Everything about Ferromagnetism totally explained
Ferromagnetism is the "normal" form of
magnetism with which most people are familiar, as exhibited in horseshoe
magnets and
refrigerator magnets. It is responsible for most of the magnetic behavior
encountered in everyday life. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today," according to a classic text on ferromagnetism.
Ferromagnetism is defined as the
phenomenon by which materials, such as
iron, in an external
magnetic field become magnetized and
remain magnetized for a period after the material is no longer in the field.
All
permanent magnets are either ferromagnetic or
ferrimagnetic, as are the
metals that are noticeably attracted to them.
Historically, the term
ferromagnet was used for any material that could exhibit spontaneous magnetization: a net
magnetic moment in the absence of an external magnetic field. This general definition is still in common use. More recently, however, different classes of spontaneous magnetization have been identified when there's more than one magnetic ion per
primitive cell of the material, leading to a stricter definition of "ferromagnetism" that's often used to distinguish it from
ferrimagnetism. In particular, a material is "ferromagnetic" in this narrower sense only if
all of its magnetic ions add a positive contribution to the net magnetization. If some of the magnetic ions
subtract from the net magnetization (if they're partially
anti-aligned), then the material is "ferrimagnetic". If the ions anti-align completely so as to have zero net magnetization, despite the magnetic
ordering, then it's an
antiferromagnet. All of these alignment effects only occur at
temperatures below a certain critical temperature, called the
Curie temperature (for ferromagnets and ferrimagnets) or the
Néel temperature (for antiferromagnets).
Ferromagnetic materials
There are a number of crystalline materials that exhibit ferromagnetism (or ferrimagnetism). The table on the right lists a representative selection of them here, along with their Curie temperatures, the temperature above which they cease to exhibit spontaneous magnetization (see below).
Ferromagnetic metal alloys whose constituents are not themselves ferromagnetic in their pure forms are called
Heusler alloys, named after
Fritz Heusler.
One can also make amorphous (non-crystalline) ferromagnetic metallic alloys by very rapid
quenching (cooling) of a liquid alloy. These have the advantage that their properties are nearly isotropic (not aligned along a crystal axis); this results in low coercivity, low
hysteresis loss, high permeability, and high electrical resistivity. A typical such material is a transition metal-metalloid alloy, made from about 80% transition metal (usually Fe, Co, or Ni) and a metalloid component (
B,
C,
Si,
P, or
Al) that lowers the melting point.
A relatively new class of exceptionally strong ferromagnetic materials are the
rare-earth magnets. They contain lanthanide elements that are known for their ability to carry large magnetic moments in well-localized f-orbitals.
Physical origin
The property of ferromagnetism is due to the direct influence of two effects from
quantum mechanics:
spin and the
Pauli exclusion principle.
The spin of an
electron, combined with its
orbital angular momentum, results in a magnetic
dipole moment and creates a magnetic field. (The classical analogue of quantum-mechanical spin is a spinning ball of charge, but the quantum version has distinct differences, such as the fact that it has discrete up/down states that are not described by a
vector; similarly for "orbital" motion, whose classical analogue is a
current loop.) In many materials (specifically, those with a filled
electron shell), however, the total dipole moment of all the electrons is zero (for example, the spins are in up/down pairs). Only atoms with partially filled shells (for example, unpaired spins) can experience a net magnetic moment in the absence of an external field. A ferromagnetic material has many such electrons, and if they're aligned they create a measurable macroscopic field.
These permanent dipoles (often called simply "spins" even though they also generally include orbital angular momentum) tend to align in parallel to an external magnetic field, an effect called
paramagnetism. (A related but much weaker effect is
diamagnetism, due to the orbital motion
induced by an external field, resulting in a dipole moment
opposite to the applied field.) Ferromagnetism involves an additional phenomenon, however: the dipoles tend to
align spontaneously, without any applied field. This is a purely quantum-mechanical effect.
According to classical
electromagnetism, two nearby magnetic dipoles will tend to align in
opposite directions (which would create an
antiferromagnetic material). In a ferromagnet, however, they tend to align in the
same direction because of the
Pauli principle: two electrons with the same
spin can't also have the same "position", which effectively reduces the energy of their electrostatic interaction compared to electrons with opposite spin. (Mathematically, this is expressed more precisely in terms of the
spin-statistics theorem: because electrons are
fermions with half-integer spin, their
wave functions are
antisymmetric under interchange of particle positions. This can be seen in, for example, the
Hartree-Fock approximation to lead to a reduction in the electrostatic potential energy.) This difference in energy is called the
exchange energy.
At long distances (after many thousands of
ions), the exchange energy advantage is overtaken by the classical tendency of dipoles to anti-align. This is why, in an equilibriated (non-magnetized) ferromagnetic material, the dipoles in the whole material are not aligned. Rather, they organize into
magnetic domains (also known as
Weiss domains) that are aligned (magnetized) at short range, but at long range adjacent domains are anti-aligned. The transition between two domains, where the magnetization flips, is called a
domain wall (for example, a
Bloch/
Néel wall, depending upon whether the magnetization rotates parallel/perpendicular to the domain interface) and is a gradual transition on the atomic scale (covering a distance of about 300 ions for iron).
Thus, an ordinary piece of iron generally has little or no net magnetic moment. However, if it's placed in a strong enough external magnetic field, the domains will re-orient in parallel with that field, and will remain re-oriented when the field is turned off, thus creating a "permanent" magnet. This magnetization as a function of the external field is described by a hysteresis curve. Although this state of aligned domains isn't a minimal-energy configuration, it's extremely stable and has been observed to
persist for millions of years in seafloor
magnetite aligned by the
Earth's magnetic field (whose poles can thereby be seen to flip at long intervals). The net magnetization can be destroyed by heating and then cooling (
annealing) the material without an external field, however.
As the temperature increases, thermal oscillation, or
entropy, competes with the ferromagnetic tendency for dipoles to align. When the temperature rises beyond a certain point, called the
Curie temperature, there's a second-order
phase transition and the system can no longer maintain a spontaneous magnetization, although it still responds paramagnetically to an external field. Below that temperature, there's a spontaneous symmetry breaking and random domains form (in the absence of an external field). The Curie temperature itself is a
critical point, where the
magnetic susceptibility is theoretically infinite and, although there's no net magnetization, domain-like spin correlations fluctuate at all lengthscales.
The study of ferromagnetic phase transitions, especially via the simplified
Ising spin model, had an important impact on the development of statistical physics. There, it was first clearly shown that
mean field theory approaches failed to predict the correct behavior at the critical point (which was found to fall under a
universality class that includes many other systems, such as liquid-gas transitions), and had to be replaced by
renormalization group theory.
Unusual ferromagnetism
It is thought that other similarly-formed materials, such as
isoelectronic compounds of
boron and nitrogen, may also be ferromagnetic.
The alloy
ZnZr2 is also ferromagnetic below 28.5 K.
Sources
- Charles Kittel, Introduction to Solid State Physics (Wiley: New York, 1996).
- Neil W. Ashcroft and N. David Mermin, Solid State Physics (Harcourt: Orlando, 1976).
- John David Jackson, Classical Electrodynamics (Wiley: New York, 1999).
- E. P. Wohlfarth, ed., Ferromagnetic Materials (North-Holland, 1980).
- "Heusler alloy," Encyclopedia Britannica Online, retrieved Jan. 23, 2005.
- F. Heusler, W. Stark, and E. Haupt, Verh. der Phys. Ges. 5, 219 (1903).
- S. Vonsovsky Magnetism of elementary particles (Mir Publishers, Moscow, 1975).
Further Information
Get more info on 'Ferromagnetism'.
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