Ferromagnetic (Ga,Mn)As Layers and ... - OPUS Würzburg

76 5. Local Anisotropy Control by Strain Engineering
6 0 0
W id th a t H a lf H e ig h t (m T )
4 0 0
2 0 0
0
-1 0 1 2 3 4 5 6
S tra in e x
(1 0 -3 )
Figure 5.10: Width of the opening at half height of the parabolic low field part of the
magnetoresistance scans over average strain in x-direction. All samples of Tbl. 5.1 with
[100]-aligned stripes are shown. The red line is a linear interpolation.
anisotropy field, with increasing average strain. There are several factors which explain
the scattering of the data points around the red linear interpolation line. As mentioned
in the previous chapter, all simulations from which the strain value is calculated consider
ideal stripes. This means that inhomogeneous etching, which may cause rough
or slanted sidewalls and therefore regions of inhomogeneous strain are not taken into
account. Also, the physical dimensions (stripe width, etch depth) are expected to
differ slightly between simulation and processed stripes, causing another uncertainty
in the strain value. Furthermore we will show in annealing experiments detailed in
the following section, that the carrier density has an impact on the strength of the
anisotropy. Since the Mn content of the stripe samples in Fig. 5.10 varies between
2.5–5.4%, differences in carrier concentration are also accountable for the scattering
of the data points. But, even taking all aforementioned uncertainties into account,
we still observe a very clear dependence of the anisotropy field on the lattice strain,
which identifies it as the driving force in determining the configuration of the magnetic
anisotropy in (Ga,Mn)As nanostructures.
[1¯10] Stripes
The situation for [1¯10] oriented stripes is notably different from what we observe for
the [100] stripes. Fig. 5.11 (a) shows magnetotransport measurements on sample I. As
with the [100] stripes, the external field sweeps are performed in the interval from B ‖
J (φ = 0 ◦ ) to B ⊥ J (φ = 90 ◦ ). In contrast to the [100] stripe measurements in Fig. 5.9,
the lowest curve at φ = 0 ◦ is not the flattest curve of the ensemble. The curvature of a
magnetotransport measurement is caused by the rotation of the magnetization to the