Ferromagnetism and its causes

Introduction

When some materials (such as iron) generate permanent magnets or are attracted to magnets, ferromagnetism’s primary mechanism, magnetism, can be classified in several ways in physics. Magnetism is caused by ferromagnetism (together with the comparable phenomena of ferrimagnetism), the most potent type of magnetism. However, these weak forces can only be observed in a laboratory using susceptible instruments because of the presence of three additional forms of magnetism: paramagnetism, diamagnetism and antiferromagnetism.

A refrigerator magnet, commonly used to hold sticky notes to the fridge door, is an everyday example of ferromagnetism. In the ancient world, “the quality of magnetism was first visible, and to us today,” describes a magnet’s attraction to the ferromagnetic material.          

Ferromagnetic materials

There are only a small number of compounds that exhibit the unique property of ferromagnetism. Alloys of transition metals such as iron, nickel, cobalt and rare earth metals are the most frequent. It is a property of the material’s crystalline structure and microstructure, not only its chemical composition. Due to Hund’s law of maximal diversity, these materials have many unpaired electrons in their d-block (iron and its cousins) or f-block (the rare earth metals), which results in their ferromagnetism. 

Heusler alloys, named after Fritz Heusler, are ferromagnetic metal alloys with non-ferromagnetic components. Ferroelectric metals, such as those found in stainless steel, are the primary constituents of non-magnetic alloys. The rapid cooling of a liquid alloy can produce amorphous (non-crystalline) ferromagnetic metallic alloys. Some examples include the transition metal-metalloid alloy (often made up of around 80% transition metal and the rest of the metalloid component) that reduces the melting point of a given material. Rare-earth magnets are a relatively new class of solid ferromagnetic materials. Lanthanide elements, which are widely recognised for carrying enormous magnetic moments, are found in these atoms. 

Causes of ferromagnetism

Ferromagnetism is caused by the alignment of permanent dipoles in atoms that result from unpaired electrons in outer shells and the interaction between the dipoles of neighboring atoms. For example, the outermost shell of ferromagnetic materials like iron and cobalt contains electrons, but the inner shell close to the extreme is still empty. As a result, ferroelectric materials have enormous spin magnetic moments due to their electronic structure, leading to atomic solid dipole moments.

Examples of ferromagnetic materials

Metals make up the vast majority of ferromagnetic materials. Ferromagnetic materials include iron, cobalt, nickel and other metals. In addition, ferromagnetic materials include metal alloys and rare earth magnets. The oxidation of iron into an oxide produces magnetite, a ferromagnetic substance. 580°C is the Curie temperature. Initially, it was thought to be a magnetic material. Magnetite possesses the most robust magnetic properties of all the naturally occurring minerals.

Properties of ferromagnetic materials

• Domains of dipole moment exist in the atoms of ferromagnetic materials.
• Ferroelectric materials have dipoles aligned in the same direction as the magnetic field outside the material.
• Magnesium has a vital dipole moment, which points in the magnetic field’s pull direction.
• When a magnetic field is applied, the intensity of magnetisation (M) increases rapidly and linearly (H). As a result, saturation is material-specific.
• The susceptibility to magnetic fields is enormous and very positive. M denotes the intensity of magnetisation, whereas H indicates how strong an applied magnetic field is. This means that the magnetic susceptibility Xm is equal to M/H.
• The material has a very high and positive magnetic flux density. Ferromagnetic materials have a high density of magnetic field lines. 0 is the magnetic permittivity of free space, H is the applied magnetic field’s strength, and M represents the magnetisation’s intensity. This gives us the magnetic flux density B = 0 (H + M).
• The field inside the material is significantly more substantial than the magnetising field. Hence the relative permeability varies linearly with the magnetising field. Many lines of force are drawn into the material as a result.
• The magnetic field has a strong attraction to ferromagnetic materials. As a result, they tend to cluster around the poles on an uneven field.
• A watch glass with a ferromagnetic powder in it will display a depression in the middle of it if it is placed on two pole pieces that are far enough apart. This is because the strongest magnetic fields are found near the poles.
• Ferromagnetic substances lose their magnetic characteristics when they’re liquefied at high temperatures.

Uses of ferromagnetic substances

Ferromagnetic materials have numerous industrial uses, for example, electric motors, generators, transformers, telephones, loudspeakers and credit cards.

Curie temperature

Temperature affects ferromagnetic properties. Ferromagnetic materials become paramagnetic when heated to a high enough temperature. Curie’s temperature is the temperature at which this transition happens. The abbreviation is TC.

Hysteresis

A ferromagnetic substance does not completely demagnetise when the external magnetic field is removed. A magnetic field in the opposite direction must be supplied to return the material to a state of zero magnetisation. Magnetism hysteresis occurs when an external magnetic field is removed from a ferromagnetic material. A loop is formed when the magnetic flux density (B) of material is plotted against an external magnetic field’s intensity (H). The term is hysteresis loop. The magnetic flux density is known as retentivity when the magnetising force is removed. A high coercivity reverse magnetising field is required to demagnetise a material thoroughly.

Domain structure

This large-scale sort of magnetic organisation for ferromagnets, dubbed domain structure, was proposed by French scientist Pierre-Ernest Weiss. There are many domains in a ferromagnetic material in which all of the magnetic moments of atoms and ions are aligned. An external magnetising force will, depending on its strength, spin one after the other of these domains into alignment with the external field and cause aligned domains to increase at the expense of nonaligned ones, resulting in a magnetised item as a whole. 

Saturation is a state in which the entire thing is contained within a single area. It is possible to see the structure of a domain directly. A magnetite-based colloidal solution is applied to the surface of ferromagnets in one method. An optical microscope can easily detect a distinct pattern of particle concentrations when surface poles are present. Polarised light, polarised neutrons, electron beams, and X-rays have all shown domains.

Conclusion 

Several ionically bonded compounds have been ferromagnetic since the 1950s, particularly since 1960. Electrical insulators can be found in some of these compounds, whereas semiconductors are found in others. Halides (compounds of fluorine and other fluorine-containing elements like chlorine and bromine) and chalcogenides (compounds of oxygen and sulphur) are examples of these compounds. Only manganese, chromium (Cr) and europium (Eu) have permanent dipole moments in these materials; the others are diamagnetic. A significant amount of spontaneous magnetisation occurs in the rare-earth metals erbium (Er) and holmium (Ho) when their nonparallel moment arrangements are present at low temperatures. In addition, ferromagnetic ordering can be found in some ionic compounds with the spinel crystal structure. For example, Thulium (Tm) spontaneously magnetises below 32 kelvins (K) due to a distinct design.

We have studied ferromagnetism and ferromagnetic materials. They are of great use in modern industries.