Magnetic field-free STEM delivered
Atomic-resolution ADF STEM images of non-magnetic GaN (left) and magnetic iron-silicon in a magnetic field-free environment. Scale bars are 0.5 nm and 0.2 nm respectively. [Shibata et al, Nature Communications 2019]
A newly developed magnetic objective lens system from researchers at the University of Tokyo and JEOL provides a magnetic-free environment for materials during electron microscopy.
By combining the lens system with a higher order aberration corrector, Professor Naoya Shibata and colleagues have performed direct, atomic-resolved imaging with sub-angstrom spatial resolution of magnetic silicon steels.
Imaging now takes place with residual magnetic fields of less than 0.2 mT at the sample position compared to the tesla-sized magnetic fields produced during conventional electron microscopy.
The Magnetic-field-free Atomic-Resolution STEM (“MARS”)
“To the best of our knowledge, this is the first time that such a goal has been achieved,” says Shibata. “This [system] is expected to be extensively used for the research and development of advanced magnetic materials.”
A critical disadvantage of today's magnetic condenser-objective-lens systems for atomic-resolution TEMs and STEMs is that samples must be inserted into very high magnetic fields of up to 2 to 3 T to realise the short focus length condition essential for atomic-resolution imaging.
Such magnetic fields can severely hamper atomic-resolution imaging of many important soft and hard magnetic materials as the field can alter, or even destroy, a sample's magnetic and physical structure.
With this in mind, Shibata and colleagues have developed a magnetic-field-free objective-lens combined with a state-of-the-art aberration corrector to simultaneously realise atomic resolution electron microscopy and a magnetic field-free sample environment.
The base electron microscope is a commercially available cold field emission type 200 kV STEM; the ARM-200CF, JEOL.
The lens system contains two round lenses positioned in an exact mirror-symmetric configuration with respect to the sample plane.
a Cross section of a conventional magnetic objective lens. Samples (purple) are placed between the upper and lower polepieces (gold). b Schematic shows the z-component magnetic field (Bz) distribution across the upper and lower polepieces when the magnetic objective lens system is excited. c Cross section of the new magnetic field-free objective lens system. d Schematic shows the Bz distribution across this new objective lens. [Shibata et al, Nature Communications 10, Article number: 2308 (2019)]
The front objective lens is located in front of the specimen and the back objective lens is located behind the specimen, with respect to the incoming electron beam.
While the magnetic polepieces and coils of the lenses are in an exact mirror symmetric configuration with respect to the sample plane, the polarities of excitations are opposite, giving an anti-symmetric magnetic field distribution across the sample plane.
As a result, the the z-component of the magnetic fields of the objective lenses can be cancelled out at the sample plane.
And thanks to the symmetry of these round lenses, the radial component of the magnetic field is minimal near the optic axis.
As the researchers explain in Nature Communications, the new lens system should realise extremely small residual magnetic fields at the sample position, while placing the strongly excited front and back-objective lenses close enough to the sample to give the short focus length condition necessary for atomic-resolution imaging.
Using the new system, Shibata and colleagues have studied the atomic structure of a grain-oriented silicon-steel sheet, a key magnetic engineering materials used in electric transformers and motors.
Crucially, the system can also be operated in the same manner as that of conventional TEMs/STEMs.
Research is published in Nature Communications.