Ferromagnetism: principles, materials and applications
Ferromagnetism is a form of magnetic order in which atomic moments align to produce a strong, persistent magnetization. This article describes its origin, properties, common materials, measurement and major applications.
Overview
Ferromagnetism is a common form of magnetic order in which a material can exhibit a spontaneous, and often strong, net magnetic moment. Such materials are familiar as permanent magnets and are attracted to other magnets. Classical examples include iron, cobalt and nickel. In the context of physics, ferromagnetism is one among several types of magnetic behavior and is notable for its strong, long-range alignment of microscopic magnetic moments.
Image gallery
3 Images
Microscopic origin
At the atomic level the relevant carriers of magnetism are electrons, which possess an intrinsic property called spin and an associated magnetic moment. In many solids quantum-mechanical exchange interactions favor parallel alignment of neighboring electronic moments; when a large number of moments align coherently the region acquires a net magnetization. This alignment can persist without an external field and is most robust at low temperatures approaching absolute zero, though it can survive to much higher temperatures in typical ferromagnets.
Domains, anisotropy and hysteresis
Ferromagnetic materials are usually subdivided into microscopic regions called domains, each with a uniform magnetization direction. The overall magnetic behavior of a piece of material depends on the size, shape and arrangement of these domains as well as material-specific magnetic anisotropy, which favors certain directions for magnetization. When an external magnetic field is applied and varied, ferromagnets exhibit hysteresis: the response depends on magnetic history, producing remanence (residual magnetization when the field is removed) and coercivity (the reverse field required to reduce the magnetization to zero). These properties distinguish "soft" magnetic materials, which are easy to magnetize and demagnetize, from "hard" magnetic materials, which retain magnetization and are used for permanent magnets.
Temperature effects and Curie point
Thermal energy tends to disorder the alignment of moments. As temperature increases the collective order weakens until a characteristic temperature, the Curie temperature, is reached and long-range ferromagnetic order disappears. Above this point the material becomes paramagnetic and only shows magnetization in the presence of an external field. The Curie temperature and how magnetization falls with temperature vary between materials and with composition in alloys.
Materials and variations
Pure metals such as iron, cobalt and nickel are canonical ferromagnets, but many alloys, oxides and intermetallic compounds also show ferromagnetic order. Rare-earth elements combined with transition metals produce high-performance permanent magnets widely used in modern technology. Some iron oxides and related minerals exhibit complex magnetic behavior; for example hematite can display weak or temperature-dependent magnetism. Materials can also display closely related orders such as ferrimagnetism or antiferromagnetism when sublattices have unequal or opposing moments.
Measurement and experimental techniques
Laboratory study of ferromagnetism uses a range of methods. Magnetometry measures magnetization as a function of field and temperature and reveals hysteresis loops and Curie points. Techniques such as ferromagnetic resonance, neutron scattering, magnetic force microscopy and electron microscopy probe microscopic domain structure, anisotropy and dynamic responses. These methods are important for both basic science and development of magnetic materials for applications.
Technological applications
Ferromagnetic materials underpin many technologies. They are essential components of electric motors and generators, transformers, inductors and solenoids. Permanent magnets and soft magnetic cores are used in sensors, actuators, loudspeakers and magnetic bearings. Magnetic recording media and the magnetic stripes on cards rely on magnetically encoded particles or layers; examples of everyday objects associated with magnetic materials include television and display systems and credit cards. Modern information storage, electric vehicles, wind turbines and many consumer electronics depend on engineered ferromagnetic materials.
Historical notes and ongoing research
The phenomenon of ferromagnetism has been observed for centuries, but its microscopic explanation emerged with quantum mechanics and the concept of exchange interactions. Ongoing research spans development of stronger and lighter permanent magnets that reduce reliance on critical elements, studies of thin films and nanostructures where reduced dimensions change magnetic behavior, and exploration of magnetism in novel materials. Advances in spintronics exploit electron spin and ferromagnetic layers to create new device functionalities.
Practical considerations and safety
Strong magnets can present hazards: they can attract ferrous objects suddenly and interfere with electronic devices or implanted medical devices. Recycling and disposal of magnetic materials, particularly those containing rare or toxic elements, is an important environmental concern. Material selection balances magnetic performance, cost, mechanical properties and environmental impact.
Further reading
Introductory texts in solid-state physics and materials science provide accessible accounts of ferromagnetism, while specialized reviews cover measurement techniques and applications in industry. For general interest and demonstrations of magnetic phenomena one may consult educational resources in magnetism and physics outreach materials. For technical information, industry standards and datasheets are available through manufacturers and professional organizations.
Related topics include the behavior of individual electrons in solids, the role of spin in electronics, and the limits of magnetic ordering near absolute zero. Materials research continues to refine our understanding and broaden the practical uses of ferromagnets.
Introduction
A material is classified or declared as a ferromagnetic material when, below the Curie temperature, the magnetic moments of the atoms align in parallel in it. This effect is due to the fact that in these materials there is an interaction between the atoms which causes the total energy of the material to be reduced by ordering compared to a disordered configuration.
This tendency of the elementary magnets to align themselves in parallel leads to a spontaneous magnetization of larger areas, the white districts, in which the elementary magnets are mostly aligned in parallel. This distinguishes ferromagnets from paramagnets, where the magnetic moments are normally disordered.
In the absence of external influences, the directions of the magnetic fields of adjacent white districts are anticorrelated. In the Bloch and Neel walls between the districts, the elementary magnets are aligned in such a way that a transition occurs between the two directions of magnetization. In this state, a body made of a ferromagnetic material does not generate an external magnetic field, since the fields of the different Weiss domains compensate each other.
When the material is subjected to an external magnetic field, the Weiss districts, which are magnetized in the opposite direction to the external magnetic field, shrink and eventually fold over. This creates a macroscopic magnetization whose field overlaps with the external one in such a way that the field lines appear to be drawn laterally into the material. In an inhomogeneous field, the material thus magnetized is attracted to sites of greater field strength, be they magnetic north or south poles. Paramagnets behave similarly, but the alignment of the magnetic moments occurs solely due to the external field and not additionally due to the parallelizing influence of the neighboring moments. Therefore, the effect is much weaker.
Ferromagnetic materials are classified according to the behaviour they exhibit when removed from a magnetic field. Generally, a residual magnetism then remains, the so-called remanence.
- In soft magnetic materials the remanence is low, i.e. most of the magnetization is lost immediately when the object is removed from the external magnetic field, especially after alternating fields have been applied.
- Hard magnetic materials are harder to magnetize but retain a greater permanent magnetization. Such materials, e.g. hardened steel, can be magnetized into permanent magnets or exist as permanent magnets from the outset, i.e. permanently assume a clearly recognizable (macroscopic) magnetization.
The remanence magnetization can be eliminated by applying a magnetic counterfield, which occurs when the coercive field strength is reached. In the case of hard magnetic materials, the level of the necessary counter-field is greater than in the case of soft magnetic materials. In the case of permanent magnets, both a high remanence and a high coercivity are desirable.
Ferromagnetism must be distinguished from ferrimagnetism (e.g. in ferrites), which has macroscopically similar properties but is microscopically related to antiferromagnetism. In this case, as in antiferromagnetism, the elementary magnets are alternately directed in opposite directions, but with different strengths in the two directions, which is why - unlike in antiferromagnetism - a magnetization remains for each pair.
Substances with ferromagnetic properties
| Substance |
|
| 1395 | |
| 1033 | |
| 627 | |
| CrO2 | 390 |
| 289 | |
| 219 | |
| 85 | |
| EuO | 70 |
| 20 | |
| 19 |
Among the elements or metals in pure form, iron, nickel and cobalt exhibit ferromagnetic properties at room temperature. In 2018, ruthenium was identified as the fourth element with ferromagnetic properties at room temperature (in the metastable body-centered tetragonal phase). At lower temperatures (see table), the lanthanides gadolinium, terbium, dysprosium, holmium and erbium also become ferromagnetic.
In practice, ferromagnetic alloys such as AlNiCo, SmCo, Nd2Fe14B, Ni80Fe20 ("permalloy"), or NiFeCo alloys ("mumetal") are often used. It is noteworthy that under certain circumstances also some compounds of generally non-ferromagnetic elements exhibit ferromagnetic behaviour, for example chromium dioxide, manganese arsenide, europium(II) oxide or the superfluid A-1 phase of He-3, furthermore the so-called Heusler alloys.
It is also worth noting that the best known ferromagnetic material, iron, is not ferromagnetic as the main component of an austenitic alloy. Austenitic microstructures are a component of many stainless steels and some grades of stainless steel. (Iron crystallizes in the body-centered cubic lattice at room temperature. Austenitic alloys, on the other hand, are predominantly face-centered).
In general, the presence of ferromagnetic properties is dependent on the presence of unpaired electrons in the electron configuration of the ground state of the metal or compound in question, which is essentially unique to transition metals and rare earths.
Ferromagnetism normally only occurs in the solid state because the Curie temperature of these materials is lower than the melting temperature. However, ferromagnetism has also been observed in a supercooled melt. Ferrofluids are suspensions of solid magnetic particles in a non-magnetic liquid.
Questions and answers
Q: What is ferromagnetism?
A: Ferromagnetism is a property of materials such as iron, where they are naturally magnetic and are attracted to other magnets.
Q: What is the strongest type of magnetism?
A: Ferromagnetism is the strongest type of magnetism.
Q: In which technological objects is ferromagnetism used?
A: Ferromagnetism is used in everyday technological objects like credit cards and television screens.
Q: What are ferromagnetic materials?
A: Ferromagnetic materials are materials that have some magnetization on their own, meaning they are still magnetic even without any magnetic field from other objects at a temperature of absolute zero.
Q: What is the reason behind the magnetization of ferromagnetic materials?
A: The electrons in a ferromagnetic material have a spin, and the electrons like to have all the spins going one way, thus giving some magnetism to the material.
Q: What happens to magnetization as temperature is increased?
A: Usually, as the temperature is increased, magnetization goes down because the high temperature causes the electrons in the material to move around more, resulting in their spins not perfectly going in the same way anymore.
Q: What is the Curie temperature?
A: The Curie temperature is the point at which there is no magnetization anymore, and it changes among different materials.
Related articles
Author
AlegsaOnline.com Ferromagnetism: principles, materials and applications Leandro Alegsa
URL: https://en.alegsaonline.com/art/34091