Study of perpendicular exchange bias in CoPt/IrMn bilayer system
Date Issued
2014
Date
2014
Author(s)
Tsai, Chau-Yi
Abstract
This present work focus on applying single-layered Co49Pt51 alloy thin films with perpendicular magnetic anisotropy(PMA) to a perpendicular exchange bias (PEB) system. Co49Pt51 alloy thin films were fabricated at absolutely room temperature (RT),i.e. without heating up substrates during whole fabricating processes. The PEB system was composed of Co49Pt51/IrMn bilayers and also fabricated at absolutely RT.
In the first part, we demonstrate that the rotational sputtering technique benefits crystal structure of antiferromagnetic layer in longitudinal exchange bias system and thereby enhances the EB value by Ta/NiFe/FeMn EB system. And the system is optimized based on this technique with the highest EB value of 350 Oe. In the second part, Co49Pt51 alloy films were fabricated on glass substrates at room temperature (RT) with significant PMA using a combination of Ta and Pt underlayers (RT-CoPt). Their properties were compared with those produced by conventional high temperature route (HT-CoPt). The XRD patterns are similar but slightly peak shifts suggest interdiffusion may exists in HT-CoPt samples. The microstructural studies revealed extremely smooth surface with root mean square roughness (Rrms) of 0.5 nm for the RT-CoPt films; while the HT-CoPt film showed wavy surface with Rrms = 2.4 nm. The out-of-plane and in-plane hysteresis measurements displayed the existence of strong PMA for both the RT-CoPt and HT-CoPt films. The squarness ratio (SQR) and magneto-crystalline anisotropy constant (Ku) are similar between two cases but coercivities and switching behavior are quite different.
In the third part, perpendicular exchange bias (PEB) effect in the as-grown Co49Pt51/IrMn bilayers was demonstrated at room temperature using single-layered Co49Pt51 alloy thin film as ferromagnetic (FM) layer. Several unusual features were observed in this system, viz.,: i) the PEB was spontaneously established without any artificial magnetic treatments; ii) single-shifted loops were obtained rather than doubleshifted ones, and iii) the spontaneous PEB effect was accompanied by a reduction in perpendicular coercivity, Hc. Training effect studies indicate that the PEB effect is stable in this system. Significant reduction in the PEB effect was found for the CoPt/IrMn films either grown under inducing field or subjected to post field-annealing. The thickness dependence of PEB effect on the FM and AFM layers were also investigated and the largest PEB value of 533 Oe was obtained for the sample grown with 3-nm CoPt and 10-nm IrMn layers.
In the fourth part, in continuation with the existence of perpendicular exchange bias (PEB) in Co49Pt51/IrMn bilayers in absence of external inducing field during deposition, the symmetry-breaking of PEB behavior in these films with respect to the inducing field direction was investigated. Not only the loop shift direction altered by reversing treating field, but different PEB values were observed in samples grown and annealed with field with direction toward film growth direction (here we define this direction as positive direction, PF) and the opposite (here we define this direction as negative direction, NF). The dissimilar PEB values obtained with different inducing field directions clearly suggest the existence of symmetry-breaking in the Co49Pt51/IrMn PEB system. The mechanism of manipulating loop shifting directions and symmetry-breaking in the system are investigated and discussed in the viewpoint of crystal and moment structures.
Finally, FeMn was introduced as antiferromagnetic (AFM) layer to couple with single-layered Co49Pt51 alloy thin film and compared with Co49Pt51/IrMn bilayer system in order to further explore the mechanism of spontaneous perpendicular exchange bias (PEB) we recently found in CoPt/IrMn bilayers. Bilayers of CoPt/IrMn and CoPt/FeMn were prepared under same conditions by sputtering at RT without any inducing field. Although PEB phenomenon was observed in as-grown CoPt/FeMn bilayers as well, the loop shape and PEB behavior were obviously different. Comparing to CoPt (5 nm)/IrMn (10 nm) system, CoPt (5 nm)/FeMn (10 nm) bilayer shows a sheared loop similar to double-shifted loop and much lower SQR (0.52) and He (180 Oe). In addition, two systems reveal entirely different dependence of PEB on AFM layer thickness. In CoPt/FeMn case, the dependence is more complicated with an unusual peak when AFM layer is 10 nm. By taking account of the dissimilar loop shapes and distinct dependence of PEB on AFM thickness, the mechanism of spontaneously-established PEB in these two systems are considered to be different. Investigations of interfacial microstructures through cross-sectional transmitting electron microscopy reveal no obvious difference between two systems. Only in scanning transmitting electron microscopy image, the interface of AFM/FM in CoPt/FeMn was observed relatively rougher than in CoPt/IrMn. Therefore, the difference in PEB behavior of these two systems are considered to not be mainly caused by difference in interfacial microstructures, but the interfacial magnetic structures. The results provide a further understanding on the mechanism of spontaneous PEB.
Subjects
薄膜
濺鍍
垂直異向性
垂直交換偏壓
鈷鉑合金
銥錳
鐵錳
Type
thesis
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