234
A. Kleibert
(E s = 1.8 J/m
2 ) have the lowest free surface energy, and interact therefore only
weakly with the nanoparticles when the contact between the nanoparticle and the
substrate is established. The higher free surface energy of NiO(110) (E s ∼ 3.0 J/m
2 )
and W(110) (E s = 3.5 J/m
2 ) lead to a stronger interaction and to a relaxation of
the high anisotropy state towards the superparamagnetic state upon impact of the
nanoparticle on the substrate.
9.3.4 Comparison with Cobalt, Nickel and Iron–Cobalt-Alloy
Nanoparticles
Finally, we address the question of whether strongly enhanced magnetic energy
barriers and metastable states are only observed in iron nanoparticles or whether
comparable behavior is found in other 3d transition metal nanoparticles as well.
For this purpose fcc cobalt and fcc nickel nanoparticles were deposited on silicon
substrates as in the case of the iron nanoparticles [5]. Surprisingly, for cobalt the
same size-independent coexistence of superparamagnetic and magnetically blocked
nanoparticles was found as observed for iron, see Fig. 9.6. Also in the case of fcc
cobalt, the observation of magnetically blocked states at room temperature show
the presence of significantly enhanced magnetic energy barriers when compared
to the fcc bulk magneto-crystalline anisotropy and surface anisotropy contributions
as discussed in detail in [5]. However, in contrast to iron nanoparticles, thermal
annealing promotes transitions from superparamagnetic states into a magnetically
blocked state in a large number of nanoparticles. In contrast to iron and cobalt, only
superparamagnetic states were found in fcc nickel nanoparticles at room temperature [5]. Also Fe 50 Co 50 alloy nanoparticles deposited on silicon substrates under
similar conditions as the iron, cobalt, and nickel nanoparticles exhibit a coexistence
of superparamagnetic and magnetically blocked nanoparticles at room temperature
[66]. These findings demonstrate that the coexistence of states with distinct magnetic
properties is not restricted to iron nanoparticles, but seems a more general phenomena
among pure and alloyed 3d transition metal nanoparticles.
9.4 Conclusions and Perspectives
In this chapter we have demonstrated that XPEEM is a powerful tool for magnetic and chemical characterization of individual 3d transition metal nanoparticles
under both UHV conditions and under reactive gas environments for in situ study of
chemical reactions. Moreover, by combining XPEEM investigations with complementary structural characterization such as SEM and AFM, one is able to correlate
directly magnetism, shape, and size of a large number of the very same nanoparticles
and probe the actual distribution of magnetic properties in extended ensembles and
A. Kleibert
(E s = 1.8 J/m
2 ) have the lowest free surface energy, and interact therefore only
weakly with the nanoparticles when the contact between the nanoparticle and the
substrate is established. The higher free surface energy of NiO(110) (E s ∼ 3.0 J/m
2 )
and W(110) (E s = 3.5 J/m
2 ) lead to a stronger interaction and to a relaxation of
the high anisotropy state towards the superparamagnetic state upon impact of the
nanoparticle on the substrate.
9.3.4 Comparison with Cobalt, Nickel and Iron–Cobalt-Alloy
Nanoparticles
Finally, we address the question of whether strongly enhanced magnetic energy
barriers and metastable states are only observed in iron nanoparticles or whether
comparable behavior is found in other 3d transition metal nanoparticles as well.
For this purpose fcc cobalt and fcc nickel nanoparticles were deposited on silicon
substrates as in the case of the iron nanoparticles [5]. Surprisingly, for cobalt the
same size-independent coexistence of superparamagnetic and magnetically blocked
nanoparticles was found as observed for iron, see Fig. 9.6. Also in the case of fcc
cobalt, the observation of magnetically blocked states at room temperature show
the presence of significantly enhanced magnetic energy barriers when compared
to the fcc bulk magneto-crystalline anisotropy and surface anisotropy contributions
as discussed in detail in [5]. However, in contrast to iron nanoparticles, thermal
annealing promotes transitions from superparamagnetic states into a magnetically
blocked state in a large number of nanoparticles. In contrast to iron and cobalt, only
superparamagnetic states were found in fcc nickel nanoparticles at room temperature [5]. Also Fe 50 Co 50 alloy nanoparticles deposited on silicon substrates under
similar conditions as the iron, cobalt, and nickel nanoparticles exhibit a coexistence
of superparamagnetic and magnetically blocked nanoparticles at room temperature
[66]. These findings demonstrate that the coexistence of states with distinct magnetic
properties is not restricted to iron nanoparticles, but seems a more general phenomena
among pure and alloyed 3d transition metal nanoparticles.
9.4 Conclusions and Perspectives
In this chapter we have demonstrated that XPEEM is a powerful tool for magnetic and chemical characterization of individual 3d transition metal nanoparticles
under both UHV conditions and under reactive gas environments for in situ study of
chemical reactions. Moreover, by combining XPEEM investigations with complementary structural characterization such as SEM and AFM, one is able to correlate
directly magnetism, shape, and size of a large number of the very same nanoparticles
and probe the actual distribution of magnetic properties in extended ensembles and