief introduction on perpendicular recording and describes some of the properties of CoCr. The preparation of our samples is explained. A short account of the apparatus used to measure the magnetic parameters is included. Chapter 2 is divided into three parts. Part I gives a general introduction on FMR. The general theory, which is elaborated further on in this chapter for specific application for CoCr and NiFe, is given. Part II describes FMR measurements carried out on single NiFe layers and NiFe layers which form part of a double layer configuration of CoCr/NiFe. An X-band spectrometer is used. Both the resonant field Hr and the linewidth AH are measured for a range of values of ~, the angle between the applied field and the normal to the film. Theoretical curves for ~(~) are calculated. A new low field, low frequency, theoretical model [2.18] for AHC~) is presented. These theoretical curves also enable us to study the effect of each individual magnetic parameter on Hr(~) , AHC~) and AHCHr). The magnetic parameter ~ can be obtained from experimental values of Hr(~). Using this model the damping parameter « can be obtained from experimental values of AH. In part III HrC~) of both CoCr and NiFe in the double layer and of single CoCr layers is measured for ~ = 00- 90°. A Q band spectrometer is used. In order to obtain information on the origin of the different CoCr peaks these measurements are also carried out on the single CoCr layers before and af ter various etching steps. Theoretical curves for H(~) are calculated and a new high field model [2.32] is presented enabling us to determine the magnetic parameters of the different layers of microstratified CoCr. FMR measurements were also carried out on single CoCr layers using an X band spectrometer [2.33]. A peak due to the uniform precession mode and a resonance peak in the zone where the film is still unsaturated was measured. In chapter 3 the hysteresis loop and in particular the perpendicular coercivity H c and the initial susceptibility or the initial slope T of single CoCr layers and CoCr/NiFe double layers are studied. This provides information on the possible interaction between the two layers, on the origin of coercivity and on the domain structure. H c of both magnetron sputtered CoCr layers and CoCr/NiFe double layers, as well as the coercivity of the CoCr layer of the double layer is measured. Hysteresis loops are calculated using a modification [3.34] of the Kooy and Enz model [3.5] to include hysteresis, using the friction force which opposes domain wall movement as a source of hysteresis. The initial susceptibility T of single and double layers is also measured. A low field approximation [3.36] of the Kooy and Enz model gives that T-l is proportional to t-l/2 where t is the thickness of the fïlm. When applying this model to RF sputtered CoCr films there is a difference in behaviour depending on the thickness of the film. Chapter 4 gives the response [4.1] of the NiFe layer to the stray field of the domains of CoCr i.e. the magnetostatic interaction in a double layer consisting of a medium with horizontal anisotropy and one with vertical anisotropy. In order to study the interaction experimentally, neutron depolarizat1on experiments [4.5] were carried out on double layers and the corresponding single layers. Neutron depolarizat1on enabled us to study the interaction between the NiFe and the domains of CoCr. Furthermore the coercivity of the NiFe of the double layer ls compared with that of the single layer.