MnAs is an original magnetic seminconductor as its ferromagnetic order disappears close to room temperature and presents a structural transition at the same time. When deposited as a thin film, peculiar magnetic configurations arise linked to the cristal stricture. In a transmission electron microscope, magnetic measurements can be done for each grain using a recent technique : the electron magnetic chiral dichroism.
Numerous studies were related to the disappearance of the ferromagnetic phase of MnAs accompanied by a structural transition close to room temperature. As a crystal, this material presnets easy, intermediate and hard magnetization axis whose signatures are studied by measuring macroscopic hysteresis cycles. The local measurement of magnetic moments by EMCD (electron magnetic chiral dichroism) in a transmission electron microscope can quantify the magnetocrystalline anisotropy in those directions.
The EMCD technique can be compared to the XMCD (X-ray Magnetic Circular Dichroism) technique proposed in 1988 to measure the magnetic moments with synchrotron radiation. During the interaction with the sample, the incident beam (electron or X-rays) transfers energy to the material enabling transitions from core to excited energy levels. The exploration of the empty electronic band provides access to crystallographic information on the chemical bonds but also on the filling of bands and thus on magnetism. X-ray absorption spectra or energy loss of electrons (EELS) spectra in a TEM are recorded to determine this information. The spatial resolution available in a TEM allows EMCD to locally probe the magnetic moments.
The structural transition of MnAs was studied by diffraction in-situ in the TEM with a heating sample object for several grains of the film. In parallel, EMCD spectra were recorded on these same grains proving that the structural transition is accompanied by a disappearance of the ferromagnetic order.
Different MnAs lamella were then prepared in cross sections according to the 3 directions corresponding to the easy, intermediate and hard axis. EMCD measurements along these 3 directions have shown, for the first time with this technique, the variation of magnetic moments all the 3 axes.
This project was supported by the ANR EMMa project (ANR12 BS10 013 01).
X. Fu et al., “In Situ Observation of Ferromagnetic Order Breaking in MnAs/GaAs(001) and Magnetocrystalline Anisotropy of Alpha-MnAs by Electron Magnetic Chiral Dichroism,” Physical Review B 93, no. 10 (March 11, 2016): 104410, doi:10.1103/PhysRevB.93.104410.