  Geometrically confined magnetic particles due to their unique response to
external magnetic fields find a variety of applications, including magnetic
guidance, heat and drug delivery, magneto-mechanical actuation, and contrast
enhancement. Highly sensitive detection and imaging techniques based on
nonlinear properties of nanomagnets were recently proposed as innovative
strong-translational potential methods applicable in complex, often opaque,
biological systems. Here we report on significant enhancement of the detection
capability using optical-lithography-defined, ferromagnetic iron-nickel alloy
disk-shaped particles. We show that an irreversible transition between a
strongly non-collinear (vortex) and single domain states, driven by an
alternating magnetic field translates into a nonlinear magnetic response that
enables ultrasensitive detection of these particles. The record sensitivity of
~ 3.5x10-9 emu, which is equivalent to ~39 pg of magnetic material is
demonstrated at room temperature for arrays of patterned disks. We also show
that unbound disks re-suspended in aqueous buffer can be successfully detected
and quantified in real-time when administrated into a live animal allowing for
tracing their biodistribution. Use of nanoscale ferromagnetic particles with
engineered nonlinear properties opens prospects for further enhancing
sensitivity, scalability and tunability of noise-free magnetic tag detection in
high-background environments for various applications spanning from biosensing
and medical imaging to anti-counterfeiting technologies.
