"Tumbling" spins as a magnetic knife

First measurement of long-lasting magnetic fluctuations in crystals

"Snapshot" of a spin wave in the antiferromagnet MnF2 MPI for solid state research
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One of the best known magnetic materials is magnetised iron. However, although many of its properties are well analyzed, the excited states of this metal have eluded previous studies. Now, with the help of neutrons, scientists have for the first time measured the lifetime of low-energy excitations in a magnetic material. The results now published in "Science" also allow conclusions about the phenomenon of the so-called spin waves in such materials.

The magnetic atoms in ferromagnetic materials such as iron have an internally rotating impulse, the so-called spin, which originates from the electrons. These spins can be thought of as very small bar magnets. In a ferromagnet, all spins point in the same direction. Once such material is exposed to a magnetic field, it acts as a magnetized object. A closely related class of compounds are the antiferromagnets, in which half of the spins point in the opposite direction. Despite the same number of magnetic moments as in the ferromagnet, such materials are consequently not magnetized by a magnetic field. Since their discovery more than 70 years ago, many antiferromagnets have been made. They have found important technological application, for example in computer hard disks.

Scientist under the direction of Prof. Dr. med. Bernhard Keimer, Director at the Max Planck Institute for Solid State Research in Stuttgart, including physicists at the Technical University of Munich and the Hahn-Meitner-Institut Berlin, recently used a neutron technique recently commissioned at the research reactor FRM II in Munich to generate such an antiferrom to investigate. MnF2, the material studied by the researchers, is an antiferromagnet. In this ionic material, every Mn2 + ion has a net spin pointing in the opposite direction to its neighbor spins.

Interaction of spins

Like electrons, neutrons also have a spin. Now if a magnetic material is irradiated with a neutron beam, the spins of the neutrons interact with the spins of the material, similar to how two small bar magnets influence each other. The interacting neutrons are deflected; From the analysis of the deflected neutron beam, the magnetic properties of the investigated material can then be explained. In addition, the neutrons penetrate deeply into a sample because they are poorly absorbed by most materials, so the technique can be used to extract information about physical properties that are not only reflected on the surface of a material,

In many antiferromagnets, including MnF2, the spins of the magnetic atoms interact strongly. In such a case, if you add a small amount of energy to this material system, it will not only be absorbed by a single ion, but split across a large volume of material. Such an excitation is called spin wave, which can also be considered as a coordinated magnetic fluctuation. display

Spin changes as a wave

To visualize this phenomenon, one can imagine that one end of a bar magnet is fixed to one apex of the cone and that the other end begins to circle periodically around the circumference of the cone. If we look at the crystal in a certain direction, a snapshot of the cones would show that each bar magnet rotates more strongly around the cone axis than its predecessor. Over time, this pattern shifts through the crystal, analogous to the propagation of a water wave. The energy spectrum of such spin waves can already be described with great accuracy in many materials today.

A spin wave moves through a solid until it is interrupted, for example, by another spin wave or atomic contamination or crystal defect. As a result of such a collision, the energy and momentum of the spin wave generally change. The spin wave life is the average time that a spin wave makes before it is interrupted. By measuring the spin wave lifetime, one learns more about the type and strength of the interactions that the spin wave experiences.

Over the last 40 years, many spin shaft lifetime theoretical calculations have been made, with particular emphasis on collision with other spin waves. But so far it has not been possible to test these predictions experimentally, since with the previous technique it was not possible to measure the lifetime sufficiently long over a broad range of spin wave impulses. Therefore, the fundamental question of how spin waves interact with each other and whether their interactions with existing physical models are well described remains unanswered.

"Tumbling" loses its lifespan

The Max Planck researchers have now introduced a new, so-called "spin-echo" technique that uses a magnetic field to mark those neutrons that hit the sample. The spins of the incident neutron beam are forced to protrude around the magnetic field in such a way that the degree of their precession depends on the neutron energy. After scattering, the neutrons move through a second magnetic field in the opposite direction, so that the precession of the spins is partially reversed. The remaining net progression then gives the lifetime of a spin wave.

Using this technique, the researchers have now measured the spin wave lifetime in MnF2 at their recently commissioned neutron spectrometer TRISP at the research reactor FRM-II in Garching near Munich. They discovered two unexpected minima in spin wave lifetime. These results represent a major challenge to existing spin-wave theories; The high resolution of the measurements and the broad scope of the data allow extensive comparisons with theoretical predictions. The spin-echo technique has thus impressively demonstrated its great potential for investigating fundamental questions in the physics of magnetic compounds. Next, the researchers want to investigate even more complex magnetic materials.

(MPG, 03.07.2006 - NPO)