What is ferromagnetism? - Chemistry
Ferromagnetism is the basic method in which a compound forms a permanent magnet or is attracted to a magnetic field. In a nonmagnetic compound, permanent magnetic dipoles tend to line up antiparallel in order to cancel each other out. In ferromagnetism, however, the opposite is true. What is Ferromagnetism? The ability of a substance to become permanently magnetized by exposure to an external magnetic field. Search the Dictionary for More Terms.
The substances which can be permanently magnetised even in the absence ferromabnetism a magnetic field are called ferromagnetic substances, and the mechanism is called ferromagnetism. Ferromagnetism: The substances that are strongly attracted by a magnetic field are called ferromagnetic substances. Ferromagnetic substances can be permanently magnetised even in the absence of a magnetic field.
In solid state, the metal ions of ferromafnetism substances are grouped together into small regions called domains and each domain acts as a tiny magnet. In an un-magnetised piece of a ferromagnetic substance, the domains are randomly-oriented and so, their magnetic moments get cancelled.
However, when the substance is placed in a magnetic field, all the domains get oriented in the direction of the magnetic field.
As a result, a strong magnetic effect is produced. This ordering of domains persists even after the removal of the magnetic field. Thus, the ferromagnetic substance becomes a permanent magnet. Question Papers. Textbook Solutions. Important Ferromagentism Question Bank Solutions Time Tables. What is ferromagnetism? Explain the following with suitable examples: Ferromagnetism. Advertisement Remove all ferromagneetism. Solution 1 Show Solution The substances ferromagnwtism can be permanently magnetised even in the absence of a magnetic field are called ferromagnetic substances, and the mechanism is called ferromagnetism.
Solution 2 Show Solution Ferromagnetism: The substances that are strongly attracted by a magnetic field are called ferromagnetic substances. Schematic alignment of magnetic moments in ferromagnetic substances. Concept: Magnetic Properties. Report Error Is there an error in this question or solution?
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Ferromagnetism is a physical phenomenon (long-range ordering), in which certain materials like iron strongly attract each other. Ferromagnets occur in rare earth materials and gadolinium. It is one of the common phenomena that is encountered in life that is responsible for magnetism in magnets. Ferromagnetism: The substances that are strongly attracted by a magnetic field are called ferromagnetic substances. Ferromagnetic substances can be permanently magnetised even in the absence of a magnetic field. Some examples of ferromagnetic substances are iron, cobalt, nickel, gadolinium, and CrO 2.
Ferromagnetism is the basic mechanism by which certain materials such as iron form permanent magnets , or are attracted to magnets. In physics , several different types of magnetism are distinguished. Ferromagnetism along with the similar effect ferrimagnetism is the strongest type and is responsible for the common phenomenon of magnetism in magnets encountered in everyday life.
An everyday example of ferromagnetism is a refrigerator magnet used to hold notes on a refrigerator door. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today".
Permanent magnets materials that can be magnetized by an external magnetic field and remain magnetized after the external field is removed are either ferromagnetic or ferrimagnetic, as are the materials that are noticeably attracted to them. Only a few substances are ferromagnetic.
The common ones are iron , cobalt , nickel and most of their alloys, and some compounds of rare earth metals. Ferromagnetism is very important in industry and modern technology, and is the basis for many electrical and electromechanical devices such as electromagnets , electric motors , generators , transformers , and magnetic storage such as tape recorders , and hard disks , and nondestructive testing of ferrous materials. Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron , which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which do.
Permanent magnets are made from "hard" ferromagnetic materials such as alnico , and ferrimagnetic materials such as ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces.
The local strength of magnetism in a material is measured by its magnetization. Historically, the term ferromagnetism was used for any material that could exhibit spontaneous magnetization : a net magnetic moment in the absence of an external magnetic field; that is any material that could become a magnet. This general definition is still in common use. One is ferromagnetism in the strict sense, where all the magnetic moments are aligned. The other is ferrimagnetism , where some magnetic moments point in the opposite direction but have a smaller contribution, so there is still a spontaneous magnetization.
In the special case where the opposing moments balance completely, the alignment is known as antiferromagnetism. Therefore antiferromagnets do not have a spontaneous magnetization.
Ferromagnetism is an unusual property that occurs in only a few substances. The common ones are the transition metals iron , nickel , cobalt and their alloys, and alloys of rare earth metals. It is a property not just of the chemical make-up of a material, but of its crystalline structure and microstructure.
There are ferromagnetic metal alloys whose constituents are not themselves ferromagnetic, called Heusler alloys , named after Fritz Heusler. Conversely there are non-magnetic alloys, such as types of stainless steel , composed almost exclusively of ferromagnetic metals. Amorphous non-crystalline ferromagnetic metallic alloys can be made 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 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. The table lists a selection of ferromagnetic and ferrimagnetic compounds, along with the temperature above which they cease to exhibit spontaneous magnetization see Curie temperature. Most ferromagnetic materials are metals, since the conducting electrons are often responsible for mediating the ferromagnetic interactions.
It is therefore a challenge to develop ferromagnetic insulators, especially multiferroic materials, which are both ferromagnetic and ferroelectric. A number of actinide compounds are ferromagnets at room temperature or exhibit ferromagnetism upon cooling. An alternate description of this distortion is to consider the length c along the unique trigonal axis after the distortion has begun and a as the distance in the plane perpendicular to c.
Below the Curie temperature. In , a team of MIT physicists demonstrated that a lithium gas cooled to less than one kelvin can exhibit ferromagnetism.
This demonstration is the first time that ferromagnetism has been demonstrated in a gas. In , a team of University of Minnesota physicists demonstrated that body-centered tetragonal ruthenium exhibits ferromagnetism at room temperature. Recent research has shown evidence that ferromagnetism can be induced in some materials by an electric current or voltage.
In July scientists reported inducing ferromagnetism in the abundant diamagnetic material iron pyrite "fool's gold" by an applied voltage.
The Bohr—Van Leeuwen theorem , discovered in the s, showed that classical physics theories are unable to account for any form of magnetism, including ferromagnetism. Magnetism is now regarded as a purely quantum mechanical effect.
Ferromagnetism arises due to two effects from quantum mechanics: spin and the Pauli exclusion principle. One of the fundamental properties of an electron besides that it carries charge is that it has a magnetic dipole moment , i.
This dipole moment comes from the more fundamental property of the electron that it has quantum mechanical spin. Due to its quantum nature, the spin of the electron can be in one of only two states; with the magnetic field either pointing "up" or "down" for any choice of up and down. The spin of the electrons in atoms is the main source of ferromagnetism, although there is also a contribution from the orbital angular momentum of the electron about the nucleus.
When these magnetic dipoles in a piece of matter are aligned, point in the same direction their individually tiny magnetic fields add together to create a much larger macroscopic field.
However, materials made of atoms with filled electron shells have a total dipole moment of zero: because the electrons all exist in pairs with opposite spin, every electron's magnetic moment is cancelled by the opposite moment of the second electron in the pair. Only atoms with partially filled shells i. Because of Hund's rules , the first few electrons in a shell tend to have the same spin, thereby increasing the total dipole moment. These unpaired 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.
Ferromagnetism involves an additional phenomenon, however: in a few substances the dipoles tend to align spontaneously, giving rise to a spontaneous magnetization , even when there is no applied field.
When two nearby atoms have unpaired electrons, whether the electron spins are parallel or antiparallel affects whether the electrons can share the same orbit as a result of the quantum mechanical effect called the exchange interaction.
This in turn affects the electron location and the Coulomb electrostatic interaction and thus the energy difference between these states. The exchange interaction is related to the Pauli exclusion principle , which says that two electrons with the same spin cannot also be in the same spatial state orbital.
This is a consequence of the spin-statistics theorem and that electrons are fermions. Therefore, under certain conditions, when the orbitals of the unpaired outer valence electrons from adjacent atoms overlap, the distributions of their electric charge in space are farther apart when the electrons have parallel spins than when they have opposite spins.
This reduces the electrostatic energy of the electrons when their spins are parallel compared to their energy when the spins are anti-parallel, so the parallel-spin state is more stable. In simple terms, the electrons, which are attracted to the nuclei, can change their spatial state so that they both are closer to both nuclei by aligning their spins in opposite directions, so the spins of these electrons tend to be antiparallel.
This difference in energy is called the exchange energy. This energy difference can be orders of magnitude larger than the energy differences associated with the magnetic dipole-dipole interaction due to dipole orientation,  which tends to align the dipoles antiparallel.
In certain doped semiconductor oxides RKKY interactions have been shown to bring about periodic longer-range magnetic interactions, a phenomenon of significance in the study of spintronic materials. The materials in which the exchange interaction is much stronger than the competing dipole-dipole interaction are frequently called magnetic materials. For instance, in iron Fe the exchange force is about times stronger than the dipole interaction.
Therefore, below the Curie temperature virtually all of the dipoles in a ferromagnetic material will be aligned. In addition to ferromagnetism, the exchange interaction is also responsible for the other types of spontaneous ordering of atomic magnetic moments occurring in magnetic solids, antiferromagnetism and ferrimagnetism.
There are different exchange interaction mechanisms which create the magnetism in different ferromagnetic, ferrimagnetic, and antiferromagnetic substances. These mechanisms include direct exchange , RKKY exchange , double exchange , and superexchange. Although the exchange interaction keeps spins aligned, it does not align them in a particular direction. Without magnetic anisotropy , the spins in a magnet randomly change direction in response to thermal fluctuations and the magnet is superparamagnetic.
There are several kinds of magnetic anisotropy, the most common of which is magnetocrystalline anisotropy. This is a dependence of the energy on the direction of magnetization relative to the crystallographic lattice.
Another common source of anisotropy, inverse magnetostriction , is induced by internal strains. Single-domain magnets also can have a shape anisotropy due to the magnetostatic effects of the particle shape. As the temperature of a magnet increases, the anisotropy tends to decrease, and there is often a blocking temperature at which a transition to superparamagnetism occurs. The above would seem to suggest that every piece of ferromagnetic material should have a strong magnetic field, since all the spins are aligned, yet iron and other ferromagnets are often found in an "unmagnetized" state.
The reason for this is that a bulk piece of ferromagnetic material is divided into tiny regions called magnetic domains  also known as Weiss domains. Within each domain, the spins are aligned, but if the bulk material is in its lowest energy configuration; i.
Ferromagnetic materials spontaneously divide into magnetic domains because the exchange interaction is a short-range force, so over long distances of many atoms the tendency of the magnetic dipoles to reduce their energy by orienting in opposite directions wins out. If all the dipoles in a piece of ferromagnetic material are aligned parallel, it creates a large magnetic field extending into the space around it.
This contains a lot of magnetostatic energy. The material can reduce this energy by splitting into many domains pointing in different directions, so the magnetic field is confined to small local fields in the material, reducing the volume of the field.
The domains are separated by thin domain walls a number of molecules thick, in which the direction of magnetization of the dipoles rotates smoothly from one domain's direction to the other. Thus, a piece of iron in its lowest energy state "unmagnetized" generally has little or no net magnetic field.
However, the magnetic domains in a material are not fixed in place; they are simply regions where the spins of the electrons have aligned spontaneously due to their magnetic fields, and thus can be altered by an external magnetic field. If a strong enough external magnetic field is applied to the material, the domain walls will move by the process of the spins of the electrons in atoms near the wall in one domain turning under the influence of the external field to face in the same direction as the electrons in the other domain, thus reorienting the domains so more of the dipoles are aligned with the external field.
The domains will remain aligned when the external field is removed, creating a magnetic field of their own extending into the space around the material, thus creating a "permanent" magnet.
The domains do not go back to their original minimum energy configuration when the field is removed because the domain walls tend to become 'pinned' or 'snagged' on defects in the crystal lattice, preserving their parallel orientation. This is shown by the Barkhausen effect : as the magnetizing field is changed, the magnetization changes in thousands of tiny discontinuous jumps as the domain walls suddenly "snap" past defects. This magnetization as a function of the external field is described by a hysteresis curve.
Although this state of aligned domains found in a piece of magnetized ferromagnetic material is not a minimal-energy configuration, it is metastable , and can persist for long periods, as shown by samples of magnetite from the sea floor which have maintained their magnetization for millions of years. Heating and then cooling annealing a magnetized material, subjecting it to vibration by hammering it, or applying a rapidly oscillating magnetic field from a degaussing coil tends to release the domain walls from their pinned state, and the domain boundaries tend to move back to a lower energy configuration with less external magnetic field, thus demagnetizing the material.
Commercial magnets are made of "hard" ferromagnetic or ferrimagnetic materials with very large magnetic anisotropy such as alnico and ferrites , which have a very strong tendency for the magnetization to be pointed along one axis of the crystal, the "easy axis".
During manufacture the materials are subjected to various metallurgical processes in a powerful magnetic field, which aligns the crystal grains so their "easy" axes of magnetization all point in the same direction.
Thus the magnetization, and the resulting magnetic field, is "built in" to the crystal structure of the material, making it very difficult to demagnetize.
As the temperature increases, thermal motion, or entropy , competes with the ferromagnetic tendency for dipoles to align. When the temperature rises beyond a certain point, called the Curie temperature , there is a second-order phase transition and the system can no longer maintain a spontaneous magnetization, so its ability to be magnetized or attracted to a magnet disappears, although it still responds paramagnetically to an external field.
Below that temperature, there is a spontaneous symmetry breaking and magnetic moments become aligned with their neighbors.