Ferromagnetic particles as ultra-sensitive non-linear response labels for magnetic particles imaging (MPI) and sensing applications
A significant enhancement of detection capabilities of the room temperature MPQ is seen using optical lithography-defined, ferromagnetic iron-nickel alloy microdisks. Irreversible transitions between strongly non-collinear (vortex) and a collinear single domain states, driven by an ac magnetic field, translate into a nonlinear magnetic response that enables ultrasensitive detection of material at relatively small magnetic fields.
1. A method of fabricating magnetic labels, the method comprising:
(a) depositing a metal-alloy material onto a non-magnetic substrate;
(b) patterning the metal-alloy material to form a disk-configuration on the non-magnetic substrate; and
(c) applying an external alternating current magnetic field to the metal-alloy material to form the magnetic labels,
wherein after applying the external alternating current magnetic field, the magnetic labels irreversibly transition from a spin-vortex (SV) state to a magnetized single-domain (SD) state.
2. The method of claim 1 , further comprising measuring a response of the magnetic labels using magnetic particle quantification (MPQ) or imaging the magnetic labels using magnetic particle imaging (MPI) techniques.
3. The method of claim 2 , further comprising detecting the magnetic labels by measuring a frequency harmonics response to the external alternating current magnetic field.
4. The method of claim 3 , wherein the harmonics parameter of the external alternating current magnetic field has at least one spectra component of frequency f 1 such that f=(2n+1)×f 1 .
5. The method of claim 3 , wherein the harmonics parameter of the external alternating current magnetic field has two spectral components of frequencies f 1 and f 2 such that f=kf 1 ±mf 2 , where k and m are integers and where either k or m may equal 0.
6. The method of claim 2 , wherein the response of the magnetic labels yields detection of the nanoparticle to at least 39 picograms.
7. The method of claim 1 , wherein the magnetic labels are microdisks comprising a ferromagnetic iron-nickel alloy core.
8. The method of claim 7 , wherein the microdisks have diameter (d) to thickness (t) ratios of d/h>2.
9. The method of claim 7 , wherein a first plurality of the microdisks have a diameter-to-height (d/h) ratio of 2<d/h≤10, a second plurality of the microdisks have a d/h ratio of 10<d/h≤30, and a third plurality of microdisks have a d/h ratio of d/h>30.
10. The method of claim 1 , wherein the spin-vortex (SV) state comprises a non-collinear vortex ground state having zero remanence.
11. The method of claim 1 , wherein the external magnetic field has a magnetic field strength H corresponding to less than 250 Oe.
12. The method of claim 1 , wherein the external magnetic field operates at frequencies ranging between 100 Hz and 150 kHz.
13. The method of claim 1 , wherein the magnetic labels have a magneto-crystalline anisotropy less than 10 3 J/m 3 in the plane of the magnetic labels.
14. The method of claim 1 , wherein the magnetic labels have a magneto-crystalline anisotropy greater than 10 3 J/m 3 and less than 10 6 J/m 3 out of the plane of the magnetic labels.
15. The method of claim 1 , wherein the magnetic labels are ferromagnetic Fe:Ni permalloy microdisks.