In this work we present the results of differential phase contrast (DPC) [1,2] measurements of micro magnetic field distributions in annealed cobalt thin films. The polycrystalline specimens with a thickness of about 50 nm were produced by thermal boat evaporation under high vacuum conditions. After annealing the average size of individual cobalt crystals is increased and their magnetic properties are changed as shown by G. Herzer in [3] and [4]. Our goal was to determine, if DPC is a suitable technique to investigate how these changes in the crystallographic structure effect the magnetic properties of our specimen. Therefore we performed single DPC measurements as well as DPC tilting series to investigate if and how the magnetic behaviour of the annealed cobalt films is changed.

We investigated 5 different samples. One being the untempered thin film and the others were annealed over one hour with increasing temperatures ranging from 520 K to 820 K. To investigate the magnetic properties we performed a DPC tilt series for each sample. By tilting the specimen relative to the magnetic field of the objective lens we change the effective external in plane field on our samples. This leads to a change of the in plane induction in our cobalt thin films. We started each tilting series at large angles of α=20°, to be sure that all magnetic moments in our specimen are aligned in one direction. Figure 1 shows a DPC measurement of such a saturated state. The homogeneous green colour of the measurement confirms that the magnetic induction is saturated along the direction indicated by the colourwheel. Figures 2-4 show an excerpt of DPC measurements performed during the tilt series of the specimen annealed at 850 K. It can be seen, that the magnetic structure changes with decreasing α and it seems, that one of the magnetic ripples is pinned to the highlighted crystallites in the center of figures 2 and 3.

By calculating the average direction and strength of the beam deflection for each individual DPC measurement we get information about the dependency between the magnetic induction and the tilting angle respectively the external magnetic field applied. We will present the results of our measurements on the different annealed specimen showing hysteretic behaviour of the magnetic induction and discuss the differences between them.

During our experiments especially the tilt series, which take about 4 hours to obtain, we encountered some difficulties with the DPC technique. In this work we will present these shortcomings and show solutions to some of the problems occured.

[1] J. N. Chapman, The Investigation of Magnetic Domain-structures In Thin Foils By Electron-microscopy, J. Phys. D: Appl. Phys. 17, 623-647 (1984)
[2] H. Rose, Phase Contrast in Scanning Transmission Electron Microscopy, Optik 39, 4, 416-436, (1974)
[3] G. Herzer et al., Grain structure and magnetism of nanocrystalline ferromagnets,IEEE Transactions on Magnetics 25, 3327-3329, (1989)
[4] G. Herzer et al., Grain size dependence of coercivity and permeability in nanocrystalline ferromagnets,IEEE Transactions on Magnetics 26, 1397-1402, (1990)

Figures:

Figure 1: Saturation of the in plane induction at large tilting angle (20 degrees) indicated by the nearly homogeneous green color of the DPC image.

Figure 2: Pinning of a magnetic ripple during a tilting series at 12 degrees on the highlighted crystallites.

Figure 3: Pinning of a magnetic ripple at 12.5 degrees. Although the external magnetic field is increased pinning of the ripple is still visible.

Figure 4: No more pinning at 12.8 degrees on the in figures 2 and 3 highlighted crystallites.

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EMC Abstracts - https://emc-proceedings.com/abstract/dpc-measurements-on-annealed-cobalt-thin-films/