2nd CHIRALTEM workshop

3rd CHIRALTEM workshop

NEW: get together at DPG conference


- Vienna - Regensburg -

- Dresden - Prag - Trieste -




Preparation and characterisation of magnetic specimens for CHIRALTEM purposes

- In the context of the CHIRALTEM project, we prepare and investigate magnetic specimen which are expected to be suitable for the measurement of a circular dichroic-like interaction between the electron beam and the specimen in a TEM (transmission electron microscope).
In the moment, we work on cobalt monocrystals with a very high saturation induction of about Ms=1.8 T and on iron garnet films with only approximately 150 mT of saturation induction. Both specimens show a perpendicular anisotropy, due to which the magnetisation in the specimen is oriented mainly out of plane.


Lorentz electron micrograph of a thin YIG film with perpendicular field of 0.05T
- Due to the low saturation field, the magnetisation of the garnet specimen will always be saturated in the magnetic field of the regular objective lens of the TEM when operated in High Magnifiaction modes, i.e. above approx. 5 kX. if the current through the objective lens is reversed this causes also a reversal of the magnetic field of the lens, followed by switching of the magnetization direction. This inturn would lead to a measurable change in the dichroic effect. Below a picture of a garnet film in a magnetic field of only 0.05T can be seen. The stripe domains are already breaking up into magnetic bubbles. If the magnetic field gets higher, the specimen will be magnetised completely out of plane along the external field direction.

dynamic of garnet domains: download movie (14MB)


- In contrast to that, the high saturation field of a cobalt monocrystal might make it possible to observe magnetic domains under certain conditions, although the objective lens is activated. A small shift of the electron beam from one domain to another would then change the magnetisation seen by the electrons and should also lead to an easily measurable dichroic effect.
Lorentz electron micrograph of a thin cobalt film without external field

LLG-simulation of cobalt domains
- Micromagnetic simulations will supplement our experiments. Here an LLG-simulation
of a thin cobalt film (50nm) is shown. The dimension is 1.28µm x 1.28µm . Red and blue colors indicate the magnetisation pointing into and out of the specimen plane. The yellow arrows indicate the in-plane component of the magnetisation in a plane located at a depth z/2 in the specimen, where z is the thickness of the thin specimen.. Obviously, there are large neighbouring areas of alternatingly up and down oriented magnetic induction, which will allow the change from one magnetic orientation within the specimen by simply shifting the beam from one area to another without changing other parameters such as specimen thickness, lens excitation, specimen height etc. which might influence the measurement.



- For further investigations, we construct a pulsed-field TEM specimen holder which will enable us to apply magnetic fields of more than 0.5T for short times (ms). It might also be used for a variation of the magnetic field perpendicular to the specimen if used with permanent current. Its main purpose, however, is tocause a remanent change of magnetic domains if used with pulsed currents. A sketch of this specimen holder is shown below.

Pulsed-field specimen holder for TEM