Magnets have fascinated humans for several thousand years and enabled the age of digital data storage. They occur in various flavors. Ferrimagnets form the largest class of magnets and consist of two types of atoms. Similar to a compass needle, each atom exhibits a little magnetic moment, also called spin, which arises from the rotation of the atom's electrons about their own axes. In a ferrimagnet, the magnetic moments point in opposite directions for the two types of atoms (see panel A). Thus, the total magnetization is the sum of all magnetic moments of type 1 (M1), blue arrows) and type 2 (M2), green arrows). Due to the opposite direction, the magnitude of the total magnetization is M1-M2.
When an insulating ferrimagnet is heated, the heat is first deposited in the atomic lattice which causes the atoms to move randomly around their cold positions. Finally, part of the heat also causes random rotation (precession) of the spins around their cold direction. Thus, magnetic order gets lost; the total magnetization (M1-M2) decreases and eventually vanishes if the temperature of the ferrimagnet exceeds a critical temperature, the so-called Curie temperature.
(A) A ferrimagnet consists of two spin sorts of opposite orientation (green and blue arrows). In the experiment, the atomic lattice of the ferrimagnet is heated by an extremely short terahertz laser pulse. This situation is analogous to heating the air (=atomic lattice) inside an oven that contains a pot with water (=spins). (B) Heat is transferred into the spin system and decreases the magnetization of each spin type by exactly the same amount. This process arises because spin is transferred from the blue to the green spin sort. Thus, the magnet is heated without changing its total magnetization! In the pot analogy, heat is transferred from the air outside the pot to the water inside. While the amount of water in the pot has not changed, an overpressure has built up. (C) Finally, the hot spins release their overpressure to the atomic lattice, thereby reducing the total magnetization. In the analogy, water overpressure is released through little leaks in the pot lid.
Credit: Fritz Haber Institute Berlin
Although this process is of fundamental importance, its dynamics are not well understood. Even for the ferrimagnet yttrium iron garnet (YIG), one of the most intensely researched ferrimagnets, it is unknown how long it takes until the heated atomic lattice and the cold magnetic spins reach equilibrium with each other. Previous estimates of this time scale differ from each other by a factor of up to one million.
A team of scientists from Berlin (Collaborative Research Center / Transregio 227 Ultrafast Spin Dynamics, Fritz Haber Institute and Max Born Institute), Dresden (Helmholtz Center), Uppsala (Sweden), St. Petersburg (Russia), and Sendai (Japan) have now revealed the elementary steps of this process. "To instantaneously and exclusively heat up the atomic lattice of a YIG film, we use a very specific and novel kind of stimulus: ultrashort bursts of laser light at terahertz frequencies.
With a subsequently arriving visible laser pulse, we can then step-by-step trace the evolution of the initially cold magnetic spins. Essentially, we record a stop-motion movie of how the magnetization evolves." says Sebastian Maehrlein, who conducted the experiments.
His colleague Ilie Radu from summarizes: "Our observations are striking. We found that sudden heating of the atomic lattice reduces the magnetic order of the ferrimagnet on two distinct time scales: an incredibly fast scale of only 1 ps and a 100,000 times slower scale of 100 ns."
These two time scales can be understood in analogy to water in a closed pot that is put into a hot oven. The hot air of the oven corresponds to the hot atomic lattice whereas the magnetic spins correspond to the water inside the pot (see panel A). Once the atomic lattice is heated by the terahertz laser burst, the enhanced random oscillations of the atoms lead to a transfer of magnetic order from spin type 1 to spin type 2.
Therefore, both the magnetic moments M1 (blue arrows in panel B) and M2 (green arrows) are reduced by exactly the same amount (red arrows). This process evolves on the fast time scale, and the atomic spins are forced to heat up while leaving the total magnetization M1-M2 unchanged, just like water in a closed pot that has to keep its volume.
We know, however, that a heated ferrimagnet not only aims at reducing M1 and M2, but also its total magnetization M1-M2. To do so, part of the spin must be released to the atomic lattice. This situation is again completely analogous to the hot water in a closed pot: the pressure inside the pot increases but is slowly released to the outside through little leaks in the lid (see panel C). This leakage of angular momentum to the atomic lattice is exactly what happens in the ferrimagnet through weak couplings between spins and lattice.
"We now have a clear picture of how the hot atomic lattice and the cold magnetic spins of a ferrimagnetic insulator equilibrate with each other." says Ilie Radu. The international team of researchers discovered that energy transfer proceeds very quickly and leads to a novel state of matter in which the spins are hot but have not yet reduced their total magnetic moment.
This "spin overpressure" is released through much slower processes that permit leakage of angular momentum to the lattice. "Our results are also relevant for applications in data storage." Sebastian Maehrlein adds. â€œThe reason is simple. Whenever we want to switch the value of a bit between 0 to 1 in a magnetic storage medium, angular momentum and energy have to finally be transferred between atomic lattice and spins."
Ilie Radu | EurekAlert!
Creating switchable plasmons in plastics
10.12.2019 | Linköping University
Ultrafast stimulated emission microscopy of single nanocrystals in Science
10.12.2019 | ICFO-The Institute of Photonic Sciences
Graphene, a two-dimensional structure made of carbon, is a material with excellent mechanical, electronic and optical properties. However, it did not seem suitable for magnetic applications. Together with international partners, Empa researchers have now succeeded in synthesizing a unique nanographene predicted in the 1970s, which conclusively demonstrates that carbon in very specific forms has magnetic properties that could permit future spintronic applications. The results have just been published in the renowned journal Nature Nanotechnology.
Depending on the shape and orientation of their edges, graphene nanostructures (also known as nanographenes) can have very different properties – for example,...
Using a clever technique that causes unruly crystals of iron selenide to snap into alignment, Rice University physicists have drawn a detailed map that reveals...
University of Texas and MIT researchers create virtual UAVs that can predict vehicle health, enable autonomous decision-making
In the not too distant future, we can expect to see our skies filled with unmanned aerial vehicles (UAVs) delivering packages, maybe even people, from location...
With ultracold chemistry, researchers get a first look at exactly what happens during a chemical reaction
The coldest chemical reaction in the known universe took place in what appears to be a chaotic mess of lasers. The appearance deceives: Deep within that...
Abnormal scarring is a serious threat resulting in non-healing chronic wounds or fibrosis. Scars form when fibroblasts, a type of cell of connective tissue, reach wounded skin and deposit plugs of extracellular matrix. Until today, the question about the exact anatomical origin of these fibroblasts has not been answered. In order to find potential ways of influencing the scarring process, the team of Dr. Yuval Rinkevich, Group Leader for Regenerative Biology at the Institute of Lung Biology and Disease at Helmholtz Zentrum München, aimed to finally find an answer. As it was already known that all scars derive from a fibroblast lineage expressing the Engrailed-1 gene - a lineage not only present in skin, but also in fascia - the researchers intentionally tried to understand whether or not fascia might be the origin of fibroblasts.
Fibroblasts kit - ready to heal wounds
03.12.2019 | Event News
15.11.2019 | Event News
15.11.2019 | Event News
10.12.2019 | Architecture and Construction
10.12.2019 | Information Technology
10.12.2019 | Life Sciences