Magnetometry and magnetoresistance measurements in MnSi thin films and
rigorous analytical solutions of the micromagnetic equations show that the
field-induced unwinding of confined helicoids occurs via discrete steps. A
comparison between the magnetometry data and theoretical results shows that
finite size effects confine the wavelength and lead to a quantization of the
number of turns in the helicoid. We demonstrate a prototypical spintronic
device where the magnetic field can push or pull individual turns into a
magnetic spring that can be read by electrical means
In epitaxial MnSi/Si(111) films, the in-plane magnetization saturation is
never reached due to the formation of specific surface chiral modulations with
the propagation direction perpendicular to the film surfaces [Wilson et al.
Phys. Rev. B 88, 214420 (2013)]. In this paper we show that the occurrence of
such chiral surface twists is a general effect attributed to all bulk and con-
fined magnetic crystals lacking inversion symmetry. We present experimental
investigations of this phenomenon in nanolayers of MnSi/Si(111) supported by
detailed theoretical analysis within the standard phenomenological model. In
magnetic nanolayers with intrinsic or induced chirality, such surface induced
instabilities become sizeable effects and play a crucial role in the formation
of skyrmion lattices and other nontrivial chiral modulations
We present Hall effect measurements on MnSi/Si(111) epilayers and find an anomalous Hall contribution that is significantly smaller than in bulk crystals, which enables the observation of an additional contribution to the anomalous signal previously overlooked in MnSi. Our measurements indicate the signal is not due to skyrmions in MnSi thin films, which are absent in out-of-plane fields, but rather are the result of scattering from the cone phase. The absence of magnetic contrast in the transmission electron microscopy (TEM) measurements are consistent with this interpretation. We provide a method to model TEM images of skyrmions lattices to determine the conditions necessary for their observation in other B20 epilayers with an anisotropy that is favourable to their formation.
The magnetic structure of the in-plane skyrmions in epitaxial MnSi/Si(111) thin films is probed in three dimensions by the combination of polarized neutron reflectometry (PNR) and small angle neutron scattering (SANS). We demonstrate that skyrmions exist in a region of the phase diagram above at temperature of 10 K. PNR shows the skyrmions are confined to the middle of the film due to the potential well formed by the surface twists. However, SANS shows that there is considerable disorder within the plane indicating that the magnetic structure is a 2D skyrmion glass.
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