Nanomagnet

In magnetism, a nanomagnet is a nanoscopic scale 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.

The ultimate limit in miniaturization of nanomagnets was achieved in 2016: individual Ho atoms present remanence when deposited on an atomically thin layer of MgO coating a silver film was reported by scientists from EPFL and ETH, in Switzerland. ^[4] Before that, the smallest nanomagnets reported, attending to the number of magnetic atoms, were double decker phthalocyanes molecules with only one rare-earth atom. ^[5] Other systems presenting remanence are nanoengineered Fe chains, deposited on Cu₂N/Cu(100) surfaces, showing either Neel ^[6] or ferromagnetic ground states ^[7] with in systems with as few as 5 Fe atoms with S=2. 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:

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

Conditions 1 and 2, but not 3, have been demonstrated in a number of nanostructures, such as nanoparticles, ^[8] nanoislands, ^[9] and quantum dots ^{[10] [11]} with a controlled number of magnetic atoms (between 1 and 10).

References

1. 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. S2CID   39584741.
2. 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. PMID   11384312. S2CID   41734831.
3. Gatteschi, Dante; Sessoli, Roberta; Villain, Jacques (2006). Molecular Nanomagnets (Reprint ed.). New York: Oxford University Press. ISBN   0-19-856753-7.
4. Donati, F.; Rusponi, S.; Stepanow, S.; Wäckerlin, C.; Singha, A.; Persichetti, L.; Baltic, R.; Diller, K.; Patthey, F. (2016-04-15). "Magnetic remanence in single atoms". Science. 352 (6283): 318–321. Bibcode:2016Sci...352..318D. doi:10.1126/science.aad9898. hdl: 11590/345616 . ISSN   0036-8075. PMID   27081065. S2CID   30268016.
5. 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. arXiv: cond-mat/0506582 . Bibcode:2005cond.mat..6582I. doi:10.1021/ja0428661. PMID   15771471. S2CID   40136392.
6. Loth, Sebastian; Baumann, Susanne; Lutz, Christopher P.; Eigler, D. M.; Heinrich, Andreas J. (2012-01-13). "Bistability in Atomic-Scale Antiferromagnets". Science. 335 (6065): 196–199. Bibcode:2012Sci...335..196L. doi:10.1126/science.1214131. ISSN   0036-8075. PMID   22246771. S2CID   128108.
7. Spinelli, A.; Bryant, B.; Delgado, F.; Fernández-Rossier, J.; Otte, A. F. (2014-08-01). "Imaging of spin waves in atomically designed nanomagnets". Nature Materials. 13 (8): 782–785. arXiv: 1403.5890 . Bibcode:2014NatMa..13..782S. doi:10.1038/nmat4018. ISSN   1476-1122. PMID   24997736.
8. 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. PMID   12750516. S2CID   5559569.
9. 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. PMID   16574821. S2CID   24061939.
10. 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" (PDF). Physical Review Letters . 97 (10) 107401. Bibcode:2006PhRvL..97j7401L. doi:10.1103/PhysRevLett.97.107401. hdl: 10045/25252 . PMID   17025852.
11. 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) 247209. arXiv: 0710.5389 . Bibcode:2007PhRvL..99x7209K. doi:10.1103/PhysRevLett.99.247209. PMID   18233484. S2CID   16664854.

Further reading

• Friedman, J. R.; Sarachik, M. P. (2010). "Single-Molecule Nanomagnets". Annual Review of Condensed Matter Physics . 1: 109–128. arXiv: 1001.4194 . Bibcode:2010ARCMP...1..109F. doi:10.1146/annurev-conmatphys-070909-104053. S2CID   118713965.