225
10–15 nm grain sizes have shown practically no hysteresis. From a
technical point of view, if the idea is to fabricate a strong permanent magnet, the coercive force should be designed to be as high
as possible.
In addition to the coercivity, the magnetization reversal mechanism
is also strongly affected by size. Within a specific range of diameters,
typically greater than the supermagnetic diameter but lower than
the critical diameter, the exchange interaction energy is sufficiently
strong to keep the spins aligned during the reversal process. Above
this range of diameters but still below the critical diameter (D cri ),
the process of magnetization reversal becomes incoherent, involving the switching of small volumes of materials within the nanoparticles or nanosize grains. The size of magnetic nanoparticles or
grains also has an effect on the saturation magnetization, namely,
the magnetization increases below a particular size. For example, for
zinc ferrite, a significant increase in saturation magnetization occurs
in these materials for particles around 20 nm. This enhancement
in magnetization helps restrict the rotation of the magnetization
vector by thermal motion. This is a key issue for magnetic recording technologies, particularly as increases in recording density are
demanded. In other words, small magnetic elements are needed,
but if the magnetization is unstable due to thermal fluctuations, it
is of no use as magnetic memory.
Another magnetic effect that has deep associations with nanoscale is the phenomenon of giant and colossal magnetoresistance.
In general, magnetoresistance is a material’s property whereby the
application of a DC magnetic field alters the electrical resistance. As
discussed in Section 4.5, the electrical resistance of a material is a
consequence of electron scattering by atoms and defects. Thereby
the resistance is associated with the electron mean-free path, which
is the average distance traveled by an electron without suffering a
collision.
Under these conditions, if a DC magnetic field is applied, the
Lorentz force may curve the electron direction within its mean-free
path, leading to an increase in resistance. Although this behavior
was discovered in metals by Lord Kelvin a long time ago, it had
few practical implications, because very strong fields were required.
However, in 1988 a much greater magnetoresistance effect, called
giant magnetoresistance (GMR), was discovered in films composed of
alternating nanoscale layers of ferromagnetic and nonferromagnetic
materials. The principle behind this phenomenon is associated
with differences in the density of states for spin-up and spin-down
Magnetic Properties
10–15 nm grain sizes have shown practically no hysteresis. From a
technical point of view, if the idea is to fabricate a strong permanent magnet, the coercive force should be designed to be as high
as possible.
In addition to the coercivity, the magnetization reversal mechanism
is also strongly affected by size. Within a specific range of diameters,
typically greater than the supermagnetic diameter but lower than
the critical diameter, the exchange interaction energy is sufficiently
strong to keep the spins aligned during the reversal process. Above
this range of diameters but still below the critical diameter (D cri ),
the process of magnetization reversal becomes incoherent, involving the switching of small volumes of materials within the nanoparticles or nanosize grains. The size of magnetic nanoparticles or
grains also has an effect on the saturation magnetization, namely,
the magnetization increases below a particular size. For example, for
zinc ferrite, a significant increase in saturation magnetization occurs
in these materials for particles around 20 nm. This enhancement
in magnetization helps restrict the rotation of the magnetization
vector by thermal motion. This is a key issue for magnetic recording technologies, particularly as increases in recording density are
demanded. In other words, small magnetic elements are needed,
but if the magnetization is unstable due to thermal fluctuations, it
is of no use as magnetic memory.
Another magnetic effect that has deep associations with nanoscale is the phenomenon of giant and colossal magnetoresistance.
In general, magnetoresistance is a material’s property whereby the
application of a DC magnetic field alters the electrical resistance. As
discussed in Section 4.5, the electrical resistance of a material is a
consequence of electron scattering by atoms and defects. Thereby
the resistance is associated with the electron mean-free path, which
is the average distance traveled by an electron without suffering a
collision.
Under these conditions, if a DC magnetic field is applied, the
Lorentz force may curve the electron direction within its mean-free
path, leading to an increase in resistance. Although this behavior
was discovered in metals by Lord Kelvin a long time ago, it had
few practical implications, because very strong fields were required.
However, in 1988 a much greater magnetoresistance effect, called
giant magnetoresistance (GMR), was discovered in films composed of
alternating nanoscale layers of ferromagnetic and nonferromagnetic
materials. The principle behind this phenomenon is associated
with differences in the density of states for spin-up and spin-down
Magnetic Properties