  Isolated magnetic skyrmions are stable, topologically protected spin textures
that are at the forefront of research interests today due to their potential
applications in information technology. A distinct class of skyrmion hosts are
rare earth - transition metal (RE-TM) ferrimagnetic materials. To date, the
nature and the control of basic traits of skyrmions in these materials are not
fully understood. We show that for an archetypal ferrimagnetic material
DyCo$_3$ that exhibits a strong perpendicular anisotropy, the ferrimagnetic
skyrmion size can be tuned by an external magnetic field. Moreover, by taking
advantage of the high spatial resolution of scanning transmission X-ray
microscopy (STXM) and utilizing a large x-ray magnetic linear dichroism (XMLD)
contrast that occurs naturally at the RE resonant edges, we resolve the nature
of the magnetic domain walls of ferrimagnetic skyrmions. We demonstrate that
through this method one can easily discriminate between Bloch and N\'eel type
domain walls for each individual skyrmion. For all isolated ferrimagnetic
skyrmions, we observe that the domain walls are of N\'eel-type. This key
information is corroborated with results of micromagnetic simulations and
allows us to conclude on the nature of the Dzyaloshinskii-Moriya interaction
(DMI) which concurs to the stabilisation of skyrmions in this ferrimagnetic
system. Establishing that an intrinsic DMI occurs in RE-TM materials will also
be beneficial towards a deeper understanding of chiral spin texture control in
ferrimagnetic materials.
