Nanomagnet
Encyclopedia
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 of ferromagnetic metals (iron
, cobalt
, and nickel
) and single-molecule magnets. The vast majority of nanomagnets feature transition metal (titanium
, vanadium
, chromium
, manganese
, iron, cobalt or nickel) or rare earth (Gd, Eu, Er) 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. 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:
Conditions (1) and (2), but not (3), have been demonstrated in a number of nanostructures, such as nanoparticles, nanoislands, and quantum dots 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 .
Magnetization
In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material...
) at zero applied magnetic field (remanence
Remanence
Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
).
The small size of nanomagnets prevents the formation of magnetic domains
Magnetic domains
A magnetic domain describes a region within a magnetic material which has uniform magnetization. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction...
(see single domain (magnetic)
Single domain (magnetic)
Single domain, in magnetism, refers to the state of a ferromagnet in which the magnetization does not vary across the magnet. A magnetic particle that stays in a single domain state for all magnetic fields is called a single domain particle . Such particles are very small...
). 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
Superparamagnetism
Superparamagnetism is a form of magnetism, which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time between two flips is called the Néel relaxation time...
) which present a limit for the use of nanomagnets for permanent information storage.
Canonical examples of nanomagnets are grains of ferromagnetic metals (iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
) and single-molecule magnets. The vast majority of nanomagnets feature transition metal (titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
, chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
, iron, cobalt or nickel) or rare earth (Gd, Eu, Er) magnetic atoms.
Attending to the number of magnetic atoms, the smallest nanomagnets reported so far are double decker phthalocyanes
Phthalocyanine
Phthalocyanine is an intensely blue-green coloured macrocyclic compound that is widely used in dyeing. Phthalocyanines form coordination complexes with most elements of the periodic table...
with only one rare earth atom. 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, nanoislands, and quantum dots 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 .