Researchers from the IBM Almaden Research Center, the ALS, Stanford University, and Arizona State University have used the new PEEM2 photoemission electron microscope at the ALS together with the technique of x-ray magnetic linear dichroism (XMLD) spectromicroscopy to overcome this obstacle. In tests with films of nickel oxide, they recorded the first images by any technique showing unambiguous antiferromagnetic contrast on a thin-film surface.

The images of nickel oxide they obtained exhibited bright lines with typical widths from 400 to 2000 nm (left), indicating that the average antiferromagnetism in the stripes was different from that in the dark background. With a spatial resolution of 50 nm, however, the PEEM2 could not resolve individual antiferromagnetic domains, which were estimated to be smaller than this. In one of several additional tests the researchers conducted to confirm that the image contrast was due to the antiferromagnetic structure of the sample, the visually observable contrast between the stripes and the background disappeared (right) as the temperature of the samples approached the Néel temperature, above which antiferromagnetism disappears.

Advancing Magnetic Data Storage

Atoms are like tiny magnets. Antiferromagnetic magnetic materials have the peculiar property that the atomic magnets are aligned along an axis, but they can point in either direction along the axis, so that the material as a whole does not appear to be magnetic. Nonetheless, antiferromagnets play a key role in the operation of advanced magnetic data storage and memory devices composed of several layers, each as thin as a few atoms. In addition to the antiferromagnetic layer, some of the layers are magnetic and some are not magnetic. To understand how these devices work in detail, with an eye to improving their performance or to devising new devices, scientists want to probe each layer separately as well as the boundaries between the layers. Unfortunately, because their net magnetism is zero, antiferromagnets are hard to study with traditional tools. In the experiments described here, the researchers used a type of x-ray microscopy to distinguish for the first time areas with different antiferromagnetic behavior, with high spatial resolution and sensitivity, thereby paving the way for future experiments on these important materials.

Research conducted by J. Stöhr and J. Lüning (IBM Almaden Research Center); A. Scholl, S. Anders, and H. A. Padmore (ALS); T. J. Regan, and R. L. White (Stanford University); and M. R. Scheinfein (Arizona State University.

Funding: U. S. Department of Energy; Office of Basic Energy Sciences; Stanford University Center for Materials Research; and the International Disk Drive Equipment and Materials Association.

Publication about this experiment: J. Stöhr et al, Phys. Rev. Lett. 83, 1862 (1999).

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