Spin Vortices: From Fundamentals to Applications

Date: 
01/28/2013 - 4:00pm to 5:15pm
Speaker: 
Dr. Valentyn Novosad, Materials Scientist & Principal Investigator, Argonne National Laboratory (ANL)
Location: 
Communicore C1-4

Vortices are observed in the statics and dynamics of a variety of physical systems, including patterned mesoscopic ferromagnets [1] . A spin vortex consists of a large region of in-plane curling magnetization and a nano-sized vortex core region where the magnetization vector points out-of-plane, Fig. 1a. When the vortex core is perturbed from its equilibrium state, it begins a gyrotropic oscillation [2]. The precession frequency does not depend on the vortex core orientation for isolated vortices, while for an array of interacting vortices the dynamic dipolar interaction eliminates this degeneracy and results in a variety of collective excitation modes [3]. Manipulation of the vortex core polarities in interacting vortex-state magnetic dots enables precise control of the dynamic response [4]. Various solid-state device applications based on magnetic vortex concept include Magnetic Random Access Memories, microwave oscillators, magnetic field sensors and magnonic crystals. 

We have recently took a new direction in our studies of magnetic vortices by developing a fabrication process to coat the microdisks with gold to prevent oxidation and removing them from their substrate (Fig. 1b) in order to suspend them in aqueous solution [5]. We then studied their magnetic behavior using laser-based magneto-optics and have found that the disks rotate under low-amplitude ac magnetic fields. In an ac field aligned with the light path, the rotation modulates the laser light transmission through the solution. The interesting light-switching rheological response prompted us to investigate what happens when these disks are biofunctionalized and attached in vitro to human brain cancer cells, which are much larger in size. It was found that application of the low-amplitude, low frequency ac magnetic field then causes tugging on the cell membrane, ultimately leading to the cancer cells self-destruction via apoptosis [6]. We believe that due to their physical properties that are unique to the spin vortex state (a high saturation magnetization value, zero remanence, strong magneto-mechanical response, excellent MRI contrast enhancement, inductive heating, an intrinsic spin resonance at MHz frequencies as well as scalability) these materials represent an interesting nanoplatform for biomedical applications [7, 8].
Fig. 1:  (a) Micromagnetic model of magnetic-vortex spin distribution, (b) Optical micrograph of the dried suspension of gold-coated microdisks.
References
[1] V. Novosad, et al., “Nucleation and annihilation of magnetic vortices in sub-micron permalloy dots”, IEEE Trans. Magn., 37 (2001) 2088.
[2] V. Novosad, et al., “Magnetic vortex resonance in patterned ferromagnetic dots,” Phys. Rev., B 72, 024455 (2005).
[3] K. Buchanan, et al., “Soliton pair dynamics in patterned ferromagnetic ellipses”, Nature Phys. 1, 172-176 (2005).
[4] S. Jain, et al., “From chaos to selective ordering of vortex cores in interacting mesomagnets”, Nature Comm.| DOI: 10.1038/ncomms2331.
[5] E. Rozhkova, et al., “Ferromagnetic microdisks as carriers for biomedical applications”, J. Appl. Phys. 105, (2009) 07B306.
[6] D.-H. Kim, et al, “Biofunctionalized magnetic-vortex microdiscs for targeted cancer-cell destruction”, Nature Mat.,  9, 165 - 171 (2010).
[7] Vitol, et al., “Multifunctional ferromagnetic disks for modulating Cell Function”, IEEE Trans. Mag. 48, 3269-3274 (2012).
[8] Vitol, et.al., “Microfabricated magnetic structures for future medicine: from sensors to cell actuators”, Nanomedicine 7(10), 1611–1624 (2012)

We have recently took a new direction in our studies of magnetic vortices by developing a fabrication process to coat the microdisks with gold to prevent oxidation and removing them from their substrate (Fig. 1b) in order to suspend them in aqueous solution [5]. We then studied their magnetic behavior using laser-based magneto-optics and have found that the disks rotate under low-amplitude ac magnetic fields. In an ac field aligned with the light path, the rotation modulates the laser light transmission through the solution. The interesting light-switching rheological response prompted us to investigate what happens when these disks are biofunctionalized and attached in vitro to human brain cancer cells, which are much larger in size. It was found that application of the low-amplitude, low frequency ac magnetic field then causes tugging on the cell membrane, ultimately leading to the cancer cells self-destruction via apoptosis [6]. We believe that due to their physical properties that are unique to the spin vortex state (a high saturation magnetization value, zero remanence, strong magneto-mechanical response, excellent MRI contrast enhancement, inductive heating, an intrinsic spin resonance at MHz frequencies as well as scalability) these materials represent an interesting nanoplatform for biomedical applications [7, 8].

Fig. 1:  (a) Micromagnetic model of magnetic-vortex spin distribution, (b) Optical micrograph of the dried suspension of gold-coated microdisks.

References

[1] V. Novosad, et al., “Nucleation and annihilation of magnetic vortices in sub-micron permalloy dots”, IEEE Trans. Magn., 37 (2001) 2088.

[2] V. Novosad, et al., “Magnetic vortex resonance in patterned ferromagnetic dots,” Phys. Rev., B 72, 024455 (2005).

[3] K. Buchanan, et al., “Soliton pair dynamics in patterned ferromagnetic ellipses”, Nature Phys. 1, 172-176 (2005).

[4] S. Jain, et al., “From chaos to selective ordering of vortex cores in interacting mesomagnets”, Nature Comm. DOI: 10.1038/ncomms2331.

[5] E. Rozhkova, et al., “Ferromagnetic microdisks as carriers for biomedical applications”, J. Appl. Phys. 105, (2009) 07B306.

[6] D.-H. Kim, et al, “Biofunctionalized magnetic-vortex microdiscs for targeted cancer-cell destruction”, Nature Mat.,  9, 165 - 171 (2010).

[7] Vitol, et al., “Multifunctional ferromagnetic disks for modulating Cell Function”, IEEE Trans. Mag. 48, 3269-3274 (2012).

[8] Vitol, et.al., “Microfabricated magnetic structures for future medicine: from sensors to cell actuators”, Nanomedicine 7(10), 1611–1624 (2012)

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