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

Monday, 16th July 2018Publication highlights

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 M1M2.

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 (M1M2) 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.

A team of scientists from Berlin (Collaborative Research Center/Transregio 227, 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 at the Fritz Haber Institute of the Max Planck Society. 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 M1M2 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 M1M2. 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.”

Press contacts:
Prof. Tobias Kampfrath, tobias.kampfrath@fu-berlin.de, +49 30 8413–5222; FHI PC Department Office: +49 30 8413–5112
Dr. Ilie Radu, radu@mbi-berlin.de, +49 30 6392 1357; Max Born Institute Berlin, Germany

Original Publication:
S. F. Maehrlein, I. Radu, P. Maldonado, A. Paarmann, M. Gensch, A. M. Kalashnikova, R. V. Pisarev, M. Wolf, P. M. Oppeneer, J. Barker, T. Kampfrath, Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation. Sci. Adv. 4, eaar5164 (2018).



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.

John B. Fenn Award for a Distinguished Contribution in Mass Spectrometry for Gert von Helden

Wednesday, 2nd May 2018Miscellaneous
Gert von Helden receives, together with Martin Jarrold and David Clemmer (both Indiana University, USA), the 2018 “John B. Fenn Award for a Distinguished Contribution in Mass Spectrometry” by the American Society for Mass Spectrometry (ASMS). The Award recognizes a focused or singular achievement in fundamental or applied mass spectrometry and is given for pioneering contributions to the development of ion mobility spectrometry (IMS).
See:  https://www.asms.org/about-asms-awards/distinguished-contribution


A comprehensive volume on chemical warfare entitled “One Hundred Years of Chemical Warfare: Research, Deployment, Consequences” has been published under the auspices of the Max Planck Society

Wednesday, 6th December 2017Publication highlights, Miscellaneous

ProductFlyer-9783319516639On April 22, 1915, the German military released 150 tons of chlorine gas at Ypres, Belgium. Carried by a long-awaited wind, the chlorine cloud passed within a few minutes through the British and French trenches, leaving behind at least 1,000 dead and 4,000 injured. This chemical attack, which amounted to the first use of a weapon of mass destruction, marks a turning point in world history. The preparation as well as the execution of the gas attack was orchestrated by Fritz Haber, the director of the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry in Berlin-Dahlem. During World War I, Haber transformed his research institute into a center for the development of chemical weapons (and of the means of protection against them).

Bretislav Friedrich and Martin Wolf (Fritz Haber Institute of the Max Planck Society, the successor institution of Haber’s institute) together with Dieter Hoffmann, Jürgen Renn, and Florian Schmaltz (Max Planck Institute for the History of Science) organized an international symposium to commemorate the centenary of the infamous chemical attack. The symposium examined crucial facets of chemical warfare from the first research on and deployment of chemical weapons in WWI to the development and use of chemical warfare during the century hence. The focus was on scientific, ethical, legal, and political issues of chemical weapons research and deployment — including the issue of dual use — as well as the ongoing effort to control the possession of chemical weapons and to ultimately achieve their elimination.

The volume consists of papers presented at the symposium and supplemented by additional articles that together cover key aspects of chemical warfare from 22 April 1915 until the summer of 2015.

The book was presented at a symposium on November 30, 2017 to the delegates of the 22nd Conference of State Parties of the Chemical Weapons Convention at the Organization for the Prohibition of Chemical Weapons in The Hague. Introduced by Paul Walker (Green Cross) and presented and moderated by Bretislav Friedrich (FHI), the symposium entitled “One Hundred Years since Ypres and Counting: Glimpses of the Past and the Present” explained the involvement of the Max Planck Society and provided a sampling of the book’s chapters by Edward Spiers (University of Leeds), Ulf Schmidt (University of Kent), Karin Mlodoch (Haukari), and Ralf Trapp (Chessenaz). Among the attendees were four survivors of the 1988 Halabja chemical attack.

Website: http://www.springer.com/de/book/9783319516639
eBook available at https://link.springer.com/book/10.1007/978-3-319-51664-6

This year’s ENI Award goes to Professor Robert Schlögl

Tuesday, 10th October 2017Preise und Auszeichnungen

Professor Schlögl received this year’s ENI Award, also known as “Nobel Prize for Energy”, in the Energy Transition category. We congratulate him on this prestigious distinction. Read more here:

The award and the awardees 2017: https://www.eni.com/en_IT/innovation/eni-award.page

Robert Schlögl: https://www.eni.com/en_IT/innovation/eni-award/2017-schlogl-energy-transition.page

ENI Press Release: https://www.eni.com/en_IT/media/2017/10/10th-eni-award-2017-prizes-awarded-for-scientific-research-in-the-field-of-the-energy-and-the-environment

ENI_Robert Schloegl

Source: ENI

(Deutsch) Azubipreis für Robert Hippmann Pena

Tuesday, 5th September 2017Miscellaneous

Sorry, this entry is only available in German.

Gert von Helden appointed as professor at Radboud University Nijmegen

Wednesday, 28th June 2017Preise und Auszeichnungen

The Radboud University has launched a press release concerning the appointment of Gert von Helden as professor of IR spectroscopy of biomacromolecules. Read the full press release.

(Deutsch) Professor Robert Schlögl erhält Ehrentitel der TU München

Thursday, 13th April 2017Preise und Auszeichnungen

Sorry, this entry is only available in German.

(Deutsch) Maschinelles Lernen erobert die klassischen Naturwissenschaften

Wednesday, 25th January 2017Publication highlights

Sorry, this entry is only available in German.

Gerard Meijer decorated as a Knight of the Order of the Netherlands Lion

Monday, 16th January 2017Preise und Auszeichnungen

GM_Radoboud_Farewell2For his exceptional achievements as President of the Executive Board of Radboud University, Gerard Meijer was knighted on the occasion of his farewell ceremony.

The university states in a press release:

“Under the presidency of Gerard Meijer, Radboud University has celebrated many successes. Researchers at the University received no less than five Spinoza Prizes—the highest scientific distinction in the Netherlands. Three Gravitation Programmes with Nijmegen consortium leadership were awarded. With the Radboud Excellence Initiative, an idea of Gerard Meijer, international scientific talent from all over the world was attracted to Nijmegen.

Throughout his tenure, the University was annually named the Best Comprehensive University in the Netherlands, most recently receiving the honour for the sixth consecutive year. The campus was made even more beautiful with the new Faculty of Law building and the purchase of the Berchmanianum. He achieved success for the joint Dutch Universities in his dealings with publishers in the field of open access, which enabled free access to scientific articles for everyone.”

Read the full press release here.