The effect of interface roughness on the perpendicular exchange bias of NiO/CoPt/Pt stacking structure
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e effect of interface roughness on the perpendicular exchange bias of NiO/CoPt/Pt stacking structure GAO Ying, HARUMOTO Takashi, NAKAMURA Yoshio & SHI Ji
*
Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8550, Japan Received May 17, 2020; accepted July 31, 2020; published online November 25, 2020
An orthogonal coupling structure of NiO/CoPt/Pt/glass with perpendicular exchange bias effect has been prepared. The exchange bias is found to be strongly dependent on the interface roughness between NiO and CoPt layer. The conventional inverse proportionality of the exchange bias field with the ferromagnetic thickness is not applicable to this top-pinned stacking structure. Moreover, an anomalous ferromagnetic thickness dependence of blocking temperature is also observed. A simulation on the basis of a spin configuration model with defects at the interface agrees well with the experimental observations. These findings suggest considerable contribution of the interface roughness to the perpendicular exchange bias. perpendicular exchange bias, interface roughness, FM thickness, blocking temperature Citation:
Gao Y, Harumoto T, Nakamura Y, et al. The effect of interface roughness on the perpendicular exchange bias of NiO/CoPt/Pt stacking structure. Sci China Tech Sci, 2020, 63, https://doi.org/10.1007/s11431-020-1700-9
1 Introduction When a ferromagnet (FM) is in contact with an antiferromagnet (AFM), the interaction at the interface between them leads to the well-known exchange bias (EB) effect. The FM hysteresis loop is shifted in the field axis and the magnitude of this shift is defined as exchange bias field (Heb). Since its discovery in 1956 [1], extrinsic control of EB has received considerable attention for various spintronic applications such as hard drive read heads [2], magnetic tunnel junction [3], magnetic sensors [4], and spintronic devices [5–8]. To clarify the mechanism and to control the magnitude of EB, factors such as FM, AFM layer thickness, spin configuration, interface roughness, and grain size, have been intensively investigated [9–12]. In realistic situation, the interface could not be perfectly flat. This issue has been treated in the literatures [13,14] and the imperfect interface is often considered as an important factor governing the EB phenomenon as pointed out by Malozemoff in his random interface field *Corresponding author (email: [email protected])
model: Random interface roughness gives rise to a random magnetic field that acts on the interface spins, yielding unidirectional anisotropy [15]. Spray and Nowak [16] theoretically estimated the relationship between the magnitude of Heb and the interface roughness by using Monte Carlo simulation. Whereas, how much the Heb is determined by the interface roughness still experimentally remains a puzzle, especially in an orthogonal coupling system. On the other hand, theoretical models propose that Heb is inversely proportional to the FM layer thickness, tFM: J Heb = FM/A
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