  The ability to tune the competition between the in-plane and out-of-plane
orientation of magnetization provides a means to construct thermal sensors with
a sharp spin re-orientation transition at specific temperatures. We have
observed such a tuneable, temperature driven spin re-orientation in
structurally amorphous, ferrimagnetic rare earth-transition metal (RE-TM) alloy
thin films using scanning transmission X-ray microscopy (STXM) and magnetic
measurements. The nature of the spin re-orientation transition in FeGd can be
fully explained by a non-equilibrium, nanoscale modulation of the chemical
composition of the films. This modulation leads to a magnetic domain pattern of
nanoscale speckles superimposed on a background of in-plane domains that form
Laudau configurations in micron-scale patterned elements. It is this speckle
magnetic structure that gives rise to a sharp two step-reversal mechanism that
is temperature dependent. The possibility to balance competing anisotropies
through the temperature opens opportunities to create and manipulate
topological spin textures.
