Iron Oxide-Based Magnetic Nanoparticles Synthesized from THE Organic Solution Phase for Advanced Biological Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 27

(a) M s

M s

M r

H c

M M

H H

(b) z z z z

M z

RF on RF off

M xy

y y y

y

x x x x

fIgure 2.2 Schematic illustrations of (a) the hysteresis loops of a group of ferromag-
netic (left) and superparamagnetic NPs (right). (b) Schematic illustration of spin orienta-
tion of the proton under a permanent magnetic field (in z -direction) and a resonant
RF perturbation field that is perpendicular to the permanent field direction. The RF field
perturbs the magnetization of spins into xy -directions. Subsequent removal of this RF irra-
diation causes spin to realign along the z -direction—a process called spin relaxation. The
process leading to moment increase in z -direction is called T 1 relaxation, while that leading
to moment decrease in xy -directions is called T 2 relaxation. (Reproduced with permission
from Ref. [6]. © Wiley.)

( M r ) after the removal of the external field. To fully demagnetize these MNPs requires
an external field in the opposite direction and in certain strength, called coercive field
or coercivity ( H c ) [11, 14]. When the size of a single-domain MNP decreases to a
certain degree, superparamagnetism occurs and the remnant M r drops to zero in the
absence of the external magnetic field. Figure 2.2a displays the representative hyster-
esis loops of a group of ferromagnetic NPs and superparamagnetic NPs. The super-
paramagnetism originates from the thermal-induced magnetization reversal and is
defined by

e KV kT

/ 2

ττ = (2.1)

u

0

in which τ is the magnetization relaxation time, K u is the particle's magnetic anisotropy
constant, V is the particle volume, k is Boltzmann's constant, and T is temperature
[3, 15]. In the critical size where the thermal energy ( kT ) is comparable to magnetiza-
tion reversal energy ( K u V ), the single-domain MNPs can be demagnetized by thermal
fluctuation without the external field reaching the superparamagnetic state. The super-
paramagnetic NPs have negligible magnetic dipole interactions and therefore can be
dispersed and stabilized in the liquid media for biological applications [3].