  Magnetic nanoparticles are important building blocks for future technologies
ranging from nano-medicine to spintronics. Many related applications require
nanoparticles with tailored magnetic properties. However, despite significant
efforts undertaken towards this goal, a broad and poorly-understood dispersion
of magnetic properties is reported, even within mono-disperse samples of the
canonical ferromagnetic 3d transition metals. We address this issue by
investigating the magnetism of a large number of size- and shape-selected,
individual nanoparticles of Fe, Co, and Ni using a unique set of complementary
characterization techniques. At room temperature only superparamagnetic
behavior is observed in our experiments for all Ni nanoparticles within the
investigated sizes, which range from 8 to 20 nm. However, Fe and Co
nanoparticles can exist in two distinct magnetic states at any size in this
range: (i) a superparamagnetic state as expected from the bulk and surface
anisotropies known for the respective materials and as observed for Ni; and
(ii) a state with unexpected stable magnetization at room temperature. This
striking state is assigned to significant modifications of the magnetic
properties arising from metastable lattice defects in the core of the
nanoparticles as concluded by calculations and atomic structural
characterization. Also related with the structural defects, we find that the
magnetic state of Fe and Co nanoparticles can be tuned by thermal treatment
enabling one to tailor their magnetic properties for applications. This work
demonstrates the importance of complementary single particle investigations for
a better understanding of nanoparticle magnetism and for full exploration of
their potential for applications.
