International research collaboration solves long-standing riddle in magnetism

Electrons in a magnetic material are aligned in specific patterns. Each of the electrons possesses a so-called spin, which resembles an inherent angular momentum. This spin can either point downwards (majority) or upwards (minority) and the imbalance between the two determines the magnetization of the material. If energy is brought into the system (e.g. by heating), the spins of the majority electrons can leave this order and the spin changes its direction.

A change in the spin direction also changes the imbalance and thus directly reduces the magnetization of the material. This describes a phase transition from an ordered magnetic into an unordered phase. Exactly this phase transition is the main focus of the research by the international physics team surrounding OPTIMAS researcher Professor Martin Aeschlimann.

Two contradicting theories existed for the description of the phenomenon: The first theory assumes that some electrons switch their spin direction completely from down to up, while the second theory assumes that the electrons only tilt their spin direction slightly, but by a large number of electrons.

The research team could prove for the first time that the second theory – the so-called Heisenberg picture – yields the main explanation for the phase transition. To study the transitions electrons in a thin layer of cobalt were excited by an intense laser pulse. This provides the energy to perform the phase transition, which happens in a time scale of just a few femtoseconds. A time scale where e.g. light only travels about one thousands of the thickness of hair. Thanks to modern state-of-the-art technology the research team was able to study this phase transition in detail. The studies allow several new conclusions to be drawn for the ultrafast demagnetization and the involved microscopic processes.

Central parts of the research were carried out within the collaborative research center of the German Research Foundation: SFB/TRR 173 Spin+X, as well as the State Research Center OPTIMAS of the University of Kaiserslautern. The participating teams are from the University of Kaiserslautern, the Forschungszentrum Jülich, the University of Göttingen, the University of Dortmund, as well as the University of Colorado and the National Institute of Standards in Boulder (Colorado, USA).

The work of the Kaiserslautern researchers and their colleagues was published in the high ranking journal "Science Advances":

TITLE: Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt

http://advances.sciencemag.org/content/3/3/e1602094.full

Contact person:

Prof. Martin Aeschlimann

Fachbereich Physik

Telefon: 0631-205 2322

E-Mail: ma[at]physik.uni-kl.de