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Standing Waves Probe Nanowedge Interfaces

Structures with nanometer-scale dimensions are ever more important in science and technology. Integrated circuits are the most familiar example, but nanostructures of a different type are also commercialized in magnetic read heads for high-density data storage and may soon appear in magnetic memory chips. With the increased importance in such nanostructures of atoms residing at buried solid­solid interfaces, characterizing buried interfaces becomes a crucial step in understanding mechanisms and developing new devices based on these state-of-the-art materials. For example, new methods to nondestructively study buried interfaces would help to clarify the nature of both the giant magnetoresistance effect and exchange biasing, two key phenomena that make magnetic nanostructures useful. To address this problem, a group led by researchers from Berkeley Lab and the University of California, Davis, has now combined the technique of generating standing waves of circularly polarized soft x rays at ALS Beamline 4.0.2 with the growth of wedge-shaped samples. In particular, the researchers demonstrated the ability to map both composition and magnetization across an iron­chromium interface by means of core-level photoelectron spectroscopy, magnetic circular dichroism, and parallel mathematical modeling.

In order to probe buried interfaces, the Berkeley­Davis group first realized that soft x-ray standing waves generated via Bragg reflection from a multilayer mirror should make it possible to spectroscopically study buried interfaces nondestructively, provided that there is a way to vary the position of the standing-wave intensity maximum around the interface. And they also knew that wedge-shaped samples can be used to investigate the thickness dependence of many types of phenomena. In their combined approach, the group grew the sample to be studied on top of the mirror in a wedge shape, and then simply by translating the sample horizontally in front of a focused x-ray beam, the standing wave could be scanned vertically through the sample.

wedge-shaped sample

Wedge-shaped sample. Scanning the sample in the direction of the wedge moves the intensity maximum of the standing wave from one side of the iron–chromium interface to the other.

What's Going On in There?

In order to see, whether with your eye or with a scientific instrument, there has to be a way to illuminate what it is you want to observe. For scientists studying so-called magnetic nanostructures comprising multiple layers of magnetic and nonmagnetic materials each only a few atoms thick, the problem is particularly challenging because most of the action takes place out of sight in the comparatively few atoms that reside at or near the boundaries (interfaces) between the layers.

Yang et al. have attacked this problem for the interface between iron and chromium by creating a so-called x-ray standing wave that penetrates vertically through the sample so that the maximum x-ray intensity, which will generate most of the signal to be measured, occurs at a particular depth below the surface. By scanning a focused x-ray beam across the surface of a wedge-shaped sample in which the position of the interfaces changes with thickness, they were able to map the changes in chemical and magnetic behavior at and around the interface. Since magnetic nanostructures are the foundation of modern magnetic data storage and memory devices, this capability should find wide use. Researchers in other areas of nanostructure science may also find it valuable.

In the experiments, strong standing waves with a period of 4.0 nm and an approximately 3:1 ratio between the maximum and minimum intensity were created from a synthetic multilayer mirror fabricated at the Center for X-Ray Optics consisting of 40 periods of B4C and tungsten: [B4C/W]40. A wedge-shaped bilayer of iron (1.6 nm) and chromium (variable thickness) was grown on top of the multilayer. By analyzing various core-level photoelectron intensities as a function of both x-ray incidence angle and beam position, the researchers could derive layer thicknesses and measure the interface mixing/roughness due to migration of atoms across the interface to form a mixture of iron and chromium.

Magnetic circular dichroism in photoemission from the 2p and 3p levels of iron and chromium resulted in identification of regions with decreased (increased) ferromagnetic alignment for iron (chromium) and derivation of the positions and widths of these regions. The magnetically altered regions in both metals were only 1­2 atomic layers in thickness. From these results, the group concluded that (1) normally antiferromagnetic chromium becomes ferromagnetic just below the center of the interface but with antiparallel alignment with respect to iron, and (2) the equal-concentration region in the center of the interface strongly inhibits magnetic alignment for both species along the direction of net magnetization that was probed (also the direction of light incidence). Spectra from the 3s levels of iron and chromium further indicated that the local spin moments on both atoms do not change on crossing the interface.

 

core-level photoemission
Analysis of the core-level photoemission and magnetic circular dichroism as the standing wave moves through the interface yields composition and magnetization profiles across the interface, including a region of intermixed iron and chromium in which the atomic magnetic moments change orientation.

The investigators expect that the standing-wave-plus-wedge method will not be limited to magnetic nanolayers but should apply equally well to the characterization of other types of nanostructures and their interfaces. Expanding the signal detected to include soft x-ray fluorescence, valence-band photoemission, or the spin of the photoelectrons will also extend the range of applications.

Research conducted by S.-H. Yang, J.B. Kortright, J. Underwood, and F. Salmassi (Berkeley Lab); B.S. Mun, N. Mannella, and M.A. Van Hove (Berkeley Lab and University of California, Davis); S.-K. Kim (Seoul National University, Korea); E. Arenholz, A. Young, and Z. Hussain (ALS); and C.S. Fadley (Berkeley Lab, University of California, Davis, and University of Hawaii).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES) and Korea Science and Engineering Foundation. Operation of the ALS is supported by BES.

Publication about this research: S.-H. Yang et al., "Probing buried interfaces with soft x-ray standing wave spectroscopy: application to the Fe/Cr interface," J. Phys.: Condens. Matter 14, L407 (2002).

ALSNews Vol. 203, July 17, 2002

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