236
A. Kleibert
While these findings contribute to a better understanding of the controversial
reports on the magnetic anisotropy of 3d transition metal nanoparticles available
in the literature, the actual microscopic origin of the enhanced magnetic anisotropy
energy in iron, cobalt and iron–cobalt-alloy nanoparticles has yet to be clarified. The
fact that the SEM and AFM characterization yield no discernible differences between
magnetically blocked and superparamagnetic nanoparticles, strongly suggests that
the origin of the enhanced magnetic energy barriers lies in the microstructure of the
nanoparticles. Since the iron and cobalt nanoparticles in [5, 38] were only found in
the well known bcc and fcc lattices, the enhanced magnetic energy barriers might
be related to structural defects, such as dislocations or stacking faults, which could
arise from the growth kinetics, for instance [31]. Dislocations give rise to local strain
and therefore to magneto-elastic contributions to the total magnetic anisotropy. For
iron and cobalt, the magnetic energy barriers due to dislocations can significantly
exceed those arising from the respective magneto-crystalline anisotropy, and might
therefore contribute to the enhanced magnetic energy barriers as discussed in more
detail in [5]. Moreover, in the case of bcc iron, dislocations are highly mobile and
might be ejected from the finite volume of iron nanoparticles upon thermal excitation.
Such mechanism could explain the observed transitions from magnetically blocked
to superparamagnetic states in individual iron nanoparticles. In the case of fcc cobalt
nanoparticles, stacking faults are frequently observed, which can give rise to local
hcp stacks within the nanoparticles [5, 67]. Based on the properties of bulk cobalt,
hcp stacking could give rise to sizable uniaxial magnetic anisotropy contributions
and lead to the observed magnetically blocked states at room temperature. Moreover,
local hcp stacking may even increase the cohesion energy of nanoparticles and might
therefore even be promoted by thermal annealing, which could explain the observed
transition from superparamagnetic to magnetically blocked states in individual cobalt
nanoparticles.
To achieve a direct correlation between microstructure and magnetic anisotropy
energy requires one to combine magnetic characterization with structural characterization with atomic resolution of the very same nanoparticle. Such correlation
can be achieved for instance by combining XPEEM with atomic resolution TEM.
First successful experiments on cobalt nanoparticles show the feasibility of such
approach, and hold the promise that the nature and orientation of the crystal lattice
and defects together with the orientation of the magnetization of the nanoparticles
can be obtained. Such results are expected to provide an unprecedented benchmark
for the development of new theoretical models for nanoparticle magnetism that take
both the impact of surface properties and structural defects on the magnetic properties into account. Experimentally, such approach will allow one to unambiguously
address the impact of the many different structural motifs found in nanoparticles
on their magnetic properties. Such motifs include multiply twinned structures, various truncated polyhedral forms, and the unique crystallographic phases, that can
be found at the nanoscale, such as the –phase in cobalt or the hcp phase in nickel
nanoparticles [40–43]. Such knowledge, will significantly improve our understanding of magnetism at the nanoscale and will enable one to achieve an increased control
over the magnetic properties of nanoparticles. Finally, we note that magnetic charac-
A. Kleibert
While these findings contribute to a better understanding of the controversial
reports on the magnetic anisotropy of 3d transition metal nanoparticles available
in the literature, the actual microscopic origin of the enhanced magnetic anisotropy
energy in iron, cobalt and iron–cobalt-alloy nanoparticles has yet to be clarified. The
fact that the SEM and AFM characterization yield no discernible differences between
magnetically blocked and superparamagnetic nanoparticles, strongly suggests that
the origin of the enhanced magnetic energy barriers lies in the microstructure of the
nanoparticles. Since the iron and cobalt nanoparticles in [5, 38] were only found in
the well known bcc and fcc lattices, the enhanced magnetic energy barriers might
be related to structural defects, such as dislocations or stacking faults, which could
arise from the growth kinetics, for instance [31]. Dislocations give rise to local strain
and therefore to magneto-elastic contributions to the total magnetic anisotropy. For
iron and cobalt, the magnetic energy barriers due to dislocations can significantly
exceed those arising from the respective magneto-crystalline anisotropy, and might
therefore contribute to the enhanced magnetic energy barriers as discussed in more
detail in [5]. Moreover, in the case of bcc iron, dislocations are highly mobile and
might be ejected from the finite volume of iron nanoparticles upon thermal excitation.
Such mechanism could explain the observed transitions from magnetically blocked
to superparamagnetic states in individual iron nanoparticles. In the case of fcc cobalt
nanoparticles, stacking faults are frequently observed, which can give rise to local
hcp stacks within the nanoparticles [5, 67]. Based on the properties of bulk cobalt,
hcp stacking could give rise to sizable uniaxial magnetic anisotropy contributions
and lead to the observed magnetically blocked states at room temperature. Moreover,
local hcp stacking may even increase the cohesion energy of nanoparticles and might
therefore even be promoted by thermal annealing, which could explain the observed
transition from superparamagnetic to magnetically blocked states in individual cobalt
nanoparticles.
To achieve a direct correlation between microstructure and magnetic anisotropy
energy requires one to combine magnetic characterization with structural characterization with atomic resolution of the very same nanoparticle. Such correlation
can be achieved for instance by combining XPEEM with atomic resolution TEM.
First successful experiments on cobalt nanoparticles show the feasibility of such
approach, and hold the promise that the nature and orientation of the crystal lattice
and defects together with the orientation of the magnetization of the nanoparticles
can be obtained. Such results are expected to provide an unprecedented benchmark
for the development of new theoretical models for nanoparticle magnetism that take
both the impact of surface properties and structural defects on the magnetic properties into account. Experimentally, such approach will allow one to unambiguously
address the impact of the many different structural motifs found in nanoparticles
on their magnetic properties. Such motifs include multiply twinned structures, various truncated polyhedral forms, and the unique crystallographic phases, that can
be found at the nanoscale, such as the –phase in cobalt or the hcp phase in nickel
nanoparticles [40–43]. Such knowledge, will significantly improve our understanding of magnetism at the nanoscale and will enable one to achieve an increased control
over the magnetic properties of nanoparticles. Finally, we note that magnetic charac-