15 Smart Platforms for Biomedical Applications
365
chain in addition to the magnetic anisotropy of the individual particle. A thorough
understanding of the effect of these dipolar interactions on the SLP and in which
regimes they are desirable is needed [136–139].
15.3.3 The Top-Down Engineering of Application Specific
Nanoparticles
Lithographically defined microdiscs made from permalloy were shown to be effective at triggering apoptotic cell death in glioblastoma cells in vitro [36] which sparked
investigation into the mechanical destruction of cancer cells as a viable cancer
therapy. The study featured here [125] began with the hypothesis that the permalloy
vortex magnetic configuration was not the most efficient method of torque generation
from an applied field, and that magnetic microdiscs with a strong uniaxial anisotropy
are more desirable for this application.
Mechanical actuation of magnetic particles in an applied field is dictated largely
by the magnetic anisotropy of the particle. The magnetic anisotropy can be thought
of as a measure of how strongly the magnetization vector of the particle is linked
to its mechanical structure. Optimizing the actuation of magnetic particles for such
an application involves engineering its magnetic anisotropy to best fit the type of
applied field being used.
Lithographically defined microdiscs are based on thin-film magnetic systems and
may have the easy magnetic axis in the plane of the film, or out of the plane of
the film. The permalloy vortex is a special case of the in-plane system, where the
shape constraints from patterning and the inherently soft material lead to a magnetic
system with an easy plane of magnetization (the plane of the microdisc). When a
field is applied in the plane of the microdisc the vortex core is displaced resulting in
a net magnetization in the direction of the applied field, and this is achieved equally
“easily” in any direction in the plane of the disc. Perpendicular magnetic thin films are
constructed by alternating layers of a magnetic material (CoFeB) with a heavy metal
(Pt) in a multilayer heterostructure. The out-of-plane magnetization is driven by spin–
orbit coupling at the CoFeB/Pt interface [120–123]. The perpendicular microdiscs
achieve a net zero remanent state by utilizing RKKY AF coupling between adjacent
magnetic layers in the multilayer stack. Both sets of discs in this study were 2 μm in
diameter. The permalloy vortex discs were 60 nm in permalloy thickness with 5 nm
gold caps, and the perpendicular microdisc contained a total of 21.6 nm of CoFeB
dispersed in a multilayer heterostructure approximately 110 nm in thickness with
5 nm gold caps. The vortex microdisc thus has a higher magnetic moment than the
perpendicular microdiscs, with an easy plane of magnetization as opposed to an easy
axis. It should be noted that the hard axis saturation field for the vortex microdiscs was
approximately 1 T, and the effective hard axis anisotropy field for the perpendicular
particles (once the RKKY coupling is accounted for) was approximately 0.5 T.
365
chain in addition to the magnetic anisotropy of the individual particle. A thorough
understanding of the effect of these dipolar interactions on the SLP and in which
regimes they are desirable is needed [136–139].
15.3.3 The Top-Down Engineering of Application Specific
Nanoparticles
Lithographically defined microdiscs made from permalloy were shown to be effective at triggering apoptotic cell death in glioblastoma cells in vitro [36] which sparked
investigation into the mechanical destruction of cancer cells as a viable cancer
therapy. The study featured here [125] began with the hypothesis that the permalloy
vortex magnetic configuration was not the most efficient method of torque generation
from an applied field, and that magnetic microdiscs with a strong uniaxial anisotropy
are more desirable for this application.
Mechanical actuation of magnetic particles in an applied field is dictated largely
by the magnetic anisotropy of the particle. The magnetic anisotropy can be thought
of as a measure of how strongly the magnetization vector of the particle is linked
to its mechanical structure. Optimizing the actuation of magnetic particles for such
an application involves engineering its magnetic anisotropy to best fit the type of
applied field being used.
Lithographically defined microdiscs are based on thin-film magnetic systems and
may have the easy magnetic axis in the plane of the film, or out of the plane of
the film. The permalloy vortex is a special case of the in-plane system, where the
shape constraints from patterning and the inherently soft material lead to a magnetic
system with an easy plane of magnetization (the plane of the microdisc). When a
field is applied in the plane of the microdisc the vortex core is displaced resulting in
a net magnetization in the direction of the applied field, and this is achieved equally
“easily” in any direction in the plane of the disc. Perpendicular magnetic thin films are
constructed by alternating layers of a magnetic material (CoFeB) with a heavy metal
(Pt) in a multilayer heterostructure. The out-of-plane magnetization is driven by spin–
orbit coupling at the CoFeB/Pt interface [120–123]. The perpendicular microdiscs
achieve a net zero remanent state by utilizing RKKY AF coupling between adjacent
magnetic layers in the multilayer stack. Both sets of discs in this study were 2 μm in
diameter. The permalloy vortex discs were 60 nm in permalloy thickness with 5 nm
gold caps, and the perpendicular microdisc contained a total of 21.6 nm of CoFeB
dispersed in a multilayer heterostructure approximately 110 nm in thickness with
5 nm gold caps. The vortex microdisc thus has a higher magnetic moment than the
perpendicular microdiscs, with an easy plane of magnetization as opposed to an easy
axis. It should be noted that the hard axis saturation field for the vortex microdiscs was
approximately 1 T, and the effective hard axis anisotropy field for the perpendicular
particles (once the RKKY coupling is accounted for) was approximately 0.5 T.