Chemistry Reference
In-Depth Information
Fig. 4.29 Schematic 2D representation of the microstructure of crystalline and grain boundaries
in nanostructured fluoride powder [
165
]
respectively). Such a disagreement partially originates from the structural disorder
of the grain boundaries which can differ from that of the amorphous phases and the
symmetry breaking through the presence of neighbouring crystalline grains.
Indeed, the polarization of the grain boundaries by the crystalline grain also
enhances their magnetic ordering temperature. (3) At high temperature, the grain
boundaries behave as a paramagnet and the single domain grains possess thus a
superparamagnetic behaviour. Indeed, one expects a progressive magnetic
decoupling between grains as effective as the thickness of the grain boundaries is
large (exceeding the antiferromagnetic length), leading to an assembly of non-
interacting or weak-interacting antiferromagnetic particles. The distributions of
both the grain size and of the thickness of grain boundaries give rise to blocking
temperature ranged from 200 up to 360 K (corresponding to the Néel temperature
of r-FeF
3
), that is confirmed by static magnetic measurements [
166
].
Some temperature dependent exchange bias effect was evidenced from the non
symmetrical hysteresis loops on some nanostructured ferric fluoride powder
recorded after zero field cooling, suggesting the existence of a strong magnetic
coupling between grain boundaries and crystalline grains up to the paramagnetic
state of grain boundaries.
A computer modelling based on a modified annealing/Metropolis atomistic
algorithm was successfully achieved to describe the structural and magnetic
properties of the nanostructured ferric fluoride, particularly the topology within the
grain boundaries. Long-range interatomic potentials were first adjusted from
experimental results of crystalline phases and atomic structures of twisted and
tilted GBs were then established as a function of the relative disorientation of the
grains without applying any periodic boundary conditions. This approach takes
into account the structure of the grains far from the interface in order to constrain
