8 Magnetic Self-Assembling of Spherical Co Nanoparticles …
201
8.6.1.2 Collective Magnetic Properties in 3D Assemblies of MNPs
Within a NP, small enough to have a single magnetic domain, all atomic spins are
aligned in the same direction, and thus, the particle can be considered as a point
dipole (or superspin) with a large magnetic moment. Some remarkable phenomena
have been observed in MNPs; the most well-known among them being that of superparamagnetism (SPM) of non-interacting NPs, where the magnetic moments of each
NPs act independendly [60]. Superparamagnetic NPs have found their use in many
fields of applied technology including, but not limited to, biomedicine [61], magnetic
resonance imaging [62] data storage [60, 63].
In an assembly of highly concentrated NPs, the materials’ magnetic properties
can be greatly influenced by the dipolar interactions between NPs. It has been found
that at sufficiently high concentrations, the interparticle dipolar interactions can
produce “collective states” below a system-dependent transition temperature T c .
The observed collective states are almost invariably “disordered” and thus called
“superspin glasses” (SSG) as they show many of the phenomenology found in
atomic spin glasses [64, 65]. Further increasing inter-particle correlations, the SSG
state is predicted to transform into long-range ordered dipolar superferromagnetic
(SFM) state. However, a clear-cut experimental evidence of a dipolar SFM state in
real 3D NP assemblies has not yet been reported. This is likely due to the stringent geometrical conditions for inducing such a complex state: i.e., long ellipsoidal
sample shape, highly ordered fcc or bcc lattice structure and parallel alignment of
anisotropy-axis [66, 67]. Due to their structural characteristics, fcc supercrystals of
Co NPs are considered as good candidates to display dipolar SFM properties [68, 69].
In the superferromagnetic transition region (from superparamagnetic to superferromagnetic), enhanced magnetocaloric effect is expected, bringing this novel class of
material one step close to application in the field of energy efficiency (refrigeration)
[70, 71].
8.6.1.3 Collective Magnetic Properties in 3D Fcc Supercrystals
of Fcc-Co Polycrystals
The first aim is here (1) to study the effect of the mesoscopic order in 3D assemblies
of fcc-Co polycrystals (characterized by a low anisotropy) on the magnetic properties
by DC susceptibility measurements. The second one is (2) to study the possibility of
superspin glass behavior in the same system by AC susceptibility measurements.
(1) The effect of the structural order of 3D assemblies on magnetic properties is
studied by considering highly ordered fcc supercrystals and disordered assemblies. Both ordered and disordered samples are made with the same batch of
7.5 nm fcc-Co polycrystals. The zero field cooled (ZFC) magnetization versus
temperature is measured by cooling the sample in zero applied field from 300
to 5 K, applying a field of 20 Oe and then measuring the magnetization as the
201
8.6.1.2 Collective Magnetic Properties in 3D Assemblies of MNPs
Within a NP, small enough to have a single magnetic domain, all atomic spins are
aligned in the same direction, and thus, the particle can be considered as a point
dipole (or superspin) with a large magnetic moment. Some remarkable phenomena
have been observed in MNPs; the most well-known among them being that of superparamagnetism (SPM) of non-interacting NPs, where the magnetic moments of each
NPs act independendly [60]. Superparamagnetic NPs have found their use in many
fields of applied technology including, but not limited to, biomedicine [61], magnetic
resonance imaging [62] data storage [60, 63].
In an assembly of highly concentrated NPs, the materials’ magnetic properties
can be greatly influenced by the dipolar interactions between NPs. It has been found
that at sufficiently high concentrations, the interparticle dipolar interactions can
produce “collective states” below a system-dependent transition temperature T c .
The observed collective states are almost invariably “disordered” and thus called
“superspin glasses” (SSG) as they show many of the phenomenology found in
atomic spin glasses [64, 65]. Further increasing inter-particle correlations, the SSG
state is predicted to transform into long-range ordered dipolar superferromagnetic
(SFM) state. However, a clear-cut experimental evidence of a dipolar SFM state in
real 3D NP assemblies has not yet been reported. This is likely due to the stringent geometrical conditions for inducing such a complex state: i.e., long ellipsoidal
sample shape, highly ordered fcc or bcc lattice structure and parallel alignment of
anisotropy-axis [66, 67]. Due to their structural characteristics, fcc supercrystals of
Co NPs are considered as good candidates to display dipolar SFM properties [68, 69].
In the superferromagnetic transition region (from superparamagnetic to superferromagnetic), enhanced magnetocaloric effect is expected, bringing this novel class of
material one step close to application in the field of energy efficiency (refrigeration)
[70, 71].
8.6.1.3 Collective Magnetic Properties in 3D Fcc Supercrystals
of Fcc-Co Polycrystals
The first aim is here (1) to study the effect of the mesoscopic order in 3D assemblies
of fcc-Co polycrystals (characterized by a low anisotropy) on the magnetic properties
by DC susceptibility measurements. The second one is (2) to study the possibility of
superspin glass behavior in the same system by AC susceptibility measurements.
(1) The effect of the structural order of 3D assemblies on magnetic properties is
studied by considering highly ordered fcc supercrystals and disordered assemblies. Both ordered and disordered samples are made with the same batch of
7.5 nm fcc-Co polycrystals. The zero field cooled (ZFC) magnetization versus
temperature is measured by cooling the sample in zero applied field from 300
to 5 K, applying a field of 20 Oe and then measuring the magnetization as the