Nanomagnet

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In

nanoscopic scale system that presents spontaneous magnetic order (magnetization) at zero applied magnetic field (remanence
).

The small size of nanomagnets prevents the formation of

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. S2CID 39584741
    .
  2. S2CID 41734831
    .
  3. ISBN 0-19-856753-7
    .
  4. S2CID 30268016
    .
  5. S2CID 40136392
    .
  6. S2CID 128108
    .
  7. PMID 24997736
    .
  8. S2CID 5559569
    .
  9. S2CID 24061939
    .
  10. PMID 17025852
    .
  11. S2CID 16664854
    .

Further reading


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