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摘要
The present study shows that antiferromagnetic spin and twin domain walls govern the hysteretic expressions of exchange anisotropy at low and high fields, respectively, using annealed NiO single crystals and Co. In the presence of twin walls, spin walls are shown to be a geometrical necessity in the antiferromagnetic NiO. A threshold field (similar to 10 000 Oe) exists below which twin walls are frozen, and rotational hysteresis is dominated by losses due to spin walls. Above the threshold field, twin walls become mobile, resulting in a sharp increase in rotational hysteresis. Remarkably, rotational hysteresis associated with spin walls is similar to that of an ordinary ferromagnet-as the field strength increases, rotational hysteresis tends toward zero. However, unlike an ordinary ferromagnet where rotational hysteresis becomes zero above its saturation field, rotational hysteresis in antiferromagnet drops but then sharply increases once the threshold field for twin wall motion is exceeded. In crystals without spin walls, low-field rotational hysteresis is zero or negligible. Domain imaging of twin walls in antiferromagnet and Weiss walls in ferromagnet reveals a one-to-one spatial correlation even though twin walls are considered to have no net dipoles. This surprising result is explained by the fact that crystallographic interfaces in real crystals are not atomically sharp or ideal, and the defective interface invariably results in net moment across the finite width of the twin wall. The field dependence of domain walls in Co film exchange coupled to NiO shows global similarities to previously reported behavior of Co films deposited on nanocrystalline NiO [H. D. Chopra, D. X. Yang, P. J. Chen, H. J. Brown, L. J. Swartzendruber, and W. F. Egelhoff, Jr., Phys. Rev. B 61, 15312 (2000)]. In both cases, domain wall motion is not the dominant mode of magnetization reversal (wall motion is entirely absent in the present study while wall motion was only occasionally observed in the previous study), reversal occurs abruptly and different regions switch at different fields, and the observed pinning sites are frozen in the applied field range. Despite vastly different underlying microstructure of NiO in these two studies (annealed NiO single crystal with a fixed orientation across the NiO-Co interface in the present study versus nanocrystalline NiO in the previous study), the observed similarities may be understood by recognizing that, just as crystallographically imperfect twin interfaces serve as pinning sites for walls in Co film, other defects can play a similar role. Practically, results provide means of tuning rotational hysteresis by varying the type and the density of domain walls in NiO and by tuning the saturation field of the ferromagnet relative to the threshold field for the twin wall motion.This work was supported by the National Science Foundation Grants No. DMR-0706074 and No. DMR-0305242, and this support is gratefully acknowledged.
- 出版日期2009-9
