As and Epitaxial-Growth MnSi Thin Films - OPUS Würzburg

98 7. Magnetotransport with H ‖ [111]
temperature. We see broadening features monotonic in field between 10 K and 45 K.
We note no similarly broad change in the temperature regions for the magnetoresistance
measurements (Figure 7.20 except for the point near T c . We also note possible changes
brought about by the thermal process. While there are observed changes in the zero-field
cooling measurements after field heating, they are not monotonic in field, which we could
attribute to differences in the temperature and measurement sweep.(Figure 7.21)
7.2 Summary
• Transport measurements with field along the Si[111]/MnSi[111] hard axis for the grown
films show good correspondence with observations of the Hall effect for both bulk and
MnSi thin films. Symmetrization ofthe measured signal with respect to themagnetic field
is done similar to [Ritz 13a] resulting in a single magnetic field dependence. The humplike
features in the extracted anti-symmetric Hall signal is consistent with results for bulk
MnSi in [Ritz 13a] at low applied pressure. However, we cannot fully claim this because
of the hysteretic behavior of the magnetoresistance, which introduces some uncertainties
into the extracted parameters and the nature of the features in the magnetotransport
signal.
Using the formula from [Li 13], we extract the anomalous, topological and ordinary Hall
parameters for both the 20 and 12 nm test films grown. For both samples, the values
of the ordinary Hall coefficient R 0 for both films are consistent with the values used by
[Neub 09a] for calculating the effective topological magnetic field and spin polarization
P at the A-phase. However, lower values have been measured at lower temperatures by
[Lee 09] and [Neub 09a]. This increase in the value of R 0 may possibly attributed to the
strain in the films, even at small induced uniaxial anisotropy, reducing the carrier concetration
through a warping of the bandstructure.
• While the extracted values for the thicker layer has a value for the coefficient for intrinsic
scattering β that is consistent with the results of [Lee 09], we measure a much higher
value for the thinner layer. One possible reason for this is the increasing importance of
surface effects in thinner films.[Gerb 02] Another possible explanation would probably be
due to the topological nature of the anomalous Hall term itself. An ehanced net chirality
in the thinner film might result in a higher anomalous signal.[Tata 02, Tagu 09] This
can probably be supported by the higher RRR for the thinner film, which is higher than
[Li 13] where they confirmed the crystallinity of their samples with TEM.
• The magnitude of the calculated topological Hall Hall contributions for both films, all
done at 4K, fall within the small value 24 nΩ·cm, which is in the same order as the
values measured by [Li 13] at low temperatures (≈ -7.8 nΩ·cm) and for [Neub 09b] near