Nanomagnet

A nanomagnet is a submicrometric system that presents spontaneous magnetic order (magnetization) at zero applied magnetic field (remanence).

The small size of nanomagnets prevents the formation of magnetic domains (see single domain (magnetic)). The magnetization dynamics of sufficiently small nanomagnets at low temperatures, typically single-molecule magnets, presents quantum phenomena, such as macroscopic spin tunnelling. At larger temperatures, the magnetization undergoes random thermal fluctuations (superparamagnetism) which present a limit for the use of nanomagnets for permanent information storage.

Canonical examples of nanomagnets are grains^[1]^[2] of ferromagnetic metals (iron, cobalt, and nickel) and single-molecule magnets.^[3] The vast majority of nanomagnets feature transition metal (titanium, vanadium, chromium, manganese, iron, cobalt or nickel) or rare earth (Gadolinium, Europium, Erbium) magnetic atoms.

Attending to the number of magnetic atoms, the smallest nanomagnets reported so far are double decker phthalocyanes with only one rare earth atom.^[4] Canonical single-molecule magnets are the so-called Mn12 and Fe8 systems, with 12 and 8 transition metal atoms each and both with spin 10 (S = 10) ground states.

The phenomenon of zero field magnetization requires three conditions:

A ground state with finite spin
A magnetic anisotropy energy barrier
Long spin relaxation time.

Conditions 1 and 2, but not 3, have been demonstrated in a number of nanostructures, such as nanoparticles,^[5] nanoislands,^[6] and quantum dots^[7]^[8] with a controlled number of magnetic atoms (between 1 and 10).

A nanomagnet can have enhanced electronic properties due to size effect, such as long spin relaxation time of conduction electron, which may be useful for nano-scale spintronic device.^[9]

References

^ Guéron, S.; Deshmukh, Mandar M.; Myers, E. B.; Ralph, D. C. (15 November 1999). "Tunneling via Individual Electronic States in Ferromagnetic Nanoparticles". Physical Review Letters. 83 (20): 4148–4151. arXiv:cond-mat/9904248. Bibcode:1999PhRvL..83.4148G. doi:10.1103/PhysRevLett.83.4148.
^ Jamet, M.; Wernsdorfer, W.; Thirion, C.; Mailly, D.; Dupuis, V.; Mélinon, P.; Pérez, A. (14 May 2001). "Magnetic Anisotropy of a Single Cobalt Nanocluster". Physical Review Letters. 86 (20): 4676–4679. arXiv:cond-mat/0012029. Bibcode:2001PhRvL..86.4676J. doi:10.1103/PhysRevLett.86.4676.
^ Gatteschi, Dante; Sessoli, Roberta; Villain, Jacques (2006). Molecular Nanomagnets (Reprint ed.). New York: Oxford University Press. ISBN 0-19-856753-7.
^ Ishikawa, Naoto; Sugita, Miki; Wernsdorfer, Wolfgang (March 2005). "Nuclear Spin Driven Quantum Tunneling of Magnetization in a New Lanthanide Single-Molecule Magnet: Bis(Phthalocyaninato)holmium Anion". Journal of the American Chemical Society. 127 (11): 3650–3651. doi:10.1021/ja0428661.
^ Gambardella, P. (16 May 2003). "Giant Magnetic Anisotropy of Single Cobalt Atoms and Nanoparticles". Science. 300 (5622): 1130–1133. Bibcode:2003Sci...300.1130G. doi:10.1126/science.1082857.
^ Hirjibehedin, C. F. (19 May 2006). "Spin Coupling in Engineered Atomic Structures". Science. 312 (5776): 1021–1024. Bibcode:2006Sci...312.1021H. doi:10.1126/science.1125398.
^ Léger, Y.; Besombes, L.; Fernández-Rossier, J.; Maingault, L.; Mariette, H. (7 September 2006). "Electrical Control of a Single Mn Atom in a Quantum Dot". Physical Review Letters. 97 (10). Bibcode:2006PhRvL..97j7401L. doi:10.1103/PhysRevLett.97.107401.
^ Kudelski, A.; Lemaître, A.; Miard, A.; Voisin, P.; Graham, T. C. M.; Warburton, R. J.; Krebs, O. (14 December 2007). "Optically Probing the Fine Structure of a Single Mn Atom in an InAs Quantum Dot". Physical Review Letters. 99 (24). arXiv:0710.5389. Bibcode:2007PhRvL..99x7209K. doi:10.1103/PhysRevLett.99.247209.
^ Hai, Pham Nam; Ohya, Shinobu; Tanaka, Masaaki (4 July 2010). "Long spin-relaxation time in a single metal nanoparticle". Nature Nanotechnology. 5 (8): 593–596. Bibcode:2010NatNa...5..593H. doi:10.1038/nnano.2010.130.

References

Further reading