| The exchange-bias effect provides unidirectional anisotropy to ferromagnetic thin-films in magnetoresistance based sensors. It has been applied widely in spintronic devices such as spin valve, magnetic tunnel junction, and read and write heads of computer hard disk, has become the focus of international research.The use of thin films with perpendicular exchange-bias(P-EB) effect could make the magnetoresistance device smaller and more stable. The key point to improve the performance of magnetoresistance devices is to have controls of the magnetic coupling and the directions of magnetic moments in magnetic multilayers. So far,the in-plane EB effect in FM/AFM interface is weak because it relies on uncompensated coupling across a ferromagnet/antiferromagnet interface. So it is hard to meet the technical requirements of modern high-density information storage. Therefore, the investigations on the magnetic coupling of Fe(Co)/Tb multilayers with high perpendicular magnetic anisotropy(PMA) and the effect on the magnetization reversal behavior of Fe(Co) layers are very important to extend the applications of Fe(Co)/Tb multilayers.Series of Fe/Tb and Fe Co/Tb multilayers were prepared by using magnetron sputtering. The micro-magnetic structure and magnetic properties of the films were characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM), atomic force microscopy and magnetic force microscopy(AFM/MFM), and vibrating sample magnetometer(VSM). The effects of the sputtering power, sub layer thickness, buffer layer, aging and annealing on the structure and magnetic properties of the films were studied systematically. Based on this work, a series of [Fe Co/Tb]NHard/Ta(t)/[Fe Co/Tb]NSoft multilayer samples were prepared. The effect of the spacer thickness of Ta sub layer on the PMA and P-EB of the films were investgated. The main conclusions are presented as follows:1. The structure and magnetic properties of GL/[Fe/Tb]5/Al multilayersSeries of GL/[Fe/Tb]5/Al multilayers were prepared at PAr = 0.1 Pa. The thickness of the Tb sub layer(t Tb) was varied from 0.6 nm to 2.0 nm, and the thickness of the Fe sub layer(t Fe) was varied from 0.4 nm to 1.8 nm. When the thickness of Fe layer in the GL/[Tb(0.8 nm)/Fe(t nm)]5/Al(5 nm) multilayer is 0.8 nm and 1.2 nm respectively, the magnetic moments of Fe and Tb are antiferromagnetically coupled, leading to saturationmagnetization(Ms) closing to zero. It indicates that within such a Fe thickness range, the Tb/Fe multilayer behaves like a single magnetic layer, a Tb Fe alloy film, the Fe and Tb moments are compensated. The effective magnetic anisotropy energy is less than zero when t Fe < 0.8 nm or t Fe > 1.2 nm, implies that the magnetization of the multilayer has an in-plane dominated easy axis. The saturation magnetization of the GL/[Tb(t nm)/Fe(1.4 nm)]5/Al(5 nm) multilayer increases significantly after annealing at Ta = 190 ℃ for t = 90 s.2. The structure and magnetic properties of GL/Ta/Tb/[Fe Co/Tb]5/Ta multilayers(1) The effects of Tb sputtering power(PTb) on PMA: Firstly, we study the influence of Tb sputtering power(PTb = 16, 13.5 w) on the perpendicular magnetic anisotropy of the GL/Ta(10 nm)/Tb(4 nm)/[Fe45Co55(t nm)/Tb(1.3 nm)]5/Ta multilayers while maintaining the sputtering power of Fe a constant of PFe = 26 w. The results show that higher sputtering power can improve interfacial PMA of Fe Co/Tb multilayer for t = 0.9 nm, the lower sputtering power can improve interfacial PMA of Fe Co/Tb multilayer when t = 1.9 nm.(2) The effect of sub layer thickness: When the thickness of Tb sub layer is 3 nm, the effective magnetic anisotropy energy of GL/Ta(10 nm)/Tb(4 nm)/[Fe45Co55(t nm)/Tb(3 nm)]5/Ta(10 nm)(t =1-2.2) multilayers decreases gradually with the increase of the thickness of Fe Co layer due to the increase of the in-plane demagnetization energy. When the thickness of Fe Co sub layer is 1.2 nm, the effective magnetic anisotropy energy of GL/Ta(10 nm)/Tb(4 nm)/[Fe45Co55(1.2 nm)/Tb(t nm)]5/Ta(10 nm)(t = 0.5- 3.0) multilayers increases gradually with increasing of Fe Co layer thickness, and reaches the saturation after t ≥ 1 nm.(3) Aging effect : The aging effect(4 months) has strong influences on the magnetic properties of the GL/Ta(10 nm)/Tb(4 nm)/[Fe Co(1.2 nm)/Tb(t)]5/Ta(10 nm) multilayers with t = 0.7, 1.3, 2, 2.4 nm, respectively. When t = 0.7 nm, the aging results in increasing of Ms and decreasing of the saturation magnetic field applied in the plane of the film. In contrast, when t Tb ≥ 1.3 nm, the aging leads to decrease of Ms due to interface diffusion.3. P-EB effect of the GL/Ta/[Tb/FeCo]5Hard/Ta(t)/[FeCo/Tb]5Soft/Ta multilayersWhen the thickness of the spacer layer(Tb or Ta) between [FeCo(1.4 nm)/Tb(1.4 nm)]5Soft soft magnetic layer and [Fe Co(1.2 nm)/Tb(3 nm)]5Hard hard magnetic layer is thin(e.g. t Tb =1.4 nm or t Ta = 0.5, 1.0 nm), the GL/Ta/[ Fe Co(1.2 nm)/Tb(3 nm)]5Hard/Ta(t)/[ Fe Co(1.4 nm)/Tb(1.4 nm)]5Soft/Ta multilayers exhibit exchange-spring coupling. The exchange-spring coupling reduces when the spacer is thick(e.g. t Tb =3 nm or t Ta = 2, 3 nm). It was also shows that the coercivity of the the sample with hard magneticlayer(HL)(GL/Ta/SL/Ta/HL/Ta) on the top part of the films is visibly greatter than that of the sample with the HL in the bottom part of the film. The perpendicular exchange bias(P-EB) effects of GL/Ta(10 nm)/[Tb(1.4 nm)/Fe Co(1.4 nm)]5/Ta(t nm)/[Fe Co(1.2 nm)/Tb(3.0 nm)]5/ Ta(10 nm),(t = 0, 1, 2, 3) multilayers are observed. It is found that the P-EB field is related to the magnitude and the orientation of the applied magnetic field. The value of P-EB field is low( HE ~ 100 Oe) when the spacer layer is thin(t = 1 nm); while the value of the P-EB field is high( HE ~ 198 Oe) when the spacer layer is thick(t = 2, 3 nm). |
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