|Direct Imaging of Antiferromagnetic Vortex States|
Magnetic materials are characterized by the ordering of electron spins, with nearest-neighbor spins parallel to each other in ferromagnetic (FM) materials and antiparallel to each other in antiferromagnetic (AFM) materials. As the size of a magnetic system is reduced to micron scale, it has been shown that the spins in an FM microstructure can curl around to form a magnetic vortex state. While there has been intensive activity in the study of vortex states in FM disks, there has been no direct observation of such states in an AFM microstructure, although theory predicts many interesting and unique properties for the AFM vortex state. Recently, a research team from Berkeley, Korea, and China has taken the first direct image of an AFM vortex in multilayered magnetic disk structures using x-ray magnetic linear dichroism (XMLD) and photoemission electron microscopy (PEEM) at ALS Beamlines 4.0.2 and 11.0.1 , respectively. The experiments observed two types of AFM vortices, one of which has no analogue in FM vortices.
In an FM microstructure, the magnetic vortex state is formed by spatially constraining a thin film enough so that the spins will curl around the center of the structure in order to minimize the magnetic charges at the microstructure's boundary. In an AFM material, however, the magnetic vortex state cannot be formed in this way because the antiparallel arrangement of spins in the microstructure diminishes the net magnetic charge. To create an AFM vortex, the researchers' idea was to imprint an FM vortex into an AFM layer through interfacial magnetic interaction in an FM/AFM bilayer microstructure. Observation of the AFM vortex state requires the application of the element-specific XMLD technique on single-crystalline AFM microstructures. To meet this requirement, high-quality single-crystalline NiO/Fe and CoO/Fe bilayers were deposited onto a Ag(001) surface by molecular-beam epitaxy (MBE) and patterned using a focused-ion beam (FIB) before measurement at ALS Beamlines 4.0.2 (XMLD) and 11.0.1 (PEEM-3).
While the FM Fe disks exhibited the expected FM vortices for both circular and square disks, the AFM NiO and CoO disks also revealed unambiguously the existence of the AFM vortex state. The most interesting observation was that, in addition to the curling vortex structure in thinner NiO and CoO films, where the AFM spins were coupled collinearly with the Fe spins, there also exists a divergent AFM vortex structure in thicker NiO and CoO films, where the AFM spins are coupled perpendicularly to the Fe spins. This type of divergent vortex is never allowed in FM microstructures because it would result in a net magnetic charge at the disk boundary.
The results further show the interplay between the FM vortex, AFM vortex, and the magnetic field generated by the surrounding Fe film. When the surrounding Fe film formed a single domain, generating a magnetic field of around 32 Oe, the cores of the FM and AFM vortices were shifted away from the center of the disk. When the surrounding Fe area was broken into two domains with opposite orientations (reducing the associated magnetic field to zero), the vortex core positions remained virtually unchanged due to the pinning of the AFM vortex. Only after the sample temperature was raised above the CoO Néel temperature, as evidenced by the disappearance of the CoO vortex domain, did the Fe vortex core position move toward the center of the disk.
To further explore the vortex effect on exchange bias, we would need to measure the FM vortex core position systematically as a function of applied field. Applying an alternating-current demagnetization field or a magnetic pulse of different strength could help to realize this. Another direction could explore imprinting other types of vortex structures (for example, antivortices and vortex lattices) from the FM layer into the AFM layer.
Research conducted by J. Wu, D. Carlton, J.S. Park, J. Bokor, and Z.Q. Qiu (University of Califoria, Berkeley); Y. Meng (UC Berkeley and Institute of Physics, Chinese Academy of Science); E. Arenholz, A. Doran, A.T. Young, and A. Scholl (ALS); C. Hwang (Korea Research Institute of Standards and Science); and H.W. Zhao (Institute of Physics, Chinese Academy of Science).
Research funding: National Science Foundation, U.S. Department of Energy, Korea Foundation for International Cooperation of Science and Technology, the Chinese Education Department, and the Western Institute of Nanoelectronics. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.
Publication about this research: J. Wu, D. Carlton, J.S. Park, Y. Meng, E. Arenholz, A. Doran, A.T. Young, A. Scholl, C. Hwang, H.W. Zhao, J. Bokor, and Z.Q. Qiu, "Direct observation of imprinted antiferromagnetic vortex states in CoO/Fe/Ag(001) discs," Nat. Phys. 7, 303 (2011).
ALS Science Highlight #235