Investigations On The Magnetic Coupling In Co/Pt(Pd) Multilayers

The use of Co/Pt(Pd) multilayers with perpendicular anisotropy could make the magnetoresistance device smaller and more stable. The key point of the applications of magnetoresistance devices which based on the GMR and TMR is to have controls of the magnetic coupling and the directions of magnetic moments in magnetic multilayers. So far, the couplings between Co layers in Co/Pt(Pd) multilayers are ferromagnetic(FM) coupling, so it is hard to control the directions of magnetic moment in Co layers and the applications are limited. Therefore, investigations on the magnetic coupling between Co layers, magnetostatic interaction in Co/Pt(Pd) multilayers and the effect on the magnetization reversal of Co layer are very important to extend the applications of Co/Pt(Pd) multilayers.Many methods were used in my works to investigate the controls of the magnetic coupling and the directions of magnetic moments in Co/Pt(Pd) multilayers, such as adjusting the configuration of the multilayers, using nano CoO or NiO capping layer instead of Pt protecting layer and preparing magnetic multilayers with MgO spacer.We had discovered a new effect named time-dependent effect in Co/Pd multilayers. After exposure of Co/Pd multilayers in air, their coercivity and saturation magnetization demonstrated a drastic time-dependent decrease. The time-dependent effect had been observed to be strongly related to the Co layer thickness and the repetition number n of [Co/Pd]n multilayer. It can be determined that the time-dependent effect is related to exposure of the sample to the air. Most possibly, it could be attributed to the hydrogen-induced effect within the Co interlayer.By adjusting the thickness of the middle Pd spacer and reducing the repetition number, we observed antiferromagnetic(AF) coupling in Pd/Co(3A)/Pd(54A)/Co(3A)/Pd multilayer. When increased the thickness of Co layers, we found the coupling between Co layers were FM coupling firstly which turned to AF coupling with time. The observed time-dependent FM-AF transition is attributed to absorption of gaseous elements during exposure of the sample to the air, which can induce the reduction of Pd-polarization. By the same ways, AF coupling were observed in Pt/Co/Pt/Co/Pt multilayers. Moreover, it had been found that interlayer exchange coupling in Co/Pt multilayers oscillates between AF and FM coupling as a function of Pt spacer thickness, and the period of oscillation is almost 2 A.The coupling in Co/Pt multilayer turned from FM to AF coupling by the present of CoO or NiO capping layer. We could control the coupling and direction of magnetic moment of the Co layers by adjusting the thickness of CoO capping layer. After field-cooling in 1 kOe, the magnetization reversal is quite asymmetric along the field decreasing and increasing branches of the loops below the blocking temperature of 220 K. It is related to the presence of the nano CoO capping layer and its thermal instability. Indirect biasing has been observed to be induced in the bottom Co layer.[Co/Pt]_n multilayer was used as FM layers, and MgO as the spacer layer. A series of sandwichlike magnetic multilayers were prepared as [Pt(6A)/Co(4A)]₄/MgO(tMgOA)/ [Co(4A)/Pt(6A)]₂. We have observed AF interlayer coupling when the spacer thickness is larger than 12 A, and a transition to FM coupling for lower MgO thickness. The thermal behavior of interlayer coupling shows a FM-AF transition at 145K. It can be mainly ascribed to the temperature-dependent enhancement of the magnetization of the FM layers.