Towards an Unambiguous Electron Magnetic Chiral Dichroism (Emcd) Measurement in a Transmission Electron Microscope (Tem)

by Christian Hurm

The intention of the ChiralTEM project (2004-2007) was the detection of electron (energy loss) magnetic chiral dichroism in a transmission electron microscope (TEM), in analogy to X-ray magnetic circular dichroism (XMCD). For the experiments, single-crystal electron transparent specimen's with magnetic induction perpendicular to the specimen's plane are required. In this thesis, different preparation techniques are evaluated regarding their usability to produce ideal specimen's for a verification of EMCD. After the demonstration of a dichroic measurement, an obvious way to prove the chiral effect is to invert the specimen's magnetization from parallel to antiparallel to the electron beam trajectory, leaving all other parameters of the experimental setup unchanged. For this case, one would - according to theory - expect a change in the dichroic signal measured. In the magnetic field of the objective lens of a 300kV TEM, typical ferromagnetic specimens will be close to saturation perpendicular to the specimen's plane. Reversing the current through the coils of the objective lens will then simply invert the magnetization of the specimen.In consequence, any magnetic chiral effect is expected to change sign. A switching unit for Tecnai microscopes has been constructed for save commutation of the lens currents. The direct sensitivity of the dichroic signal to the direction of the magnetization gives evidence to the magnetic origin of the effect.