International research collaboration identifies mechanism of the ferromagnetic-paramagnetic phase transition in cobalt

Ferromagnetic materials, such as the elements cobalt, nickel and iron, are characterized by a spontaneous magnetization below their phase transition temperature TC. On a microscopic level, the properties of a magnetic material are essentially described by the intrinsic angular momentum of the electrons, the so-called spin. In the simplest picture, all spins are aligned with respect to each other leading to an overall magnetic order of the ferromagnetic material. In reality, however, the spins of the electrons arrange preferentially in two anti-parallel orientations, i.e., their spins are oriented downwards with respect to the magnetization axis (majority electrons) or upwards (minority electrons). The imbalance in the number of majority and minority electrons results in the macroscopic magnetization of the material which is well known from rod magnets or from the needle of a compass. In contrast, the electron spins in the paramagnetic phase are disordered. If energy, e.g. by heating, is transferred to the ferromagnetic system some majority electrons will break out of this order and the orientation of their spin is changed. This modification in the spin orientation now leads to a reduction of the imbalance between electrons with spin up and electrons with spin down. As a consequence, the macroscopic magnetization of the material decreases. This results in a phase transition from the ordered ferromagnetic to the disordered paramagnetic state, which was the focus of the researcher team.

Theoretically, the phase transition from the ferro- to the paramagnetic state can be described by two extreme cases: One theory assumes that the force coupling neighboring spins with each other, the so-called exchange interaction, spontaneously disappears and therefore their spins are no longer aligned (Stoner-picture). The other theory states that the electron spins are precessing around the magnetization axis and thus lead to a reduction of the magnetization (Heisenberg-picture).

The physicists were able to prove for the first time that the second theory provides the explanation for the phase transition in cobalt. Using an ultrafast laser, they excited the electrons in a thin cobalt film, resulting in a phase transition that occurs within a few femtoseconds. In a femtosecond, light travels just one thousandth of the thickness of a hair. Thanks to the state-of-the-art experiment, the team of scientists could see exactly what is happening in this extremely short time span.

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).

To article in „Science Advances“:
http://advances.sciencemag.org/content/3/3/e1602094.full

Contact persons:

Prof. Martin Aeschlimann
Fachbereich Physik
Telefon: 0631-205 2322
E-Mail: ma[at]physik.uni-kl.de

Dr. Steffen Eich
Fachbereich Physik
Telefon: 0631-205 3576
E-Mail: seich[at]physik.uni-kl.de