As and Epitaxial-Growth MnSi Thin Films - OPUS Würzburg

8.1. Magnetotransport Measurements in MnSi epitaxial thin films 107
% Magnetoresistance
17.9 nm with c-Si cap
0.8
J || B(90 o )
B(0 o __
) || MnSi[211]
0.6
0.4
0.2
0.0
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
Magnetic Field (T)
% Magnetoresistance
0.2
0.1
0.0
-0.1
-0.2
17.1 nm with c-Si cap
_
J || B(-90 o ) || MnSi[211]
B(0 o _
) || MnSi[011]
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
Magnetic Field(T)
Fig. 8.6: Longitudinal magnetoresistance measurements for 17.1 and 17.9nm-nm thick MnSi
home-grown films, presented here as points of comparison for the 20-nm measurement. The
percent change in magnetoresistance is calculated from the zero-field resistance value and both
samples show very low magnetoresistance which is positive at very low fields. The current for
both films are along MnSi[¯211] (B(0 ◦ ) ‖MnSi[01¯1]).
From the magnetoresistance polar plots of both layers, it is pretty difficult to conclude
the strength of the uniaxial anisotropy with the lack of sharp magnetization switches for
materials such as (Ga,Mn)As. We might attribute this to the demagnetization corrections
from the shape anisotropy of the Hall bar.[Ahar 98] Indeed, sample shape has been
studied and observed to reduce and shift the transition fields. [Baue 12]
Also as a test for the possibility of in-plane transport contributions of an out-of-plane
field, magnetic fields of 0 to 200 mT was applied along MnSi[111] while varying the
in-plane magnetic field. The plots are shown in Figure 8.8. For helical magnets, an applied
in-plane field distorts the helix into a helicoidal structure until the magnetization
structure breaks into isolated domains at saturation.[Karh 12] For our measurements, we
do not reach this critical value H D in our applied fields. For an out-of-plane field, the
helical structure is pulled into a conical state until the applied field is high enough to
reach an induced ferromagnetic state. From measurements done on MnSi thin films, the
helical-conical transition magnetic field is estimated to be around 180 mT for fields applied
along MnSi[111].[Karh 11] Below a certain field, the helical structure re-orients to
the easy axis.[Plum 81]
Comparing the polar plots for the 0 mT and 200mT applied fields, it seems that there is a
change in the low-field fluctuations at an out-of-plane applied field near 200 mT. (Figure
8.8) However, as with Figure 8.7, this difference could possibly just arise from the demagnetization
effects. This is supported by the plots with other out-of-plane magnetic field
strengths where the observed evolution of the magnetic properties appear to be random