Researchers have found that the spins in an antiferromagnet perform a version of “The Twist,” turning one way and then the other, challenging a model that has been a cornerstone of exchange-bias theory for 27 years. Exchange bias is the mechanism that couples the magnetization of an antiferromagnetic (AFM) layer to that of an adjacent ferromagnetic (FM) layer. Discovered over five decades ago, it plays a key role in magnetic device technology but is still not fully understood because of the complexity that emerges from many interacting parts. In 1987, Mauri et al. proposed a popular model stating that, in the strong FM/AFM coupling limit, reversing the FM magnetization by 180 degrees should twist the AFM spins into a domain wall with a 180-degree turn.
At ALS Beamline 4.0.2, researchers used x-ray magnetic linear dichroism (XMLD) to measure the spin structure in FM-AFM-FM nanolayers. They were surprised to discover that the AFM spins twisted in one direction (right-handed chirality) as the magnetization of the top FM layer rotated from 0 to 90 degrees, then switched to the opposite chirality (left-handed chirality) at 90 degrees before unwinding from 90 to 180 degrees. This clarified a critical issue that had long puzzled scientists: why the same FM/AFM interfacial coupling can lead to a strong uniaxial magnetic anisotropy but very weak exchange bias. The results rule out Mauri’s exchange-bias mechanism for perpendicular FM/AFM coupling, and future theoretical models will need to find new pinning mechanisms for the exchange bias other than Mauri’s 180-degree domain-wall mechanism.
Work performed on ALS Beamline 4.0.2.
Citation: J. Li, A. Tan, S. Ma, R.F. Yang, E. Arenholz, C. Hwang, and Z.Q. Qiu, “Chirality Switching and Winding or Unwinding of the Antiferromagnetic NiO Domain Walls in Fe/NiO/Fe/CoO/Ag(001),” Phys. Rev. Lett. 113, 147207 (2014).