Nanomagnetic Imaging

The microscopic arrangement of the magnetic structure within a thin metal film plays a fundamental role in many technological applications ranging from information storage in computer hard drives to anti-lock brake sensors in automobiles.  NIST researchers have developed a technique, called scanning electron microscopy with polarization analysis (SEMPA), that can reveal the magnetic structure of such materials with nanoscale resolution and without disturbing the magnetization.  The technology is useful for understanding and developing a wide range of electronics systems that rely on magnetic properties in addition to electric charge to process and store information. 

Scanning electron microscopy is widely used to examine the surfaces of materials used in computers and other electronic devices. Detecting how a beam of electrons scatters off the surface gives a topographical picture of its peaks and valleys. In addition, measurement of electrons released from the surface by the incoming beam reveals information about the characteristics of electrons within the material's atomic structure. And for a ferromagnetic material such as iron, scanning electron microscopy can map out surface magnetism.

A ferromagnetic material, such as iron, becomes permanently magnetized when its electrons all align in the same direction. Electrons behave like little magnets, with alignment corresponding to the orientation of their spins. When a beam of electrons interacts with a magnetic surface, the spins of the electrons emitted from the surface reflect the surface's magnetic orientation.

Project researchers have developed a method to detect the spins of these ejected electrons from which they can create a magnetic map of the surface with different colors corresponding to different magnetic directions. In other magnetic imaging techniques, such as magnetic force microscopy, a tiny magnetic probe interacts with the surface, potentially altering its magnetization. The technique, called scanning electron microscopy with polarization analysis (SEMPA), does not disrupt the magnetic properties of the surface being measured.

Ferromagnetic surfaces and thin films with increasingly small-scale structures are important for building smaller circuits and storing data at higher densities. SEMPA can examine films just a few atoms thick at a resolution of ten nanometers. As a diagnostic tool, it allows industry researchers to get detailed magnetic maps of surfaces, a capability that aids in device design and improvement.

More generally, use of SEMPA to better understand magnetic properties at the nanoscale has a wide range of applications. One example being actively investigated by NIST researchers is magnetic random access memory (MRAM) for computers, which uses electronic spin to store and read data. SEMPA is helping NIST researchers measure the magnetic properties of oxides and other emerging magnetic materials, and to probe how magnetic interactions within materials differ on the nanoscale compared to the macroscale.

SEMPA is also contributing to a growing field called spintronics, which uses electron spins rather than charge to encode the 1s and 0s of binary data. With SEMPA, NIST researchers have provided a versatile tool that can refine today’s technology and will help develop future devices that make use of magnetic properties.

• Optimization of spin-triplet supercurrent in ferromagnetic Josephson junctions, C. Klose, T. S. Khaire, Y. Wang, W. P. Pratt, N. O. Birge, B. J. McMorran, T. P. Ginley, J. A. Borchers, B. J. Kirby, B. B. Maranville, and J. Unguris, Physical Review Letters 108, 127002 (2012).
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• Electron vortex beams with high quanta of orbital angular momentum, B. J. McMorran, A. Agrawal, I. M. Anderson, A. A. Herzing, H. J. Lezec, J. J. McClelland, and J. Unguris, Science 331, 192 -195 (2011).
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• Direct imaging of current-driven domain walls in ferromagnetic nanostripes, W. C. Uhlig, M. J. Donahue, D. T. Pierce, and J. Unguris, Journal of Applied Physics 105, (2009).
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• SEMPA imaging for spintronics applications, J. Unguris, S. H. Chung, and D. T. Pierce, in Characterization and Metrology for Nanoelectronics: 2007 International Conference on Frontiers of Characterization and Metrology (2007), p. 472-476.
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• Exchange coupling in magnetic multilayers grown on iron whiskers, J. Unguris, R. Celotta, D. Tulchinsky, and D. Pierce, Journal of Magnetism and Magnetic Materials 198–199, 396–401 (1999).
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