What happens when we heat the atomic lattice of a magnet with terahertz bursts all of a sudden?

https://www.nanowerk.com/nanotechnology-news2/newsid=50687.php

(Nanowerk News) 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. 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.

Fig. 1 (A-C): Heating a magnet without changing its magnetization. (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. (Image: Fritz Haber Institute)

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 (Science Advances, "Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation").

"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 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-M2unchanged, 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. “The 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."

Manipulating ultrafast spin at terahertz frequencies

http://www.rdmag.com/news/2013/04/manipulating-ultrafast-spin-terahertz-frequencies

An ultrafast spin current triggers the emission of Terahertz-Radiation. Graphics: H. D. Wöhrle/Universität GöttingenThe demands for ever increasing speed of information storage and data processing have triggered an intense search for finding the ultimately fast ways to manipulate spins in a magnetic medium. In this context, the use of femtosecond light pulses—the fastest man-made event—with photon energies ranging from X-rays (as used for instance at the HZB femto-slicing facility) to THz spectral range proved to be an indispensable tool in ultrafast spin and magnetization dynamics studies.

In a paper in Nature Nanotechnology, HZB-scientist Ilie Radu and his colleagues from Fritz-Haber-Institut Berlin, Uppsala, Göttingen and Forschungzentrum Jülich demonstrate a simple but very powerful way of manipulating the spins at unprecedented speeds within the so far unexplored THz range (1 THz=1012 Hz). They use a femtosecond laser pulse to photo-excite the spins from a magnetic material to a non-magnetic one that is chosen to either trap or release the electrons carrying the spins. By this method they are able to generate ultrashort spin currents with tailor-made shapes and durations, which are detected using an ‘ultrafast amperemeter’ (based on the Inverse Spin Hall Effect) that converts the spin flow into a terahertz electromagnetic pulse.

These findings will possibly allow us to develop and design novel material with tailor-made characteristics, which might boost the magnetic recording rates of the magnetic bits to unprecedented speeds at THz frequencies.  

Terahertz spin current pulses controlled by magnetic heterostructures

Source: Helmholtz Association

Computer data storage at Terahertz speeds more feasible with observation of magnetic reversal

Top, centre: While the magnetization of gadolinium (red arrow) has not yet changed, the magnetization of iron (blue arrow) has already reversed. Large picture: The laser pulse (pink) triggers magnetic reversal, while the X-ray pulse (blue) measures it. (Credit: HZB/Radu)

MY NOTE:  THIS IS ONE OF THOSE VERY INTERESTING PIECES OF INFORMATION, WHILE NOT DIRECTLY RELATED TO CURRENT TERAHERTZ SCANNING OR ASTRONOMY, IS INCLUDED HERE AS IT APPEARS IT WOULD BE OF INTEREST TO MANY READERS.

http://www.sciencedaily.com/releases/2011/04/110413101910.htm
ScienceDaily (Apr. 13, 2011) — A newly discovered magnetic phenomenon could accelerate data storage by several orders of magnitude.

With a constantly growing flood of information, we are being inundated with increasing quantities of data, which we in turn want to process faster than ever. Oddly, the physical limit to the recording speed of magnetic storage media has remained largely unresearched. In experiments performed on the particle accelerator BESSY II of Helmholtz-Zentrum Berlin, Dutch researchers have now achieved ultrafast magnetic reversal and discovered a surprising phenomenon.
In magnetic memory, data is encoded by reversing the magnetization of tiny points. Such memory works using the so-called magnetic moments of atoms, which can be in either "parallel" or "antiparallel" alignment in the storage medium to represent to "0" and "1."
The alignment is determined by a quantum mechanical effect called "exchange interaction." This is the strongest and therefore the fastest "force" in magnetism. It takes less than a hundred femtoseconds to restore magnetic order if it has been disturbed. One femtosecond is a millionth of a billionth of a second. Ilie Radu and his colleagues have now studied the hitherto unknown behaviour of magnetic alignment before the exchange interaction kicks in. Together with researchers from Berlin and York, they have published their results in Nature.
For their experiment, the researchers needed an ultra-short laser pulse to heat the material and thus induce magnetic reversal. They also needed an equally short X-ray pulse to observe how the magnetization changed. This unique combination of a femtosecond laser and circular polarized, femtosecond X-ray light is available in one place in the world: at the synchrotron radiation source BESSY II in Berlin, Germany.
In their experiment, the scientists studied an alloy of gadolinium, iron and cobalt (GdFeCo), in which the magnetic moments naturally align antiparallel. They fired a laser pulse lasting 60 femtoseconds at the GdFeCo and observed the reversal using the circular-polarized X-ray light, which also allowed them to distinguish the individual elements. What they observed came as a complete surprise: The Fe atoms already reversed their magnetization after 300 femtoseconds while the Gd atoms required five times as long to do so. That means the atoms were all briefly in parallel alignment, making the material strongly magnetized. "This is as strange as finding the north pole of a magnet reversing slower than the south pole," says Ilie Radu.
With their observation, the researchers have not only proven that magnetic reversal can take place in femtosecond timeframes, they have also derived a concrete technical application from it: "Translated to magnetic data storage, this would signify a read/write rate in the terahertz range. That would be around 1000 times faster than present-day commercial computers," says Radu.