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Open Access Publications from the University of California

Structure and properties of Co-based thin films and multilayers with perpendicular magnetic anisotropy


The effects of growth temperature, TG, and ion-irradiation on the magnetic properties and microstructure of Co/Pt multilayers with perpendicular anisotropy were investigated. The columnar-grain size and the <111> texture were found to increase with an increase in TG. Up to a critical temperature, Tcrit, a monotonic increase in coercivity, HC, with TG was measured, followed by a decrease in coercivity above Tcrit. The magnetic domains of films grown below Tcrit were irregular and paisley-like and their sub-micron size decreased with increasing TG. Above Tcrit, sub-100 nm domains were found. A decrease in HC and domain size was measured when multilayers were exposed to ion-irradiation. The ion-induced changes in the magnetic properties are associated with a decrease in interfacial anisotropy, KS. Transmission electron microscopy, the ion-irradiation simulations and the calculations of interface anisotropy found the ion-induced changes of magnetic properties not to be caused by the large microstructural changes but to be associated with localized ion-beam induced atomic disordering at the Co/Pt interfaces. At high enough doses, KS becomes comparable to the shape anisotropy and a transition to in-plane magnetization results. The principles of ion-modification were applied to pattern the magnetic properties of the Co/Pt multilayers down to below 100 nm periodic arrays by irradiation through a stencil mask or direct focused ion beam writing. The magnetic domain confinement and changes in the magnetic reversal processes due to the ion-irradiation patterning were studied for two types of magnetic arrays: (a) in-plane magnetized dots surrounded by out-of plane matrix and (b) of out-of-plane bits surrounded by in-plane lines. Two complementary magnetic imaging techniques were utilized: Lorentz transmission electron microscopy sensitive to in-plane magnetization and magnetic transmission x-ray microscopy sensitive to perpendicular magnetization. The boundaries of the patterns, defined by the transition from out-of-plane to in-plane magnetization, were determined by the irradiation pattern. Softening of the in-plane regions (HC < 300 Oe) as compared to the out-of-plane coercivity was found. The perpendicular reversals were found to always originate at the pattern boundaries at substantially reduced nucleation fields and to be influenced by the microstructure. Bit patterns larger than ~100 nm islands were found to be multi-domain and below 100 nm, single-domain bits were recorded.C-axis oriented CoCr films were grown and the magnetic domain structure was investigated as a function of sample thickness and temperature. A transition thickness from large in-plane domains to up-down stripe domains was found to increase with temperature.

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