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Direct Imaging of Antiferromagnetic Vortex States Print

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.

A New Spin
on Magnetic Vortices

When you constrain the spins in a magnetic material to approach two dimensions, as in a thin film, the spins will tend to lie flat. When you further constrain the material to approach even lower dimensions, as in a magnetic "dot" of micron or submicron scale, the spins will form vortices with the spins in the core pointing out of the plane, either "up" or "down." This phenomenon, first observed a decade ago, has intrigued scientists as a path to a possible breakthrough in how we store and manipulate binary information in today's digital world.

But the path from initial discovery to workable model is long and branching, with many detailed questions that need to be answered. For example: How do you switch the "polarity" of a vortex core from up to downWhat is the mechanism by which this switching occursWhat is the effect of small defects introduced in production? In this work, Wu et al. directly observe, for the first time, vortex states in antiferromagnetic (AFM) materials and how they relate to underlying ferromagnetic (FM) layers. The coupling that occurs in AFM/FM bilayer systems is critical to maintaining and manipulating spin-encoded information and is therefore a key aspect of magnetic data-storage applications.

Sample geometry

The samples under study consisted of single-crystalline NiO or CoO (AFM) thin films grown on top of a 12-nm Fe (FM) layer, all on a silver substrate. The layers in the central disc experience a magnetic field produced by the surrounding Fe film. This localized field has limited effect on photemitted electrons, enabling PEEM studies of the effects of an "external" magnetic field on the vortex state. Top: Scanning electron microscope image. Bottom: Line profile and schematic drawing.

 

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.

Curling and divergent vortex structures

Element-specific XMLD images of the NiO and CoO magnetic domains exhibit curling vortices at a film thickness (d) of 0.6 nm (left) and divergent vortices at a thickness of 3.0 nm (right). The divergent vortex is forbidden in an FM disk.

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.

Effects of external magnetic field

Magnetic domain images of a 2-μm-diameter CoO(3-nm)/Fe(12-nm)/Ag(001) disk under different conditions. (a) The local magnetic field produced by the Fe film surrounding the disk (H ~ 32 Oe) shifted the vortex core position from the disk center. (b) After the application of an external 50-Oe magnetic-field pulse, the Fe film changed from a single-domain state to a multidomain state, and the local magnetic field produced by the Fe film went from ~32 Oe to ~0 Oe. The Fe vortex core remained in its off-center position, showing the exchange-bias effect of the CoO vortex. (c) After the sample was warmed to 300 K (above the CoO Néel temperature), the exchange-bias effect vanished, and the Fe vortex core moved back to the center.

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.

Photos of researchers

Some members of the researcher team: J. Wu, D. Carlton, E. Arenholz, J. Bokor, C. Hwang, Z.Q. Qiu, A. Scholl, and H.W. Zhao.

 


 

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

 

ALSNews Vol. 324