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
[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 Cu2 N/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:
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 ,
[8] nanoislands,
[9] and
quantum dots
[10]
[11] with a controlled number of magnetic atoms (between 1 and 10).
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