Until today we knew about the existence of ferromagnetic materials e antiferromagnetic. The news of epochal importance, released after the publication of an important academic study in Nature, is that there is also a third type of materials. They highlight the behavior underlying thealter magnetism, a discovery that opens the way to boundless horizons, hitherto never explored, but only theorized. Let’s try to understand what a altermagnetic material and because the mid-February 2024 announcement is truly revolutionary.
What is magnetism, in short
Il magnetism it is a physical phenomenon that involves the interaction between materials and magnetic fields. From a physical point of view, magnetism is closely linked to the properties of materials at the atomic and subatomic level.
At the atomic level, atoms are composed of electrical charges, including electrons which orbit around the nucleus. The movement of electrons creates a magnetic field associated with each electron. In non-magnetic materials, the magnetic spins of the electrons are arranged randomly. In magnetic ones, the spins of the electrons are organized in such a way that their magnetic fields add, resulting in macroscopic magnetization. This can happen in various ways, such asalignment of the spins in certain directions or the formation of “magnetic domains”, regions in which the spins are aligned. The spin it is associated with the intrinsic angular momentum of the particles: it is a quantum characteristic that describes their intrinsic orientation.
When a magnetic material is exposed to a external magnetic field, the spins of the electrons can respond by aligning with the field or opposing it. The behavior generates an interaction between the magnetic material and the external magnetic field, which can manifest itself as attraction o repulsiondepending on the relative orientation of the spins.
Electricity and magnetism are closely related, as demonstrated by Maxwell’s equations. A time-varying magnetic field can generate an electric field and vice versa, giving rise to electromagnetic induction and the note postulates Faraday’s laws e di Ampere.
The main difference between ferromagnetic and antiferromagnetic materials lies in the arrangement of the magnetic moments of the atoms or molecules that make up the material.
Ferromagnetism and antiferromagnetism
In ferromagnetic materials, the magnetic spins of atoms or molecules are aligned parallel to each other, generating a magnetic moment netto.A room temperature, these types of material can maintain spontaneous magnetization, even in the absence of an external magnetic field. When subjected to an external magnetic field, atoms or molecules align even more, increasing their magnetization properties.
Conversely, in antiferromagnetic materials, the magnetic spins are arranged to cancel each other out. Pairs of spins aligned in opposite directions cause an orientation of the magnetic moment which is overall zero.
Despite the natural tendency of spins to be antiferromagnetic, a very low temperatures these materials may be susceptible to a phase transition. In these situations the spins orient in a specific direction.
The discovery ofantiferromagnetism it dates back to 1932, the result of the studies of the French physicist Louis Néel.
The great discovery of early 2024: altermagnetism
Until the 20th century, ferromagnetic materials were thought to be the only ones permanent magnetsthat is, capable of maintaining their magnetic properties for long periods without the need for an external magnetic field or the application of an electric current flow.
In 2019, following years of investigations, a group of scientists spoke about the Crystal Hall effect in antiferromagnetic materials. In solid state physics, theCrystalline Hall effect occurs in materials with a specific arrangement of atoms that breaks time-reversal symmetry, leading to anomalous electrical behavior. In particular, in the collinear antiferromagnets, this effect is linked to the presence of two copies of the elementary magnetic moment inside the unit cell. This leads to a complex structure of the Hall constant and unique electrical phenomena.
By analyzing the crystalline structure that causes the effect, the existence of a new type of magnetism called alter magnetism. Until now, however, researchers have remained in the realm of hypotheses.
The behavior of altermagnetic materials, combining ferromagnetism and antiferromagnetism
A research team led by Juraj Krempaskyresearcher at the Paul Scherrer Institute (Switzerland) focused on the electronic structure of manganese telluride crystals, previously considered antiferromagnetic, and was able to confirm the “altermagnetic” behavior.
The Swiss experts observed the behavior of light on manganese telluride, also measuring the energies and speeds of the electrons inside the crystal. The results showed a surprising correspondence with the simulations for altermagnetic materials.
The research team also highlighted that the division of electrons into two groups allows them to move anomalously within the crystal, underlining that this is precisely the main characteristic of altermagnetism.
Richard Evans, of the University of York in the UK, commented that altermagnetic materials not only allow electrons to move more freely compared to antiferromagnetic materials, but combine the characteristic of not requiring an external magnetic field, unlike ferromagnetic materials. By exploiting this property, it would be possible to create new magnetic devices which do not interfere with each other.
The new frontiers that can be explored with altermagnetism
Altermagnetism, as we have seen, is a type of persistent magnetic state. Altermagnetic structures are collinear and compensated by crystalline symmetry, being evident in the absence of net magnetism.
Although it had been theorized previously, the work published in mid-February 2024 on Nature, opens up really interesting scenarios. First of all, altermagnetism could lead to concrete applications in the field of spintronica.
Spintronica
Spintronics is a branch of technological research that exploits the intrinsic property of electrons called spindescribed previously, for transport and manipulate informationin addition to the electric charge used in conventional electronics.
While classical electronics is mainly based on electric charge of electrons for the transmission and processing of information, spintronics also exploits the spin property of electrons. The intrinsic angular momentum of electronics can then be used to represent a binary state (0 or 1) in a similar way to the way electric currents represent the on/off status in classical logic.
Better performing and more capacious hard disks. In the background the first magnetic computer
By leveraging spintronics, it is possible to design and create devices that are more energy efficient (less heat dissipation requirements) as well as more performing and compact. Altermagnetism suggests that the magnetic properties of some materials can be harnessed in innovative ways to control the spin of electrons.
Any concrete examples? Thanks to altermagnetism it may be possible to increase the storage capacity of hard disks. The devices available today are made of ferromagnetic materials. By taking advantage of the fact that altermagnetic materials do not require the application of an external magnetic field, magnetic materials with a higher density than those currently available could be produced. Also because they do not interfere with each other.
Not only. With the advent of altermagnetic materials, we are at this point much closer to the realization of a “magnetic computer” which uses magnetic spin instead of electric current to perform measurements and calculations.