Le phénomène mis en lumière par les chercheurs est contre-intuitif : l'augmentation de la température devrait mener à plus d'agitation thermique, et non l'inverse. © rost9, Adobe Stock

Physical curiosity: Magnetic spins that freeze with increasing temperature

The disorder traditionally increases with temperature: the latter is also only an indicator of the so-called thermal movement of the particles. The more we heat, the more we giveenergyand the energy changes to, among other things movement. But in the case observed by the researchers and described in a study published in natural physics, It is the opposite. The rise in temperature has brought some kind of order under the atoms that make up the tested material. Or more precisely, they are turns magnetic that are frozen, i.e. oriented in the same direction.

Spin Glasses’ frustrated interactions

Whatever the material we are studying in detail, it is made up of atoms on a small scale, more or less aligned with each other depending on the type of solid. Every atom or particle that makes up this material has a magnetic spin, an intrinsic property also known as “momentum of”. magnetic spin “. It characterizes the ability of the particle to align with the surrounding magnetic field. The interaction of materials with the Maxwell showed that electromagnetic waves, including light, allow electric and magnetic fields to propagate at finite speeds. The existence of an absolute medium called Ether necessary for this propagation has been refuted by the theory of relativity.” data-url=”https://news.google.com/sciences/definitions/matiere-champ-electromagnetic- 3879/” data-more=”read definition”>electromagnetic field varies according to the atoms that make it up and their organization. So-called ferroelectric materials, for example, are insulators and therefore do not conduct Electrical power. On the other hand, if they are The electric field is a concept that dates back to the work of Coulomb (1785), it is the force field associated with a unit electric charge. It is derived from the electrical potential.
The…” data-url=”https://news.google.com/sciences/definitions/matiere-champ-electric-3880/” data-more=”Read more”>electric fieldshe dipole moments Electrostatics of the atoms that make it up align with this field. Likewise, ferromagnetic materials are made up of elements…” data-url=”https://news.google.com/sciences/definitions/physics-ferromagnetic-2080/” data-more=” Read more”>ferromagnetic can become magnetized under the action of an external magnetic field, so adjust its unit: the ampere per square meter (A/m2).” data-url=”https://news.google.com/sciences/definitions/matter-magnetic -moment-3884/” data-more=”Read the definition”>magnetic moments with the field, and then keep part of it. It arises from the existence of a density of microscopic magnetic moments carried by the electrons in matter (ampere currents…” data-url=”https:/ /news.google.com/ sciences/definitions/matter magnetization-3882/ ” data-more=”Read more”>magnetization.

That glasses of the spin, materials discussed in this study are not glasses in the traditional sense of the term. They correspond to metal alloys, mixtures of metals to which some magnetic impurities are added: elements with a different magnetic behavior than the others. So with several magnetisms different within the same material, the spin magnetic moment is randomly oriented. Easier, ” they are often referred to as A magnet has a north pole and a south pole. Like poles repel, different poles attract. So magnets are dipoles…” data-image=”https://cdn.futura-sciences.com/buildsv6/images/midioriginal/5/a/1/5a1b5e0814_50034606_champ-magnetic-aimant-droit.jpg” data -url =”https://news.google.com/sciences/definitions/magnet-matter-3883/” data-more=”Read more”>magnets chaotic ‘ says the study. those are materials Called “frustrated”: each atom experiences contradictory interactions originating from impurities or elements of the matrix. The spin of the atom cannot stabilize and is content to vary randomly.

A counterintuitive phenomenon

But the material the researchers used is what they call “self-induced spin glass.” It consists only of generalities
Symbol:…” data-image=”https://cdn.futura-sciences.com/buildsv6/images/midioriginal/6/c/1/6c113c6be6_79956_neodyme1.jpg” data-url=”https://news. google.com/sciences/definitions/chimie-neodyme-15194/” data-more=”Read more”>neodymium, a In contact with an acid, a metal gives a salt; in contact with oxygen it gives an oxide (difficult for “noble metals” like gold, …” data-url=”https://news.google.com/sciences/definitions/matiere -metal-3877/ ” data-more =”Read more”>metal belonging Rare earth. Mixed with iron, it behaves like a magnet. But on its own, everything is different, it behaves like a spin glass: the spins form in it like a “swirling spiral”She explains in a press release: Your direction is constantly changing. Questionable, “Competing interactions arising from the structure of the network itself; […] these competing exchange interactions result in a variety of low-energy states,” thus causing frustration in spin orientation. Indeed, in pure neodymium, the researchers found hexagonal and cubic structures within the material, in which the neodymium atoms interact differently. In order to, “At low temperature, the two sublattice magnetizations have different tendencies towards magnetic order; that is, they would prefer two different magnetic orders. It is the competition between these ordering tendencies that effectively leads to the behavior of spin glass.explain the researchers.

In addition to this amazing property, another special feature of this material is its behavior towards temperatures: when it was heated from -268°C to -265°C, the spins didn’t wiggle any more, they froze! The neodymium then transitions from the spin-glass state to that of spin-glass ferromagnetic ! Then, as the temperature dropped, the random distribution returned. “This ‘freezing’ of the pattern does not normally occur with magnetic materials.explains Alexander Khajetoorians, co-author of the study and Professor of Scanning Probe Microscopy at Radboud University. The magnetic behavior of neodymium that we have observed is actually the opposite of what “normal” happens. It’s pretty counterintuitive, like water turning into an ice cube when heated. » This behavior, which is very rare in nature, can be found in the Rochelle salt, for example. There are two Curie temperatures in this material, ie temperatures that limit its permanent polarization. Rochelle salt only becomes ferroelectric between these two temperatures. Outside this interval it is only paraelectric: but the dipole moments are present have no preferred orientation.

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