nanomagnet

{{short description|Magnetism at the molecular scale}}

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{{cite journal |last1=Guéron |first1=S. |last2=Deshmukh |first2=Mandar M. |last3=Myers |first3=E. B. |last4=Ralph |first4=D. C. |title=Tunneling via Individual Electronic States in Ferromagnetic Nanoparticles |journal=Physical Review Letters |date=15 November 1999 |volume=83 |issue=20 |pages=4148–4151 |doi=10.1103/PhysRevLett.83.4148|arxiv = cond-mat/9904248 |bibcode = 1999PhRvL..83.4148G |s2cid=39584741 }}{{cite journal |last1=Jamet |first1=M. |last2=Wernsdorfer |first2=W. |last3=Thirion |first3=C. |last4=Mailly |first4=D. |last5=Dupuis |first5=V. |last6=Mélinon |first6=P. |last7=Pérez |first7=A. |title=Magnetic Anisotropy of a Single Cobalt Nanocluster |journal=Physical Review Letters |date=14 May 2001 |volume=86 |issue=20 |pages=4676–4679 |doi=10.1103/PhysRevLett.86.4676|arxiv = cond-mat/0012029 |bibcode = 2001PhRvL..86.4676J |pmid=11384312|s2cid=41734831 }} of ferromagnetic metals (iron, cobalt, and nickel) and single-molecule magnets.{{cite book |last1=Gatteschi |first1=Dante |last2=Sessoli |first2=Roberta |last3=Villain |first3=Jacques |title=Molecular Nanomagnets |date=2006 |publisher=Oxford University Press |location=New York |isbn=0-19-856753-7 |edition=Reprint}} 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.{{Cite journal|last1=Donati|first1=F.|last2=Rusponi|first2=S.|last3=Stepanow|first3=S.|last4=Wäckerlin|first4=C.|last5=Singha|first5=A.|last6=Persichetti|first6=L.|last7=Baltic|first7=R.|last8=Diller|first8=K.|last9=Patthey|first9=F.|date=2016-04-15|title=Magnetic remanence in single atoms|journal=Science|language=en|volume=352|issue=6283|pages=318–321|doi=10.1126/science.aad9898|issn=0036-8075|pmid=27081065|bibcode=2016Sci...352..318D|s2cid=30268016|hdl=11590/345616|hdl-access=free}} Before that, the smallest nanomagnets reported, attending to the number of magnetic atoms, were double decker phthalocyanes molecules with only one rare-earth atom.{{cite journal |last1=Ishikawa |first1=Naoto |last2=Sugita |first2=Miki |last3=Wernsdorfer |first3=Wolfgang |title=Nuclear Spin Driven Quantum Tunneling of Magnetization in a New Lanthanide Single-Molecule Magnet: Bis(Phthalocyaninato)holmium Anion |journal=Journal of the American Chemical Society |date=March 2005 |volume=127 |issue=11 |pages=3650–3651 |doi=10.1021/ja0428661|pmid=15771471 |arxiv=cond-mat/0506582 |bibcode=2005cond.mat..6582I |s2cid=40136392 }} Other systems presenting remanence are nanoengineered Fe chains, deposited on Cu₂N/Cu(100) surfaces, showing either Neel {{Cite journal|last1=Loth|first1=Sebastian|last2=Baumann|first2=Susanne|last3=Lutz|first3=Christopher P.|last4=Eigler|first4=D. M. |author-link4= Don Eigler |last5=Heinrich|first5=Andreas J. |author-link5= Andreas J. Heinrich |date=2012-01-13|title=Bistability in Atomic-Scale Antiferromagnets|journal=Science|language=en|volume=335|issue=6065|pages=196–199|doi=10.1126/science.1214131|issn=0036-8075|pmid=22246771|bibcode=2012Sci...335..196L|s2cid=128108}} or ferromagnetic ground states{{Cite journal|last1=Spinelli|first1=A.|last2=Bryant|first2=B.|last3=Delgado|first3=F.|last4=Fernández-Rossier|first4=J.|last5=Otte|first5=A. F.|date=2014-08-01|title=Imaging of spin waves in atomically designed nanomagnets|journal=Nature Materials|language=en|volume=13|issue=8|pages=782–785|doi=10.1038/nmat4018|pmid=24997736|issn=1476-1122|arxiv=1403.5890|bibcode=2014NatMa..13..782S}} 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,{{cite journal |last1=Gambardella |first1=P. |title=Giant Magnetic Anisotropy of Single Cobalt Atoms and Nanoparticles |journal=Science |date=16 May 2003 |volume=300 |issue=5622 |pages=1130–1133 |doi=10.1126/science.1082857|bibcode = 2003Sci...300.1130G |pmid=12750516|s2cid=5559569 |url=http://infoscience.epfl.ch/record/135806 }} nanoislands,{{cite journal |last1=Hirjibehedin |first1=C. F. |title=Spin Coupling in Engineered Atomic Structures |journal=Science |date=19 May 2006 |volume=312 |issue=5776 |pages=1021–1024 |doi=10.1126/science.1125398|pmid=16574821 |bibcode = 2006Sci...312.1021H |s2cid=24061939 }} and quantum dots{{cite journal |last1=Léger |first1=Y. |last2=Besombes |first2=L. |last3=Fernández-Rossier |first3=J. |last4=Maingault |first4=L. |last5=Mariette |first5=H. |title=Electrical Control of a Single Mn Atom in a Quantum Dot |journal=Physical Review Letters |date=7 September 2006 |volume=97 |issue=10 |doi=10.1103/PhysRevLett.97.107401|bibcode = 2006PhRvL..97j7401L |pmid=17025852 |page=107401|hdl=10045/25252 |url=http://rua.ua.es/dspace/bitstream/10045/25252/1/2006_PhysRevLett.97.107401.pdf |hdl-access=free }}{{cite journal |last1=Kudelski |first1=A. |last2=Lemaître |first2=A. |last3=Miard |first3=A. |last4=Voisin |first4=P. |last5=Graham |first5=T. C. M. |last6=Warburton |first6=R. J. |last7=Krebs |first7=O. |title=Optically Probing the Fine Structure of a Single Mn Atom in an InAs Quantum Dot |journal=Physical Review Letters |date=14 December 2007 |volume=99 |issue=24 |doi=10.1103/PhysRevLett.99.247209|arxiv = 0710.5389 |bibcode = 2007PhRvL..99x7209K |pmid=18233484 |page=247209|s2cid=16664854 }} with a controlled number of magnetic atoms (between 1 and 10).