On high-resolution transmission electron microscopy in an unstable magnetic environment.

Owing to the particular environment of the JEM4000EX HREM at the University of Pennsylvania, it became necessary to control the magnetic environment around the microscope. The unstable magnetic environment included two components, a slowly wandering vertical DC magnetic field and an AC magnetic field. The effects of these fields on the microscope performance were to introduce an uncertainty in the objective lens defocus value over a series of images and to reduce the attainable resolution of the microscope, respectively. A solution is presented in which these fields are stably reduced well within the limits of sensitivity of the JEOL, Ltd. JEM4000EX for high-resolution imaging. This solution is based on a feedback loop using a pseudo-Helmholtz coil to generate a well-defined vertical magnetic field.