Nanomagnetic Imaging

A computer hard drive stores information by means of the magnetic alignment of microscopic bits that stud its surface. NIST researchers have developed a modified form of scanning electron microscopy that can reveal the magnetic characteristics of such a surface without disturbing the magnetization. The technology is useful for understanding and building a wide range of computer memory devices as well as other electronics systems that rely on magnetic properties rather than electric charge.

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.